WO1999049708A1 - Method for making electrical connections between conductors separated by a dielectric - Google Patents

Method for making electrical connections between conductors separated by a dielectric Download PDF

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Publication number
WO1999049708A1
WO1999049708A1 PCT/US1998/019859 US9819859W WO9949708A1 WO 1999049708 A1 WO1999049708 A1 WO 1999049708A1 US 9819859 W US9819859 W US 9819859W WO 9949708 A1 WO9949708 A1 WO 9949708A1
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WO
WIPO (PCT)
Prior art keywords
separation layer
contact pad
contact
circuit
tip
Prior art date
Application number
PCT/US1998/019859
Other languages
French (fr)
Inventor
Kevin Y. Chen
Original Assignee
Minnesota Mining And Manufacturing Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to AU94041/98A priority Critical patent/AU9404198A/en
Application filed by Minnesota Mining And Manufacturing Company filed Critical Minnesota Mining And Manufacturing Company
Publication of WO1999049708A1 publication Critical patent/WO1999049708A1/en

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    • H—ELECTRICITY
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    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L24/00—Arrangements for connecting or disconnecting semiconductor or solid-state bodies; Methods or apparatus related thereto
    • H01L24/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
    • H01L24/81—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected using a bump connector
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    • 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 at least one potential-jump barrier or surface barrier, e.g. 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
    • H01L21/485—Adaptation of interconnections, e.g. engineering charges, repair techniques
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    • 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 at least one potential-jump barrier or surface barrier, e.g. 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
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    • H01L21/486—Via connections through the substrate with or without pins
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    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/40—Forming printed elements for providing electric connections to or between printed circuits
    • H05K3/4038—Through-connections; Vertical interconnect access [VIA] connections
    • H05K3/4084—Through-connections; Vertical interconnect access [VIA] connections by deforming at least one of the conductive layers
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    • H01L2224/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
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    • H01L2224/818—Bonding techniques
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    • 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
    • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
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    • H05K2201/091—Locally and permanently deformed areas including dielectric material
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    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/01—Tools for processing; Objects used during processing
    • H05K2203/0195—Tool for a process not provided for in H05K3/00, e.g. tool for handling objects using suction, for deforming objects, for applying local pressure
    • H—ELECTRICITY
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    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/11—Treatments characterised by their effect, e.g. heating, cooling, roughening
    • H05K2203/1189—Pressing leads, bumps or a die through an insulating layer

Abstract

A method for making electrical connections between conductive contact pads separated by a dielectric separation layer (12) is disclosed. In the method of the present invention, a structural separation layer is provided that separates at least two aligned conductive contact pads (14a, 14b). The electrical connection between the contact pads (14a, 14b) is made by using a tool (20) to apply pressure, heat, mechanical vibration, or a combination thereof to one of the contact pads (14a). In this manner, a contact pad is deformed and pressed through the separation layer (12), thereby displacing the separation layer material between the contact pads (14a, 14b). In this manner, no prefabricated hole is required in the separation layer (12).

Description

METHOD FOR MAKING ELECTRICAL CONNECTIONS BETWEEN CONDUCTORS SEPARATED BY A DIELECTRIC
Field of the Invention
The present invention relates generally to a method and apparatus for making electrical connections. More specifically, the present invention relates to a method and apparatus for making connections in circuits between conductors separated by an insulating, or dielectric, layer.
Background of the Invention
Contemporary electronic devices often employ multiple components that require precise mechanical and electrical connections in order for the devices to function properly. The components may be semiconductor integrated circuits, resistors, capacitors, connectors, etc. The semiconductor components are often first packaged in so-called first level packages that connect the input/output (I/O) electrodes of the integrated circuit (IC) to pads on the packaging substrate. The first level package is then connected to a second level package such as a printed circuit board.
For first level packaging, techniques such as wire bonding, tape automated bonding (TAB), and solder or adhesive flip-chip bonding have been developed for electrically connecting IC I/O to package substrate pads. In wire bonding, an ultrasonically vibrating tool is used to bond one end of an individual wire to the IC I/O pad and the other end of the wire to the substrate bond pad. In this manner, the IC is electrically connected to the substrate. The surfaces of the IC I/O pads and the substrate bond pads must be clean and free from organic or other contamination in order to achieve good metallurgical bonding between the wire and the bonding pads.
TAB uses a flexible circuit to provide a wiring frame having conductor wires arranged in a specific pattern to match the IC I/O pads and the substrate pads. A thermal compression technique is then employed to bond the wires to the pads either one at a time or many simultaneously. Again, the surfaces of the bonding pads on the IC chip and substrate must be free from contamination to ensure good metallurgical connections.
Extensive discussions exist in the technical literature on the subject of wire bonding and TAB bonding. A detailed description of these techniques may be found in "Microelectronics Packaging Handbook," edited by Roa R. Tummala and Eugene J. Rymaszewski, and references therein.
