KR0169502B1 - Surface mount assembly and method of device using adcon interconnections - Google Patents

Abstract

The present invention relates to an apparatus and method for batch assembly of various leaded or padded devices that are encapsulated in a printed wiring board (PWB) and that are not encapsulated, with greatly improved manufacturing capabilities and reliability. The pressure frame assembly allows the uniform distribution of pressure to be applied to various lead packages and pad semiconductor chips during the curing of the conductive adhesive (AdCon), and also reduces the variation in initial interconnect resistance, thereby allowing adcon interconnection. Enhance the reliability of the connection. Pressure is applied to the device by externally applying fluid under pressure to a flexible resilient stretchable membrane that entirely covers the contour of the device and the area of the neighboring PWB. The external application of pressure enhances the thermal conductivity required to cure the adcon, broadens the range of processing parameters available for this technique, and reduces assembly time. Assembly yields of 100% were observed for the various surface mount packages, indicating that the system would be very reliable.

Description

[Name of invention]

Device and Method for Surface Mounting Devices Using Adcon Interconnect

[Scope of Invention]

The present invention relates to a surface mounting apparatus and method for a device using Adcon interconnection.

[Background of invention]

Electrically conductive adhesives are widely used in the electronics industry. An important application is die bonding and assembly of printed wiring boards (PWBs). An illustration of a device 2 is schematically illustrated in FIG. 1 and corresponds to a plurality of conductor pads 5 connected to the conductors 6 in a printed wiring board 4 forming part of the large assembly 1. Thus, the leads 3 surface-mounted on the printed wiring board are arranged in a square. As the size of the device continues to shrink, the number of leads of the device that need to be connected to the PWB continues to increase, the distance between adjacent leads or pad centers (i.e., the pitch) continues to decrease, and the isotropic conductive adhesive is applied without shortening the adjacent leads. It is becoming more difficult to place correctly. Another approach is to use an anisotropic conductive adhesive that only conducts in a direction perpendicular to the wiring board (Z-direction).

Anisotropic conductive adhesives (hereinafter referred to as adcons) (for adhesive connectors) are a type of electrically conductive adhesive material formulated with the concept of bridging. This material is made by dispersing electrically conductive particles in an insulating polymer matrix forming an adhesive mixture. Typically, the adhesive mixture is applied to the surface of the PWB by stencil printing, screen-printing or laminating a film of conductive adhesive. It is applied to the surface of the PWB having the conductor pads 5. A device 2 having a portion of the PWB 4 having the conductor pads 5, an area of the adcon 7 on the PWB, and a lead 3 to be connected to the conductor pads is schematically shown in FIG. 2. Is shown. After the device is placed in the PWB and the placement force between the conductor pads at the PWB substrate and the lead of the device is removed from the adcon, a layer of edcone having a single particle thickness remains between each lead and conductor pad. This is schematically shown in FIG. Individual particles fill the gaps between the devices and form electrical interconnects. In a similar manner, a semiconductor chip with conductor pads can be surface mounted on a PWB.

Conventionally, the formation of adcon interconnects has typically been done via a series of processes in which each package is aligned, placed under pressure, and individually cured in a curing furnace. As an example of such a process, James R. James R, Clements et al., Published in US Patent Nos. 4,667, 401 and 4,868,637, issued May 26, 1987, and Connection Technology, issued August 1988. There is a technique known in the text of Brian Sun as the paste connector-vertical conductive adhesive described on page 31, 32. However, this type of process can result in insufficient formation of non-conductive joints due to inadequate control of the applied pressure, resulting in damage to leads and interconnects. This method was difficult to implement for high performance input / output (I / O) surface mount packages and small chips, because of the low throughput, the assembly process also required the PWB, package and assembly tool to be coplanar. Because. Without planarity in the system, productivity and reliability can cause problems. In addition, very small semiconductor chips do not have a risk of damaging the chip and therefore need to apply a large force to a small area. Therefore, there is a need not only for a method of applying a uniform pressure across one device or a device of different dimensions and height, but also an apparatus for implementing the method, and an effective method for overcoming the above problem.

