WO2008114465A1 - Method of forming solder bumps and solder bump-forming assembly - Google Patents
Method of forming solder bumps and solder bump-forming assembly Download PDFInfo
- Publication number
- WO2008114465A1 WO2008114465A1 PCT/JP2007/056531 JP2007056531W WO2008114465A1 WO 2008114465 A1 WO2008114465 A1 WO 2008114465A1 JP 2007056531 W JP2007056531 W JP 2007056531W WO 2008114465 A1 WO2008114465 A1 WO 2008114465A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- solder
- workpiece
- holes
- heat
- bumps
- Prior art date
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Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
- H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
- H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
- H01L21/48—Manufacture or treatment of parts, e.g. containers, prior to assembly of the devices, using processes not provided for in a single one of the subgroups H01L21/06 - H01L21/326
- H01L21/4814—Conductive parts
- H01L21/4846—Leads on or in insulating or insulated substrates, e.g. metallisation
- H01L21/4853—Connection or disconnection of other leads to or from a metallisation, e.g. pins, wires, bumps
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
- H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
- H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
- H05K3/3457—Solder materials or compositions; Methods of application thereof
- H05K3/3485—Applying solder paste, slurry or powder
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/02—Bonding areas; Manufacturing methods related thereto
- H01L2224/04—Structure, shape, material or disposition of the bonding areas prior to the connecting process
- H01L2224/05—Structure, shape, material or disposition of the bonding areas prior to the connecting process of an individual bonding area
- H01L2224/0554—External layer
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/02—Bonding areas; Manufacturing methods related thereto
- H01L2224/04—Structure, shape, material or disposition of the bonding areas prior to the connecting process
- H01L2224/05—Structure, shape, material or disposition of the bonding areas prior to the connecting process of an individual bonding area
- H01L2224/0554—External layer
- H01L2224/0556—Disposition
- H01L2224/0557—Disposition the external layer being disposed on a via connection of the semiconductor or solid-state body
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/02—Bonding areas; Manufacturing methods related thereto
- H01L2224/04—Structure, shape, material or disposition of the bonding areas prior to the connecting process
- H01L2224/05—Structure, shape, material or disposition of the bonding areas prior to the connecting process of an individual bonding area
- H01L2224/0554—External layer
- H01L2224/0556—Disposition
- H01L2224/05571—Disposition the external layer being disposed in a recess of the surface
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/02—Bonding areas; Manufacturing methods related thereto
- H01L2224/04—Structure, shape, material or disposition of the bonding areas prior to the connecting process
- H01L2224/05—Structure, shape, material or disposition of the bonding areas prior to the connecting process of an individual bonding area
- H01L2224/0554—External layer
- H01L2224/05573—Single external layer
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/10—Bump connectors; Manufacturing methods related thereto
- H01L2224/15—Structure, shape, material or disposition of the bump connectors after the connecting process
- H01L2224/16—Structure, shape, material or disposition of the bump connectors after the connecting process of an individual bump connector
- H01L2224/161—Disposition
- H01L2224/16151—Disposition the bump connector connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive
- H01L2224/16221—Disposition the bump connector connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked
- H01L2224/16225—Disposition the bump connector connecting between a semiconductor or solid-state body and an item not being a semiconductor or solid-state body, e.g. chip-to-substrate, chip-to-passive the body and the item being stacked the item being non-metallic, e.g. insulating substrate with or without metallisation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
- H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
- H01L2224/10—Bump connectors; Manufacturing methods related thereto
- H01L2224/15—Structure, shape, material or disposition of the bump connectors after the connecting process
- H01L2224/17—Structure, shape, material or disposition of the bump connectors after the connecting process of a plurality of bump connectors
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
- H01L2924/0001—Technical content checked by a classifier
- H01L2924/00014—Technical content checked by a classifier the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
- H05K2203/03—Metal processing
- H05K2203/0338—Transferring metal or conductive material other than a circuit pattern, e.g. bump, solder, printed component
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
- H05K2203/04—Soldering or other types of metallurgic bonding
- H05K2203/043—Reflowing of solder coated conductors, not during connection of components, e.g. reflowing solder paste
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
- H05K2203/14—Related to the order of processing steps
- H05K2203/1476—Same or similar kind of process performed in phases, e.g. coarse patterning followed by fine patterning
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/22—Secondary treatment of printed circuits
- H05K3/28—Applying non-metallic protective coatings
- H05K3/282—Applying non-metallic protective coatings for inhibiting the corrosion of the circuit, e.g. for preserving the solderability
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
- H05K3/00—Apparatus or processes for manufacturing printed circuits
- H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
- H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
- H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
- H05K3/3489—Composition of fluxes; Methods of application thereof; Other methods of activating the contact surfaces
Definitions
- This invention relates to a method of forming solder bumps on electrical contacts of a workpiece. It also relates to an assembly for use in forming solder bumps by this method.
- a solder bump is a rounded mound of solder which is commonly used to form an electrical connection between components of electronic parts.
- a plurality of solder bumps are formed on the electrical contacts of a first component.
- a second component having a plurality of electrical contacts is then placed opposite the first component, with each of the electrical contacts of the second component contacting one of the solder bumps formed on the first component.
- the two components and the solder bumps are then heated in a furnace to melt the solder bumps and fuse them to the electrical contacts of the second component, resulting in a mechanical and electrical connection between the first and second components through the solder bumps.
- solder bumps are widely used in the manufacture of electrical equipment because of advantages including low material costs, high reliability, efficiency, and compactness.
- Various methods can be used to form solder bumps on components, which will be referred to in general as workpieces.
- One common method forms solder bumps from solder paste, while another common method forms solder bumps from solder balls.
- a solder paste is a mixture of a solder powder and a flux (typically containing a vehicle and an activator in a solvent), the flux giving the mixture a pasty consistency.
- the solder paste is usually transferred to electrical contacts such as electrodes of a workpiece by printing through a screen or mask or by application with a dispenser.
- electrical contacts such as electrodes of a workpiece
- the workpiece is placed into a furnace and heated to a temperature above the melting temperature of the solder powder in the solder paste.
- the solder powder melts, it congeals to form a molten mass of solder, which forms into rounded shapes atop the electrical contacts due to the surface tension of the molten solder.
- the mounds of solder solidify as bumps on the electrical contacts.
- the substances forming the flux usually either vaporize during the heating process or else remain atop the workpiece as residue, which can be cleaned off with a suitable cleaning fluid.
- solder bumps on a workpiece using solder balls flux is applied to the electrical contacts of the workpiece as an adhesive, and then a solder ball is placed atop each contact and held in place atop the contact by the tack of the flux.
- the workpiece and the solder balls are then placed into a furnace and heated to a temperature sufficient for the solder balls to at least partially melt and wet the surface of the contacts.
- the workpiece and the solder balls are then allowed to cool, and upon solidification of the solder balls, they are electrically and mechanically bonded to the contacts to form solder bumps.
- solder bumps from solder paste has the problem that careful control of the viscosity of the solder paste is required to allow precise application of the solder paste to a workpiece. If the viscosity of a solder paste is too high, it is difficult to transfer the paste through the minute holes of a screen or mask to a workpiece, while if the viscosity of the paste is too low, the paste may experience excessive slump after it has been transferred to a workpiece. Slumping may cause the paste to spread between electrical contacts to cause bridging.
- solder bumps from solder balls has problems with respect to the difficulty of positioning minute solder balls on a workpiece.
- the process of placing solder balls on the electrical contacts of a workpiece is typically performed using a device called a vacuum chuck.
- This device includes a plate having a large number of holes formed therein in a pattern corresponding to the pattern of electrical contacts on a workpiece. Each hole can be connected to a suction device. When a negative pressure is applied to the holes by the suction device, each hole can hold a solder ball therein. When the chuck is positioned over a workpiece and the negative pressure is replaced by a positive pressure, the solder balls are discharged from the holes and transferred to the electrical contacts of the workpiece.
- Vacuum chucks have a number of drawbacks, particularly when used with solder balls of very small diameter.
- One drawback is that a vacuum chuck sometimes has difficulty holding solder balls containing minute surface irregularities, so some of the holes in the vacuum chuck may be missing a solder ball, resulting in one or more solder balls not being transferred to a workpiece and causing the workpiece to be defective.
- Another drawback is that a solder ball may become stuck in a hole in a vacuum chuck and not be released when a positive pressure is applied to the hole, or the ball may be discharged from the hole with such force that it does not remain on the workpiece in the desired location.
- Yet another drawback is that a vacuum chuck is expensive to manufacture.
- a solder ball sheet comprises a ball-holding sheet of a heat-resistant material having a large number of holes formed therein in a pattern corresponding to the pattern of electrical contacts on a workpiece on which solder balls are to be mounted.
- a solder ball is placed into each hole and is held in the hole by an adhesive.
- the solder ball sheet and the workpiece are placed against each other with each of the solder balls opposing one of the electrical contacts.
- solder ball sheet and the workpiece are then heated while being pressed against each other to fuse the solder balls to the electrical contacts of the workpiece. After cooling, the ball-holding sheet can be peeled off the workpiece, leaving the solder balls attached to the electrical contacts of the workpiece.
- a solder ball sheet overcomes a number of the disadvantages of a vacuum chuck. All of the holes in the solder ball sheet can reliably hold a solder ball therein even if the solder ball has surface imperfections, and it is nearly impossible for a solder ball to become trapped in a hole in a solder ball sheet during transfer to a workpiece, so the problem experienced with vacuum chucks that solder balls are not transferred to a workpiece can be avoided. In addition, a solder ball sheet can be manufactured very inexpensively.
- solder ball sheets also have a number of drawbacks.
- One drawback is that the adhesive which is used to hold the solder balls inside the holes of the ball- holding sheet may be damaged or suffer degradation during the process of heating the solder balls to transfer them to a workpiece, so it is generally not practical to reuse a solder ball sheet, resulting in a waste of resources.
- Another drawback is that each of the solder balls disposed inside a solder ball sheet is typically of identical diameter, so each of the solder bumps which is formed on a workpiece is also of identical size, and it is not possible to form solder bumps of different sizes on different electrical contacts.
- solder balls are nearly perfectly round, they easily roll around and scatter during the process of inserting solder balls into the holes in a solder ball sheet, and some trouble is required to collect stray solder balls.
- use of a solder ball sheet involves the expense of manufacturing or purchasing solder balls for use in filling the ball-holding sheet.
- WO 2006/043377 discloses a sheet for use in forming solder bumps from a solder powder.
- the sheet comprises a perforated layer, a substrate, and an adhesive layer sandwiched between the perforated layer and the substrate.
- a solder powder is placed into holes in the perforated layer and is held in place in the holes by the adhesive layer to form a single layer of particles of solder powder in each hole.
- the solder powder is then covered with a flux.
- the solder powder in each hole forms into a solder ball which can be transferred to an electrical contact of a workpiece to form a solder bump on the workpiece.
- solder powder rather than solder balls, it is possible to form solder bumps of different sizes using a single sheet by varying the diameters of the holes in the perforated layer and thereby varying the amount of solder powder in different holes.
- solder powder since solder powder is not perfectly round, it does not roll around in the same manner as solder balls, resulting in less scattering of the solder powder when it is being inserted into the holes in the perforated layer.
- solder powder is generally cheaper to manufacture than are solder balls.
- this sheet has the drawback that it cannot easily be reused on account of employing an adhesive layer to maintain solder powder in the sheet. Furthermore, if dirt or other contaminants enter into a hole in the perforated layer and adhere to the adhesive layer prior to insertion of solder powder into the holes, solder powder cannot adhere to the spots occupied by the dirt or other contaminants, resulting in a decrease in the number of particles of solder powder in the holes and a decrease and variation in the size of the solder bumps which are formed.
- the only way to increase the size of bumps formed by melting the powder is to increase the area of the holes and/or increase the particle diameter of the powder.