Flip chip bonding refers to making direct IC pad to substrate pad connections by flipping the IC over so that the I/O pads face the substrate pads. In solder flip chip, solder bumps are added to the IC I/O pads, and then the chip is flipped over, aligned and placed onto the substrate. The entire assembly is then passed through a reflow oven to soften the solder until it wets the surfaces. Subsequent cooling hardens the solder and bonds the chip I/O to the substrate pads.
In adhesive flip chip bonding, a z-axis conductive adhesive is placed between the chip and substrate, and then the chip is flipped, aligned and bonded to substrate using heat and pressure. Often, protruding bumps of solder or gold may be added to the IC pads before bonding to help make better connections. For example, U.S. Patent No. 4,749,120 (Hatada) describes a mechanical connection between a semiconductor device having an array of conductive bumps and corresponding pads on a wiring board, with a non-conducting adhesive. During the bonding process, the bumps are forced through the adhesive, making electrical contact with their respective pads.
Second level package wiring circuits have traditionally been achieved by the use of printed rigid circuit boards, flexible circuits (flex circuits), or ceramic multichip modules. Typically, these circuits require multiple wiring layers in order to connect all components. The multiple trace layers are supported and electrically separated by dielectric materials such as polymer resins, ceramics, or composites of fiber-reinforced resins. A key feature in these multilayer circuits is the via - a conducting pathway through the insulating dielectric material that connects wiring traces in separated layers. Numerous approaches have been taken to make electrical connections through vias more reliable and more efficient at a lower cost. These approaches have met with varying degrees of success. One basic approach involves plated-through-hole (PTH) printed wiring board fabrication processes for making the mechanical and electrical connections in separate processing steps. The typical process of making circuits having two wiring layers, often referred to as double sided circuits, includes first laminating copper sheets to both sides of an adhesive-resin-impregnated glass fiber mat. Electrical connections between the two sides may be achieved through the PTH process. The PTH process typically includes drilling holes to create vias in the circuit board where electrical contact between layers is desired. These vias include walls that are cleaned and plated with conductive metallurgy. Finally, the copper sheet is patterned to form circuit traces using conventional patterning processes known in the art. The process may be repeated in a similar fashion using additional double-sided circuits to build up many circuit layers. Clearly, in such a configuration, multiple process steps are required for making the PTH vias. Processes that form the mechanical and electrical connections simultaneously potentially simplify the manufacturing process and reduce costs associated therewith. One method of simultaneously forming mechanical and electrical connections employs an adhesive ply with conductive buttons to attach circuit elements. For example, U.S. Patent No. 5,282,312 (DiStefano et al.) discloses two metal-seeded flexible circuits patterned with PTH vias. The connection between circuit layers is made by an adhesive bond ply having patterned conductive buttons. The buttons are placed in the bond ply at locations where connections are desired between the circuit layers. The adhesive bond ply has sufficient rigidity such that conduction through the bond ply is only permitted at the conductive buttons. One drawback to this construction is the required complex patterning process and subsequent registration of the bond ply to the circuit layers, which adds cost and decreases yield.
A similar process was developed for using an anisotropic (z-axis- conductive) adhesive to bond the circuit layers together. These anisotropic adhesives include adhesive films loaded with conductive particles. In operation, the anisotropic adhesive is disposed between the conductor pads of separate circuit elements such that when the anisotropic adhesive film is compressed, the adhesive is forced out of the way of the conductor pads while the conductive particles remain trapped between the conductors, thereby forming electrical contacts therebetween, even though the conductor pads never make direct contact with each other. It is important for these anisotropic adhesive films to have sufficiently dispersed conductive particle loading densities to prevent the particles from shorting out in the plane of the circuit.
U.S. Patent No. 5,502,889 (Casson et al.) discloses a method for fabricating a multilayer circuit board using an anisotropically conducting adhesive to connect multiple layers of double-sided circuitry. However, due to the possible electrical shorting between contacts, the use of a random dispersion of conductive particles in the resultant circuit prevents a high density of interconnections as is required for high performance circuits. In addition, the random dispersion of particles requires patterned masking of the circuit layers to prevent shorting between layers.
U.S. Patent No. 5,401,913 (Gerber et al.) discloses multiple circuit layers having metal columns, or bumps. The circuit layers are mechanically connected by a non-conducting adhesive layer placed between successive circuit layers. The circuit layers are subsequently laminated in the presence of heat and pressure, such that the bumps are forced through the adhesive layer and contact their respective pads on the adjacent circuit layer.
Another approach, commonly referred to as the build-up process, employs a sequential layer-by-layer circuit patterning process. In this process, a printed wiring board (PWB) circuit substrate is used as a foundation to support the buildup circuit. First, a layer of dielectric material is deposited on the PWB substrate by coating out of a solution. Holes are then made in the dielectric film at locations where electrical connections with the PWB substrate are needed. Metal is then deposited into the holes and patterned on the surface of the dielectric film to complete the first layer of build-up circuitry. The process can be repeated to build more layers. A similar method may be employed to build up circuit layers by laminating a pre-metalized dielectric film to the PWB board, then making holes in the film, metalizing the walls of the holes, and patterning the circuit. The key steps in these approaches are making and aligning the holes.
