Patent Publication Number: US-6709895-B1

Title: Packaged microelectronic elements with enhanced thermal conduction

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 08/962,988, filed Oct. 31, 1997 now U.S. Pat. No. 6,127,724 the disclosure of which is hereby incorporated by reference herein. The present application claims benefit of U.S. Provisional Application No. 60/030,541, filed Oct. 31, 1996 and U.S. Provisional Application No. 60/038,129, filed Mar. 3, 1997, the disclosures of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the art of electronic packaging and more specifically relates to assemblies incorporating microelectronic devices such as semiconductor chips, methods of making such assemblies, and components for making the same. 
     Modern electronic devices utilize semiconductor integrated circuits or “chips” which incorporate numerous electronic components. These chips are mounted on substrates which physically support the chips and electrically interconnect each chip with other elements of the circuit. The substrate may be a part of a discrete chip package used to hold a single chip. Alternatively, in a so-called “hybrid circuit” or “module”, one or more chips are mounted to a substrate forming a circuit panel arranged to interconnect the chips and the other circuit elements mounted on the substrate. In each case, the chip must be securely held on the substrate and must be provided with reliable electrical interconnection to the substrate. The electrical interconnections may include numerous input/output or “I/O” connections for carrying signals and/or power to the chip. 
     The structures utilized to provide the “first level” interconnection between a chip and the substrate should accommodate all of the electrical interconnections in a relatively compact region of the substrate. Moreover, the first level interconnection structures ordinarily are subject to strain caused by thermal cycling as temperatures in the assembly change. The electrical power dissipated within the chip tends to heat the chip and the substrate, so that temperatures of the chip and substrate rise and fall as the device is turned on and off. Differential thermal expansion of the chip and substrate cause the contacts on the chip to move relative to the connected electrical contact pads on the substrate during each heating and cooling cycle of the chip (“thermal cycling”). This tends to cause fatigue of the elements connecting individual contacts and contact pads. 
     The chip package should be capable of dissipating the heat generated by operation so as to limit the temperature rise of the chip. Additionally, the chip package should provide physical protection to the chip and to the electrical connections between the chip and substrate. Thus, the package should protect the chip and the electrical interconnections from corrosion or other chemical damage. The cost of the chip and the substrate assemblies is also a concern. All of these factors, taken together, present a considerable engineering challenge. 
     One common approach to the chip mounting problem is referred to as a “die attach/wire bond package.” The die attach/wire bond package, along with other alternatives, is generally described in Multichip Module-Technologies and Alternatives—The Basics: Doane and Franzon, eds. pp. 56-69. In the die attach/wire bond process, also referred to as a “face up” wire bond package or simply as a “wire bond” package the chip is placed on a substrate with the contact-bearing front face of the chip facing upwardly, away from the substrate. The rear face of the chip is bonded to the substrate by a material commonly referred to as a die bond material. Fine gold or aluminum wires are connected between individual contacts on the chip and individual contact pads on the substrate. The entire assembly may be contained in an outer package which is filled with an encapsulant or “potting” compound such as a relatively soft silicone or other elastomer. Because the rear face of the chip is connected to the substrate by the die bond adhesive, the wire bond package provides reasonable thermal dissipation from the chip to the substrate. The individual chips must be handled in a “bare” or unpackaged condition during assembly to the substrate. This special handling, and the wire bonding process itself, require considerable care and special operations. However, typical die attach materials having very high thermal conductivity, such as silver or ceramic filled epoxies, are relatively stiff and noncompliant. When a continuous layer of such a material is applied between the chip and the substrate, it can fail during thermal cycling. Further, voids can occur in a layer of die attach material. Gases trapped in voids can cause failure of the package during service. Moreover, it is difficult to test any individual chip at full operating speed before the chip is mounted to the substrate. 
     As described in PCT International Publication WO 92/05582 and in U.S. Pat. No. 5,347,159, the disclosures of which are incorporated by reference herein, a semiconductor chip assembly may be provided with a flexible, sheet-like backing element overlying the rear face of the chip. Terminals on the backing element are connected to contacts on the front face of the chip, as by leads extending alongside the edges of the chip. A compliant layer may be provided between the terminals on the backing element and the chip itself, so that the terminals remain movable with respect to the chip. A cover may overlie the front of the chip and may be filled with an encapsulant. The entire assembly may be handled and mounted using essentially the same techniques as are used for surface mounting of conventional components on printed circuit boards. Thus, the terminals of the package can be bonded to contact pads on the substrate using masses of solder disposed between the terminals and contact pads. Prior to mounting, the chip assembly can be tested by engaging a test fixture with the terminals. Such a package typically provides relatively good thermal conductivity between the chip and the package so that the package can serve as a heat sink. However, such a package typically provides limited thermal conductivity between the chip and the substrate. 
