Abstract:
A method of making a semiconductor chip assembly, including providing a dielectric element with a plurality of electrically conductive terminals, disposing an expander ring over the dielectric element so that a semiconductor chip on the dielectric layer is disposed in a central opening in the expander ring, and disposing an encapsulant in the gap between the expander ring and the semiconductor chip. The size of the gap is controlled to minimize the pressure exerted on the leads by the elastomer as it expands and contracts in response to changes in temperature. The semiconductor chip and expander ring may also be connected to a heat sink or thermal spreader with a compliant adhesive.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application with Ser. No. 60/073,843, filed Feb. 5, 1998; and U.S. provisional patent application with Ser. No. 60/084,377, filed on May 6, 1998 and entitled “Compliant Semiconductor Chip Package with Fan-out Leads and Method of Making Same”, the disclosures of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the art of electronic packaging, and more specifically to assemblies incorporating microelectronic components and methods of making such assemblies. 
     2. Description of the Related Art 
     In attempting to use the area on printed wiring boards more efficiently, semiconductor chip manufacturers have switched some of their production from larger more cumbersome interconnection conventions, such as pin grid arrays and perimeter leaded quad flat packs, to smaller conventions such as ball grid arrays (“BGA”) and chip scale packages (“CSP”). 
     Using BGA technology, semiconductor chips are typically interconnected to an external substrate, such as a printed circuit board, using solder connections, such as with “flip-chip” technology. However when solder alone is used to interconnect the chip contact to the external substrate, the columns of solder are generally designed to be short to maintain the solder&#39;s structural integrity. This results in minimal elastic solder connections properties which further results in increased susceptibility to solder cracking due to mechanical stress caused by the differential coefficient of thermal expansion (“CTE”) of the chip relative to the external substrate thereby reducing the reliability of the solder connection. In other words, when the chip heats up during use, both the chip and the external substrate expand; and when the heat is removed, both the chip and the external substrate contract. The problem that arises is that the chip and the external substrate expand and contract at different rates and at different times, thereby stressing the interconnections between them. As the features of the 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 each of these chips will have a greater effect on the thermal mismatch problem. This further increases the need for a highly compliant scheme for interconnecting each chip to the external substrate. 
     Such an interconnection scheme must also be capable of accommodating a large number of interconnection between a single semiconductor chip and an external substrate, such as a printed circuit board. Complex microelectronic devices such as modem semiconductor chips require numerous connections to other electronic components. For example, a complex microprocessor chip may require many hundreds of connections to an external substrate. 
     Semiconductor chips commonly have been connected to electrical traces on mounting substrates by one of three methods: wire bonding, tape automated bonding and flip-chip bonding. In wire bonding, the semiconductor chip is positioned on a substrate with one surface of the chip abutting the substrate and the face or contact bearing surface of the chip facing upward, away from the substrate. Individual gold or aluminum wires are connected between the contacts on the semiconductor chip and the current conducting pads on the substrate. In tape automated bonding, a flexible dielectric tape with a prefabricated array of leads thereon is positioned over the semiconductor chip and substrate, and the individual leads are bonded to the contacts and pads. In both wire bonding and conventional tape automated bonding, the current conducting pads on the substrate are arranged outside the area covered by the semiconductor chip, so that the wires or leads “fan-out” from the chip to the surrounding current conducting pads. The area covered by the subassembly is considerably larger than the area covered by chip. Because the speed with which a semiconductor chip package can operate is inversely related to its size, this presents a serious drawback. Moreover, the wire bonding and tape automated bonding approaches are generally most workable with semiconductor chips having contacts disposed in rows extending along the periphery of the chip. They generally do not lend themselves to the use of chips having contacts disposed in a so-called area array, i.e., a grid-like pattern covering all or a substantial portion of the chip face surface. 