While in the past mechanical drilling was used to make the holes, more advanced technologies are now employed, including plasma etching, chemical etching, photoimaging, laser drilling, and the like. These processes are capable of producing much smaller holes on a tighter geometry. However, each technique still requires separate steps for making the holes, metalizing the holes, and aligning the holes with the contact areas.
Summary of the Invention
The present invention provides a simple process for making electrical connections for electronic packages.
Accordingly, the present invention provides a method for making electrical contact between separated conductive contact pads either within a multilayer circuit or between two circuits to thereby electrically connect the two contact pads. In the case of bonding separated contact pads in a multilayer circuit, the first step is to provide a multilayer circuit comprising an organic separation layer and at least two conductive contact pads aligned with each other and positioned on opposite surfaces of the separation layer. At least one of the contact pads is adhered to a surface of the separation layer. The separation layer functions to maintain electrical separation between the contact pads and to provide the structural integrity of the multilayer circuit.
The next step is to take a bonding tool and press the tip of the tool against one of the contact pads. Under the pressure, the contact pad is deformed towards the second contact pad. In this manner, the insulating material of the separation layer that lies between the two contact pads is displaced. The first contact pad continues to be deformed and the separation layer material continues to be displaced until contact is made between the two contact pads. In addition to pressure, heat or mechanical vibration, or combinations thereof, may be applied to the tip of the tool to aid the bonding process by deforming the contact pad and displacing the insulating material. Thusly, contact between the pads may be achieved without first making a hole in the separation layer.
The present invention also provides a method for making electrical contact between separate circuits. Here a first circuit is provided having a first conductive contact pad. A second circuit is also provided which has a second conductive contact pad. In addition, an organic separation layer is provided and positioned between the contact pads. The separation layer may be part of the first circuit or second circuit (such as an insulating substrate), or separate from either circuit (such as an adhesive layer). The circuits are then positioned such that the first contact pad of the first circuit is aligned with the second contact pad of the second circuit, and wherein the separation layer lies between the contact pads. Then the tip of a bonding tool is pressed against the first contact pad to deform the first contact pad towards the second contact pad to thereby displace the insulating material therebetween. The first contact pad is deformed and the insulating material is displaced until the first and second contact pads come into electrical contact. The bonding may be aided by applying mechanical vibration, heat, or a combination thereof.
In this aspect of the present invention, electrical connections may be made to an integrated circuit. The method includes the steps of providing a packaging substrate circuit (circuit board) and an IC chip, positioning the circuits with their respective contact pads in registry and with an organic separation layer therebetween, and pressing the contact pads through the insulating material to form electrical connections. Heat, pressure, or mechanical vibration may be applied to the bonding tool separately or in combination to assist the penetration and bonding process. In addition, this aspect of the present invention relates to a chip package having conductive wires on the package substrate connected to chip I/O pads through polymeric insulating layers. The wire in connection with the chip I/O is part of a continuous strip on one side of the polymeric film. This construction is characterized by visible deformation in the conductor trace at and around the bonding point, visible deformation or bulging of the polymeric material at and around the bonding point, and a crater-like depression on the substrate circuit side of the connection.
The tip of the tool provided in the method of the present invention is preferably shaped to minimize the chances of completely puncturing through the conducting pad material. Because the contact pad tends to conform to the shape of the tip during deformation, sharp edges on the tip tend to cause more severe deformation that may lead to puncturing through portions of the contact pad. Preferably, the tip has at least a partially rounded profile so that the contact pad under that portion would not be punctured through.
A distinctive feature of the method of the present invention is the ability to make electrical connection through an insulating material without first making a hole in the insulating material. With this technique, a via is made in one single step, thereby eliminating the multiple process steps of hole making and hole metalization as in the conventional approaches discussed above.
Brief Description of the Drawings Fig. 1 (a) is a perspective view and a cross-sectional view of a circuit before making a connection according to the present invention.
Fig. 1(b) is a perspective view and cross-sectional view of the circuit in Fig. 1(a) after making a connection according to the present invention.
Figs. 2(a) and (b) are cross-sectional views of layers in a circuit before and after making connections according to the present invention.
Figs. 3(a)-(c) are cross-sectional representations of the method of the present invention used to connect two circuits.
Figs. 4(a)-(c) are cross-sectional representations of the method of the present invention used to connect two circuits. Figs. 5(a)-(c) are cross-sectional representations of the method of the present invention used to connect two circuits.
Figs. 6(a)-(c) are cross-sectional representations of the method of the present invention used to connect two circuits.
Fig. 7 is a cross-sectional view of a construction used in the present invention.
Figs. 8(a)-(c) are cross-sectional views of the method of the present invention used to make a connection in a particular circuit.