     A chip package sold under the designation Micro-Star BGA by Sharp Corporation and by Texas Instruments, Inc. also includes a chip in a face-up disposition. A flexible polyimide film overlies the back surface of the chip. Wire bonds connect terminals on the front face of the chip to trace terminals on the flexible polyimide film. These terminals in turn can be bonded to a chip by solder balls. A die attach adhesive and a solder mask layer are interposed between the rear face of the chip and polyimide layer. The wire bonds and the chip are covered by a molded epoxy cover. Voids and delaminations in the die attach layer can cause rupture of such a package during thermal cycling. Moreover, such packages typically provide relatively low thermal conductivity from the chip to the substrate. 
     Additionally, encapsulated metallic conductors such as wire bonds are susceptible to fatigue failures. Typical wire bonds incorporate sharp corners and sharp changes in cross-sectional area at junctions between the fine bonding wire and the connected parts. For example, in a “ball-bond,” each fine wire joins a relatively massive ball of wire material at one end. These features tend to create stress concentrations at the junctures. If the wire is flexed repeatedly during service, it can fail at such stress concentrations. Where the wire is encapsulated in a rigid material, differential thermal expansion of the chip and encapsulant can cause repeated flexure of the wire and fatigue failure. Attempts have been made to avoid such fatigue failures by using very soft encapsulants such as soft elastomers or gel. However, these attempts have not been entirely successful; fatigue failure of encapsulated wire bonds remains a significant problem, even with relatively soft encapsulants. 
     Accordingly, further improvements in microelectronic packaging would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention provides such further improvements. One aspect of the present invention provides a semiconductor chip assembly which includes a dielectric element, having top and bottom surfaces such as a flexible, sheetlike element. A semiconductor chip is mounted above the top surface of the dielectric element with the rear surface of the chip facing downwardly towards the dielectric element and with the front surface, bearing contacts, facing upwardly away from the dielectric element. A plurality of thermally conductive elements such as thermally conductive posts are disposed between the top surface of the dielectric element and the rear surface of the chip and most preferably these posts extend all the way from the top surface of the dielectric element to the rear surface of the chip. The dielectric element has terminals which are accessible from the bottom surface of the dielectric element so that the terminals can be connected to a substrate disposed beneath the dielectric element. Thus, the terminals may be disposed on the bottom surface, or may be accessible from the bottom surface through holes in the dielectric element. Appropriate means are provided for electrically connecting the contacts on the chip to the terminals. For example, flexible leads may extend between the contacts on the chip and conductive traces formed on the dielectric element. Thus, when the terminals are connected to contact pads on the substrate, the chip contacts are electrically connected to the contact pads of the substrate. The posts of the dielectric element and the terminals cooperatively provide a thermal conduction path from the rear face of the chip to the substrate. 
     Most preferably, the assembly includes a flexible rear encapsulant disposed between the rear surface of the chip and the top surface of the dielectric element, so that the rear encapsulant extends between the posts. The rear encapsulant and the posts preferably fill the space between the rear surface of the chip and the top surface of the dielectric element completely, so that there are no voids in such space. The posts desirably have some flexibility. Thus, the posts may be formed from a material such as a silver-filled epoxy or other metal-filled polymer. Even where a relatively stiff metal-filled polymer or other relatively stiff material is used, the configuration of the posts can be selected to provide reasonable flexibility and hence to permit movement of the dielectric element relative to the rear face of the chip. As discussed below, such relative movability allows compensation for thermal expansion and limits mechanical stresses in the system. 
     In a particularly preferred arrangement, some or all of the individual posts on the top surface of the dielectric element are disposed directly above corresponding terminals on the bottom surface of the dielectric element, so that each of these posts is directly aligned with the corresponding terminal. The aligned posts and terminals may be electrically connected to one another, or else may be separated from one another by a relatively thin dielectric layer such as a solder mask layer. In either case, the aligned posts and terminals provide a straight, low-resistance thermal path from the rear surface of the chip to the substrate when the terminals are bonded to contact pads on the substrate. In further preferred embodiments, a layer of a highly thermally conductive metal, such as copper, may be provided over the top surface of the dielectric element between the dielectric element and the individual posts to provide enhanced thermal dissipation from the chip to the substrate by providing an additional thermal conduction path. The dielectric element becomes relatively less flexible as the thickness of the layer of highly thermally conductive metal increases. The thermally conductive layer makes alignment of the posts and the solder masses less important because the layer provides horizontal conduction therebetween. The assembly thus provides good thermal dissipation from the chip to the substrate. This thermal dissipation is further aided by the rear encapsulant, which provides further thermal conductivity between the rear surface of the chip and the top surface of the dielectric element. The rear encapsulant has a thermal conductivity of approximately between about 1.0 to about 10.0 W/cm-°K. 
     A further aspect of the present invention provides a semiconductor chip assembly which also includes a dielectric element having top and bottom surfaces, the dielectric element having electrically conductive traces thereon. Here again, a semiconductor chip is disposed atop the dielectric element, with the rear surface of the chip facing the top surface of the dielectric element. Flexible leads extend from contacts on the front surface of the chip downwardly along side edges of the chip and join the traces on the dielectric element at bond points. The bond points are disposed in at least one edge region of the dielectric element. Each edge region is disposed alongside one edge of the chip. A dielectric encapsulant referred to herein as the “lead encapsulant” surrounds the leads and the chip. Thus, portions of the lead encapsulant are disposed between the edges of the chip and the leads. 