The present invention relates to an apparatus and method for batch assembly of various leaded or padded devices that are encapsulated in a printed wiring board (PWB) and that are not encapsulated, with greatly improved manufacturing capabilities and reliability. The pressure frame assembly allows the uniform distribution pressure to be applied to various lead packages and pad semiconductor chips during the curing of the conductive adhesive (AdCon), and also reduces the variation in the initial interconnect resistance, thereby adduct interconnection To enhance the reliability. The pressure is applied to the device by externally applying a fluid under pressure to a flexible elastically stretchable membrane that entirely covers the contour of the device and the area of the neighboring PWB. External application of pressure enhances the thermal conductivity needed to cure Edcon, broadens the range of processing parameters available for this technique, and shortens assembly time. An assembly yield of 100% was observed for the various surface fixation packages, indicating that the system would be very reliable.

[Brief Description of Drawings]

1 is a schematic perspective view of a portion of a PWB and a square device surface mounted to the PWB.

2 is a schematic of a portion of a PWB in which a thin layer of adcon is placed and a lead conductive element prior to assembly.

3 is a schematic view of a layout having conductive elements (pads) on the PWB which are in electrical contact with the leads of the device via conductive particles of edcone after application of pressure in the arrangement shown in FIG.

4 is an exploded front view of a cross section of a pressure frame device used for batch assembly of an adcon interconnect.

5 is a front schematic view of a cross section of the pressure frame device of FIG. 4 in a closed operating position.

6 is an exploded perspective view of the device shown in FIG.

7 is a schematic perspective view of the apparatus shown in FIG.

8 is a schematic perspective view of a portion of a PWB having a square device surface mounted to the PWB and a cable pigtail connection connected to the device.

9 is a schematic perspective view of the apparatus of FIG. 6 including a cable pigtail connection.

Detailed description of the invention

The assembly of electronic circuits with electrically anisotropic conductive adhesives, compared to soldering techniques, offers a number of advantages for the fabrication of PWBs and environmental effects. The low temperature required to cure the adhesive also minimizes thermal and physical damage to the package and PWB. The adhesive system allows for the assembly of good pitch components without forming insufficient soldering, and also reduces assembly costs. Environmentally, anisotropic conductive adhesives eliminate the need to flux wash residues with environmentally harmful compounds such as chlorofluorocarbons (CFC _S ) and expose workers to melt wave solder baths or reflow ovens. Reduce the degree

An apparatus for practicing the present invention is schematically illustrated in FIGS. 4 and 5. A perspective view of this device is also shown in FIGS. 6 and 7, respectively. For simplicity, the elements of the device and the device are not to scale. The device 14, shown in exploded perspective view in FIG. 4, has a base 12, a spacer 13, a membrane 14, and an inlet for inhaling a suitable fluid, such as air, under pressure at the top of the membrane. 16 includes a cover 15 installed. Membrane 14 is a flexible resilient material, such as polysiloxane rubber, fixed around the spacer and the cover. When the fluid is introduced and pressurized in a confined space surrounded by a membrane and a cover, the membrane conforms reliably to the surface of the device and the lid and the contour of the neighboring surface of the PWB at the base 12. . Before applying pressure, it is important to note that the membrane must cover the space on the device uniformly in a well-organized manner that avoids twisting and wrinkling, which can adversely affect the PWB and the device and the cover and adaptability. The spacer 13 was provided with at least one discharge port 18 through which the air is discharged under the membrane 14. The spacer 13 is basically provided as a removable unit for the purpose of conveniently cleaning the floor 19 of the base 12. However, the spacer 13 may be fixed permanently or removably to the base 12. Optionally, a grid (not shown) may be provided on the floor 19 of the base 12. The grid may be in the form of a grid of screens or rods on the floor or a grid formed on the floor at the base, or may be in other forms. The main purpose of the grid is to reduce the possibility of air pockets appearing between the bottom of the pwb and the floor of the base, and to facilitate the removal of the PWB from the base after application of pressure and heat.