     In the flip-chip mounting technique, the contact-bearing surface of the semiconductor chip faces towards the substrate. Each contact on the semiconductor chip is joined by a solder bond to the corresponding current carrying pad on the substrate, as by positioning solder balls on the substrate or contacts on the semiconductor chip, juxtaposing the chip with the substrate in the face-down orientation and momentarily melting or reflowing the solder. The flip-chip technique yields a compact assembly, which occupies an area of the substrate no larger than the area of the chip itself. However, flip-chip assemblies suffer from significant problems with thermal stress. The solder bonds between the contacts on the semiconductor chip and the current carrying pads on the substrate are substantially rigid. Changes in the size of the chip and the substrate due to thermal expansion and contraction in service create substantial stresses in these rigid bonds, which in turn can lead to fatigue failure of the bonds. Moreover, it is difficult to test the semiconductor chip before attaching it to the substrate and hence difficult to maintain the required outgoing quality level in the finished assembly, particularly where the assembly includes numerous semiconductor chips. 
     Numerous attempts have been made to solve the foregoing problems. Useful CSP solutions are disclosed in commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; 5,455,390; 5,477,611; 5,518,964; 5,688,716; and 5,659,952, the disclosures of which are incorporated herein by reference. 
     In preferred embodiments, the structures disclosed in U.S. Pat. Nos. 5,148,265 and 5,148,266, incorporate flexible, sheet-like structures referred to as “interposers” or “chip carriers”. The preferred chip carrier has a plurality of terminals disposed on a flexible, sheet-like top layer. In use, the interposer is disposed on the contact-bearing surface of the chip with the terminals facing upwardly, away from the chip. The terminals are then connected to the contacts on the chip. Most preferably, this connection is made by bonding prefabricated leads on the interposer to contacts on the chip, using a tool engaged with the leads. The completed assembly is then connected to a substrate, as by bonding the terminals of the chip carrier to the substrate. Because the leads and the dielectric layer of the chip carrier are flexible, the terminals on the chip carrier can move relative to the contacts on the chip without imposing significant stresses on the bonds between the leads and the contacts on the chip or on the bonds between the terminals of the chip carrier and the substrate. Thus, the assembly can compensate for thermal effects. Moreover, the assembly most preferably includes a compliant layer disposed between the terminals on the chip carrier and the face of the semiconductor chip itself as, for example, an elastomeric layer incorporated in the chip carrier and disposed between the dielectric layer of the chip carrier and the semiconductor chip. Such a compliant structure permits displacement of the individual terminals independently towards the chip and also facilitates movement of the terminals relative to the chip in directions parallel to the chip surface. The compliant structure further enhances the resistance of the assembly to thermal stresses during use and facilitates engagement between the subassembly and a test fixture during manufacturing. Thus, a test fixture incorporating numerous electrical contacts can be engaged with all of the terminals in the subassembly despite minor variations in the height of the terminals. The substrate can be tested before it is bonded to a substrate so as to provide a tested, known-good part to the substrate assembly operation. This in turn provides very substantial economic and quality advantages. 
     U.S. Pat. No. 5,455,930 describes a further improvement. Components according to preferred embodiments of the &#39;930 patent use a flexible, dielectric top sheet. A plurality of terminals are mounted on the top sheet. A support layer is disposed underneath the top sheet, the support layer having a bottom surface remote from the top sheet. A plurality of electrically conductive, elongated leads are connected to the terminals on the tip sheet and extend generally side by side downwardly from the terminals through the support layer. Each lead has lower end at the bottom surface of the support layer. The lower ends of the leads have conductive bonding materials as, for example, eutectic bonding metals. The support layer surrounds and supports the leads. Components of this type can be connected to microelectronic elements, such as semiconductor chips or wafers by juxtaposing the bottom surface of the support layer with the contact-bearing surface of the chip so as to bring the lower ends of the leads into engagement with the contacts on the chip, and then subjecting the assembly to elevated temperature and pressure conditions. All of the lower ends of the leads bond to the contacts on the semiconductor chip substantially simultaneously. The bonded leads connect the terminals on the top sheet with the contacts on the chip. The support layer desirable is either formed from a relatively low-modulus, compliant material, or else is removed and replaced after the lead bonding step with such a compliant material. In the finished assembly, the terminals on the relatively flexible dielectric top sheet desirably are moveable with respect to the contacts on the semiconductor chip to permit testing of and to compensate for thermal effects. The component and the methods of the &#39;930 patent provide further advantages, including the ability to make all of the bonds to the chip or other component in a single lamination-like process step. 