- increasing the area of the holes necessarily decreases the distance between adjacent holes, increasing the likelihood of molten solder flowing from one hole to another to form bridges or of cracks forming in the sheet between adjacent holes, while increasing the particle diameter increases the variation in size from one bump to another and makes it difficult to form bumps of uniform size.
- the present invention provides a method of forming solder bumps on the electrical contacts of a workpiece which can overcome the drawbacks of existing methods of forming solder bumps.
- the present invention also provides a solder bump-forming assembly for use in carrying out the method.
- a method of forming solder bumps on a workpiece includes placing a heat-resistant sheet opposite a workpiece, the sheet having a plurality of holes therein each containing a solder paste which comprises a solder powder and a flux, heating the solder paste in each hole to at least the liquidus temperature of the solder powder in the paste to form a mass of molten solder adhering to one of the contacts of the workpiece, cooling the molten solder, and removing the heat-resistant sheet from the workpiece.
- a method of fabricating a solder bump-forming assembly includes disposing a solder paste in holes of a heat- resistant sheet. The method may further include drying the surface of the solder paste disposed in the holes to reduce or eliminate surface tackiness.
- a solder bump-forming assembly for use in forming solder bumps on a workpiece by placing it on the workpiece as it is includes a heat-resistant sheet having a plurality of holes formed therein in a pattern matching a pattern of electrical contacts on a workpiece, and a solder paste disposed in each of the holes.
- the exposed surface of the solder paste disposed in the holes in the heat- resistant sheet is preferably dried to reduce the surface tack of the solder paste prior to heating the solder paste to form solder bumps.
- the heat-resistant sheet comprises a backing layer and a perforated layer formed atop the backing layer.
- a photoresist layer having holes formed therein by photolithography is particularly suitable as the perforated layer.
- the solder paste is preferably a lead-free solder paste.
- the bump-forming assembly may further include a protective layer which covers the heat-resistant sheet and melts at a temperature below the liquidus temperature of the solder powder in the solder paste.
- a method of forming solder bumps according to the present invention does not involve printing, so the careful control of viscosity of a solder paste, which is required when printing a solder paste onto a workpiece, is unnecessary. Furthermore, the method employs a solder paste instead of solder balls or loose solder powder, so it does not entail the problems of methods using loose materials which can easily scatter about the workplace.
- a bump-forming assembly according to the present invention does not require an adhesive layer, so it does not experience the above-described problem of cracking of a heat-resistant sheet. As a result, solder bumps can be formed without the occurrence of bridging between adjoining contacts of a workpiece. In addition, since there is no adhesive layer which undergoes thermal degradation during heating to form solder bumps, the heat-resistant sheet of the bump-forming assembly has good reusability.
- the solder paste employed in a bump-forming assembly according to the present invention comprises a solder powder and a flux.
- the solder paste may be a commercially available solder paste which is employed for reflow soldering of electronic parts.
- a typical commercially available lead-free solder paste containing solder powder made of a lead-free solder alloy contains approximately 11 weight % (more than 10 weight % and up to 12 weight %) of flux.
- a solder paste used in the present invention contains a lower proportion of flux and a higher proportion of solder powder than in a typical commercially available solder paste.
- a lead-free solder paste containing 10 weight % or less of flux can be used.
- solder paste used in the present invention does not require printability, a lower proportion of flux (and therefore a higher viscosity of the solder paste) than a solder paste for use by printing can be employed. It was found that when the flux content of the solder paste is reduced, the number and size of voids which occur in solder bumps formed from the paste are significantly decreased. The reasons for this decrease thought to be as follows.
- Voids in solder bumps formed from solder paste are caused partly by gas which evolves when an oxide film on solder powder in the paste or on electrical contacts on which solder bumps are formed is removed by the activity of flux in the solder paste.
- Voids are also caused by the vaporization of solvents contained in the flux when solder paste is heated during reflow. A decrease in the flux content of a solder paste decreases the amount of solvents and therefore leads to a decrease in the size and number of voids which are formed.
- the method by which bumps are formed according to the present invention also results in less formation of voids.
- a solder paste is directly applied to electrical contacts of a workpiece by screen printing according to a conventional method, the entire top surface of each electrical contact is suddenly and simultaneously wet by molten solder during heating of the workpiece.
- the solder paste initially melts and forms into a ball in each hole, and then it wets and is transferred to an opposing electrical contact on the workpiece.
- the electrical contacts of the workpiece are wet by molten solder in a gradual manner, which is thought to contribute to a decrease in the size and number of voids formed in the resulting solder bumps.
- solder paste in which the proportion of flux can be decreased provides another advantage to the present invention. Namely, the height of solder bumps which are formed can be increased by increasing the proportion of solder powder in the solder paste without increasing the particle size of the solder powder or increasing the size of holes..
- a bump-forming assembly according to the present invention can employ inexpensive materials and inexpensive manufacturing techniques, so it is extremely economical.
- the holes in the heat-resistant sheet of the bump-forming assembly may be uniform in size, or their size may vary from one hole to another to enable solder bumps of different size to be formed at the same time on a workpiece.
- solder bumps There are no particular restrictions on the type of workpiece on which solder bumps can be formed using the method of the present invention.
- the method can be used to form solder bumps on a substrate of a surface mounted device, such as a substrate defining the bottom surface of a BGA device, or it can be used to form solder bumps on a semiconductor chip for use inside a surface mounted device or on a semiconductor wafer to be sliced into separate chips.
- Figure 1 is a schematic vertical cross-sectional view of a portion of an embodiment of a solder bump-forming assembly according to the present invention.
- Figure 2 is a schematic plan view of an embodiment of a solder bump-forming assembly having a pattern of holes for a single surface mounted device.
- Figure 3 is a schematic plan view of an embodiment of a solder bump-forming assembly having a plurality of patterns of holes for a plurality of surface mounted devices.
- Figures 4A - 4F are schematic vertical cross-sectional views of an embodiment of a solder bump-forming assembly according to the present invention at different stages of an example of a method of forming solder bumps according to the present invention.
- FIG. 1 is a schematic vertical cross-sectional view of a portion of an embodiment of a solder bump-forming assembly 10.
- this embodiment includes a heat-resistant solder-receiving sheet 11 having a plurality of holes 14 formed therein in a pattern matching the pattern of electrical contacts on a workpiece on which solder bumps are to be formed, and a solder paste 15 disposed in each hole in the solder-receiving sheet 11.
- the assembly 10 may further include a protective layer 16 formed atop the solder-receiving sheet 11 so as to cover the solder paste 15 in the holes 14.
- the solder-receiving sheet 11 is preferably made of a material or materials which are able to maintain their shape without undergoing degradation at the temperature to which the sheet 11 is heated in order to melt the solder powder in the solder paste 15 housed in the holes 14 of the sheet 11 for reflow, since a significant change in the shape or degradation of the sheet 11 during heating may interfere with the ability to form solder bumps on a workpiece and make it difficult to remove the sheet 11 from the workpiece at the completion of bump formation.
- the sheet 11 preferably has sufficient heat resistance that it can be used a plurality of times before being replaced.
- the solder-receiving sheet 11 is preferably made of a material having poor wettability by molten solder so that molten solder will not adhere to the sheet 11 during the process of forming solder bumps.
- the dimensions of the solder-receiving sheet 11 as viewed in plan are at least as large as the dimensions of the region of a workpiece on which solder bumps are to be formed.
- the solder-receiving sheet 11 may have a size such that the pattern of holes 14 corresponds to the pattern of electrical contacts for a single electronic device, or it may have a size such that it contains a plurality of patterns of holes 14, each corresponding to electrical contacts for a separate electronic device.
- Figure 2 is a plan view of a solder-receiving sheet 11 having a single group of holes 14 corresponding to the contacts of a single electronic device
- Figure 3 is a plan view of a solder-receiving sheet 11 containing a plurality of groups 17 of holes 14, each group 17 arranged in a pattern corresponding to a separate electronic device.
- the solder-receiving sheet 11 of Figure 3 can be used to form solder bumps on a plurality of workpieces simultaneously, each workpiece corresponding to a single electronic device, or it can be used to form solder bumps on a single large workpiece, which can then be sliced into smaller workpieces for use in forming individual electronic devices.
- the solder-receiving sheet 11 may comprise a single layer of a single material. However, it is often advantageous if the solder-receiving sheet comprises a plurality of layers, which may be made of different materials so as to take advantage of the different properties of the different materials.
- the solder-receiving sheet 11 comprises a backing layer 12 and a perforated layer 13 which is formed atop the backing layer 12 and in which the holes 14 in the solder- receiving sheet 11 are formed. No adhesive layer is used to form the solder-receiving sheet 11 in this embodiment.
- the backing layer 12 provides strength and stiffness to the solder-receiving sheet 11.
- the backing layer 12 is preferably sufficiently stiff that the solder- containing sheet 11 can be handled without significant bending under its own weight during use so that the positional relationship among the holes 14 in the perforated layer 13 can be maintained constant.
- the backing layer 12 can be made from a variety of materials, including resins, metals, ceramics, paper, and combinations of two or more of these materials.
- suitable metals are stainless steel, aluminum, aluminum alloys, and iron-nickel alloys such as Alloy 42.
- suitable ceramics are alumina and aluminum nitride.
- suitable plastics are polyimide resins and polyetherimide resins.
- the thickness of the backing layer 12 will depend upon the desired stiffness of the solder-receiving sheet. There is no particular restriction on the thickness, but a typical thickness of the backing layer 12 when it is made of a glass-epoxy resin or stainless steel is around 100 - 200 ⁇ m.
- the backing layer 12 will usually be flat so as to provide a flat support surface.
- the perforated layer 13 is made of a material capable of being processed to form holes 14 for receiving solder paste 15 in a desired pattern. There are no particular restrictions on a material used to form the perforated layer 13, but a particularly preferred material is a photoresist, i.e., a resist which is capable of being processed by photolithography.
- a resist is advantageous because it typically has good heat resistance, it can be formed on a variety of surfaces, and it can be easily processed to simultaneously form a large number of holes by standard photolithographic techniques.
- a resist is not limited to any particular form and can be either a dry film resist or a liquid resist, although a dry film resist is typically more convenient from the standpoint of handling.
- the resist is also not restricted to any particular type. For example, it can be an etching resist, a plating resist, a solder resist, or a sand blasting resist.
- the resist preferably has sufficient heat resistance so as not to undergo significant deformation at the melting temperature of the solder powder contained in the solder paste. Specifically, it preferably has a glass transition temperature (abbreviated below as Tg) in a cured state of at least approximately 100° C.
- Tg glass transition temperature
- Some specific examples of dry film photoresists which are suitable for use in the present invention are SR-FZ (Tg of 102° C) manufactured by Hitachi Chemical Co., Ltd. and PDF 300G (Tg of at least 180° C) manufactured by Nippon Steel Chemical Co., Ltd., both of Japan.
- An example of a liquid resist which can be employed is PSR-4000 series (Tg of approximately 120° C) manufactured by Taiyo Ink Manufacturing Co., Ltd. of Japan.
- the resist may also melt and fuse to the surface of the solder or to any solder resist which may be present on the surface of the workpiece on which solder bumps are being formed, making it difficult to remove the solder- receiving sheet after forming solder bumps on the workpiece.
- a resist can be provided atop the backing layer by any method appropriate for the material of which the resist is made.
- the resist is in the form of a dry film photoresist
- an example of a suitable method of applying it to the top surface of the backing layer is lamination under heat and pressure.
- the resist is formed from a liquid photoresist, it can be applied to the top surface of the backing layer using a curtain coater, a spray coater, or by screen printing.