Fig. 9 is a cross-sectional photograph of an electrical connection of the present invention. Detailed Description
The present invention is directed towards a method for making electrical connections between conductive contact pads or electrodes through an insulating separation layer without providing a pre-made, metalized, and aligned hole in the separation layer material. While it is illustrative to describe the method of the present invention in terms of various electronic assemblies, the examples given herein are not meant to limit the scope of the present invention or the claims recited hereinafter. Rather, the method of the present invention extends to the bonding of any two or more conductive layers through an insulating separation layer. In general terms, the method of the present invention begins by providing an electronic pre-assembly. As discussed below, this pre-assembly may take on many configurations such as, for example, a single multilayer circuit, an IC chip and packaging substrate, a flexible circuit and electronic device, and combinations and variations thereof. Common to each of these configurations is the existence of at least two aligned conductive contact elements separated by a structural separation layer made from an organic insulating, or dielectric, material. The separation layer is structural in the sense that it provides mechanical support to at least one of the conductive contact elements. The other contact element, aligned with the contact element supported by the separation layer, need not be supported by the separation layer, but should be positioned proximate to the separation layer during alignment with the supported contact element.
The next step of the method of the present invention is to provide a bonding tool having a tip that will be used to deform and bond the conductive contact elements. The tool is typically constructed from tungsten carbide or similar hard materials and has a tip at the bonding end that tapers to an endvill. The endvill preferably has at least a partially rounded profile to prevent puncturing or tearing of the contact pad during deformation. The rounded edge gradually slopes into a flat portion at the base of the tip. This flat portion of the tip is the portion that makes the first contact with the conductive contact element. The width of the flat portion of the endvill is preferably between about 10 μm and 500 μm, but the actual dimensions of the tip will depend greatly upon the size of the conductive contact elements. The next step of the method of the present invention is to deform one of the conductive contact elements by applying pressure from the tip of the bonding tool. By so deforming one of the conductive contact elements, the insulating material of the separation layer that is disposed directly between the contact elements is displaced. Heat or mechanical vibration may also be applied from the tip of the bonding tool during deformation to aid contact element deformation and separation layer displacement. The deformation of the contact element and displacement of the insulating material continues until contact is made between the contact elements. At this point, pressure, heat, mechanical vibration, or a combination thereof, may be applied from the tip of the bonding tool to bond the contact elements to form an electrical connection.
The conductive contact element to be deformed is preferably made from a ductile metal. A ductile metal is more likely to deform without fracturing or cleaving under the pressure of the bonding tool. If the contact element fractures such that it is completely severed from its conductive trace or wire, the connection will fail. Suitable ductile metals include copper, aluminum, silver, gold, tin, zinc, and alloys made therefrom. Dimensional considerations are also a factor in the conductive contact elements to be deformed. In particular, the conductive contact elements to be deformed should not be so thin relative to the distance between the contact elements to be bonded that the contact element is incapable of being deformed without being severed from the conductive trace to which it is connected. Typically, the dimensional considerations that go into the contact elements will depend on the material of the contact element, the material of the separation layer, and the thickness of the separation layer. For example, suitable thicknesses of conductive contact elements may range from about 0.5 μm to about 100 μm.
The separation layer is made from an organic material that is electrically insulating, and is preferably a polymeric material. Aside from its insulating properties, the separation layer provides mechanical support for one or more of the contact elements to be bonded. While providing mechanical support, the separation layer material is preferably flowable under the local application of a certain amount of pressure, heat, and/or mechanical vibration such as from the tip of an ultrasonic welding tool. For example, the separation layer may comprise an adhesive. The separation layer may also comprise a thin, flexible polymer film. Suitable films include polyimide films and polyester films.
There is a trade-off between the thickness of the separation layer and the size of the conductor pads being deformed. The separation layer thickness is limited by the amount of deformation allowed in a conductor pad before it fractures. Because the thickness of the separation layer defines the separation distance between the contact elements to be bonded, a thicker separation layer requires the conductor pad to be "stretched" farther during deformation in order to reach the opposing conductor pad. Smaller conductor pads will thus require thinner separation layers because smaller conductor pads tend to fracture with less deformation. Thus, the thickness of the separation layer is preferably thin enough that the distance between opposing contact elements allows full penetration of the separation layer by one contact element without the contact element being fractured or severed from its conductive trace or wire. In general, the thickness of the separation layer is preferably in the range of about 2 μm to about 500 μm.
The properties of the contact elements (i.e., material, thickness, and size) and the separation layer (i.e., material and thickness) may vary widely depending on the desired application. However, some general guidelines may be defined using specific examples. For instance, the following sets of properties are adequate: (1) a 50 μm thick polyester separation layer having copper pads that are about 100 μm or greater in diameter and 3 μm or greater in thickness, and (2) a 50 μm thick polyimide separation layer having copper pads that are about 100 μm or greater in diameter and 10 μm or greater in thickness. In general, the ratio of the thickness of the contact element to be deformed (tc) to the thickness of the separation layer (ts), tc:ts, is preferably in the range of about 1:50 to about 10:1, and more preferably in the range of about 1 : 10 to about 1:1. However, ratios outside this range may be acceptable given, for example, a contact element that is particularly ductile and a separation layer that is particularly flowable under the deformation conditions. An electrical connection made by the method of the present invention has unique characteristics. Due to the local displacement of the separation layer material during the deformation of the contact element, the electrical connection made under the present invention may be characterized by visible deformation in the contact element at and around the bonding point, visible deformation or bulging of the separation layer material at and around the bonding point, and a crater-like depression in the area of the electrical connection. The bulging may be seen on either or both sides of the circuit in differing amounts depending on the relative deformability of the respective contact elements. Thus, a connection made using the present invention is readily distinguishable from connections made through a pre-made metalized hole in a circuit board, for example.