     U.S. Pat. No. 5,518,964 discloses still further improvements. Preferred methods according to the &#39;964 patent, include the step of providing a dielectric connection component having a plurality of terminals and a plurality of leads. Each lead has terminal-end attached to one of the terminals and a tip end (or contact-end) attached to a contact on a chip. Preferred methods also include the step of simultaneously forming all of the leads by moving all of the tip ends of the leads relative to the terminal-ends thereof and relative to the dielectric connection component so as to bend the tip ends away from the dielectric connection component. The dielectric connection component and the chip desirably move in vertical and horizontal directions relative to each other so as to deform the leads towards positions in which the leads extend generally vertically downward, away from the dielectric connection component. The method may also include the step of injecting a flowable compliant dielectric material around the leads. The terminals can be connected to an external substrate, such as a printed circuit board, to thereby provide electrical current communication to the contacts on the chip. Each terminal structure is movable with respect to the contacts on the chip in horizontal directions parallel to the chip, as well as in vertical directions towards and away from the chip, to accommodate differences in thermal expansion and contraction between the chip and the external substrate and to facilitate testing and assembly. The finished assembly can be mounted within an area of an external substrate substantially the same as that required to mount a bare chip. 
     U.S. Pat. No. 5,477,611 discloses a method of creating an interface between a chip and chip carrier including spacing the chip a give distance above the chip carrier and introducing a liquid in the gap between the chip and the carrier. Preferably, the liquid is an elastomer that 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 the chip. 
     U.S. Pat. No. 5,688,716 discloses a method of making a semiconductor chip assembly having fan-out leads. The method includes the step of providing a semiconductor chip and a package element attached to the chip. The peripheral region of the package element projects beyond the outer edge of the chip. A dielectric element having terminals on its top surface is positioned over the chip and package element such that a central region of the dielectric element overlies the chip and a peripheral region of the dielectric having at least some of the terminals thereon overlies the peripheral region of the package element. The assembly also has leads that are attached to contacts on the chip and to the terminals on the dielectric element. The method also comprises the step of moving the dielectric element and chip relative to one another such that the leads are bent into a flexible configuration. The method also comprises the step of injecting a liquid beneath the dielectric element and curing such liquid to form a compliant layer. 
     U.S. Pat. No. 5,659,952 discloses a method of fabricating a compliant interface for a semiconductor chip. The method includes the steps of providing a first support structure, such as a flexible dielectric sheet, having a porous resilient layer thereon. The resilient layer may be a plurality of compliant pads or compliant spacers. A second support element, such as a semiconductor chip, is abutted against the resilient layer and a curable liquid is disposed within the porous resilient layer. The curable liquid may then be cured to form a compliant layer. 
     Despite the positive results of the aforementioned commonly owned inventions, still further improvements would be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention relates to compliant semiconductor chip packages and to methods of making such packages. The semiconductor chip package according to one embodiment of the present invention comprises a dielectric element with a plurality of electrically conductive terminals, an expander ring connected to the dielectric element, a semiconductor chip disposed within a central opening in the expander ring, and fan-in and fan-out leads connecting the terminals to contacts on the semiconductor chip. Semiconductor chip packages having fan-in leads are disclosed in commonly assigned U.S. Pat. No. 5,258,330, the disclosure of which is incorporated herein by reference. Semiconductor chip packages having fan-out leads and semiconductor chip packages having both fan-in and fan-out leads are disclosed in commonly assigned U.S. Pat. No. 5,679,977, the disclosure of which is incorporated herein by reference. The package also comprises an encapsulant disposed in the gap between the expander ring and the semiconductor chip. The size of the gap is controlled to minimize the pressure exerted on the leads by the encapsulant as it expands and contracts in response to changes in temperature. The semiconductor chip and expander ring may also be connected to a heat sink or thermal spreader with a compliant adhesive. 
