US 20050139986 A1
An assembly includes a structure, a plurality of terminals and a plurality of compliant pads disposed between said terminals and said structure. The terminals are aligned with at least some of said pads, with the pads providing a standoff between the structure and the terminals. The compliant pads are preferably made of a non-conductive material such as a silicone elastomer.
1. An assembly comprising:
(a) a structure;
(b) a plurality of terminals; and
(c) a plurality of compliant pads disposed between said terminals and said structure, said terminals being aligned with at least some of said pads, said pads providing a standoff between said structure and said terminals.
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This application is a continuation of U.S. patent application Ser. No. 10/123,547, filed Apr. 16, 2002, which application is a continuation of U.S. patent application Ser. No. 09/517,852, filed Mar. 2, 2000, now U.S. Pat. No. 6,525,429, which is a divisional of U.S. patent application Ser. No. 08/842,313, filed Apr. 24, 1997, now U.S. Pat. No. 6,133,639, which in turn is a divisional of U.S. patent application Ser. No. 08/365,699, filed Dec. 29, 1994, now U.S. Pat. No. 5,659,952, which in turn is a continuation-in-part of U.S. patent application Ser. No. 08/309,433, filed Sep. 20, 1994, now abandoned, the disclosures of which are hereby incorporated by reference herein.
The present invention relates, generally, to providing a compliant interface for a semiconductor chip, and more particularly relates to a method and apparatus for providing a compliant interface to accommodate for differences in the thermal coefficient of expansion mismatch between a semiconductor chip and a support structure, such as a printed wiring board.
In attempting to use the area on printed wiring boards more efficiently, semiconductor chip manufacturers have recently been switching from larger, more cumbersome interconnection conventions, such as pin grid arrays (“PGAs”) and the perimeter leaded quad flat packs (“QFPs”), to smaller conventions, such as ball grid arrays (“BGAs”). Using BGA technology, semiconductor chips are typically interconnected to their supporting substrates using solder connections, such as with “flip-chip” technology. However, when solder alone is used to interconnect the chip contacts to the substrate, the columns of solder are generally designed to be short to maintain the solder's structural integrity. This results in minimal elastic solder connection properties which further results in increased susceptibility to solder cracking due to the mechanical stress of the differential thermal coefficient of expansion (“TCE”) of the chip relative to the supporting substrate thereby reducing the reliability of the solder connection. In other words, when the chip heats up during use, both the chip and the substrate expand; and when the heat is removed, both the chip and the substrate contract. The problem that arises is that the chip and the substrate expand and contract at different rates and at different times, thereby stressing the interconnections between them. As the features of semiconductor chips continue to be reduced in size, the number of chips packed into a given area will be greater and the heat dissipated by the each of these chips will have a greater effect on the thermal mismatch problem. This further increases the need for a highly compliant interconnection scheme for the chips.
The solder cracking problem is exacerbated when more than one semiconductor chip is mounted in a package, such as in a multichip module. Multichip modules continue to grow in popularity; however, as more chips are packaged together, more heat will be dissipated by each package which, in turn, means the interconnections between a package and its supporting substrate will encounter greater mechanical stress due to thermal cycling. Further, as more chips are integrated into multichip modules, each package requires additional interconnections thereby increasing overall rigidity of the connection between the module and its supporting substrate.
An interconnection solution put forth in U.S. Pat. No. 4,642,889, entitled “Compliant Interconnection and Method Therefor” issued to Grabbe seeks to alleviate the aforementioned solder cracking problem by embedding wires within each solder column to reinforce the solder thereby allowing higher solder pedestals and more elasticity. Another solution includes spirally wrapping wire around the outside of the solder. A further solution put forth includes providing a combination of solder and high lead solder, as found in U.S. Pat. No. 5,316,788, entitled “Applying Solder to High Density Substrates” issued to Dibble et al.
Still other prior art solutions make use of a underfill material disposed between the chip and the supporting substrate in an attempt to reduce the stress caused by TCE mismatch. Without the underfill material, this stress is typically concentrated at the weakest part of the solder balls. The underfill material allows this stress to be more uniformly spread out over the entire surface of the solder balls. Examples of the use of underfill materials may be found in U.S. Pat. Nos. 5,194,930, 5,203,076 and 5,249,101. All of these prior art solutions are aimed at reducing the shear stress endured by the interconnections caused by thermal cycling. However, each of these solutions also encounters significant problems such as insufficient compliance and process cost.