Introducing fluid under pressure tends to separate components of the device, such as the base 12 and the lid 15, from each other. Therefore, the device must be provided with some means for holding the parts together. However, such means may be inconvenient to use and assemble and may present a risk of breakage. As a useful method, it is possible to use a simple press, such as a bookbinder press, which can be operated in a suitable way with a vertical screw or lever, and as a further alternative, a press such as a laminating press may be more useful. will be.

In a preferred embodiment, the device 11 may be located between the upper press plate 20 and the lower press plate 21 of a typical laminating press (not shown). The pressure developed by the press in the transverse direction of the device maintains the parts of the device together and prevents the leakage of fluid between the membrane and the cover, while the membrane 14 disposed between the spacer and the adjacent part of the cover. ) Should not be so much damage to the part of). The base 12, spacer 13 and flexible membrane 14 are made removable from the press for the purpose of placing the product to be treated in the base and removing the treated product. The lid 15 may also be removable from the press along with the rest of the apparatus. Alternatively, the lid 15 may be fixed to the upper press plate 20 of the press and should be capable of vertical movement with the upper press plate. In addition, the base 12 may be fixed to the lower pressing plate 21. In the preferred embodiment, the spacer 13 is removable from the base 12 but may be secured to the base 12. Alternatively, the spacer 13 may be attached to the upper press plate 20 and may be moved vertically with the lid. In the latter case, a means should be made to fix the membrane 14 between the spacer and the cover or to reposition the membrane when necessary. This may necessitate attaching the spacer 13 removably to the lid.

The size of the device 11 is sufficient to accommodate only one PWB. Alternatively, it can be a PWB sheet having multiple standard PWBs of various sizes or a size that can accommodate several PWBs, which is limited only by the size of the laminating press.

In operation, an assembly of a PWB, at least one device (or semiconductor chip) having an adcon at a contact pad of the PWB and a lead (or pad) in contact with the adcon, is provided on the floor 19 of the base 12. ), A spacer 13 is disposed on the periphery of the base 12, the membrane 14 is disposed on the spacer 13, surrounding the space formed as the base and the spacer, the lid 15 ) Is covered by membrane 14 while fixing the peripheral area of the membrane between the spacer and the cover. The device 11 is then placed on the lower press plate 21 and the press is operated to move the lower press plate 20 in contact with the lid. Appropriate pressurized fluid, such as air, enters the top of the membrane 14 via the inlet 16 to bring the cover 14 into contact with the device and to ensure good registration with the exposed top surface of the PWB and the base. do. Preselection of the pressure exerting force on the device directed to the PWB removes excess adhesive between the lead or pad of the device and the contact surface of the pad 5 of the PWB, while leaving a thin layer of particles as the thickness of a single particle, while simultaneously To form an electrical connection between the contact surfaces. However, the pressure should not be exaggerated enough to cause any damage to the device, such as flattening, bending or breaking the lead.

Fluid is applied onto membrane 14 to force the membrane towards the PWB and the device through an inlet 16 connected to a suitable source of pressurized fluid such as air (not shown).

The pressure exerted by the membranes in the device causes the leads (or pads) to adsorb the metal particles of the adcon to establish a continuous conductive path between the leads of the device without any damage to the assembly and to achieve metallization in the PWB. There must be sufficient pressure to move to contact with. Commercially useful air compressors that can provide the necessary pressure for bonding purposes will be useful as an air pressure source. Hydraulic pressure in the range of 5 to 500 psi can be applied to the membrane. The actual pressure acting on the device via the membrane is somewhat greater than the pressure applied because the pressure applied combines with the force developed by the extension of the membrane. Hydraulics, typically in the range of 5 to 25 psi, preferably 15 psi, are sufficient for most IC packaging applications. Exceptionally, extremely small packages, such as, for example, 1/8 inch squared or smaller unencapsulated semiconductor chips, may require the application of very large pressures ranging from 200 to 300 spi.

In the experiments, the polymer matrix for adcon was prepared from diglycidyl ether of bisphenol F (Dainippon Ink and Chemical Co., Epiclon 830®).