     The present invention also relates to a method of making a semiconductor chip package. The method comprises the steps of providing a dielectric element, disposing a compliant layer over the dielectric element, disposing a semiconductor chip over the compliant layer, disposing an expander ring over the compliant layer such that a gap is formed between the inner diameter of a central opening in the expander ring and the outer periphery of the semiconductor chip, and electrically interconnecting terminals on the dielectric element to contacts on the semiconductor chip. If the package is to include a thermal spreader, such thermal spreader can be attached to the semiconductor chip and/or the expander ring with an adhesive. If the coefficient of thermal expansion (hereinafter “CTE”) of the thermal spreader and the CTE of the semiconductor chip are not matched, then the adhesive should be a compliant adhesive. In preferred embodiments, the thermal spreader is attached before the contacts and the terminals are electrically interconnected. In preferred embodiments, the semiconductor chip package is encapsulated by injecting a liquid composition, which is curable to an elastomeric encapsulant, into the open spaces between the dielectric element, the semiconductor chip, the expander ring and the optional thermal spreader, including the gap between the outer periphery of the semiconductor chip and the inner diameter of the central opening of the expander ring. The compliant adhesive, the compliant layer and the encapsulant may be comprised of the same or different materials. Prior to injecting the liquid composition, it is desirable to seal the package by adhering a coverlay to the bottom surface of the dielectric element. The coverlay preferably has a plurality of holes which are dispose over and aligned with the terminals on the dielectric element. If a thermal spreader is used and the thermal spreader has relief slots, it is also desirable to adhere a protective film over the thermal spreader to seal such slots. A plurality of solder balls may be attached to the terminals. The semiconductor chip package can be connected to an external circuit via such solder balls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of one embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 2 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 3 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 4 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 5 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 6 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIG. 7 is a side view of another embodiment of the semiconductor chip assembly of the present invention. 
     FIGS. 8A-8S show views of a plurality of semiconductor chip packages in progressive steps in a manufacturing process according to one embodiment of the method of the present invention. FIGS. 8A,  8 B,  8 C,  8 E are top plan view of such packages in various steps in such manufacturing process. FIG. 8D is a top plan view of a component used in such manufacturing process. FIG. 8F is a bottom plan view of another component used in such manufacturing process. FIG. 8G is a top plan view of such packages after the component of FIG. 8F has been adhered to such packages. FIG. 8H is a bottom plan view of the packages in progress after the manufacturing step described in FIG. 8G has been completed. FIG. 8I is an exploded view of a portion of FIG.  8 H. FIG. 8J is a bottom plan view of the packages in progress after another manufacturing process step has been completed. FIG. 8K is an exploded view of a portion of FIG.  8 J. FIG. 8L is a bottom plan view of another component used in such manufacturing process. FIG. 8M is a bottom plan view of the packages in process after the component of FIG. 8L has been adhered to such packages. FIG. 8N is a top plan view of another component used in such manufacturing process. FIG. 8O is a top plan view of the packages in process after the component of FIG. 8N has been adhered to such packages. FIGS. 8P-8S are bottom plan view of such packages in various steps in such manufacturing process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As depicted in FIG. 1, the semiconductor chip assembly  1  according to one aspect of the present invention includes a semiconductor chip  2 , an expander ring  7  and a dielectric element  5 . Semiconductor chip  2  has a face surface  15 , a back surface  16  opposite the face surface, and four side surfaces  17  (two of which are visible in FIG. 1) which connect the face surface to the back surface. The four side surfaces form the outer perimeter of semiconductor chip  2 . Expander ring  7  has a first surface  20 , a second surface  21  opposite the first surface, and four inner side walls  22  (two of which are visible in FIG. 1) which define a central opening. Dielectric element  5  has a top surface  18 , a bottom surface  19  opposite top surface  18 , and a plurality of apertures  6 . Top surface  18  is comprised of a central region, which is disposed beneath the face surface  15  of semiconductor chip  2 , and a peripheral region that surrounds the central region. Descriptors such as “top”, “bottom”, “beneath”, etc, should be understood to refer to the drawing in FIG.  1  and not to any gravitational frame of reference. In preferred embodiments dielectric element  5  is flexible. Expander ring  7  is disposed over dielectric element  5  such that second surface  21  confronts the peripheral region of the top surface  18  of dielectric element  5 . The CTE of the dielectric element is preferably from 15 to 22 ppm/° C., inclusive. The CTE of the expander ring is preferably from 5 to 30 ppm/° C., inclusive. Semiconductor chip  2  is disposed within the central opening of expander ring  7  such that a gap  8  is formed between the outer perimeter of semiconductor chip  2  and the four inner side walls  22  of the central opening of expander ring  7 . A compliant layer  11  is disposed between face surface  15  of semiconductor chip  2  and top surface  18  of dielectric element  5 . The CTE of the compliant layer is preferably from 100 to 300 ppm/° C., inclusive. An adhesive  27  is disposed between the expander ring  7  and the dielectric element  5 . An encapsulant  3  is disposed within gap  8 . In preferred embodiments, 
     W≧(CTE expander ring −CTE chip )X c )/(CTE encapsulant (1+2p)); 
     where w is the width of gap  8 ; CTE expander ring  is the coefficient of thermal expansion of the expander ring; CTE chip  is the coefficient of thermal expansion of the semiconductor chip; X c  is the shortest distance between the outer edge of the chip and the center of the chip (See FIG.  1 ); CTE encapsulant  is the coefficient of thermal expansion of the encapsulant; and p is the Poisson ratio for the encapsulant. In preferred embodiments, the encapsulant is elastomeric, has a modulus of 0.5 to 600 MPa. and is comprised of a silicone gel, a silicone elastomer, a filled silicone elastomer, a urethane, an epoxy, or a flexiblized epoxy. In particularly preferred embodiments, the elastomeric encapsulant is comprised of a silicone elastomer. 