Several inventions, commonly assigned to the assignee of the present invention, deal effectively, but specifically differently, with the thermal cycling problem. For example, U.S. Pat. No. 5,148,266 discloses improvements in semiconductor chip assemblies and methods of making the same. As set forth in the '266 patent, a semiconductor chip can be connected to a substrate using a sheet-like, and preferably flexible, interposer. The interposer overlies the top, contact-bearing surface of the chip. A first surface of the interposer faces towards the chip whereas a second surface faces away from the chip. Electrical terminals are provided on the second surface of the interposer, and the interposer is provided with apertures extending through it. Flexible leads extend through these apertures between contacts on the chip and the terminals on the second surface of the interposer. The terminals can be bonded to a substrate. Because the terminals are movable relative to the contacts on the chip, the arrangements described in the '266 patent provide excellent resistance to differential expansion of the chip relative to the substrate caused by thermal cycling. The interposer disclosed in the '266 patent may also include a compliant layer disposed between the terminals and the chip.
Co-pending, commonly assigned U.S. patent application Ser. No. 08/123,882, filed Sep. 20, 1993, now U.S. Pat. No. 5,477,611, the disclosure of which is hereby incorporated herein by reference, discloses a method for creating an interface between a chip and chip carrier including spacing the chip a given distance above the chip carrier, and introducing a liquid in the gap between the chip and carrier. Preferably, the liquid is an elastomer which is cured into a resilient layer after its introduction into the gap. In another preferred embodiment, the terminals on a chip carrier are planarized or otherwise vertically positioned by deforming the terminals into set vertical locations with a plate, and a liquid is then cured between the chip carrier and chip.
Despite the positive results of the aforementioned commonly owned inventions, still further improvements would be desirable.
The present invention provides a method and apparatus for providing a compliant interface for semiconductor chips to accommodate for the typically large thermal expansion mismatch between a chip and its support structure.
More specifically, one aspect of the present invention provides a method of fabricating a compliant interface for a semiconductor chip, typically comprised of a compliant encapsulation layer having a controlled thickness. A first support structure, such as a flexible, substantially inextensible dielectric film, having a surface is provided. A porous resilient layer, such as a layer of a plurality of compliant pads, is attached to the first surface of the first support structure, any two adjacent compliant pads defining a channel therebetween. Attaching the compliant pads to the first support structure may be accomplished a number of different ways. In one embodiment, a stencil mask having a plurality of holes extending therethrough is placed on top of the first surface of the support structure. The holes in the mask are then filled with a curable liquid elastomer. Desirably, liquid elastomer has a thick enough consistency so that the mask may then be removed before curing the elastomer. After the mask has been removed, the elastomer is at least partially cured, such as by heating or exposing to ultra-violet light. The filling step may be accomplished by screening the liquid elastomer across an exposed surface of the mask such that the elastomer is deposited into the holes of the mask. Other methods are also suitable, such as dispensing the elastomer into each of the holes individually. Thus, there is provided an assembly which includes an array of compliant pads further having channels between substantially all of the adjacent pads. At this stage of the process, the vertical height of the pads need not be extremely uniform.
In a further stage, the assembly including the array of pads is used with a second support structure. In one embodiment, the second support structure is a semiconductor chip having a plurality of contacts on a first surface. The first surface of the chip is abutted against the array of compliant pads and the contacts are electrically connected to a corresponding plurality of terminals on a second side of the support structure. Typically, the first surface of the chip is pressed against the pads to compress them, thus ensuring the chip is uniformly supported across its first surface and further ensuring the planarity of the first support structure, or flexible dielectric film, with respect to the first surface of the chip. Where the dielectric film has terminals thereon, the terminals desirably are held coplanar with one another during this step. For example, the dielectric film and the chip may be held between a pair of opposed platens, so that the terminals bear on one platen and are brought into a substantially coplanar condition. A compliant filler, such as a curable liquid elastomer, is then injected into the channels between the chip and the support structure and around the compliant pads while the chip and support structure are held in place. The elastomer may then be cured to form a substantially uniform, planar, compliant layer between the chip and the support structure.