     The bond points on each edge region are disposed in a row or rows parallel to the adjacent edge of the chip and parallel to a medial plane of the chip passing through the geometric center of the chip. The distance between each row of bond points and the adjacent edge of the chip is selected so that the sum of: (1) the product of (a) the coefficient of expansion of the lead encapsulant and (b) the distance between the edge of the chip and the ray of bond points and (2) the product of the coefficient of expansion of the chip and the distance from the medial plane of the chip to the edge of the chip is substantially equal to the product of the coefficient of expansion of the trace-bearing dielectric element and the distance between the medial plane of the chip and the bond points. As further discussed below, this relationship tends to minimize relative movement between the bond points and the immediately adjacent regions of the lead encapsulant upon thermal expansion or contraction of the assembly and thus tends to minimize repetitive stress in the leads at the bond points. 
     A further aspect of the invention provides a microelectronic assembly incorporating a chip or other microelectronic element having a front surface with contacts thereon and having flexible leads such as wire bonds extending from contacts on the front surface of the chip, the leads being connected to the contacts of joints on the front surface. Here again, a lead encapsulant, preferably a flexible encapsulant surrounds the leads and the chip. A spreader is disposed over the front surface of the chip and overlies the contacts and the adjacent regions of the lead encapsulant. Thus, the lead encapsulant extends between the chip front surface and the spreader. The spreader has a coefficient of thermal expansion close to the coefficient of thermal expansion of the chip or other microelectronic element and preferably substantially equal to the coefficient of thermal expansion of the chip. The spreader may serve as a physically protective packaging element and may also help to dissipate heat from the microelectronic element. 
     The spreader tends to suppress shear strain in the encapsulant in the vicinity of the contacts. Stated another way, expansion and contraction of the encapsulant are substantially constrained by the confronting surfaces of the microelectronic element and spreader. Upon heating or cooling of the assembly, the encapsulant surrounding each lead in the vicinity of the chip contact tends to move along with the contact. This minimizes shear stress in the lead at the contact. By contrast, if the encapsulant were constrained between a microelectronic element and a packaging element having coefficient of thermal expansion markedly different from that of the microelectronic element, significant shearing can occur in the encapsulant. As the encapsulant deforms in shear, it can impose significant flexual loads on the leads embedded therein. Such substantial loads can occur even if the encapsulant itself is soft. Indeed, substantial shearing loads can occur even if a soft encapsulant is unconstrained, if the encapsulant itself has a coefficient of thermal expansion substantially different from that of the microelectronic element. 
     In certain instances, it may be desirable to modify the shear strain which is present in the lead encapsulant. Thus, in preferred embodiments of the present invention, changes in the shear strain in the encapsulant in the vicinity of the contacts may be induced by changing the surface area of the spreader or changing the thermal mass of the spreader. For example, the surface area of the spreader confronting the chip contacts may be reduced which will, in turn, reduce the ability of the spreader to constrain the encapsulant in the vicinity of the contacts. Conversely, the thermal mass of the spreader may be increased to enhance the spreader&#39;s ability to constrain the encapsulant in the vicinity of the contacts. Alternatively, the shear strain in the encapsulant in the vicinity of the contacts may be modified by changing the geometry or shape of the lead encapsulant itself, e.g. forming the encapsulant layer with sloped sidewalls. 
     A package according to the most preferred embodiments of the present invention incorporates the features of all of the aspects discussed above. 
     A further aspect of the invention provides a method of enhancing the reliability of electrical connections in a semiconductor package during operation of the chip. The method includes the steps of providing a semiconductor chip having a front contact bearing surface and a rear surface; providing flexible leads extending from the contacts on the front surface of the chip, the flexible leads being connected to the contacts at joints on the front surface; juxtaposing a spreader above the front surface, the spreader having a coefficient of thermal expansion substantially equal to the coefficient of thermal expansion of the chip; and disposing a liquid encapsulant between the front surface and the spreader and around the leads and curing the encapsulant, whereby the motion of the leads during thermal cycling is constrained. 
     The foregoing and other objects, features and advantages of the present invention would be more readily apparent from the detailed description of the preferred embodiments set forth below, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic, partially sectional view of a microelectronic assembly in accordance with one embodiment of the invention. 
     FIG. 2 is a fragmentary, diagrammatic sectional view depicting portions of an assembly in accordance with a further embodiment of the invention. 
     FIG. 3 is a view similar to FIG. 2 but depicting portions of an assembly in accordance with yet another embodiment of the invention. 
     FIG. 4 is a diagrammatic sectional view depicting an assembly in accordance with yet another embodiment of the invention. 
     FIG. 5 is a view similar to FIG. 4 but depicting an assembly in accordance with another embodiment of the invention. 
     FIG. 6 is a diagrammatic, partially sectional view of a microelectronic assembly in accordance with another embodiment of the present invention. 
     FIG. 7 is a diagrammatic, partially sectional view of a microelectronic assembly in accordance with yet another embodiment of the present invention. 