     A plurality of leads  4  interconnect contacts on the semiconductor chip  2  to terminals on the dielectric element  5 . Leads  4  may be formed by any method, including the methods disclosed in commonly assigned U.S. Pat. Nos. 5,390,844; 5,398,863; 5,489,749; 5,491,302; and 5,536,909, the disclosures of which are incorporated herein by reference. Leads  4  may also be formed by wire bonding. In preferred embodiments, the leads are comprised of gold, copper or alloys thereof or combinations thereof. 
     The leads  4  are used to electrically interconnect terminals on the dielectric element to contacts on the semiconductor chip or to electrically interconnect the terminals to an external circuit. The apertures  6  may be used to provide access for a bonding tool to the leads so that such electrically interconnections can be made. The apertures are optional and may be replaced with other means for making such electrical interconnections. One such means is an electrically conductive path disposed within such dielectric element. 
     In another embodiment of the present invention, and as depicted in FIG. 2, compliant layer  11  may include a plurality of compliant spacers  11   a . One or more such compliant spacers  11   a  may also be disposed between second surface  21  of expander ring  7  and the peripheral region of top surface  18  of dielectric element  5 . Compliant spacers  11   a  preferably have a modulus of 0.5 to 600 MPa. In preferred embodiments, the compliant spacers  11   a  are comprised of a silicone gel, a silicone elastomer or a flexiblized epoxy. In particularly preferred embodiments, the compliant spacers are comprised of a silicone elastomer. 
     In order to dissipate heat from the assembly, a thermal spreader  10  may be connected to back surface  16  of semiconductor chip  2  with a first adhesive  9 , as depicted in FIG.  3 . Thermal spreader  10  may also be connected to the first surface  20  of expander ring  7  with a second adhesive or ring adhesive  26 . The second adhesive may also be used to accommodate for differences and tolerances between the semiconductor chip and the expander ring. First adhesive  9  and second adhesive  26  may be comprised of the same or different materials. In preferred embodiments, the first and second adhesives have a modulus between 0.5 to 600 MPa. The first and second adhesives are preferably comprised of a silicone gel, a silicone elastomer, a polyimide siloxane, or a flexiblized epoxy. The first and second adhesives may further comprise one or more fillers. In preferred embodiments, at least one of such fillers has a high thermal conductivity. Such highly thermally conductive fillers may be metallic or non-metallic. In preferred embodiments the second adhesive is comprised of a silicone elastomer. For semiconductor chip packages that will be used in low power applications, the preferred first adhesive is selected from the group consisting of filled flexiblized epoxies and filled silicone elastomers. Filled flexiblized epoxies are particularly preferred. For semiconductor chip packages which will be used in medium power applications, the preferred first adhesive is selected from the group consisting of filled flexiblized epoxies, filled polyimide siloxanes and filled silicone elastomers. For semiconductor chip packages which will be used in high power applications, the preferred first adhesive is an epoxy filled with silver/glass, an epoxy filled with gold/geranium alloys, or an epoxy filled with gold/silicon alloys. 