- the holes 14 can be formed by standard photolithographic techniques. These techniques typically include exposure of the resist to light, developing of the exposed resist with a developing solution, and post-treatment such as post-baking. The holes are formed in the resist in a pattern matching the pattern of electrical contacts on a workpiece on which solder bumps are to be formed. Suitable photolithographic techniques are well known to those skilled in the art. It is possible to form holes in a resist layer disposed on a surface other than the backing layer and then laminate the resulting perforated layer atop the backing layer, but it is generally simpler to form the holes after the resist has been applied to or formed on the backing layer.
- the holes 14 can be formed using a variety of conventional methods, depending upon the material of which the perforated layer is made, including drilling, electric spark machining, punching, and laser machining. Similar methods can be used to form holes when the solder-receiving sheet 11 comprises a single layer rather than a plurality of layers.
- the holes 14 in the perforated layer 13 it is possible for the holes 14 in the perforated layer 13 to extend only partway through the thickness of the layer, but it is easier from the standpoint of ease of manufacture as well as uniformity of the holes 14 if they extend all the way through the thickness of the perforated layer 13 to the top surface of the backing layer 12.
- the shape of the holes 14 in the perforated layer 13 is not critical. From a manufacturing standpoint, it is easiest to form holes which are curved (such as circular or elliptical) as viewed in plan and have constant dimensions over their depth, but it is also possible to form holes having a polygonal shape as viewed in plan or having dimensions which vary over their depth. It is not necessary for the shape of a hole 14 as viewed in plan to match the shape of an electrical contact on a workpiece on which a solder bump is to be formed.
- each hole 14 at its upper end as viewed in plan will depend upon the desired size of the bump which is to be formed by the solder paste 15 contained in the hole.
- the area of a hole as viewed in plan does not need to match the area of the contact on which a solder bump is to be formed.
- the holes 14 may all have the same area as each other as viewed in plan, or the area may differ from one hole 14 to another to enable solder bumps of different size to be formed on a workpiece at the same time, as shown in Figure 1.
- the solder paste 15 which is received in the holes 14 in the perforated layer 13 includes a solder powder and a flux which is mixed with the solder powder to form a substantially uniform mixture. When the solder paste is initially applied to the solder- receiving sheet 11 5 it is preferably in a spreadable form to enable it to be easily inserted into the holes 14 in the sheet 11.
- the solder paste 15 which is used in the present invention may be a conventional solder paste used for reflow soldering of surface mounted parts.
- solder paste for use in the present invention does not need to be capable of being applied to a surface by printing or stenciling, its viscosity can vary over a wider range than is permissible with conventional solder pastes, As discussed previously, a solder paste having a reduced flux content compared to a typical commercially available solder paste is advantageous in that the number and size of voids which are formed during reflow of the solder paste can be reduced. For example, when the solder paste 15 is a lead-free solder paste, the proportion of flux in the solder paste may be decreased to 10 weight % or lower.
- the flux may contain the same classes of components as a conventional solder paste, such as resins as vehicles, activators which may be selected from organic acids, amines, and salts thereof, solvents, and surfactants, and each component may be the same as that employed in a conventional flux for use in solder paste.
- a water- soluble flux or a water-insoluble flux can be used. In view of ease of removal of flux residue after bump formation, it is often more convenient to use a water-soluble flux since it is made possible to remove flux residue by washing with water.
- the solvents in the flux of a conventional solder paste requiring printability are usually primarily solvents having a boiling point in the vicinity of 250° C to prevent the solvent from substantially evaporating below the melting point of the solder powder in the solder paste.
- a solder paste used in the present invention does not require printability, it can employ solvents having a significantly lower boiling point (such as a boiling point below 200° C and preferably between 100 and 200° C) than that of solvents used in conventional solder pastes.
- the solvents in the flux of the solder paste can be primarily low boiling point solvents such as propylene glycol monomethyl ether (boiling point of 121° C) or dipropylene glycol monomethyl ether (boiling point of 187° C).
- low boiling point solvents such as propylene glycol monomethyl ether (boiling point of 121° C) or dipropylene glycol monomethyl ether (boiling point of 187° C).
- Use of such lower boiling point solvents can result in a decreased formation of voids in the resulting solder bumps since the solvents are evaporated nearly completely before the solder powder begins to melt during reflow.
- the solder powder in the solder paste can be formed by conventional methods for manufacturing solder powder.
- the particle size of the powder can be the same as that used in conventional solder pastes and can be selected in accordance with the desired size and pitch of the solder bumps to be formed.
- the solder paste 15 can be inserted into the holes 14 in the perforated layer 13 of the solder-receiving sheet 11 by any convenient method.
- a suitable method is to apply a dab of the solder paste to the top surface of the sheet 11 and then to spread the paste over the top surface with a squeegee.
- a roller can be used to spread the solder paste 15 over the surface of the sheet 11 and force the solder paste into the holes 14, and then a squeegee can be used to scrape off any excess solder paste from the surface of the sheet 11.
- the surface of the solder paste may be tacky, particularly when the solder paste 15 employs a conventional flux.
- the solder paste In conventional reflow soldering with a solder paste, the solder paste generally needs to have surface tack in order to hold components in place on a workpiece prior to reflow.
- the solder paste In the present invention, the solder paste is not used to hold components in place on a workpiece, so it does not require any surface tack. A lack of surface tack is in fact preferable since dust and other contaminants readily adhere to a tacky surface.
- the solder paste 15 is preferably dried to evaporate solvents from at least the surface of the solder paste 15 and reduce or eliminate surface tackiness. Drying can be carried out by drying at room temperature, but to reduce time, drying is preferably carried out by baking in an oven, for example. A preferred baking temperature depends on the boiling points of solvents contained in the flux, but it is usually in the range of 100 - 150° C. The baking time can be selected in accordance with the type and amount of the solvents. Baking is preferably performed long enough to substantially eliminate surface tackiness. As a result of baking, the surface of the solder paste 15 is hardened, but below the surface, the solder paste 15 remains pasty.
- the evaporation of solvents from the solder paste not only reduces surface tack but also reduces the occurrence of voids in the resulting solder bumps.
- the formation of voids during reflow soldering is a result of the generation of gas during reflow, which is related to the amount of solvents present in solder paste.
- the top surface of the sheet 11 is preferably covered with a protective layer 16.
- the protective layer 16 can serve a number of functions. If the protective layer 16 is substantially impervious to air, it can be used to shield the solder paste from the atmosphere and thereby protect the solder paste 15 against oxidation. If the protective layer 16 is substantially impervious to water, it can prevent the solder paste 15 from absorbing water vapor in the air during storage or protect the solder paste against spills of water. The protective layer 16 can also serve to shield the solder paste 15 from dust and other contaminants. In addition, if the protective layer 16 has a fluxing action when heated, it can be used as a source of flux when forming solder bumps on electrical contacts of a workpiece.
- the material of which the protective layer 16 is formed will depend upon the desired functions.
- the protective layer 16 may be in the form of a removable film which can be peeled off the perforated layer 13 prior to the start of bump formation, but preferably it remains attached to the perforated layer 13 at the start of bump formation and melts at a temperature below the melting temperature of the solder powder in the solder paste 15.
- the protective layer 16 is preferably substantially tack-free.
- An example of a suitable material for the protective layer 16 is a resin which has a film-forming capability and which is in the form of a solid film at room temperature but quickly melts to a low viscosity or softens at a temperature below the melting point of solder power in the solder paste 15, preferably at a temperature of at most 70° C such as appximately 60 - 70° C.
- the resin is preferably a water-soluble resin to enable the protective layer 16 or remnants thereof to be easily removed by water from the surface of a workpiece after solder bumps have been formed on the contacts of the workpiece.
- suitable water-soluble resins which can be used are polyvinyl alcohol (PVA) and its derivatives such as adducts of ethylene oxide to PVA, polyester polyols, polyether polyols, polyethylene glycols, polyvinylpyrrolidone, and co-polymers of polyvinylpyrrolidone and polyvinyl acetate.
- PVA polyvinyl alcohol
- the protective layer 16 can be given a fluxing action by including a material having a fluxing action in the protective layer 16.
- suitable materials for this purpose are organic acids, amines, glycerine, and alcohols.
- the protective layer 16 may also include various other substances, such as a surfactant, a defoaming agent, and a leveling agent.
- the protective layer 16 may be formed atop the perforated layer 13 by any convenient method.
- the protective layer is water soluble, the components of the protective layer are first dissolved in water, alcohol, or other aqueous substance to form a solution, which is then applied to the top surface of the perforated layer 13 by spray coating or other convenient coating method and dried to form a film.
- the thickness of the protective layer 16 There are no strict limits on the thickness of the protective layer 16, but it will typically be in the range of 0.3 to 10 ⁇ m. For example, a thickness of around 5 ⁇ m has been found to be suitable from the standpoints of performance and economy.
- the solder-receiving sheet 11 may include a plurality of alignment marks for use in aligning the holes 14 in the perforated layer 13 with the contacts of a workpiece by optical recognition equipment.
- alignment marks may be in the form of round marks formed in the resist constituting the perforated layer 13 in locations spaced outwards from the pattern of holes 14.
- alignment marks may be formed in two diagonally opposed corners of the solder-receiving sheet 11 in positions corresponding to alignment marks on a workpiece.
- Figures 4A - 4F are schematic cross-sectional views of a workpiece and the bump-forming assembly 10 of Figure 1 at different steps of the method.
- Figure 4A is a cross-sectional view of a portion of a typical workpiece 20.
- the workpiece 20 includes a substrate 21, a plurality of electrical contacts 22 formed atop the substrate 21, and a resist layer 23 covering the top surface of the substrate 21 except where the contacts 22 are located.
- a flux 24 Prior to the start of bump formation, at least the contacts 22 of the workpiece 20 may be coated with a flux 24 in order to remove oxides formed on the surface of the contacts 22 and facilitate wetting of the contacts by molten solder.
- the flux 24 may be applied by customary methods, such as spraying, printing, or roll coating.
- a solder bump-forming assembly 10 like that shown in Figure 1 having holes 14 containing solder paste 15 therein and arranged in a pattern matching the pattern of the contacts 22 on the workpiece 20 and having a protective layer 16 is positioned opposite the top surface of the workpiece 20, with each of the holes 14 in the assembly opposing one of the contacts 22 on the workpiece 20.
- the bump-forming assembly 10 and the workpiece 20 are then heated to a temperature sufficient to melt the solder powder in the solder paste 15.
- the heating temperature is at least the liquidus temperature of the solder paste 15 and preferably 10 to 30° C higher than the liquidus temperature.
- the heating temperature is preferably on the order of around 225-250° C.
- the heating time will depend upon the heating temperature and factors such as the thermal capacity of the bump-forming assembly 10 and the workpiece 20.
- the heating atmosphere is not restricted and it may be air or an inert atmosphere such as a nitrogen atmosphere.
- the bump-forming assembly 10 and/or the workpiece 20 often have a tendency to warp if not restrained.
- pressure is preferably applied to the bump-forming assembly 10 and the workpiece 20 during heating in the direction normal to their top and bottom surfaces.
- the magnitude of a suitable pressing force will depend upon the materials of which the bump-forming assembly 10 and the workpiece 20 are made, which determines their tendency to warp when heated. In general, a pressure on the order of 10 N per cm 2 applied to the electrical contacts 22 of the workpiece 20 is suitable.
- Heating while applying a controlled pressure to the bump-forming assembly and a workpiece 20 can be easily carried out using conventional equipment such as a flip chip bonder.
- the orientation of the bump-forming assembly 10 and the workpiece 20 with respect to each other during heating is not critical.
- the primary factors controlling the formation of the solder bumps on the contacts 22 of the workpiece 20 are surface tension and the wettability of the contacts 22 by molten solder, with gravity being less important. Therefore, it is possible to form solder bumps on the workpiece 20 whether the bump-forming assembly 10 is disposed atop or underneath the workpiece 20 during heating.
- the bump-forming assembly 10 is preferably disposed closer to the source of heat than is the workpiece 20.