Fig. 9 is a photograph showing a cross-section of an electrical connection made according to the present invention. The details of the circuit shown in Fig. 9 are set forth below in Example 1. In general, the circuit has at least a first contact element 210, a second contact element 211, and a separation layer 212 positioned between the contact elements. The average thickness of the separation layer is Ts. Fig. 9 shows that an electrical connection of the present invention is characterized by a contact region 200, a deficiency region 202, and an annular excess region 204. Because Fig. 9 is a cross-section, annular excess region 204 appears in two parts. In contact region 200, the contact elements 210 and 211 are in electrical contact, and the thickness of the separation layer 212 is zero. The deficiency region 202 contains the entire contact region and is defined by that region where the thickness of the separation layer is less than Ts. The annular excess region 204 surrounds the deficiency region and is defined by the region where the thickness of the separation layer is greater than Ts. The excess region is formed during bonding when material from the deficiency region is displaced into surrounding areas. The excess volume of separation layer material in the excess region equals the amount of material displaced from the deficiency region during bonding.
The characteristics of the deformed separation layer near and around the contact point may be quantified in terms of a separation layer density function, T(r). T(r) gives the thickness of the separation layer at a position r measured from the center of the bonding point where r = 0. After bonding according to the present invention, T(r) = 0 at r = 0 because the contact elements are in electrical contact at the center of the bonding point. T(r) increases as r increases (away from the contact region) until a maximum thickness is reached. This maximum is seen as a visible bulge around the bonding site. T(r) then decreases with further increase in r until the thickness of the separation layer reaches its normal thickness (thickness before bonding). The point at which the separation layer thickness returns to normal after the bulge defines the boundary of the bonding area, and is denoted rmax. Thus, the outer diameter of the excess region as shown in Fig. 9 is 2rmax. To understand T(r), first consider a separation layer that has a uniform thickness prior to bonding of Ts. The volume of the separation layer material within the bonding area can be found by integrating T(r) with respect to r from r = 0 to r = rmax. For a separation layer of uniform thickness, the volume within the bonding area before bonding is Tsπrm . Now, an electrical connection is made according to the present invention with the center of the bonding point at r - 0. During bonding, a portion of the separation layer is displaced from the bonding point to the surrounding areas. However, no separation layer material is removed from the bonding area. Therefore, the volume of separation layer material within the bonding area remains Tsτtrm . The conservation of separation layer material within the bonding area upon displacement during bonding is the tell-tale sign of an electrical connection made according to the present invention.
A. Multilayer Circuit
Fig. 1(a) shows a two-sided circuit 10, the simplest of multilayer circuits, both schematically and in cross-section. The circuit is characterized by a main separation layer 12, a first pair of aligned conductive contact pads 14a, 14b, a second pair of aligned conductive contact pads 15a, 15b, and a conductive trace 16 connected to contact pad 14a. The contact pads 14a and 14b are positioned on opposite surfaces of the separation layer.
Separation layer 12 is an organic insulating material capable of being locally displaced upon the application of pressure from the tip of a bonding tool. Many classes of organic insulating materials are suitable. The particular material chosen will depend on the desired dimensions and structural properties of the circuit, the material and dimensions of the conductive contact pad, and other such factors that will be readily apparent to the ordinarily skilled artisan. In general, adhesives and polymers normally employed as structural layers in multilayer circuits will be suitable as separation layer materials.
Conductive contact pads 14a, 14b and 15a, 15b are made of a ductile metal or alloy. The contact pads are capable of being deformed under the application of pressure, heat, ultrasonic vibration, or a combination thereof while still maintaining electrical contact with conductive leads such as wires or circuit traces. Suitable metals include copper, aluminum, silver, gold, tin, zinc, or other ductile metals and alloys thereof. Preferably the conductive contact pads have a substantial copper content. Copper is ductile and inexpensive, and its use as contact pads in circuits is common.
Multilayer circuits such as that shown in Fig. 1(a) and (b) may be fabricated by laminating metal sheets onto each surface of a polymeric resin film, or by sputter or vapor deposition of a metal onto both surfaces of a polymeric resin film. The metal film on each surface of the polymeric film is then patterned to form circuit traces and contact pads according to specific designs using conventional patterning techniques such as photoimaging followed by subtractive etching or photoimaging, circuit plating and etching. The resultant multilayer circuits have patterned conductor traces and contact pads on each surface of the dielectric separation layer, and the two patterns are positioned so that pairs of contact pads are in alignment. No electrical connection exists between the two surfaces at this point of the process.