     FIG. 8 is a diagrammatic, partially sectional view of a microelectronic assembly in accordance with still another embodiment of the present invention. 
     FIG. 9 is a fragmentary process flow chart. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A microelectronic assembly in accordance with one embodiment of the present invention incorporates a dielectric element  20  having a top surface  22  and a bottom surface  24 . Dielectric element  20  is formed as a flexible sheet of a dielectric material as, for example, a sheet of a polyimide between about 15 and about 40 microns thick and desirably about 25 microns thick. The dielectric element further includes metallic conductive traces  26  disposed on the top surface of the dielectric element. The conductive traces define terminals  28  which are exposed to the bottom surface through holes  30  in the dielectric element. Only some of terminals  28  and holes  30  are visible in FIG.  1 . The terminals and holes are distributed in an array over substantially the entire sheet or dielectric element  20 . Also, only some of the metallic traces  26  are visible in FIG. 1. A microelectronic element, in this case a conventional semiconductor chip  32  is disposed above the top surface of the dielectric element. Chip  32  has a top surface  34 , bottom surface  36  and edges  38  extending between such surfaces. The chip is substantially rectangular and hence has additional edges (not shown) perpendicular to edges  38 . Contacts  40  are disposed on the top surface  34  of the chip, in rows parallel to edges  38  and in other rows (not shown) parallel to the additional edges of the chip. The chip also has an electrically conductive potential plane  42  on back surface  36 . The potential plane is arranged to serve as a ground or power potential reference plane for the internal electronic elements (not shown) incorporated in the chip. 
     A thin, insulating film such as a conventional solder mask  46  overlies the top surface of the dielectric element and traces  26 . Film  46  desirably is less than about 40 microns thick. The film incorporates apertures  48  aligned with a few of the terminals, such as terminal  28   a , visible in FIG.  1 . 
     A plurality of thermally conductive flexible posts  50  are disposed between the bottom surface of the chip and the top surface  22  of the dielectric element. Each of the posts  50  is aligned with a terminal  28 . For example, post  50   b  is aligned with terminal  28   b . The majority of the posts are supported on insulating film  46 . Post  50   a , however, is aligned with aperture  48  and hence makes direct electrical contact with the aligned terminal  28   a . Posts  50  are formed from a thermally and electrically conductive material. For example, a die attach adhesive such as a silver-filled epoxy of the type sold by Creative Materials, of Tyngsboro, Mass. under the designation 100-49 can be used. Posts  50  should be flexible enough to accommodate displacements of the magnitude which will be caused by thermal expansion of the chip. That is, the posts  50  should be capable of flexing through a range of movement equal to the maximum expansion of the chip relative to surrounding components. A conservative measure of such maximum movement is the product of the coefficient of thermal expansion of the chip, multiplied by the maximum distance along the lower surface of the chip from the center of the lower surface to the most remote corner; multiplied again by the maximum temperature difference to be encountered in the entire life cycle of the assembly, including manufacturing operations, testing, storage and service. The movement calculated according to this rule typically will be greater than the actual movement encountered in service. The desirable elastic modulus of the material in the posts will vary with the height of the posts. Preferably, however, the posts are between about 50 and about 250 microns high and the elastic modulus of the material in the posts is less than about one GigaPascals. Preferably, the material in the posts has a thermal conductivity of at least about 1.0 and more preferably at least about 10.0 W/cm-°K. The posts desirably occupy between about 20 to about 60% of the available area on the back surface of the chip. 
     A plurality of flexible leads  54  extend from contacts  40  on the chip to bond points  56  on the traces  26  of dielectric element  20 . The bond points  56  are disposed in rows, in edge regions of the dielectric element disposed outboard of the edges  38  of the chip and outboard of the other edges (not shown). Thus, each row of contacts  40  along an edge of the chip is connected to a row of bond points  56  along the adjacent edge region of the dielectric element. Leads  54  may be conventional bonding wires, typically gold or aluminum bond wires. Alternatively, leads  54  may be flat, strip-like leads such as those used in tape automated bonding processes, or leads as disclosed in the aforementioned &#39;582 International Publication. Other types of flexible leads may also be used. 
     A flexible rear encapsulant  52  occupies the space between the rear surface  36  of the chip and the top surface  22  of the dielectric layer, completely filling any voids left unoccupied by the posts, insulating layer  46 , and traces  26 . The rear encapsulant has a thermal conductivity of approximately between about 1.0 to about 10.0 W/cm-°K. A dielectric lead encapsulant  58  intimately surrounds the leads or bonding wires  54  and covers the top surface  34  of the chip as well as the top surface  22  of the dielectric layer. Lead encapsulant  58  may be of the same composition as rear encapsulant  52 , or else may be a separate material. For example, rear encapsulant  52  may be loaded with a thermally conductive filler, and may be electrically conductive whereas lead encapsulant  58  may be left unloaded. Desirably, the lead encapsulant and the rear encapsulant are both flexible. The encapsulants may be a gel, an elastomer or other soft, flexible material. The encapsulants desirably have tensile modulus of elasticity or young&#39;s modulus less than 100,000 pounds per square inch (less than 670 MegaPascals) and most desirably between about 100 pounds per square inch and about 10,000 pounds per square inch (between 0.67 and 67 MegaPascals). The encapsulants preferably have hardness below about 60 on the Shore D durometer scale, more preferably between about Shore A  20  and about Shore D  60  and most preferably between about Shore A  20  and about Shore A  90 . Many conventional soft potting compounds utilized in the semiconductor industry can be employed. For example, gels, elastomers, soft epoxies and the like may be used. A silicone gel of the type sold under the designation 577 by the Dow Corning Corporation of Midland, Mich. may be used. A silicone gel such as the Dow Corning DC6810 silicone gel may be used. Alternatively, a Thermoset  323  flexible epoxy encapsulant may be used. 