- the materials forming the protective layer 16 of the bump-forming assembly 10 and the flux layer 24 of the workpiece 20 are selected so as to melt and flow before the temperature of the solder paste 15 reaches the liquidus temperature of the solder powder in the solder paste 15.
- the electrical contacts 22 are made of a material having good wettability by molten solder, so the molten solder in the holes 14 in the perforated layer 13 will wet the contacts 22 and form into rounded masses under surface tension.
- solder-receiving sheet 11 and the workpiece 20 are allowed to cool to solidify the molten masses of solder and form them into solder bumps 25, as shown in Figure 4C.
- the solder-receiving sheet 11 is then removed from the workpiece 20, leaving the solder bumps 25 adhered to the contacts 22 of the workpiece 20, as shown in Figure 4D.
- the backing layer 12 and the perforated layer 13 are both preferably made of materials having poor wettability by solder, so the solder-receiving sheet 11 can be easily removed from the workpiece 20 without adhering to the solder bumps 25.
- a bump-forming assembly 10 is very tolerant of misregistration between the bump-forming assembly 10 and a workpiece 20.
- solder bumps 25 may have a flattened upper surface, as shown in Figure 4D. If it is desired for the solder bumps 25 to have a more rounded shape, they may be further processed by heating to at least the liquidus temperature of the solder forming the solder bumps 25 in a state in which the solder bumps 25 are not restrained from above to allow the solder bumps 25 to form into a rounded shape under surface tension. In this reheating step, the bumps 25 are typically heated to a temperature on the order of 10 - 30° C above the liquidus temperature for at least 30 seconds.
- the reheating is preferably carried out in an inert atmosphere such as a nitrogen atmosphere in order to prevent the wettability of the solder bumps from worsening.
- an inert atmosphere such as a nitrogen atmosphere
- the solder bumps 25 are preferably coated with a typical post-flux 26 prior to being placed into a reflow furnace for reheating.
- a water-soluble post-flux is often more convenient from the standpoint of ease of cleaning. The post-flux 26 melts and flows during reheating.
- solder bumps 25 After the solder bumps 25 have been formed into the desired shape and allowed to cool, the solder bumps 25 and the workpiece 20 can be cleaned by washing with a suitable fluid to remove any residue, such as flux residue.
- Figure 4F illustrates the workpiece 20 and the solder bumps 25 at the completion of the reheating and cleaning. The workpiece 20 can then be subjected to further processing to manufacture an electronic device.
- a method of forming solder bumps according to the present invention and a bump-forming assembly for use in the method will be further described with reference to the following examples.
- a bump-forming assembly like the one illustrated in Figure 1 was prepared in the following manner.
- a stainless steel sheet measuring 30 x 30 mm and having a thickness of 150 ⁇ m was used as a backing layer.
- the backing layer was coated with a solder resist (Taiyo Ink PSR-4000 SC02) using a curtain coater, and the resist layer was dried.
- the resist layer was then exposed to light using a parallel ray exposure apparatus, developed, and then heat treated at 15O 0 C for 30 minutes to form holes in the resist layer and obtain a perforated layer having a thickness of 50 ⁇ m.
- the perforated layer contained 3600 circular holes each having a diameter of 135 ⁇ m and arranged in a 60 x 60 lattice with a pitch of 150 ⁇ m between holes.
- Each of the holes in the perforated layer extended through the thickness of the layer to the top surface of the backing layer to form a solder-receiving sheet.
- solder paste having the composition listed below using a urethane rubber squeegee.
- the solder paste contained 9.7 weight % of a water-soluble flux having the composition listed below and 90.3 weight % of a solder powder of a Sn-3Ag-0.5Cu alloy having a liquidus temperature of 217° C.
- the solder-receiving sheet was baked at 120 0 C for 10 minutes for drying. The resulting solder-receiving sheet had no surface tack.
- solder power Sn-3Ag-0.5Cu
- particle diameter 5 to 15 ⁇ m
- PEG 4000 polyethylene glycol with a molecular weight of approximately 4000
- anonionic surfactant 0.5 wt% of anonionic surfactant
- the top surface of the solder-receiving sheet was then spray coated with the following composition, which was dried to form a protective layer having a thickness of 3 ⁇ m, thereby forming a bump-forming assembly.
- the diglycolic acid in the composition is effective as an activator for flux, so the resulting protective layer had a fluxing action.
- Composition Used to form Protective Layer (parts by weight) 93 parts of PEG 4000;
- the resulting bump-forming assembly was then used to form solder bumps on a workpiece comprising a printed circuit board having 3600 electrical contacts (electrodes) formed on its top surface in the same pattern as the holes in the perforated layer in the bump-forming assembly.
- the top surface of the workpiece was coated with a layer of a water-soluble flux (Sparkle Flux WF2050 sold by Senju Metal Industry Co. Ltd.) using a spray fluxer.
- the bump-forming assembly was positioned atop the workpiece in the manner shown in Figure 4B using a device referred to as a transfer apparatus, which is similar in structure to a commercially available flip chip bonder and which is capable of pressing two members against each other with a controlled temperature and pressure.
- the bump-forming assembly and the workpiece disposed in the transfer apparatus were then pressed against each other for one minute with a force of 30 N while the temperature of the apparatus was set at 250° C to melt the solder powder in the solder paste and form solder bumps on all 3600 contacts of the workpiece.
- the workpiece and the solder-receiving sheet of the bump-forming assembly were then removed from the transfer apparatus, and after cooling, the solder-receiving sheet was removed from atop the workpiece.
- the top surface of the workpiece having the solder bumps which were formed thereon was then coated with a water- soluble flux (Sparkle Flux WF2050), and the workpiece and the solder bumps were then heated in a nitrogen atmosphere in a reflow furnace for 4 minutes at a maximum temperature of 250 0 C to form the solder bumps into a more rounded shape. After removal from the furnace and cooling, the workpiece was washed with water at 50 0 C to remove any residue.
- the resulting solder bumps had an average height of 60 ⁇ m with a standard deviation of 1.5 ⁇ m.
- the occurrence and size of voids in the solder bumps was evaluated by X-ray analysis in which X-ray images of 100 bumps at a time were displayed on a monitor, and the images were visually observed to count the number of bumps in which voids were found and measure the approximate average size of voids in the bumps. This procedure was repeated by selecting 100 bumps at random. As shown in Table 1, the occurrence of voids was low and their size was small.
- solder-receiving sheet used in the above procedure was subjected to ultrasonic washing with hot water at approximately 60 0 C to remove substantially all residue of the solder paste, the protective layer, and the flux which had been applied to the workpiece. From external appearances, there were no major changes in the solder-receiving sheet compared to before it was used.
- the washed solder-receiving sheet was again filled with a solder paste, baked, and coated with a protective layer in the same manner as described above to prepare a bump-forming assembly.
- the assembly was then used to form solder bumps on a workpiece like that used above in the same manner as described above.
- the resulting solder bumps which were formed on the workpiece were of good quality, with an average height of 62 ⁇ m and a standard deviation of 1.6 ⁇ m. When the sheet was used 5 times in this manner, acceptable bumps could be formed each time.
- a bump-forming assembly was prepared in the same manner as in Example 1, but instead of the holes in the perforated layer being filled with a solder paste containing 9.7 weight % of flux, they were filled with a commercially available solder paste (M705A(6)-10-l 1 available from Senju Metal Industry Co., .Ltd.) containing 11 weight % of flux and containing solder powder (Sn-3Ag-O.5Cu) having a particle diameter of 5 - 15 ⁇ m. The solder-receiving sheet was then baked at 100° C for 15 minutes for drying. The surface condition of the solder paste after baking was nearly the same as in Example 1.
- Example 2 a protective layer with a thickness of 3 ⁇ m was formed atop the solder-receiving sheet in the same manner as in Example 1 to obtain a bump-forming assembly. The assembly was then used to form solder bumps atop a workpiece by the same procedure as in Example 1.
- the resulting solder bumps had an average height of 55 ⁇ m with a standard deviation of 2.0 ⁇ m. As shown in Table 1, the occurrence of voids was more frequent and their size was larger than in Example 1.
- solder-receiving sheet of the bump forming assembly had the same structure as the solder-receiving sheet of Example 1, it had good reusability. Specifically, when tested for reusability in the same manner as in Example 1, acceptable bumps could be formed when the sheet was used 5 times.
- This example illustrates a standard method for forming solder bumps from solder paste by a printing technique.
- solder paste was printed on the electrical contacts (electrodes) of a workpiece using a metal squeegee.
- the metal mask had a thickness of 75 ⁇ m and had 3600 through-holes with a diameter of 80 ⁇ m formed therein in a pattern matching the pattern of the electrical contacts of the workpiece. All the contacts had solder paste transferred thereon, indicating that the solder paste had good printability.
- the workpiece was then heated in a nitrogen atmosphere in a reflow furnace for 4 minutes at a maximum temperature of 250 0 C to form solder bumps on the contacts.
- the resulting solder bumps had an average height of 40 ⁇ m with a standard deviation of 3.0 ⁇ m.
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Abstract
A bump-forming assembly for use in forming solder bumps on a workpiece includes a heat-resistant sheet having a plurality of holes formed therein and a solder paste disposed in the holes. The assembly can be used to form solder bumps by placing the assembly opposite a workpiece with each of the holes in the heat-resistant sheet aligned with an electrical contact on the workpiece, and heating the assembly to melt the solder paste and form mounds of molten solder adhering to the contacts. The molten solder is then cooled, and the heat-resistant sheet is removed from the workpiece.
Description
Description
METHOD OF FORMING SOLDER BUMPS AND SOLDER BUMP-FORMING ASSEMBLY
Technical Field This invention relates to a method of forming solder bumps on electrical contacts of a workpiece. It also relates to an assembly for use in forming solder bumps by this method.
Background Art
A solder bump is a rounded mound of solder which is commonly used to form an electrical connection between components of electronic parts. In a typical method of connection using solder bumps, a plurality of solder bumps are formed on the electrical contacts of a first component. A second component having a plurality of electrical contacts is then placed opposite the first component, with each of the electrical contacts of the second component contacting one of the solder bumps formed on the first component. The two components and the solder bumps are then heated in a furnace to melt the solder bumps and fuse them to the electrical contacts of the second component, resulting in a mechanical and electrical connection between the first and second components through the solder bumps.
A common use of solder bumps is to connect surface mounted devices (such as semiconductor devices) to printed circuit boards and to form electrical connections between a semiconductor chip and a substrate of a surface mounted device employing the chip. Solder bumps are widely used in the manufacture of electrical equipment because of advantages including low material costs, high reliability, efficiency, and compactness. Various methods can be used to form solder bumps on components, which will be referred to in general as workpieces. One common method forms solder bumps from solder paste, while another common method forms solder bumps from solder balls.
A solder paste is a mixture of a solder powder and a flux (typically containing a vehicle and an activator in a solvent), the flux giving the mixture a pasty consistency. The solder paste is usually transferred to electrical contacts such as electrodes of a workpiece by printing through a screen or mask or by application with a dispenser. After application of the solder paste to the contacts, the workpiece is placed into a furnace and heated to a temperature above the melting temperature of the solder powder in the solder paste. As the solder powder melts, it congeals to form a molten mass of solder, which forms into rounded shapes atop the electrical contacts due to the surface tension of the molten solder. When the workpiece is then cooled, the mounds of solder solidify as bumps on the electrical contacts. The substances forming the flux usually either vaporize during the heating process or else remain atop the workpiece as residue, which can be cleaned off with a suitable cleaning fluid. In a method of forming solder bumps on a workpiece using solder balls, flux is applied to the electrical contacts of the workpiece as an adhesive, and then a solder ball is placed atop each contact and held in place atop the contact by the tack of the flux. The workpiece and the solder balls are then placed into a furnace and heated to a temperature sufficient for the solder balls to at least partially melt and wet the surface of the contacts. The workpiece and the solder balls are then allowed to cool, and upon solidification of the solder balls, they are electrically and mechanically bonded to the contacts to form solder bumps.