Fig. 1(a) and (b) also show a bonding tool tip 20. Tip 20 is used to apply pressure, heat, vibration, or any combination thereof to contact pad 14a. Under the forces applied by the tool tip, contact pad 14a is deformed towards contact pad 14b. Contact pad 14a' is fully deformed when it makes electrical contact with contact pad 14b as shown in Fig. 1(b). The act of deforming the contact pad necessarily causes the local displacement of the separation layer material from the area between the contact pads. The displaced material tends to accumulate in the immediate area around the contact pads, typically forming bumps or protrusions of excess material as shown in the cross-sectional view of Fig. 1(b). By pushing contact pad 14a through the separation layer in such a manner, there is no need to provide a hole in the separation layer prior to bonding. When fully deformed contact pad 14a' makes electrical contact with contact pad 14b, continued pressure, heat, vibration, or a combination thereof applied from the tip of the bonding tool may be used to weld the contact pads together, thereby creating a permanent or semi-permanent electrical connection. While Fig. 1(a) and (b) shows only one tip, multiple tips may be used to simultaneously bond any number of pairs of aligned contact pads in a multilayer circuit. In addition, the method of the present invention may be used to bond contact pads in multilayer circuits having more than two conductor layers separated by polymer films or adhesive materials. For example, Fig. 2(a) shows a multilayer circuit having four separate conductor layers. Such a circuit may be fabricated by laminating together two two-sided circuits (such as shown in Fig. 1(a)) with a third dielectric layer in-between. If each of the separation layers is the same material, the construction shown in Fig. 2(a) is obtained. However, the separation layers between each conductor layer need not be the same materials. Figs. 2(a) and (b) show three bonding scenarios in a four-layer circuit to illustrate that connections may be made between any pairs or sets of aligned contact pads. In each case, the contact pads are separated by separation layer material 12. As with a two-sided circuit, a tip 20 of a bonding tool is used to deform the contact pads and locally displace the separation layer material therebetween. For example, adjacent separated contact pads 22a and 22b may be bonded. In this case, both contact pads 22a and 22b are deformed and the separation layer material between and below the contact pads is displaced until the contact pads reach a rigid surface (not shown). The rigid surface prevents further deformation of the contact pads. At this point, fully deformed contact pads 22a' and 22b' may be welded by continued pressure, heat, or mechanical vibration from tip 20 of the bonding tool as shown in Fig. 2(b).
Similarly, three aligned and separated contact pads 24a, 24b, and 24c may be bonded. As before, all three contact pads are deformed and the separation layer material therebetween is displaced until a rigid surface is reached. At this point, the fully deformed contact pads 24a', 24b' and 24c' may be welded to form an electrical connection as shown in Fig. 2(b). Finally, an electrical connection may be made among all the conductor layers in a multilayer circuit as shown in Figs. 2(a) and 2(b). Again, tip 20 is used to deform contact pads 26a, 26b, and 26c while displacing the separation layer material therebetween until contact pad 26d is reached. At this point, fully deformed contact pads 26a', 26b' and 26c' are welded together and to contact pad 26d to form the electrical connection.
B. Separate Circuits
The method of the present invention may also be used for making connections between contact pads on separate circuits. Particularly illustrative examples are those involving bonding integrated circuit chips to packaging substrates. This may be accomplished in several ways.
Figs. 3(a)-(c) illustrate one way of bonding an IC chip to a packaging substrate. There an IC chip 34 having I/O contact pads 36a, 36b is attached to a packaging substrate 30 having conductive traces 32a, 32b. Chip 34 may be held in place using an adhesive layer 38. When properly positioned, the I/O contact pads are in alignment with the corresponding contact pads on the packaging substrate. At this point, tips 20a and 20b are used according to the present invention to push conductive traces 32a and 32b through the dielectric layer of packaging substrate 30 and through adhesive layer 38 as shown in Fig. 3(b). Fully deformed conductive traces 32a' and 32b' are then welded to I/O contact pads 36a and 36b respectively to form the finished electronic assembly shown in Fig. 3(c).
Whereas Figs. 3(a)-(c) illustrate attaching a chip to the non-conductor side of a packaging substrate according to the present invention, Figs. 4(a)-(c) illustrates attaching an IC chip to the conductor side of a packaging substrate according to the present invention. Here, chip 44 has I/O contact pads 46a and 46b that are aligned with conductive traces 42a and 42b of packaging substrate 40. Here, the dielectric separation layer between the aligned contact sites is an adhesive layer 48. In this case, tips 20a and 20b are pressed onto the dielectric layer of the packaging substrate to push the conductive traces of the packaging substrate. Conductive traces 42a and 42b are then deformed and pushed through adhesive layer 48 to reach the corresponding contact pads 46a and 46b on the IC chip. The electronic assembly is completed by welding deformed conductive traces 42a' and 42b' to I/O contact pads 46a and 46b as shown in Fig. 4(b). The dielectric layer above the conductor pads may also be deformed or displaced during the bonding process.
An IC chip may also be bonded to the conductor side of a packaging substrate having exposed contact pads as shown in Figs. 5(a)-(c). IC chip 54 having I/O contact pads 56a and 56b is attached to packaging substrate 50 having conductive traces 52a and 52b using an adhesive 58 that separates the contact pads of the chip from the conductive traces of the packaging substrate. According to the method of the present invention, tips 20a and 20b are used to apply pressure, heat, mechanical vibration, or a combination thereof to conductive traces 52a and 52b, thereby deforming the traces and displacing the adhesive separation layer 58 until electrical contact with the I/O pads is achieved. Tips 20a and 20b are then used to weld the deformed conductive traces 52a' and 52b' to the I/O pads of the chip to form the final electronic assembly.