     A spreader  60  overlies the lead encapsulant  58  and overlies the front surface  34  of the chip. Spreader  60  is formed from a material having a coefficient of thermal expansion close to that of the chip. Desirably, the linear coefficient of thermal expansion of spreader  60  is between about 50% and about 200% of the linear coefficient of thermal expansion of the chip. For a conventional silicon chip having a linear coefficient of thermal expansion of about 3×10 −6 cm/cm-°C., the linear coefficient of thermal expansion of spreader  60  desirably is between about 1.5 and about 6×10 −6 cm/cm-°C. Spreader  60  desirably has a relatively high thermal conductivity. Metal alloys such as those commonly referred to as Invar, Copper/Invar, Alloy  42 , Tungsten/Copper, Molybdenum and the like can be employed. Preferably, the lead encapsulant  58  is bonded to spreader  60  and chip  32 . The lead encapsulant as would be appreciated may, and typically does, have a coefficient of thermal expansion in its free or unconstrained state substantially different from that of the chip. For example, typical silicone encapsulants have linear coefficients of thermal expansion on the order of about 400-600cm/cm-°C. While it is preferable that the spreader be metallic, other CTE matched materials may be used as well such as silicon and alumina, as well as others. 
     Electrically and thermally conductive bonding material  62 , preferably a metallic bonding material such as conventional solder masses, may be provided on the bottom surfaces of terminals  28  so that the bonding material masses  62  protrude from the bottom surface  24  of the dielectric element. In use, the assembly can be handled, stocked and transported as a unit. Spreader  60  and encapsulant  58  protect the delicate leads  54  and chip  32 . The assembly can also be tested as by engaging the solder masses  62  with contacts of a test fixture and actuating the chip. The assembly can be mounted on a conventional substrate such as circuit panel  66  by juxtaposing the solder masses  62  with bonding pads  68  on the substrate and heating the assembly in the conventional manner to melt the solder and then cooling the assembly. These steps may be performed by the same procedures used to mount a conventional surface mounted component. Once mounted in this fashion, terminals  28  and hence traces  26 , leads  54 , and contacts  40  are electrically connected to the contact pads  68  of the substrate. Conductive posts  50   a  and other, similar posts having contact with terminals  28  electrically connect the ground or potential plane  42  of the chip with the appropriate ground or power contact pads of the substrate. The remaining posts  50  do not make electrical connections. However, all of the aligned posts  50 , terminals  28  and solder masses  62  provide short, low resistance thermal conduction paths from the bottom surface of the chip to the substrate and thus help to provide effective heat transfer from the chip to the substrate. This limits the temperature rise of the chip. 
     Several features of the design tend to limit thermal stresses applied to the leads during mounting and service. Because spreader  60  and chip  32  have similar coefficients of thermal expansion, both tend to expand and contract by approximately the same amounts during temperature changes. That portion of lead encapsulant  58  surrounding contacts  40  and surrounding the adjacent portions of leads  54  is constrained between the spreader and the chip top surface  34 . The lead encapsulant in this region thus tends to move with the chip and the spreader. Therefore, there is little, if any, shear strain in the lead encapsulant in the vicinity of contacts  40  and the adjacent regions of leads  54 . Accordingly, the relatively delicate joints between the leads and contacts  40  are effectively protected from flexure during thermal cycling. 