Although both of these methods of forming solder bumps are successfully used in the electronics industry, each method has a number of drawbacks. Forming solder bumps from solder paste has the problem that careful control of the viscosity of the solder paste is required to allow precise application of the solder paste to a workpiece. If the viscosity of a solder paste is too high, it is difficult to transfer the paste through the minute holes of a screen or mask to a workpiece, while if the viscosity of the paste is too low, the paste may experience excessive slump after it has been transferred to a workpiece. Slumping may cause the paste to spread between electrical contacts to cause bridging. It is also difficult to accurately apply a solder paste to a workpiece when the spacing between adjacent electrical contacts is extremely small and when an electrical contact to which solder paste is to be applied has a very small area.
Forming solder bumps from solder balls has problems with respect to the difficulty of positioning minute solder balls on a workpiece. The process of placing solder balls on the electrical contacts of a workpiece is typically performed using a device called a vacuum chuck. This device includes a plate having a large number of holes formed therein in a pattern corresponding to the pattern of electrical contacts on a workpiece. Each hole can be connected to a suction device. When a negative pressure is applied to the holes by the suction device, each hole can hold a solder ball therein. When the chuck is positioned over a workpiece and the negative pressure is replaced by a positive pressure, the solder balls are discharged from the holes and transferred to the electrical contacts of the workpiece.
Vacuum chucks have a number of drawbacks, particularly when used with solder balls of very small diameter. One drawback is that a vacuum chuck sometimes has difficulty holding solder balls containing minute surface irregularities, so some of the holes in the vacuum chuck may be missing a solder ball, resulting in one or more solder balls not being transferred to a workpiece and causing the workpiece to be defective. Another drawback is that a solder ball may become stuck in a hole in a vacuum chuck and not be released when a positive pressure is applied to the hole, or the ball may be discharged from the hole with such force that it does not remain on the workpiece in the desired location. Yet another drawback is that a vacuum chuck is expensive to manufacture.
As a lower-cost alternative to a vacuum chuck for mounting solder balls on a workpiece, in recent years, an arrangement referred to as a solder ball sheet has been developed. A solder ball sheet comprises a ball-holding sheet of a heat-resistant material having a large number of holes formed therein in a pattern corresponding to the pattern of electrical contacts on a workpiece on which solder balls are to be mounted. A solder ball is placed into each hole and is held in the hole by an adhesive. In order to mount the solder balls on the electrical contacts of a workpiece, the solder ball sheet and the workpiece are placed against each other with each of the solder balls opposing one of the electrical contacts. The solder ball sheet and the workpiece are then heated while being pressed against each other to fuse the solder balls to the electrical contacts of the workpiece. After cooling, the ball-holding sheet can be
peeled off the workpiece, leaving the solder balls attached to the electrical contacts of the workpiece. A solder ball sheet overcomes a number of the disadvantages of a vacuum chuck. All of the holes in the solder ball sheet can reliably hold a solder ball therein even if the solder ball has surface imperfections, and it is nearly impossible for a solder ball to become trapped in a hole in a solder ball sheet during transfer to a workpiece, so the problem experienced with vacuum chucks that solder balls are not transferred to a workpiece can be avoided. In addition, a solder ball sheet can be manufactured very inexpensively.
However, solder ball sheets also have a number of drawbacks. One drawback is that the adhesive which is used to hold the solder balls inside the holes of the ball- holding sheet may be damaged or suffer degradation during the process of heating the solder balls to transfer them to a workpiece, so it is generally not practical to reuse a solder ball sheet, resulting in a waste of resources. Another drawback is that each of the solder balls disposed inside a solder ball sheet is typically of identical diameter, so each of the solder bumps which is formed on a workpiece is also of identical size, and it is not possible to form solder bumps of different sizes on different electrical contacts. A further drawback is that since solder balls are nearly perfectly round, they easily roll around and scatter during the process of inserting solder balls into the holes in a solder ball sheet, and some trouble is required to collect stray solder balls. In addition, use of a solder ball sheet involves the expense of manufacturing or purchasing solder balls for use in filling the ball-holding sheet.
WO 2006/043377 discloses a sheet for use in forming solder bumps from a solder powder. The sheet comprises a perforated layer, a substrate, and an adhesive layer sandwiched between the perforated layer and the substrate. A solder powder is placed into holes in the perforated layer and is held in place in the holes by the adhesive layer to form a single layer of particles of solder powder in each hole. The solder powder is then covered with a flux. When the sheet and the solder powder are heated, the solder powder in each hole forms into a solder ball which can be transferred to an electrical contact of a workpiece to form a solder bump on the workpiece. This sheet overcomes a number of the above-described problems of solder ball sheets. Because it uses solder powder rather than solder balls, it is
possible to form solder bumps of different sizes using a single sheet by varying the diameters of the holes in the perforated layer and thereby varying the amount of solder powder in different holes. In addition, since solder powder is not perfectly round, it does not roll around in the same manner as solder balls, resulting in less scattering of the solder powder when it is being inserted into the holes in the perforated layer. Furthermore, solder powder is generally cheaper to manufacture than are solder balls.
However, like the above-described solder ball sheet, this sheet has the drawback that it cannot easily be reused on account of employing an adhesive layer to maintain solder powder in the sheet. Furthermore, if dirt or other contaminants enter into a hole in the perforated layer and adhere to the adhesive layer prior to insertion of solder powder into the holes, solder powder cannot adhere to the spots occupied by the dirt or other contaminants, resulting in a decrease in the number of particles of solder powder in the holes and a decrease and variation in the size of the solder bumps which are formed.
In addition, during heating as part of the process of forming the perforated layer, cracks sometimes form in the perforated layer due to a large difference between the coefficient of thermal expansion of the adhesive layer and that of the perforated layer. When the spacing between adjacent holes in the perforated layer is small, this cracking can cause molten solder in adjacent holes to flow together, resulting in bridging of solder between electrical contacts of a workpiece. Even though solder powder does not roll around, it can be easily scattered due to its small size, so handling of loose solder powder when inserting it into the holes in the sheet can still be messy. Furthermore, since the particles of solder powder are formed into a single layer, the only way to increase the size of bumps formed by melting the powder is to increase the area of the holes and/or increase the particle diameter of the powder. For a given pitch of the holes, increasing the area of the holes necessarily decreases the distance between adjacent holes, increasing the likelihood of molten solder flowing from one hole to another to form bridges or of cracks forming in the sheet between adjacent holes, while increasing the particle diameter increases the variation in size
from one bump to another and makes it difficult to form bumps of uniform size.
Disclosure of Invention
The present invention provides a method of forming solder bumps on the electrical contacts of a workpiece which can overcome the drawbacks of existing methods of forming solder bumps. The present invention also provides a solder bump-forming assembly for use in carrying out the method.
According to one form of the present invention, a method of forming solder bumps on a workpiece includes placing a heat-resistant sheet opposite a workpiece, the sheet having a plurality of holes therein each containing a solder paste which comprises a solder powder and a flux, heating the solder paste in each hole to at least the liquidus temperature of the solder powder in the paste to form a mass of molten solder adhering to one of the contacts of the workpiece, cooling the molten solder, and removing the heat-resistant sheet from the workpiece.
According to another form of the present invention, a method of fabricating a solder bump-forming assembly includes disposing a solder paste in holes of a heat- resistant sheet. The method may further include drying the surface of the solder paste disposed in the holes to reduce or eliminate surface tackiness.
According to yet another form of the present invention, a solder bump-forming assembly for use in forming solder bumps on a workpiece by placing it on the workpiece as it is includes a heat-resistant sheet having a plurality of holes formed therein in a pattern matching a pattern of electrical contacts on a workpiece, and a solder paste disposed in each of the holes.
The exposed surface of the solder paste disposed in the holes in the heat- resistant sheet is preferably dried to reduce the surface tack of the solder paste prior to heating the solder paste to form solder bumps.
In preferred embodiments, the heat-resistant sheet comprises a backing layer and a perforated layer formed atop the backing layer. A photoresist layer having holes formed therein by photolithography is particularly suitable as the perforated layer. The solder paste is preferably a lead-free solder paste. The bump-forming assembly may further include a protective layer which
covers the heat-resistant sheet and melts at a temperature below the liquidus temperature of the solder powder in the solder paste.
A method of forming solder bumps according to the present invention does not involve printing, so the careful control of viscosity of a solder paste, which is required when printing a solder paste onto a workpiece, is unnecessary. Furthermore, the method employs a solder paste instead of solder balls or loose solder powder, so it does not entail the problems of methods using loose materials which can easily scatter about the workplace.
A bump-forming assembly according to the present invention does not require an adhesive layer, so it does not experience the above-described problem of cracking of a heat-resistant sheet. As a result, solder bumps can be formed without the occurrence of bridging between adjoining contacts of a workpiece. In addition, since there is no adhesive layer which undergoes thermal degradation during heating to form solder bumps, the heat-resistant sheet of the bump-forming assembly has good reusability.
The solder paste employed in a bump-forming assembly according to the present invention comprises a solder powder and a flux. The solder paste may be a commercially available solder paste which is employed for reflow soldering of electronic parts. A typical commercially available lead-free solder paste containing solder powder made of a lead-free solder alloy contains approximately 11 weight % (more than 10 weight % and up to 12 weight %) of flux. Preferably, however, a solder paste used in the present invention contains a lower proportion of flux and a higher proportion of solder powder than in a typical commercially available solder paste. For example, a lead-free solder paste containing 10 weight % or less of flux can be used. Since a solder paste used in the present invention does not require printability, a lower proportion of flux (and therefore a higher viscosity of the solder paste) than a solder paste for use by printing can be employed. It was found that when the flux content of the solder paste is reduced, the number and size of voids which occur in solder bumps formed from the paste are significantly decreased. The reasons for this decrease thought to be as follows.
Voids in solder bumps formed from solder paste are caused partly by gas
which evolves when an oxide film on solder powder in the paste or on electrical contacts on which solder bumps are formed is removed by the activity of flux in the solder paste. The greater the amount and activity of the flux in a solder paste, the greater is the tendency for voids to increase in size and number. Voids are also caused by the vaporization of solvents contained in the flux when solder paste is heated during reflow. A decrease in the flux content of a solder paste decreases the amount of solvents and therefore leads to a decrease in the size and number of voids which are formed.
The method by which bumps are formed according to the present invention also results in less formation of voids. When a solder paste is directly applied to electrical contacts of a workpiece by screen printing according to a conventional method, the entire top surface of each electrical contact is suddenly and simultaneously wet by molten solder during heating of the workpiece. In contrast, in accordance with the present invention, when a heat-resistant sheet containing solder paste in its holes is placed opposite a workpiece and heated, the solder paste initially melts and forms into a ball in each hole, and then it wets and is transferred to an opposing electrical contact on the workpiece. As a result, the electrical contacts of the workpiece are wet by molten solder in a gradual manner, which is thought to contribute to a decrease in the size and number of voids formed in the resulting solder bumps.
The use of a solder paste in which the proportion of flux can be decreased provides another advantage to the present invention. Namely, the height of solder bumps which are formed can be increased by increasing the proportion of solder powder in the solder paste without increasing the particle size of the solder powder or increasing the size of holes..
There is no strict lower limit of the flux content of the solder paste, but preferably it is high enough to give the solder paste a viscosity such that all the holes in the heat-resistance sheet can be easily filled with solder paste using a squeegee, for example. A bump-forming assembly according to the present invention can employ inexpensive materials and inexpensive manufacturing techniques, so it is extremely
economical.
The holes in the heat-resistant sheet of the bump-forming assembly may be uniform in size, or their size may vary from one hole to another to enable solder bumps of different size to be formed at the same time on a workpiece. There are no particular restrictions on the type of workpiece on which solder bumps can be formed using the method of the present invention. For example, the method can be used to form solder bumps on a substrate of a surface mounted device, such as a substrate defining the bottom surface of a BGA device, or it can be used to form solder bumps on a semiconductor chip for use inside a surface mounted device or on a semiconductor wafer to be sliced into separate chips.