     In an alternative embodiment, and as depicted in FIG. 4, a plurality of compliant spacers  11   b  may be disposed between thermal spreader  10  and the first surface  20  of expander ring  7 . In preferred embodiments, the compliant spacers  11   b  are comprised of a silicone gel, a silicone elastomer or a flexiblized epoxy. In particularly preferred embodiments, the compliant spacers are comprised of a silicone elastomer. 
     In preferred embodiments and as depicted in FIG. 5, semiconductor chip  2  is connected to dielectric element  5  with a compliant layer comprised of compliant spacers  11   a . Expander ring  7  is connected to the peripheral region of the top surface  18  of dielectric element  5  with a plurality of compliant spacers  11   a  and to thermal spreader  10  with a plurality of compliant spacers  11   b . Compliant spacers  11   a  and  11   b  may have similar dimensions or, as depicted in FIG. 5, different dimensions. Compliant spacers  11   a  and  11   b  may be comprised of the same or different materials. 
     As depicted in FIG. 6, terminals  23  on the dielectric element  5  may be disposed on the top surface  18  of the dielectric element  5 . Leads  4  connect contacts (not shown) on semiconductor chip  2  with terminals  23 . A plated via  24  disposed in dielectric element  5  is connected to each terminal  23 . An electrically conductive mass  13  is disposed within each via  24 . In preferred embodiments each electrically conductive mass  13  is a solder ball. 
     As depicted in FIG. 7, the semiconductor chip assembly  1  of the present invention may have both fan-in leads  4   a  and fan-out leads  4   b . Dielectric element  5  has apertures  6  which accommodate both fan-in leads  4   a  and fan-out leads  4   b . In preferred embodiments the fan-in and fan-out leads are arranged interstitially such that every other lead in a row of leads is a fan-in lead and the remaining leads are fan-out leads. Assembly  1  also has a solder mask or coverlay  14 . Coverlay  14  is disposed over the bottom surface  19  of dielectric element  5 . Coverlay  14  has a plurality of holes  25  which are aligned with terminals  23 . Assembly  1  further comprises a plurality of electrically conductive masses  13  which are disposed in such holes  25 . Masses  13  can be used to electrically and physically connect the assembly to an external circuit, such as a printed circuit board. 
     The dielectric element described with reference to the above semiconductor chip packages and methods for making the same preferably is a flexible dielectric element. In particularly preferred embodiments, the dielectric element is a thin sheet of a polymeric material such as a polyimide, a fluoropolymer, a thermoplastic polymer, or an elastomer, with polyimide being a particularly preferred material for use as the flexible dielectric element. In preferred embodiments, the flexible dielectric element is from 10 to 100 microns and more preferably from 25 to 75 microns thick. 
     Each expander ring is used to support the solder balls which are attached to the terminals of the fan-out leads and to add structural stability to the package. The strip of expander rings may be made of any material which will support the solder balls. The expander rings may be made a conductive or a non-conductive material. The expander rings may be made of a metal, a plastic, or a paper based material. In preferred embodiments, the expander rings are comprised of a material selected from alloy 42, copper, invar, steel, polypropylene, epoxy or paper phenolic, or alloys thereof, or combinations thereof. In particularly preferred embodiments, the expander rings are comprised of a material selected from copper, copper alloys, steel and combinations thereof. The expander ring may be thicker or thinner than the associated semiconductor chip. In preferred embodiments however, the thickness of the expander ring is less than or equal to the thickness of the semiconductor die. The CTE of the expander ring is preferably intermediate between the CTE of the semiconductor chip and the CTE of the dielectric element. If the package contains a thermal spreader, the CTE of the thermal spreader is preferably low, close to the CTE of the semiconductor chip, and the CTE of the expander ring is preferably intermediate between the CTE of the thermal spreader and the CTE of the dielectric: element. In preferred embodiments, the CTE of the thermal spreader is from 5 to 30 ppm/° C., inclusive. One or more capacitors, transistors, and/or resistors may be embedded in the expander ring and/or on the dielectric element and electrically connected, via wire bonds, solder or a conductive adhesive, to one or more terminals on the dielectric element. 