     Moreover, the dimensions of the parts are selected so that movement of the bond points  56  on the dielectric layer is substantially equal to the movement of the immediately adjacent portions of the lead encapsulant during thermal expansion and contraction. Movement of the parts of the assembly is discussed herein with reference to the center of the chip or microelectronic element. Thus, movement of elements in directions to the right and left as seen in FIG. 1 is taken with reference to the medial plane  70 , equidistant from edges  38 . For the bond points  56  lying alongside edges  38  of the chip, movement of the lead encapsulant surrounding bond points  56  during a given change in temperature is substantially equal to the sum of the movement of edge  38  and the thermal expansion of that portion of the encapsulant lying between edges  38  and bond points  56 . That sum is given by 
     
       
           D   C ε C   +D   E ε E   (1) 
       
     
     where: D C  is the distance from a given edge  38  to medial plane  70  of the chip; ε C  is the linear thermal coefficient of expansion of the chip material; D e  is the distance from edge  38  of the chip to bond point row  56 ; and ε e  is the linear coefficient of expansion of the lead encapsulant. The movement of the bond points  58  on the dielectric layer and traces is the product of D D , the distance from medial plane  70  to the bond points and ε D , the linear coefficient of expansion of the dielectric layer and traces. Typically, the thermal expansion of the metallic traces controls the thermal expansion of the dielectric layer. That is, the coefficient of thermal expansion of the dielectric layer in directions parallel to traces  26  can be taken as equal to the thermal coefficient of expansion of the copper material in the traces, i.e., about 150×10 −6  cm/cm-°C. Thus, the dimensions are selected so that 
     
       
           D   D ε D   =D   C ε C   +D   E ε E   (2) 
       
     
     Similar selections are made with respect of the rows bond points adjacent the other edges of the chip (not shown). In each case, the medial plane used as the base of measurement is the plane parallel to the particular edge of the chip and passing through the geometric center of the chip bottom surface. Thermal expansion matching achieved by selection of dimensions in the foregoing manner does not assure absolute absence of shear at bond points  56 . For example, temperatures in the system may vary in a non-uniform manner. Also, when the assembly has been bonded to a substrate, the dielectric layer is bound to the substrate so that expansion and contraction of the substrate may dominate expansion and contraction of the dielectric layer. Preferably, however, the substrate has a coefficient of thermal expansion close to that of the dielectric layer  20  with lead traces thereon. For example, substrate  66  may be another copper and polyimide structure. Moreover, the most severe thermal cycling in the history of the device typically occurs during solder bonding, before the dielectric layer is physically bound to the substrate. Therefore, selection of dimensions in the manner discussed above will provide useful reductions in thermal stress during this critical time. 
     As shown in FIG. 2, an assembly in accordance with a further embodiment of the invention includes a chip  32 ′ similar to that discussed above having contacts  40 ′ on its top surface, as well as a spreader  60 ′ overlying the contact-bearing top surface of the chip as discussed above. The assembly further includes a dielectric element  20 ′ generally similar to that discussed above with reference to FIG. 1, except that the traces  26 ′ of layer  20 ′ are embedded within the dielectric layer. For example, dielectric element  20 ′ may be a composite structure including two or more dielectric layers laminated together. Traces  26 ′ are exposed to the top surface of the dielectric element at bond points  56 ′ through holes or slots  57 ′ in the top portion of the dielectric element. Accordingly, there is no need for a separate solder mask layer such as the solder mask layer  46  used in the embodiment of FIG.  1 . Here again, posts  50 ′ are aligned with terminals  28 ′ and hence aligned with the bonding material or solder masses  62  applied to the terminals. The aligned posts and terminals provide efficient paths for heat conduction to the bottom of the assembly. Desirably, the thickness of dielectric element  20 ′ is limited so that the thermal resistance between each post  50 ′ and the aligned terminal  28 ′ is small. For example, the thickness of a polyimide layer intervening between posts  50 ′ and terminals  28 ′ desirably is less than 40 microns. 
     The embodiment illustrated in FIG. 3 is similar, except that the dielectric layer  20 ″ has traces  26 ″ both on its top surface and on its bottom surface. Some of the terminals, such as terminal  28   a ″ are flat structures disposed on the bottom surface  24 ″ of the dielectric element, whereas other terminals  28   b ″ include metallic structures such as via liners extending through the thickness of the dielectric element. These metallic structures also enhance the thermal conductivity of the structure in the vertical direction, from the top surface towards the bottom surface. As in the embodiments discussed above with reference to FIG. 1, an additional thin dielectric layer  46 ″ overlies the top surface of the dielectric element. Here again, apertures can be provided in such layer to connect some of the posts  50 ″ to traces and/or terminals on the top surface. In each edge region of the dielectric element, the bond points  56  where the flexible leads  54 ″ join the traces  26 ″ are disposed in two rows  56   a ″ and  56   b ″ extending parallel to edge  38 ″ of the chip. Thus, some of the leads extending from the contacts  40 ″ join the traces  26 ″ on the top surface of the dielectric element at a first row of bond points  56   a ″, whereas other leads  54  and traces  26 ″ on the bottom surface  24 ″ at bond points  56 ″. Here again, the dielectric element is provided with one or more apertures, such as one or more elongated slots, to accommodate each row of bond points  56   a ″ for the lower surface traces. In other respects, the embodiment of FIG. 3 is similar to those discussed above. The multilayer structure of FIG. 3 may incorporate more than two layers of traces and more than two rows of bond points associated with each edge of the chip. For example, a layer of traces extending in the middle of the dielectric element, such as traces  26 ′ discussed above with reference to FIG. 2, may be provided. As would be appreciated, in any such structure which incorporates multiple rows of bond points positioned at different distances D A  and D B  from the adjacent edge  38 ″ of the chip, it is impossible to select both distances to match precisely the desired distance D E  calculated above. Preferably, both such distances D A  and D B  approximate the desired distance D E . 