The method of the present invention may also be used to make connections between two circuit boards. In this case, one circuit may be an IC packaging substrate whereas the other circuit is next level packaging. As an alternative, circuit boards may be connected by using a flex circuit as a bridge. In such a construction, the method of the present invention may be employed to bond each end of a flex circuit to one of the circuit boards. For example, Fig. 6(a) shows a circuit board 66 having various conductor layers 70 and a contact pad 68. A flex circuit 60 having contact pads 62 and 64 is then attached to the circuit board with an adhesive layer 72 so that contact pad 62 of the flex circuit is aligned with contact pad 68 of the circuit board. A tip 20 may then be used to deform contact pad 62, thereby displacing the flexible dielectric substrate layer of the flex circuit and the adhesive layer until contact is made with contact pad 68 as in Fig. 6(b). Fully deformed contact pad 62' may then be welded to contact pad 68 to form an electrical connection as in Fig. 6(c). This procedure is repeated for each pair of flex circuit and circuit board contact pads on each end of the flex circuit to electrically connect the two circuit boards. Example 1
Two tape ball grid array (TBGA) parts were used for making electrical connections between Cu traces and a Ni-coated Cu stiffener according to the present invention. The TBGA parts used were grounded stiffener TBGA parts available from 3M Company, St. Paul, MN, under the part identification number 80-6104-6447-3. The structure of the circuit is shown in Fig. 8(a). A flexible circuit consisting of Cu circuitry 102, 104 and supporting polyimide film 100 is laminated onto a Ni-coated Cu stiffener 108 about 0.02 inch (0.51 mm) thick. In this example, Cu circuitry pad 102 was bonded to the Ni-coated stiffener through a polyimide adhesive 106.
Two variations of the bonding method of the present invention were used to make connection between Cu circuitry pad 102 and Ni stiffener 108. In the first method, only pressure was used to make bond connection. A tungsten carbide tip was fastened to a metal block weighing about 200 to 300 g. The tip had an asymmetric profile, having a sharp edge and a rounded edge. The tip assembly was moved toward the bond pad using a stepper motor. The momentum carried by the moving tip assembly caused the tip to push the Cu pad through the polyimide adhesive and cold weld the pad to the Ni-coated Cu stiffener. Some fracturing of the Cu pad resulted during the process in the area contacted by the sharp edge portion of the tip. The Cu pad under the rounded portion of the tip remained intact. Fig. 9 shows a cross-sectional photograph of one of the vias. A total of 2816 connections were made, and all showed electrical connections when measured using an ohmmeter.
In the second bonding method, in addition to pressure, heat and ultrasonic vibration were applied to the tool to assist the welding process. A conventional wire bonding tool sold by Gaiser Tool Co. of Ventura, CA was used. The tip of the tool was modified by grinding the tungsten carbide tip into an asymmetric shape as described previously. The temperature of the tip was held at about 600°C, and the ultrasonic energy setting of the bonding tool was about 5 watts. Following bonding, all the vias were tested for electrical continuity. The bonded parts were then tested for liquid-to-liquid thermal shock by cycling the temperature of the parts from -55°C for 3 minutes to 125°C for 3 minutes with a 10 second transition time between temperatures. After 500 cycles, all the vias were again measured for electrical continuity. Two different samples were bonded this way, with only the bonding time changed between the two (0.25 seconds bonding time for the first and 0.10 seconds bonding time for the second). In the first sample, 4 out of 1408 vias were open after 500 cycles, and in the second sample, 16 out of 1341 vias were open after 500 cycles.
Example 2
In this example, a series of parallel copper traces having thicknesses of about 3 μm were formed on each of two polyester films having thicknesses of about 25 μm. One polyester film was then stacked with the other polyester film such that their respective surfaces having the copper traces faced outward. The films were oriented so that the series of copper traces on one film was perpendicular to the series of copper traces on the other film. This created a matrix of individual areas where the copper traces were in registry. Such an area is shown in Fig. 7 where copper trace 82 of polymer film 80 lies in registry with copper trace 86 of polymer film 84. The area of overlap between the copper traces measured about 400 μm by 400 μm. The stack was then placed on a piece of glass for support. A commercial ultrasonic bonding tool was modified to include a heating element for tip heating. A molybdenum tip 20 was used as the bonding tool. The tip had a bonding area of about 75 μm by 75 μm. The tip was heated to about 300°C. To make a connection between overlapping copper traces, the hot tip was pressed onto a copper trace on the top of the stack for about 1 second and ultrasonic energy was applied to the tool at a frequency of about 61 kHz using a power of about 0.5 W. As a result, the top copper trace was locally deformed and penetrated the polyester layers to form a bond with the overlapping lower copper trace. After an approximately 1 second duration, the tip was lifted and moved to the next bonding location. The resistance across each of the bonded areas was measured using the four-point probe method. About 50 bonded areas were measured. The resistance measurements ranged from about 5 mΩ to about 12 mΩ. Environmental tests were also performed on the circuit. One test compared freshly bonded areas to those on a circuit exposed to an environment at 85°C and 85% relative humidity for 573 hours. There was no statistically noticeable difference between the resistance measured on the control sample and the resistance measured on the exposed sample. The second test compared a freshly bonded circuit to one subjected to a thermal cycling from -55°C to 125°C with a period of about 500 hours. Again, no statistically noticeable difference could be discerned between the control sample and the exposed sample.