In a further embodiment, the second support structure includes a plurality of chips. The array of compliant pads is large enough to overly several chips simultaneously. This method may be used to form a compliant interface for multiple chips which then may either be cut into individual chips or may be used as a multi chip module. The above method may also be used to form a compliant interface for a semiconductor wafer before the individual chips are separated. After the compliant interface is formed, the wafer may be cut into individual chips or into multi-chip modules.
A further embodiment provides for an array of compliant pads held in place using a holding element. The pads are then fixably placed to either a surface of the chip or the supporting substrate. The holding element is then removed and the pads are compressed between the chip and the supporting substrate. Liquid elastomer is injected into the channels between the adjacent pads, as described in the above embodiments.
The pads or the pad/injected elastomer combination provide a compliant, planar interface which effectively accommodates for the thermal coefficient of expansion mismatch between the chip and a supporting substrate thereby alleviating much of the stress on the connections therebetween. Further, the combination provides an effective encapsulation barrier against moisture and contaminants.
The foregoing and other objects and advantages of the present invention will be better understood from the following Detailed Description of Preferred Embodiments, taken together with the attached figures.
FIGS. 4A-C show a top plan view of several different pad shapes and registrations in relation to the terminal's positions, according to further embodiments of the invention.
A porous layer, such as a layer of a plurality of compliant pads 110, is attached to the first surface of the film 100. The compliant pads 110 are typically made of an curable liquid elastomer material, such as the Dow Corning silicone elastomer 577, known as “Sylgard™”, with about 5-10% of fumed silica in order to obtain a stiff, thixotropic consistency. The height of the compliant pads 110 is nominally uniform, as defined by the measurement from the base of the pads, at the first surface of the film, to the apexes of the pads. However, exact uniformity in height is not critical, as discussed below. The ratio of width to height of each pad 110 is desirably about two to one (2:1) or greater in order to maintain good pad structural integrity. Each pad desirably has width or diameter, measured at its juncture with first surface of the film 100, between about 300 μm, to 2000 μm. The pads are arranged in a grid pattern with center-to-center distance or “pitch” greater than the minimum width of each pad and preferably greater than the maximum width of each pad. Thus, each pair of mutually adjacent pads defines a channel 117 between them, and all of the channels are interconnected with one another to define a substantially continuous grid of channels 117. The pad pitch and hence the width of the channels 117 must be large enough to allow a compliant filler material 170 to be injected into the channels 117, as described in greater detail below.
As shown in
The mask 80 may also be removed after the curing step. In this case, the elastomer will conform to the shape of the hole 90 in the mask as it begins to cure. Thus, the pads 110 in
Where the pads 110 are only partially cured during the pad forming process, described above, they are tacky and adhere to the chip 120 when it is compressed thereagainst. In a preferred embodiment, the chip 120 would be heated to about 1000 to 18° C. and then compressed against the pads 110 so that the partially cured pads 110 fully cure due to the heat of the chip 120 and in so doing adhere to the face surface of the chip. Alternatively, the apex of each pad may be made tacky by providing an adhesive, a silicone gel, or a film of uncured silicone elastomer thereon so that the chip 120 adheres thereto. Similarly, the chip surface may be coated with an adhesive in the regions remote from the contacts 130 so that the tips of the pads 110 engage such adhesive. While adherence of the pads 110 to the chip 120 helps maintain the alignment between the chip 120 and the film 100, it is not imperative that the pads 110 are adhered to the chip 120 so long as the chip can be held in place by compressing it against the pads 110. The heat and pressure applied during this step of the process permanently deform or “set” the pads to the heights and shapes achieved when the film and chip are squeezed between the platens. Once set in this manner, the pads tend to retain the film and chip in the same relative positions as achieved during the squeezing step, and thereby tend to retain the terminals 140 in coplanar alignment with one another.
The contacts 130 on the chip 120 are then attached to the terminals 140 on the film 100 through any suitable means. In this embodiment, each contact 130 is electrically connected to one terminal 140 through a flexible lead 150 extending therebetween. The terminal 140 is comprised of a conductive blind via (a via extending from the second to the first surface of the film 100 and having a closed bottom surface substantially in a plane with the first surface of the dielectric film 100) and a solder ball (shown in dashed lines), which is typically attached after the package is complete.