Brief Description of the Drawings
Figure 1 is a schematic vertical cross-sectional view of a portion of an embodiment of a solder bump-forming assembly according to the present invention.
Figure 2 is a schematic plan view of an embodiment of a solder bump-forming assembly having a pattern of holes for a single surface mounted device.
Figure 3 is a schematic plan view of an embodiment of a solder bump-forming assembly having a plurality of patterns of holes for a plurality of surface mounted devices.
Figures 4A - 4F are schematic vertical cross-sectional views of an embodiment of a solder bump-forming assembly according to the present invention at different stages of an example of a method of forming solder bumps according to the present invention.
Best Mode for Carrying Out the Invention
An embodiment of a method of forming solder bumps and a solder bump- forming assembly according to the present invention will be described while referring to the accompanying drawings.
Figure 1 is a schematic vertical cross-sectional view of a portion of an embodiment of a solder bump-forming assembly 10. As shown in this figure, this embodiment includes a heat-resistant solder-receiving sheet 11 having a plurality of
holes 14 formed therein in a pattern matching the pattern of electrical contacts on a workpiece on which solder bumps are to be formed, and a solder paste 15 disposed in each hole in the solder-receiving sheet 11. The assembly 10 may further include a protective layer 16 formed atop the solder-receiving sheet 11 so as to cover the solder paste 15 in the holes 14.
The solder-receiving sheet 11 is preferably made of a material or materials which are able to maintain their shape without undergoing degradation at the temperature to which the sheet 11 is heated in order to melt the solder powder in the solder paste 15 housed in the holes 14 of the sheet 11 for reflow, since a significant change in the shape or degradation of the sheet 11 during heating may interfere with the ability to form solder bumps on a workpiece and make it difficult to remove the sheet 11 from the workpiece at the completion of bump formation. In addition, the sheet 11 preferably has sufficient heat resistance that it can be used a plurality of times before being replaced. Furthermore, the solder-receiving sheet 11 is preferably made of a material having poor wettability by molten solder so that molten solder will not adhere to the sheet 11 during the process of forming solder bumps.
The dimensions of the solder-receiving sheet 11 as viewed in plan are at least as large as the dimensions of the region of a workpiece on which solder bumps are to be formed. The solder-receiving sheet 11 may have a size such that the pattern of holes 14 corresponds to the pattern of electrical contacts for a single electronic device, or it may have a size such that it contains a plurality of patterns of holes 14, each corresponding to electrical contacts for a separate electronic device. Figure 2 is a plan view of a solder-receiving sheet 11 having a single group of holes 14 corresponding to the contacts of a single electronic device, while Figure 3 is a plan view of a solder-receiving sheet 11 containing a plurality of groups 17 of holes 14, each group 17 arranged in a pattern corresponding to a separate electronic device. The solder-receiving sheet 11 of Figure 3 can be used to form solder bumps on a plurality of workpieces simultaneously, each workpiece corresponding to a single electronic device, or it can be used to form solder bumps on a single large workpiece, which can then be sliced into smaller workpieces for use in forming individual electronic devices.
The solder-receiving sheet 11 may comprise a single layer of a single material. However, it is often advantageous if the solder-receiving sheet comprises a plurality of layers, which may be made of different materials so as to take advantage of the different properties of the different materials. In the embodiment of Figure 1, the solder-receiving sheet 11 comprises a backing layer 12 and a perforated layer 13 which is formed atop the backing layer 12 and in which the holes 14 in the solder- receiving sheet 11 are formed. No adhesive layer is used to form the solder-receiving sheet 11 in this embodiment.
The backing layer 12 provides strength and stiffness to the solder-receiving sheet 11. The backing layer 12 is preferably sufficiently stiff that the solder- containing sheet 11 can be handled without significant bending under its own weight during use so that the positional relationship among the holes 14 in the perforated layer 13 can be maintained constant. The backing layer 12 can be made from a variety of materials, including resins, metals, ceramics, paper, and combinations of two or more of these materials. Some examples of suitable metals are stainless steel, aluminum, aluminum alloys, and iron-nickel alloys such as Alloy 42. Some examples of suitable ceramics are alumina and aluminum nitride. Some examples of suitable plastics are polyimide resins and polyetherimide resins. An example of a suitable composite is a glass-epoxy resin. The thickness of the backing layer 12 will depend upon the desired stiffness of the solder-receiving sheet. There is no particular restriction on the thickness, but a typical thickness of the backing layer 12 when it is made of a glass-epoxy resin or stainless steel is around 100 - 200 μm. The backing layer 12 will usually be flat so as to provide a flat support surface. The perforated layer 13 is made of a material capable of being processed to form holes 14 for receiving solder paste 15 in a desired pattern. There are no particular restrictions on a material used to form the perforated layer 13, but a particularly preferred material is a photoresist, i.e., a resist which is capable of being processed by photolithography. A resist is advantageous because it typically has good heat resistance, it can be formed on a variety of surfaces, and it can be easily processed to simultaneously form a large number of holes by standard
photolithographic techniques. A resist is not limited to any particular form and can be either a dry film resist or a liquid resist, although a dry film resist is typically more convenient from the standpoint of handling. The resist is also not restricted to any particular type. For example, it can be an etching resist, a plating resist, a solder resist, or a sand blasting resist.
The resist preferably has sufficient heat resistance so as not to undergo significant deformation at the melting temperature of the solder powder contained in the solder paste. Specifically, it preferably has a glass transition temperature (abbreviated below as Tg) in a cured state of at least approximately 100° C. Some specific examples of dry film photoresists which are suitable for use in the present invention are SR-FZ (Tg of 102° C) manufactured by Hitachi Chemical Co., Ltd. and PDF 300G (Tg of at least 180° C) manufactured by Nippon Steel Chemical Co., Ltd., both of Japan. An example of a liquid resist which can be employed is PSR-4000 series (Tg of approximately 120° C) manufactured by Taiyo Ink Manufacturing Co., Ltd. of Japan. If the Tg of the cured resist is significantly less than approximately 100° C, at the time of melting the solder powder in the solder paste housed in the holes in the perforated layer, the resist may also melt and fuse to the surface of the solder or to any solder resist which may be present on the surface of the workpiece on which solder bumps are being formed, making it difficult to remove the solder- receiving sheet after forming solder bumps on the workpiece.
A resist can be provided atop the backing layer by any method appropriate for the material of which the resist is made. When the resist is in the form of a dry film photoresist, an example of a suitable method of applying it to the top surface of the backing layer is lamination under heat and pressure. When the resist is formed from a liquid photoresist, it can be applied to the top surface of the backing layer using a curtain coater, a spray coater, or by screen printing.
When the perforated layer 13 comprises a photoresist, the holes 14 can be formed by standard photolithographic techniques. These techniques typically include exposure of the resist to light, developing of the exposed resist with a developing solution, and post-treatment such as post-baking. The holes are formed in the resist in a pattern matching the pattern of electrical contacts on a workpiece on which solder
bumps are to be formed. Suitable photolithographic techniques are well known to those skilled in the art. It is possible to form holes in a resist layer disposed on a surface other than the backing layer and then laminate the resulting perforated layer atop the backing layer, but it is generally simpler to form the holes after the resist has been applied to or formed on the backing layer.
When the perforated layer 13 is made of a material other than a photoresist, the holes 14 can be formed using a variety of conventional methods, depending upon the material of which the perforated layer is made, including drilling, electric spark machining, punching, and laser machining. Similar methods can be used to form holes when the solder-receiving sheet 11 comprises a single layer rather than a plurality of layers.
It is possible for the holes 14 in the perforated layer 13 to extend only partway through the thickness of the layer, but it is easier from the standpoint of ease of manufacture as well as uniformity of the holes 14 if they extend all the way through the thickness of the perforated layer 13 to the top surface of the backing layer 12. The shape of the holes 14 in the perforated layer 13 is not critical. From a manufacturing standpoint, it is easiest to form holes which are curved (such as circular or elliptical) as viewed in plan and have constant dimensions over their depth, but it is also possible to form holes having a polygonal shape as viewed in plan or having dimensions which vary over their depth. It is not necessary for the shape of a hole 14 as viewed in plan to match the shape of an electrical contact on a workpiece on which a solder bump is to be formed.
Usually the area of each hole 14 at its upper end as viewed in plan will depend upon the desired size of the bump which is to be formed by the solder paste 15 contained in the hole. The area of a hole as viewed in plan does not need to match the area of the contact on which a solder bump is to be formed. The holes 14 may all have the same area as each other as viewed in plan, or the area may differ from one hole 14 to another to enable solder bumps of different size to be formed on a workpiece at the same time, as shown in Figure 1. The solder paste 15 which is received in the holes 14 in the perforated layer 13 includes a solder powder and a flux which is mixed with the solder powder to form a
substantially uniform mixture. When the solder paste is initially applied to the solder- receiving sheet 115 it is preferably in a spreadable form to enable it to be easily inserted into the holes 14 in the sheet 11.
The solder paste 15 which is used in the present invention may be a conventional solder paste used for reflow soldering of surface mounted parts.
However, because a solder paste for use in the present invention does not need to be capable of being applied to a surface by printing or stenciling, its viscosity can vary over a wider range than is permissible with conventional solder pastes, As discussed previously, a solder paste having a reduced flux content compared to a typical commercially available solder paste is advantageous in that the number and size of voids which are formed during reflow of the solder paste can be reduced. For example, when the solder paste 15 is a lead-free solder paste, the proportion of flux in the solder paste may be decreased to 10 weight % or lower.
The flux may contain the same classes of components as a conventional solder paste, such as resins as vehicles, activators which may be selected from organic acids, amines, and salts thereof, solvents, and surfactants, and each component may be the same as that employed in a conventional flux for use in solder paste. Either a water- soluble flux or a water-insoluble flux can be used. In view of ease of removal of flux residue after bump formation, it is often more convenient to use a water-soluble flux since it is made possible to remove flux residue by washing with water.
The solvents in the flux of a conventional solder paste requiring printability are usually primarily solvents having a boiling point in the vicinity of 250° C to prevent the solvent from substantially evaporating below the melting point of the solder powder in the solder paste. However, since a solder paste used in the present invention does not require printability, it can employ solvents having a significantly lower boiling point (such as a boiling point below 200° C and preferably between 100 and 200° C) than that of solvents used in conventional solder pastes. For example, the solvents in the flux of the solder paste can be primarily low boiling point solvents such as propylene glycol monomethyl ether (boiling point of 121° C) or dipropylene glycol monomethyl ether (boiling point of 187° C). Use of such lower boiling point solvents can result in a decreased formation of voids in the resulting solder bumps
since the solvents are evaporated nearly completely before the solder powder begins to melt during reflow.
The solder powder in the solder paste can be formed by conventional methods for manufacturing solder powder. The particle size of the powder can be the same as that used in conventional solder pastes and can be selected in accordance with the desired size and pitch of the solder bumps to be formed.
The solder paste 15 can be inserted into the holes 14 in the perforated layer 13 of the solder-receiving sheet 11 by any convenient method. One example of a suitable method is to apply a dab of the solder paste to the top surface of the sheet 11 and then to spread the paste over the top surface with a squeegee. By repeatedly passing the squeegee over the surface of the sheet 11, each of the holes 14 can be filled with the solder paste 15, and any excess solder paste adhering to the portions of the top surface between the holes 14 can be wiped off by the squeegee. Alternatively, a roller can be used to spread the solder paste 15 over the surface of the sheet 11 and force the solder paste into the holes 14, and then a squeegee can be used to scrape off any excess solder paste from the surface of the sheet 11.