     The assembly of FIG. 4 is generally similar to the assembly of FIG. 1 discussed above. However, dielectric element  120  has terminals  128   a  disposed in peripheral regions of the dielectric element, outboard of bond rows  156  in addition to terminals  128   b , in the central region beneath chip  132 . Thus, the terminals of dielectric element  120  provide a “fan-in” and “fan-out” arrangement. Some of the traces  126   a  extend outwardly from their bond points  156  to their connected terminals  128   a , whereas other traces  126   b  extend inwardly from their bond points to their connected terminals  128   b . A capacitor  110  having a conductive bottom plate  112 , conductive top plate  114  and dielectric layer  116  between the top and bottom plates is disposed over the central region of the dielectric element. Thus, thermally conductive posts  150  are separated from the dielectric element and from terminals  128   b  by the intervening capacitor. However, the capacitor does not provide a large thermal resistance between the posts and the terminals. Conductive layers  112  and  114  desirably are metallic and have relatively high thermal conductivity. Dielectric layer  116  typically is less than about 40 microns thick and hence adds little thermal resistance to the structure. A thin dielectric film (not shown) may be provided atop the top conductive layer  114  of the capacitor to electrically insulate posts  150  from the capacitor if desired. Preferably however, posts  150  remain electrically connected to the top layer  114  of the capacitor so that all of the posts electrically connect the top layer of the capacitor to the potential plane layer  136  on the bottom surface on chip  132 . The conductive layers  112  and  114  of the capacitor are connected to one or more of terminals  128  and desirably are also connected to one or more of the flexible leads  154  which serve as ground and power leads to the chip. For example, trace  126   a , as seen in FIG. 4, is electrically connected to the bottom plate  112 . This provides a compact package with on-board capacitance to compensate for inductances in the leads. Two or more capacitors may be provided in the package. These may be stacked atop one another or, preferably, provided alongside one another. Additional capacitors may be provided over the peripheral regions of the dielectric element, outboard of bond points  156  and overlying the outboard terminals  128   a.    
     In the assembly of FIG. 4, posts  150  are aligned with terminals  128   b  of the central region in the manner discussed above, to provide straight, vertical heat conduction paths from chip  132  to the terminals. However, some or all of the posts can be out of alignment with the terminals. The degree of increase in thermal resistance caused by such misalignment depends on the degree of misalignment or horizontal distance, in the plane of dielectric element  120 , from each post to the nearest terminal. The greater such misalignment, the greater the increase in thermal resistance. Preferably, the degree of misalignment or horizontal distance from each post  150  to each terminal  128  is less than about 100 microns. Where metallic elements of the structure, such as the conductive layers  112  and  114  of a capacitor or traces  126  provide substantial thermal conductivity in horizontal directions, the effects of terminal-to-post misalignment are reduced. For example, the capacitor can be provided with a relatively thick metallic top or bottom plate to enhance the thermal conductivity of the dielectric element in horizontal directions parallel to the surfaces of the element. 
     The assembly of FIG. 4 further includes a thermally conductive plug  161  disposed between the top surface of chip  132  and spreader plate  160 . Plug  161  is bonded to both the top surface of the chip and the spreader plate and provides a low-resistance thermal path from the chip to the spreader. This enhances heat dissipation from the chip and also tends to equalize the temperatures of the spreader and chip. This further reduces shear in those regions of the lead encapsulant disposed between the top surface of the chip and the spreader and thus further reduces bending stress in those portions of leads  156  overlying the top surface, such as the lead portions adjacent contacts  140  of the chip. 
     Packages according to the foregoing embodiments can be fabricated by a process which starts with a preformed dielectric element having the traces and terminals discussed above. If capacitors as discussed above with reference to FIG. 4 are to be provided in the assembly, they may be included in the prefabricated dielectric element. The dielectric element typically is provided in the form of a tape with features such as sprocket holes and alignment masks (not shown). Posts, such as posts  50  (FIG.  1 ), are formed by silk screening a curable material such as the die attach adhesives discussed above. The chip or die  132  is then applied on the posts and the conductive material is cured. Leads  54  are applied by conventional bonding techniques such as conventional wire bonding. Spreader  60  is positioned above the front surface of the chip and a flowable, curable material is injected between the spreader and the dielectric element so that the curable liquid completely fills the spaces between the spreader  60  and dielectric element  20 , including the spaces beneath chip  32 , between posts  50 . Complete filling, without voids, can be enhanced by application of pressure to the liquid during the filling process. For example, after application of the liquid but prior to curing, the assembly can be subjected to a pressure of about 10 to about 1000 pounds per square inch. As described in greater detail in copending, commonly assigned U.S. patent application Ser. No. 08/610,610, the disclosure of which is hereby incorporated by reference herein, application of such pressure can cause any voids which are initially present in a confined layer of liquid encapsulant to shrink and disappear. Although the present invention and the &#39;610 application are not limited by any theory of operation, it is believed that such shrinkage results in part from dissolution of the gases contained in the voids in the liquid material. After the liquid material is applied, it is cured to form the lead encapsulant  58  and a rear encapsulant  52  of the same composition. In an alternative procedure, where a rear encapsulant  52  of different composition from the lead encapsulant  58  is desired, a liquid material adapted to form the rear encapsulant is applied after posts  50  are cured but before bonds  54  are installed. 