As shown in
It is important to understand that the function of the compliant pads is to provide a uniformly supported, planar standoff between the chip and the dielectric film, while allowing the compliant filler to be injected between the adjacent pads 110 to create a compliant encapsulation layer having a controlled thickness. So long as there is uniformity of support, it does not matter whether the pads 110 are in direct registration with single or multiple terminals 140, as shown in
In use, a series of chips can be bonded to the various regions of the tape by abutting each chip against the compliant pads of one region and compressing and bonding the chip and pads in the same manner as discussed above. Each pad 220 may be only partially cured, or else may have a tacky layer 230 (
Preferably, the bonding tool captures the bond region of the lead and brings it into more precise alignment with the chip contact. After bonding the leads, the compliant filler is introduced into the channels 203 between the pads, in substantially the same way as described above. Typically, the film 200 and the chip are compressed while the liquid material is introduced. These operations may be performed at separate stations of a processing line. At any time after the pads of a region have been bonded to a chip, the chip can be transported to the next station of the processing line by moving the tape using the sprocket holes 210. Each region of the tape can be severed from the remainder of the tape after all of the process steps have been completed to provide an individual chip and film assembly. Alternatively, each region can be severed before the final process step, and a portion of the process can be completed after severance. In a further embodiment of the invention, a plurality of individual regions of the tape may remain connected to one another.
Typically, each of these leads 225 will be detached within or near an edge of the bonding window 215 when such lead is connected to a respective chip contact pad. If the leads and the terminals are arranged in a fan-out pattern, this action detaches the tape center portion 205, generally centered within the loop defined as the bonding window 215 from the outer portions of the tape. However, the compliant pads 220 disposed on the tape center portion 205 provides support for the leads 225 during the bonding phase. Conversely, in a fan-in arrangement, the exterior portion of each region may optionally be removed after connecting the leads.
In the alternative embodiment shown in
In a further alternative arrangement, a plurality of individual regions, each adapted to form a support structure for an individual chip, are formed side-by-side in a two-dimensional array on a unitary film in the shape of a panel. Plural chips are attached to these regions in the same manner as discussed above, so as to form a multi-chip assembly. In either arrangement where plural chips are employed, the steps of abutting and compressing the pads and injecting the liquid compliant filler can be performed simultaneously for all of the chips, using platens large enough to accommodate the entire array, or seriatim so that each chip is processed separately.
Before engagement with the wafer 350, compliant pads 320 defining channels 325 therebetween are formed on the first surface of the film. Here again, each pad 320 may be only partially cured, or may have a curable tacky material on its apex. The film 300 is stretched taut and bonded to a rigid ring structure 310. The film 300 is then aligned with the wafer so that each region of the film is aligned with one chip of the wafer, and so that the leads are aligned with the contacts. The wafer is then abutted against the compliant pads 320. A platen 360 is applied on the back surface of the wafer 350. Another platen 370 is applied on the second surface of the film so as to engage the terminals 305. In the same manner as described above, the platens are forced toward one another, thereby compressing pads 320 and bringing the contacts 303 into substantially coplanar alignment with one another. While the contacts and pads are held in this position, the pads may be fully cured or set by heating the platen 360/wafer 350, thereby securing the film in position on the wafer. Alternately, a tacky adhesive may be applied to the first surface of the wafer around the contacts thereon to ensure the adherence of the pads 320 to the wafer 350. After this operation, the leads are bonded to the wafer contacts, followed by injection of the liquid curable elastomer 340 into the channels 325. While the contacts are again held in position by the platens, the liquid elastomer is cured to form a complete assembly encompassing the entire wafer and all regions of the film.
Individual, fully encapsulated parts may then be cut from the wafer. These separated parts will have a compliant interface to accommodate for the thermal coefficient of expansion mismatch between the parts and their eventual supporting substrates, such as printed wiring boards. Further, the encapsulant protects the individual parts from moisture and contaminants.
The embodiment shown in
Having fully described several embodiments of the present invention, it will be apparent to those of ordinary skill in the art that numerous alternatives and equivalents exist which do not depart from the invention set forth above. It is therefore to be understood that the present invention is not to be limited by the foregoing description, but only by the appended claims.
Although the present invention has been described with reference to particular preferred embodiments, it is to be understood that the embodiments are merely illustrative of the principles and application of the present invention. It is therefore understood that numerous modifications may be made to the preferred embodiments of the present invention without departing from the spirit and scope of the present invention as defined by the claims.