At the time that the solder paste 15 is inserted into the holes 14 in the perforated layer 13, the surface of the solder paste may be tacky, particularly when the solder paste 15 employs a conventional flux. In conventional reflow soldering with a solder paste, the solder paste generally needs to have surface tack in order to hold components in place on a workpiece prior to reflow. In the present invention, the solder paste is not used to hold components in place on a workpiece, so it does not require any surface tack. A lack of surface tack is in fact preferable since dust and other contaminants readily adhere to a tacky surface. Therefore, after the solder paste 15 has been inserted into the holes 14 in the perforated layer 13, the solder paste 15 is preferably dried to evaporate solvents from at least the surface of the solder paste 15 and reduce or eliminate surface tackiness. Drying can be carried out by drying at room temperature, but to reduce time, drying is preferably carried out by baking in an oven, for example. A preferred baking temperature depends on the boiling points of solvents contained in the flux, but it is usually in the range of 100 - 150° C. The baking time can be selected in accordance
with the type and amount of the solvents. Baking is preferably performed long enough to substantially eliminate surface tackiness. As a result of baking, the surface of the solder paste 15 is hardened, but below the surface, the solder paste 15 remains pasty. The evaporation of solvents from the solder paste not only reduces surface tack but also reduces the occurrence of voids in the resulting solder bumps. The formation of voids during reflow soldering is a result of the generation of gas during reflow, which is related to the amount of solvents present in solder paste. By evaporating solvents during baking, a smaller amount of gas is generated during reflow, so fewer voids are formed at that time, and the voids which form are of smaller size.
After baking the solder-receiving sheet 11 is allowed to cool (such as to room temperature), the top surface of the sheet 11 is preferably covered with a protective layer 16. The protective layer 16 can serve a number of functions. If the protective layer 16 is substantially impervious to air, it can be used to shield the solder paste from the atmosphere and thereby protect the solder paste 15 against oxidation. If the protective layer 16 is substantially impervious to water, it can prevent the solder paste 15 from absorbing water vapor in the air during storage or protect the solder paste against spills of water. The protective layer 16 can also serve to shield the solder paste 15 from dust and other contaminants. In addition, if the protective layer 16 has a fluxing action when heated, it can be used as a source of flux when forming solder bumps on electrical contacts of a workpiece.
Thus, the material of which the protective layer 16 is formed will depend upon the desired functions. The protective layer 16 may be in the form of a removable film which can be peeled off the perforated layer 13 prior to the start of bump formation, but preferably it remains attached to the perforated layer 13 at the start of bump formation and melts at a temperature below the melting temperature of the solder powder in the solder paste 15. For ease of handling of the solder-receiving sheet 11, the protective layer 16 is preferably substantially tack-free.
An example of a suitable material for the protective layer 16 is a resin which has a film-forming capability and which is in the form of a solid film at room temperature but quickly melts to a low viscosity or softens at a temperature below the
melting point of solder power in the solder paste 15, preferably at a temperature of at most 70° C such as appximately 60 - 70° C. The resin is preferably a water-soluble resin to enable the protective layer 16 or remnants thereof to be easily removed by water from the surface of a workpiece after solder bumps have been formed on the contacts of the workpiece. Some examples of suitable water-soluble resins which can be used are polyvinyl alcohol (PVA) and its derivatives such as adducts of ethylene oxide to PVA, polyester polyols, polyether polyols, polyethylene glycols, polyvinylpyrrolidone, and co-polymers of polyvinylpyrrolidone and polyvinyl acetate. If desired, the protective layer 16 can be given a fluxing action by including a material having a fluxing action in the protective layer 16. Some examples of suitable materials for this purpose are organic acids, amines, glycerine, and alcohols.
The protective layer 16 may also include various other substances, such as a surfactant, a defoaming agent, and a leveling agent.
The protective layer 16 may be formed atop the perforated layer 13 by any convenient method. When the protective layer is water soluble, the components of the protective layer are first dissolved in water, alcohol, or other aqueous substance to form a solution, which is then applied to the top surface of the perforated layer 13 by spray coating or other convenient coating method and dried to form a film. There are no strict limits on the thickness of the protective layer 16, but it will typically be in the range of 0.3 to 10 μm. For example, a thickness of around 5 μm has been found to be suitable from the standpoints of performance and economy.
The solder-receiving sheet 11 may include a plurality of alignment marks for use in aligning the holes 14 in the perforated layer 13 with the contacts of a workpiece by optical recognition equipment. For example, alignment marks may be in the form of round marks formed in the resist constituting the perforated layer 13 in locations spaced outwards from the pattern of holes 14. For example, alignment marks may be formed in two diagonally opposed corners of the solder-receiving sheet 11 in positions corresponding to alignment marks on a workpiece.
Next, an example of a method according to the present invention of forming solder bumps on a workpiece will be described while referring to Figures 4A - 4F, which are schematic cross-sectional views of a workpiece and the bump-forming
assembly 10 of Figure 1 at different steps of the method.
Figure 4A is a cross-sectional view of a portion of a typical workpiece 20. The workpiece 20 includes a substrate 21, a plurality of electrical contacts 22 formed atop the substrate 21, and a resist layer 23 covering the top surface of the substrate 21 except where the contacts 22 are located. Prior to the start of bump formation, at least the contacts 22 of the workpiece 20 may be coated with a flux 24 in order to remove oxides formed on the surface of the contacts 22 and facilitate wetting of the contacts by molten solder. Even when the protective layer 16 on the solder-receiving sheet 11 has a fluxing action, it is often desirable to apply a flux directly to the contacts 22 of the workpiece 20 because the fluxing action of the protective layer 16 is generally relatively weak. The flux 24 may be applied by customary methods, such as spraying, printing, or roll coating.
As shown in Figure 4B, a solder bump-forming assembly 10 like that shown in Figure 1 having holes 14 containing solder paste 15 therein and arranged in a pattern matching the pattern of the contacts 22 on the workpiece 20 and having a protective layer 16 is positioned opposite the top surface of the workpiece 20, with each of the holes 14 in the assembly opposing one of the contacts 22 on the workpiece 20.
The bump-forming assembly 10 and the workpiece 20 are then heated to a temperature sufficient to melt the solder powder in the solder paste 15. The heating temperature is at least the liquidus temperature of the solder paste 15 and preferably 10 to 30° C higher than the liquidus temperature. For example, in the case of a lead- free solder paste containing a Sn-3.0Ag-0.5Cu solder powder, the heating temperature is preferably on the order of around 225-250° C. The heating time will depend upon the heating temperature and factors such as the thermal capacity of the bump-forming assembly 10 and the workpiece 20. The heating atmosphere is not restricted and it may be air or an inert atmosphere such as a nitrogen atmosphere.
During heating, the bump-forming assembly 10 and/or the workpiece 20 often have a tendency to warp if not restrained. In order to prevent warping and maintain the bump-forming assembly 10 and the workpiece 20 planar and in contact with each other, pressure is preferably applied to the bump-forming assembly 10 and the workpiece 20 during heating in the direction normal to their top and bottom surfaces.
The magnitude of a suitable pressing force will depend upon the materials of which the bump-forming assembly 10 and the workpiece 20 are made, which determines their tendency to warp when heated. In general, a pressure on the order of 10 N per cm2 applied to the electrical contacts 22 of the workpiece 20 is suitable. Heating while applying a controlled pressure to the bump-forming assembly and a workpiece 20 can be easily carried out using conventional equipment such as a flip chip bonder. The orientation of the bump-forming assembly 10 and the workpiece 20 with respect to each other during heating is not critical. The primary factors controlling the formation of the solder bumps on the contacts 22 of the workpiece 20 are surface tension and the wettability of the contacts 22 by molten solder, with gravity being less important. Therefore, it is possible to form solder bumps on the workpiece 20 whether the bump-forming assembly 10 is disposed atop or underneath the workpiece 20 during heating. In order to efficiently melt the solder powder in the solder paste 15, during heating, the bump-forming assembly 10 is preferably disposed closer to the source of heat than is the workpiece 20.
The materials forming the protective layer 16 of the bump-forming assembly 10 and the flux layer 24 of the workpiece 20 are selected so as to melt and flow before the temperature of the solder paste 15 reaches the liquidus temperature of the solder powder in the solder paste 15. When the solder powder 15 in a hole 14 in the perforated layer 13 reaches its liquidus temperature, it will melt to form molten solder which congeals as a molten mass like a ball. The electrical contacts 22 are made of a material having good wettability by molten solder, so the molten solder in the holes 14 in the perforated layer 13 will wet the contacts 22 and form into rounded masses under surface tension. After heating has been carried out for a prescribed length of time, the solder-receiving sheet 11 and the workpiece 20 are allowed to cool to solidify the molten masses of solder and form them into solder bumps 25, as shown in Figure 4C. The solder-receiving sheet 11 is then removed from the workpiece 20, leaving the solder bumps 25 adhered to the contacts 22 of the workpiece 20, as shown in Figure 4D. The backing layer 12 and the perforated layer 13 are both preferably made of materials having poor wettability by solder, so the solder-receiving sheet 11 can be easily removed from the workpiece 20 without adhering to the solder bumps
25.
When the solder powder in the solder paste 15 melts and coalesces as a molten mass in each hole 14, the molten mass tends to center itself with respect to the corresponding electrical contact 22 of the workpiece 20. Therefore, even if the holes 14 in the bump-forming assembly 10 are not perfectly centered with respect to the contacts 22 of the workpiece 20, the resulting solder bumps 25 will be properly positioned atop the contacts 22. Thus, a bump-forming assembly 10 according to the present invention is very tolerant of misregistration between the bump-forming assembly 10 and a workpiece 20. During the heating process to melt the solder paste 15, the molten solder is restrained from one side by the backing layer 12 of the solder-receiving sheet 11, so the resulting solder bumps 25 may have a flattened upper surface, as shown in Figure 4D. If it is desired for the solder bumps 25 to have a more rounded shape, they may be further processed by heating to at least the liquidus temperature of the solder forming the solder bumps 25 in a state in which the solder bumps 25 are not restrained from above to allow the solder bumps 25 to form into a rounded shape under surface tension. In this reheating step, the bumps 25 are typically heated to a temperature on the order of 10 - 30° C above the liquidus temperature for at least 30 seconds. When the solder bumps 25 are made of a lead-free solder alloy, the reheating is preferably carried out in an inert atmosphere such as a nitrogen atmosphere in order to prevent the wettability of the solder bumps from worsening. As shown in Figure 4E, in order to remove oxides which formed during the previous heating step in which the solder bumps 25 were formed, the solder bumps 25 are preferably coated with a typical post-flux 26 prior to being placed into a reflow furnace for reheating. A water-soluble post-flux is often more convenient from the standpoint of ease of cleaning. The post-flux 26 melts and flows during reheating.
After the solder bumps 25 have been formed into the desired shape and allowed to cool, the solder bumps 25 and the workpiece 20 can be cleaned by washing with a suitable fluid to remove any residue, such as flux residue. Figure 4F illustrates the workpiece 20 and the solder bumps 25 at the completion of the reheating and cleaning. The workpiece 20 can then be subjected to further processing
to manufacture an electronic device.
Examples
A method of forming solder bumps according to the present invention and a bump-forming assembly for use in the method will be further described with reference to the following examples.
Example 1
A bump-forming assembly like the one illustrated in Figure 1 was prepared in the following manner. A stainless steel sheet measuring 30 x 30 mm and having a thickness of 150 μm was used as a backing layer. The backing layer was coated with a solder resist (Taiyo Ink PSR-4000 SC02) using a curtain coater, and the resist layer was dried. The resist layer was then exposed to light using a parallel ray exposure apparatus, developed, and then heat treated at 15O0C for 30 minutes to form holes in the resist layer and obtain a perforated layer having a thickness of 50 μm. The perforated layer contained 3600 circular holes each having a diameter of 135 μm and arranged in a 60 x 60 lattice with a pitch of 150 μm between holes. Each of the holes in the perforated layer extended through the thickness of the layer to the top surface of the backing layer to form a solder-receiving sheet.