     Numerous variations and combinations of the features discussed above may be utilized without departing from the present invention as defined by the claims. For example, the shape and configuration of the spreader may be varied as desired. The spreader may be provided with features such as fins to facilitate dissipation of heat from the spreader. Also, the spreader may be formed as a “can” or “shroud” extending downwardly adjacent the edges of the dielectric element so that sidewalls of the spread protect the edges of the encapsulant-filled space. As shown in FIG. 5, posts  50  may be replaced by compliant thermal connectors as described in U.S. Pat. No. 5,557,501, the disclosure of which is hereby incorporated by reference herein. Numerous flexible thermal conductors formed from elongated metallic strips as described in the &#39;501 patent are interposed between the bottom surface  236  of the chip  234  and the top surface  222  of dielectric element  220 . As described in greater detail in the &#39;501 patent, such thermal connectors can be formed by prefabricating striplike elements in flat configuration, connecting these elements between a pair of opposed elements and then moving the opposed elements away from one another to bend the striplike elements into the desired curved, final configuration. A platelike carrier  237  may be provided with some of the flat striplike elements, whereas others of the flat striplike elements may be formed on the top surface  222  of dielectric element  220 . The flat striplike elements may be deformed to the curved configurations, to form the flexible thermal connectors  250  as depicted in FIG. 5 either before or after mounting chip  234  to platelike element  237 . 
     The structure of FIG. 5 also has leads  254  provided as traces on a flexible dielectric sheet  253  similar to a conventional tape automated bonding or “TAB” tape. Leads  254  may be bonded to the contacts  240  of chip  234 , and to bond points  256  on dielectric element  220  by processes such as employed in conventional TAB bonding. Where chip  234  is attached to platelike carrier  237  prior to the moving step discussed above, the TAB bonding step can be performed before the moving step, while platelike element  237  and chip  234  are positioned immediately above the dielectric element  220 . Following this TAB bonding step, the chip may be moved away from dielectric element  220  so as to deform the striplike elements and form the thermal conductors  250  while also deforming the flexible tab tapes  253  to the condition illustrated in FIG.  5 . Some “slack” may be provided in the TAB leads to accommodate this movement and bending, as by providing a fold or bend in the TAB tapes or the leads. Alternatively, the tab bonding step may be performed after positioning the chip in its final relationship to the dielectric element. 
     The spreader  260  depicted in FIG. 5 incorporates sidewalls  261  extending downwardly adjacent the edges of the package so as to protect the encapsulant  258  chip and leads from mechanical damage. In this embodiment, the same encapsulant  258  fills the entire package, including the spaces between thermally conductive elements  250 , beneath chip  234 . 
     FIG. 6 shows another embodiment of the present invention whereby the level of shear strain in the encapsulant in the vicinity of the contacts has been modified by changing the surface area of the spreader  360 . For example, the spreader  360  shown in FIG. 6 is substantially smaller in surface area than the spreader  60  shown in FIG. 1 so that the encapsulant extends beyond the spreader. Thus, the constraining force exerted by the spreader  360  is reduced, which in turn changes the shear stress in the encapsulant  358  in the vicinity of the contacts  340  on the chip  332 . In alternative embodiments, the thermal mass of the spreader  360  may be modified to change the shear stress in the encapsulant  358  in the vicinity of the contacts  340 . 
     FIG. 7 shows further embodiments for modifying the shear stress in the encapsulant. In these preferred embodiments, the shear stress is modified by changing the geometry or shape of the encapsulant layer  458 . For example, in FIG. 7 the side walls  459  of the encapsulant  458  are sloped outwardly from the top to the bottom. Also, the spreader extends outwardly beyond the encapsulant at the top of the assembly. It is contemplated that other modifications to the geometry of the encapsulant are possible to modify the level of shear stress, e.g. concave sidewalls. 
     FIG. 8 shows a still further embodiment of the present invention which is similar to the assembly shown in FIG.  1  and described above; however, the assembly shown in FIG. 8 includes a layer  580  of a highly thermally conductive metal which is provided between the top surface  522  of the dielectric element  520  and the individual thermally conductive posts  550  disposed between the bottom surface of the chip  532  and the dielectric element  520 . The highly conductive metal layer  580  preferably includes copper, however, other highly conductive metals may be utilized. The layer  580  preferably has a thickness between approximately one (1) to seven (7) mils. The thickness of the conductive metal layer  580  shown in FIG. 8 is greatly exaggerated for purposes of clarity. The highly conductive metal layer  580 , which preferably overlies the top surface  522  of the dielectric element  520 , provides enhanced thermal dissipation from the chip  532  to the substrate  566  by providing an additional thermal conduction path. The thermally conductive layer  580  makes alignment of the posts  550  and the solder masses  562  less important because the layer  580  provides horizontal conduction 
     FIG. 9 shows a portion of a process flow chart showing the process by which a curable liquid penetrates between the rear surface of the semiconductor chip and the top surface of the dielectric element. Step  1  shows placing a curable liquid on the top surface of the dielectric element at the edges of the chip. Step  2  involves applying gas under pressure to the curable liquid. Step  3  shows maintaining of the gas pressure.