Aromatic silica thixotrope (Cabot, TS-720) was immersed in the epoxy resin in a Waring® mixer at a concentration of 5 phrs (parts per hundred resin), which is useful for concentrations ranging from 1 to 10 phr. To the aliquot of the mixture, 10 phr of 2-ethyl, 4-methylimidazole hardener (Pacific Anchor Chemical, EMI-24) was added along with the conductive particles. The concentration of the metal particles should be controlled to ensure that the particles of P and B appear to ensure reliable electrical conductivity between the PWB and the leads or pads of the device (Z-direction) while maintaining electrical insulation between neighboring conductor pads and leads (xy direction). In practice, the required volume of metal particles that will provide the maximum amount of conductive particles without causing any short circuit between adjacent leads 3 or adjacent conductor pads 5. Efforts should be made to obtain the concentration in the adhesive mixture, and proper adhesive performance is required to maintain a mechanically strong interconnect The formations are mixed manually, degassing under vacuum (15 μmHg) for 30 minutes before use In the experiments, the particles were silver-plated glass spheres with a size of 8 to 20 μm and an average diameter of 14 μm (available from Potters Induystries). The particles were 10 to 15 vol% (26 to 36 wt%), preferably An amount of 12.5% by volume (29% by weight) was added.

The assembly of the package in the test PWB was made through the following multi-step process. Mix adcon, degas, PWB to approximately 2 mils thick with a doctor blade and shim or about 5 mils as a manual stencil printer (Henry Mann AP-810) Stencil treatment was carried out on. Some packages were placed manually in a stenciled PWB and some were placed using a suitable tool (eg, a manics or single site solder machine [SSSM]) located between the platens 20 and 21. The plate assembled in (11) was placed, where the adhesive interconnect was strengthened under heat and pressure as shown in FIG.

The PWB is basically heated from the bottom of the base 12 at a temperature in the range of 100 to 350 ° C., preferably at 125 ° C. Higher temperatures may be used depending on component parts, adhesive matrix formation, and cure rate conditions. The heat may be, among other methods, a) a heating plate disposed between the base 12 and the lower press plate 21: b) a thermal coil embedded in the base, or c) a suitable heating installed in the lower press plate to provide heat within a predetermined range. It can be supplied by means, for example by means of a hot coil, an electric coil, heated water or steam heating. Additional heating may be provided from the top of the device in a similar way to heating the bottom of the device.

However, overhead heating requires only a small amount of heat required for floor heating, and basically the base by means of parts of the device, for example the lid 15 and the upper press plate 20, to avoid lengthening the curing time. This is to avoid cooling of the 12 and the lower press plate 21. The overhead heating temperature is in the range of 30 to 350 ° C, preferably 55 ° C. Alternatively, the heating structure can be reversed in an arrangement that applies a higher degree of goodness above and below the device 11.

Pressure is applied to the assembly on the base by a flexible extensible membrane 14 disposed over the space formed as the base 12 and the spacer 13. The pressure exerted on the membrane 14 by the fluid extends the membrane 14 and forces it into mating contact with the package. The membrane must be able to stretch sufficiently to match under pressure to individual package dimensions, thus ensuring a uniform tangential application of the PWB. The membrane must be made of such an elastic material that can be returned to its original unextended position when the force exerted by the fluid is removed. The wire screen used as a grid to separate the PWB from the heated base protects the membrane from access to the outlet 18 of the spacer 13. During the curing operation of each assembly, the curing time ranged from 30 seconds to 60 minutes with an average of 2 to 4 minutes.

The applicability of the apparatus was tested using devices having various leads or interconnects. The device features a double-lead disconnect resistor, (12 14-lead Small Outline Integreted Circuits (SOIC) with soldered lead wings of 50 mil pitch, and 45 132-leads with solder coated gullwing leads of 25 mil pitch). A daisy-chained Plastic Quad Flat Package (PQFP) (3,148 total interconnects) was included.

The PWB used was a board of refractory grade 4 (FR-4) made of glass fiber reinforced epoxy covered with copper foil and applied by soldering.