Example 3
Vias were made according to the present invention between two sides of a double-sided copper-on-polyimide flex circuit. The double sided circuit was made by conventional sputter deposition and electroplating processes. The dielectric was a 2 mil (about 0.05 mm) thick polyimide film. One side of the circuit was patterned with conductive pads having widths of about 400 μm and thicknesses of about 60 μm, and the other side of the circuit was a solid thin copper film having a thickness of about 6 μm. According to the method of the present invention, each pad was pushed through the polyimide one by one and bonded with the solid copper plane. Heat and ultrasonic vibration were used to assist penetration and bonding. A total of 544 vias were made and they all showed electrical connection.

Claims

WHAT IS CLAIMED IS:
1. A method for making electrical contact in a multilayer circuit comprising the steps of: providing a multilayer circuit comprising: an organic separation layer having a first major surface and a second major surface; a first conductive contact pad adhered to the first major surface of the separation layer; and a second conductive contact pad proximal the second major surface of the separation layer, wherein the first and second contact pads are in registry through the separation layer; providing a bonding tool having a tip; and pressing the tip of the bonding tool against the first contact pad to deform the first contact pad from a first, open position separated from the second contact pad to a second, closed position in electrical contact with the second contact pad, wherein deforming the first contact pad displaces at least a portion of the separation layer from between the contact pads.
2. A method for making electrical contact between circuits comprising the steps of: providing a first circuit comprising a first conductive contact pad; providing a second circuit comprising a second conductive contact pad; providing an organic separation layer; positioning the organic separation layer between the first contact pad of the first circuit and the second contact pad of the second circuit; aligning the first contact pad of the first circuit with the second contact pad of the second circuit; providing a bonding tool having a tip; and pressing the tip of the tool against the first contact pad to deform the first contact pad from a first, open position separated from the second contact pad to a second, closed position in electrical contact with the second contact pad, wherein deforming the first contact pad displaces at least a portion of the separation layer from between the contact pads.
3. An electrical connection comprising: an organic separation layer having a first major surface and a second major surface, wherein the distance between the first and second major surfaces defines a separation layer thickness, and wherein the average distance between the first and second major surfaces is Ts; a first conductive contact pad positioned proximate the first major surface of the separation layer; a second conductive contact pad positioned proximate the second major surface of the separation layer; and a bonding area consisting of: a contact region defined by an area in which the first contact pad is in electrical contact with the second contact pad, wherein the thickness of the separation layer inside the contact region is zero, a deficiency region surrounding and including the contact region, wherein the thickness of the separation layer everywhere inside the deficiency region is less than Ts, thereby defining a deficient amount of separation layer material within the deficiency region, and an annulus-shaped excess region surrounding and excluding the deficiency region, wherein the thickness of the separation layer everywhere inside the excess region is greater than Ts, thereby defining an excess amount of separation layer material within the excess region, wherein the excess amount of separation layer material within the excess region is equal to the deficient amount of separation layer material in the deficiency region.
4. The method of claim 1 or the method of claim 2, wherein the step of pressing the tip of the tool against the first contact pad further comprises applying heat and/or mechanical vibration from the tip of the tool to deform the first contact pad.
5. The method of claim 1 or the method of claim 2, further comprising the step of welding the contact pads by applying pressure, heat, ultrasonic vibration, or a combination thereof from the tip of the tool.
6. The method of claim 1 , or the method of claim 2, or the electrical connection of claim 3, wherein the first conductive contact pad is a metal selected from the group consisting of copper, aluminum, silver, gold, tin, zinc, and alloys comprising copper, aluminum, silver, gold, tin, or zinc.
7. The method or electrical connection of claim 6, wherein the first conductive contact pad is a copper pad having a thickness in the range of about 0.5 ╬╝m to about 100 ╬╝m.
8. The method of claim 1 or the method of claim 2, wherein the first conductive contact pad has a thickness, tc, and the organic separation layer has a thickness, ts, wherein the ratio of the thicknesses, tc:ts is in the range of about 1:50 to about 10:1.
9. The method of claim 8 or the electrical connection of claim 3, wherein the organic separation layer comprises an adhesive.
10. The method of claim 8 or the electrical connection of claim 3, wherein the organic separation layer comprises a polyimide or a polyester layer having a thickness in the range of about 2 ╬╝m to about 500 ╬╝m.
PCT/US1998/019859 1998-03-27 1998-09-23 Method for making electrical connections between conductors separated by a dielectric WO1999049708A1 (en)

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