Each of the holes in the perforated layer of the solder-receiving sheet was then filled with a solder paste having the composition listed below using a urethane rubber squeegee. The solder paste contained 9.7 weight % of a water-soluble flux having the composition listed below and 90.3 weight % of a solder powder of a Sn-3Ag-0.5Cu alloy having a liquidus temperature of 217° C. After the holes were filled with the solder paste, the solder-receiving sheet was baked at 1200C for 10 minutes for drying. The resulting solder-receiving sheet had no surface tack.
Composition of Solder Paste:
90.3 wt% of solder power (Sn-3Ag-0.5Cu) (particle diameter: 5 to 15 μm); 3.5 wt% of PEG 4000 (polyethylene glycol with a molecular weight of approximately 4000) (resin as a vehicle);
0.5 wt% of anonionic surfactant;
1 wt% of diglycolic acid (activator); and
4.7 wt% of ethylene glycol monohexyl ether (solvent) (b.p. 208° C).
The top surface of the solder-receiving sheet was then spray coated with the following composition, which was dried to form a protective layer having a thickness of 3 μm, thereby forming a bump-forming assembly. The diglycolic acid in the composition is effective as an activator for flux, so the resulting protective layer had a fluxing action.
Composition Used to form Protective Layer (parts by weight) 93 parts of PEG 4000;
3 parts of polyvinyl alcohol derivative (adduct of ethylene oxide to PVA);
3 parts of diglycolic acid;
1 part of a nonionic surfactant;
300 parts of water; and 100 parts of isopropyl alcohol.
The resulting bump-forming assembly was then used to form solder bumps on a workpiece comprising a printed circuit board having 3600 electrical contacts (electrodes) formed on its top surface in the same pattern as the holes in the perforated layer in the bump-forming assembly. The top surface of the workpiece was coated with a layer of a water-soluble flux (Sparkle Flux WF2050 sold by Senju Metal Industry Co. Ltd.) using a spray fluxer. Then, the bump-forming assembly was positioned atop the workpiece in the manner shown in Figure 4B using a device referred to as a transfer apparatus, which is similar in structure to a commercially available flip chip bonder and which is capable of pressing two members against each other with a controlled temperature and pressure. The bump-forming assembly and the workpiece disposed in the transfer apparatus were then pressed against each other for one minute with a force of 30 N while the temperature of the apparatus was set at 250° C to melt the solder powder in the solder paste and form solder bumps on all
3600 contacts of the workpiece.
The workpiece and the solder-receiving sheet of the bump-forming assembly were then removed from the transfer apparatus, and after cooling, the solder-receiving sheet was removed from atop the workpiece. The top surface of the workpiece having the solder bumps which were formed thereon was then coated with a water- soluble flux (Sparkle Flux WF2050), and the workpiece and the solder bumps were then heated in a nitrogen atmosphere in a reflow furnace for 4 minutes at a maximum temperature of 250 0C to form the solder bumps into a more rounded shape. After removal from the furnace and cooling, the workpiece was washed with water at 500C to remove any residue. The resulting solder bumps had an average height of 60 μm with a standard deviation of 1.5 μm.
The occurrence and size of voids in the solder bumps was evaluated by X-ray analysis in which X-ray images of 100 bumps at a time were displayed on a monitor, and the images were visually observed to count the number of bumps in which voids were found and measure the approximate average size of voids in the bumps. This procedure was repeated by selecting 100 bumps at random. As shown in Table 1, the occurrence of voids was low and their size was small.
To investigate its reusability, the solder-receiving sheet used in the above procedure was subjected to ultrasonic washing with hot water at approximately 60 0C to remove substantially all residue of the solder paste, the protective layer, and the flux which had been applied to the workpiece. From external appearances, there were no major changes in the solder-receiving sheet compared to before it was used.
After drying, the washed solder-receiving sheet was again filled with a solder paste, baked, and coated with a protective layer in the same manner as described above to prepare a bump-forming assembly. The assembly was then used to form solder bumps on a workpiece like that used above in the same manner as described above. The resulting solder bumps which were formed on the workpiece were of good quality, with an average height of 62 μm and a standard deviation of 1.6 μm. When the sheet was used 5 times in this manner, acceptable bumps could be formed each time.
Example 2
A bump-forming assembly was prepared in the same manner as in Example 1, but instead of the holes in the perforated layer being filled with a solder paste containing 9.7 weight % of flux, they were filled with a commercially available solder paste (M705A(6)-10-l 1 available from Senju Metal Industry Co., .Ltd.) containing 11 weight % of flux and containing solder powder (Sn-3Ag-O.5Cu) having a particle diameter of 5 - 15 μm. The solder-receiving sheet was then baked at 100° C for 15 minutes for drying. The surface condition of the solder paste after baking was nearly the same as in Example 1. Then a protective layer with a thickness of 3 μm was formed atop the solder-receiving sheet in the same manner as in Example 1 to obtain a bump-forming assembly. The assembly was then used to form solder bumps atop a workpiece by the same procedure as in Example 1.
The resulting solder bumps had an average height of 55 μm with a standard deviation of 2.0 μm. As shown in Table 1, the occurrence of voids was more frequent and their size was larger than in Example 1.
Since the solder-receiving sheet of the bump forming assembly had the same structure as the solder-receiving sheet of Example 1, it had good reusability. Specifically, when tested for reusability in the same manner as in Example 1, acceptable bumps could be formed when the sheet was used 5 times.
Comparative Example 1
This example illustrates a standard method for forming solder bumps from solder paste by a printing technique.
The same commercially available solder paste as was used in Example 2 was printed on the electrical contacts (electrodes) of a workpiece using a metal squeegee. The metal mask had a thickness of 75 μm and had 3600 through-holes with a diameter of 80 μm formed therein in a pattern matching the pattern of the electrical contacts of the workpiece. All the contacts had solder paste transferred thereon, indicating that the solder paste had good printability. The workpiece was then heated in a nitrogen atmosphere in a reflow furnace for 4 minutes at a maximum temperature of 250 0C to form solder bumps on the contacts.
The resulting solder bumps had an average height of 40 μm with a standard deviation of 3.0 μm. When the occurrence of voids was observed by the above- described method, nearly all the bumps had voids. Namely, as indicated in Table 1, voids were found in 100 out of 100 bumps observed at a time. The size of voids was greater than in Example 2 in which the same solder paste was used.
Table 1
*Frequency of occurrence of voids: Number of bumps in which voids were found out of 100 bumps observed.
Claims
1. A solder bump-forming assembly for use in forming solder bumps on a workpiece by placing it on the workpiece as it is comprising: a heat-resistant sheet having a plurality of holes formed therein; and a solder paste comprising a solder powder and a flux disposed in each hole in the heat-resistant sheet.
2. A solder bump-forming assembly as claimed in claim 1 which further comprises a protective layer which melts or softens at a temperature lower than the liquidus temperature of the solder powder disposed atop the heat-resistant sheet.
3. A solder bump-forming assembly as claimed in claim 1 or 2 wherein the heat-resistant sheet comprises a backing layer and a perforated layer disposed atop the backing layer.
4. A solder bump-forming assembly as claimed in claim 3 wherein the perforated layer comprises a photoresist layer.
5. A solder bump-forming assembly as claimed in claim 1 or 2 wherein the heat-resistant sheet has poor wettability by the solder powder when the solder powder is in molten state.
6. A solder bump-forming assembly as claimed in claim 2 wherein the protective layer is water soluble.
7. A solder bump-forming assembly as claimed in claim 2 wherein the protective layer contains a material having a fluxing action.
8. A method of forming an assembly for use in forming solder bumps on a workpiece comprising disposing a solder paste comprising a solder powder and a flux in holes formed in a heat-resistant sheet.
9. A method as claimed in claim 8 which further comprises covering the heat- resistant sheet with a protective layer which melts or softens at a temperature lower than the liquidus temperature of the solder powder after disposing the solder paste in the holes in the sheet.
10. A method as claimed in claim 8 or 9 wherein the heat-resistant sheet comprises a backing layer and a perforated layer disposed atop the backing layer.
11. A method as claimed in claim 10 wherein the perforated layer comprises a photoresist layer.
12. A method as claimed in claim 8 including drying the surface of the solder paste after disposing it in the holes.
13. A method as claimed in claim 9 including drying the surface of the solder paste after disposing it in the holes and before covering the sheet with the protective layer.
14. A method as claimed in claim 12 or 13 wherein the drying is carried out by heating.
15. A method of forming solder bumps on a workpiece comprising: placing a heat-resistant sheet having a plurality of holes therein each containing a solder paste opposite a workpiece with each of the holes opposing an electrical contact of the workpiece, the solder paste comprising a solder powder and a flux; heating the solder paste to at least the liquidus temperature of the solder powder to form the solder paste in each hole into a mass of molten solder adhering to the opposing contact; cooling the molten solder; and removing the heat-resistant sheet from the workpiece.
16. A method as claimed in claim 15 wherein the surface of the heat-resistant sheet opposing the workpiece is covered with a protective layer which melts or softens at a temperature lower than the liquidus temperature of the solder powder.
17. A method as claimed in claim 15 or 16 wherein the workpiece comprises a substrate for a BGA device.
18. A method as claimed in claim 15 or 16 wherein the workpiece comprises a semiconductor chip.
19. A method as claimed in claim 15 or 16 including pressing the heat- resistant sheet towards the workpiece while heating.
20. A method as claimed in claim 15 or 16 which further comprises reheating the solder bumps on the workpiece to at least the liquidus temperature of the solder bumps after removing the heat-resistant sheet.
21. A method as claimed in claim 20 which further comprises coating the solder bumps with a flux prior to reheating.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/JP2007/056531 WO2008114465A1 (en) | 2007-03-20 | 2007-03-20 | Method of forming solder bumps and solder bump-forming assembly |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| PCT/JP2007/056531 WO2008114465A1 (en) | 2007-03-20 | 2007-03-20 | Method of forming solder bumps and solder bump-forming assembly |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| WO2008114465A1 true WO2008114465A1 (en) | 2008-09-25 |
| WO2008114465A8 WO2008114465A8 (en) | 2008-12-11 |
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ID=39765585
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| PCT/JP2007/056531 WO2008114465A1 (en) | 2007-03-20 | 2007-03-20 | Method of forming solder bumps and solder bump-forming assembly |
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| Country | Link |
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Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH08316235A (en) * | 1995-05-22 | 1996-11-29 | Fujitsu Ltd | Manufacture of semiconductor device |
| JPH09246324A (en) * | 1996-03-08 | 1997-09-19 | Hitachi Ltd | Method for forming electronic component and bump thereof |
| JP2003264255A (en) * | 2002-03-11 | 2003-09-19 | Ngk Spark Plug Co Ltd | Method of manufacturing wiring board |
| WO2006043377A1 (en) * | 2004-10-19 | 2006-04-27 | Senju Metal Industry Co., Ltd. | Sheet for forming solder bump and method for manufacture thereof |
-
2007
- 2007-03-20 WO PCT/JP2007/056531 patent/WO2008114465A1/en active Application Filing
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH08316235A (en) * | 1995-05-22 | 1996-11-29 | Fujitsu Ltd | Manufacture of semiconductor device |
| JPH09246324A (en) * | 1996-03-08 | 1997-09-19 | Hitachi Ltd | Method for forming electronic component and bump thereof |
| JP2003264255A (en) * | 2002-03-11 | 2003-09-19 | Ngk Spark Plug Co Ltd | Method of manufacturing wiring board |
| WO2006043377A1 (en) * | 2004-10-19 | 2006-04-27 | Senju Metal Industry Co., Ltd. | Sheet for forming solder bump and method for manufacture thereof |
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|---|---|
| WO2008114465A8 (en) | 2008-12-11 |
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