The resistors, SOIC and PQFP assembled and cured as described above were tested for shorts and interconnects. An assembly yield of 100% of the useful interconnects was observed in all packages. Useful interconnects are defined as interconnects with resistances less than 100mΩ for double-lead resistors and 14-lead SOICs, and resistance values less than 1Ω for each 16-lead for 132-lead PQFPs. No short circuit was observed between adjacent leads and pads.

The invention can be used to connect a flexible flat cable (or cable pigtail) to a conductor in a PWB. For example, as shown in FIGS. 8 and 9, the ends of the flat cable 17 provided with contacts (not shown) may be connected to the conductors 6 of the PWB by means of adcons deposited on the PWB. Can be. Such a connection to a single device is shown in FIG. 8 and in FIG. 9 a cable is connected to at least two devices. When the cable has a length that extends beyond the periphery of the device 11, the spacer 13 is provided with an elongated hole 22 in addition to or instead of the outlet 18, so that the cable passes through the hole 22. Allow to extend past the device (11).

Claims (21)

A device for surface mounting of a plastic encapsulation device, comprising a base, the spacer being located at the periphery of the base to define a space with the base, the space having a printed wiring board and a conductive adhesive (adcon) and the adcon. An assembly with at least one device having leads or pads that will be electrically conductively interconnected to a pad or conductor on the PWB via a resilient, extensible membrane surrounds the space and engages with the spacer and the periphery And the rigid cover covers the membrane and secures the membrane between the spacer and the peripheral portion of the cover and has an inlet for introducing pressurized fluid between the cover and the membrane. Device. 2. A surface mounting apparatus according to claim 1, wherein a fixing means is provided for preventing the parts of the apparatus from being separated under the influence of the pressurized fluid. 3. The surface mounting apparatus according to claim 2, wherein the fixing means comprises a press device having a pressure plate for preventing the parts of the device from being separated under the pressure applied at the top of the membrane. The surface mounting apparatus of claim 1, wherein the material of the flexible elastic membrane is made of polysiloxane. 2. A surface mounting apparatus according to claim 1, wherein heating means are provided for heating at least the base to promote curing of the adcon. The surface mounting apparatus according to claim 1, wherein the heating means heats the base at a temperature in the range of 100 to 350 ° C. 7. The surface mounting apparatus of claim 6, wherein the temperature is about 125 ° C. The surface mounting apparatus according to claim 1, wherein heating means for heating the lid is provided at a temperature in the range of 30 to 350 캜. The surface mounting apparatus of claim 8, wherein the lid heating temperature is about 55 ° C. 10. The surface mounting apparatus of claim 1, wherein the fluid is supplied at a pressure in the range of 5 to 500 psi. The surface mounting apparatus of claim 10, wherein the pressure is in the range of 5 to 25 psi. 2. The surface mounting apparatus of claim 1, wherein the fluid consists of air. A method of surface mounting at least one device having conductive leads or pads to a printed wiring board, the method comprising applying a conductive adhesive over a conductive pad on a PWB, and at least one device having a conductive lead or pad, wherein Contacting and assembling the leads or pads of the device in alignment with the conductor pads on the PWB, placing the assembly in a space consisting of a base and a spacer, and placing the space in a resilient stretchable material Enclosing the membrane, applying an evenly distributed hydraulic pressure at the top of the membrane to stretch the membrane to force the mating contact with the at least one device, and electrically conducting conductors in the device and the PWB. Curing adcon to keep connected Surface mounting method comprising the system. The method of claim 13, wherein the membrane consists of polysiloxane. The method of claim 13, wherein the curing is facilitated by heating the assembly. The surface mounting method of claim 15, wherein the heating is performed by heating the assembly from the bottom in a temperature range of 100 to 350 ° C. 16. The method of claim 16, wherein the temperature is about 125 ° C. 18. The method of claim 15, wherein the heating is performed by heating the assembly from above in the range of temperature 30 to 350 ° C. 16. 19. The method of claim 18, wherein the temperature is about 55 ° C. The surface mount chamber of claim 13, wherein the hydraulic pressure is applied at a pressure within a range of 5 to 500 psi. 21. The method of claim 20, wherein said pressure is in the range of 5 to 25 psi.