     The thermal spreader is made from a material having a high thermal conductivity. In preferred embodiments, the CTE of the thermal spreader is close to the CTE of the semiconductor chip. For semiconductor chip packages which will be used in low power applications, the thermal spreader is preferably made from a material selected from the group consisting of copper, copper alloys, nickel plated copper alloys, aluminum, aluminum alloys, anodized aluminum alloys, and steel. For semiconductor chip packages which will be used in medium power applications, the thermal spreader is preferably made from a material selected from the group consisting of copper, copper alloys, alloy 42 and multi-layered laminates containing copper coated invar. The preferred multi-layer laminate is copper-invar-copper. For semiconductor chip packages which will be used in high power applications, the thermal spreader is preferably made of a material selected from the group consisting of aluminum nitride and tungsten copper. 
     The coverlay may be a temporary coverlay or a permanent coverlay. The coverlay material must be capable of being bonded, at least temporarily, to the dielectric element and of sealing any apertures or holes in such element. The coverlay is preferably½ mil to 10 mils thick, more preferably ½ mil to 5 mils thick, most preferably less than 2.5 mils thick. The coverlay material is preferably comprised of polypropylene, polyester, polyimide or combinations thereof, with polyimide being particularly preferred for use as a permanent coverlay and polypropylene being particularly preferred for applications using a temporary coverlay. Materials which are commonly used as solder masks, such as solder masks sold under Dupont&#39;s brand name Pyralux® may also be used as a coverlay. Dupont&#39;s Pyralux® solder mask are generally photoimageable, dry film solder masks which are based on acrylic, urethane and -imide based materials. The coverlay may also comprise an adhesive layer. The adhesive layer is preferably comprised of an acrylic, epoxy or silicone adhesive, with acrylic adhesives being particularly preferred. Prior to the step in which the coverlay is laminated to the dielectric element, the adhesive layer must be tacky or must be in a form that is heat and/or pressure activated. In preferred embodiments, the coverlay used in the present invention is a permanent coverlay. The coverlay may have a plurality of apertures. If the coverlay is comprised of a photoimageable material, the apertures may be formed in the coverlay after it is attached to the dielectric element. 
     The semiconductor chip package of the present invention can be made according to the method of the present invention. FIGS. 8A-8S depict various steps in one method of the present invention. As depicted in FIG. 8A, a dielectric element  101  is provided. In preferred embodiments, dielectric element  101  is flexible. Dielectric element  101  is in a strip form and has a top surface  102 , a bottom surface (not shown) opposite top surface  102 , and a plurality of apertures  104 . Apertures  104  are sometimes also referred to as bond windows. The flexible dielectric element described with reference to the above semiconductor chip packages and methods for making the same is preferably a thin sheet of a polymeric material such as a polyimide, a fluoropolymer, a thermoplastic polymer, or an elastomer, with polyimide being a particularly preferred material for use as the flexible dielectric element. In preferred embodiments, the flexible dielectric element is from 10 to 100 microns and more preferably from 25 to 75 microns thick. Polyimide in strip form is generally supplied with a plurality of sprocket holes  105 . Although such sprocket holes may be used as an alignment aid in the method of the present invention, such sprocket holes are not required to practice the present method. 
     Flexible dielectric element  101  has a plurality of electrically conductive traces  106 . Only a portion of each trace is visible through the bond windows  104 . Each trace  106  has a contact end and a terminal-end. The contact-end will eventually be connected to a contact on the face surface of semiconductor chip  108 . Neither the tip nor the terminal-ends are visible in FIG.  8 A. Traces  106  may be disposed on either the top surface  102  or the bottom surface  103  of the flexible dielectric element  101 . In the embodiment pictured in FIGS. 8A-8S, traces  106  are disposed on the bottom surface  103  (See FIG.  8 H). 
     As depicted in FIG. 8B, a plurality of compliant spacers  107  are disposed on the top surface  102  of flexible dielectric element  101 . Some methods of disposing such compliant spacers or resilient elements are described in commonly assigned U.S. Pat. No. 5,659,952 and U.S. patent application with Ser. No. 08/879,922 and a filing date of Jun. 20, 1997, the disclosures of which are incorporated herein by reference. In preferred embodiments, the compliant spacers  107  are comprised of a silicone gel, a silicone elastomer or a flexiblized epoxy. The compliant spacers preferably have a modulus of 0.5 to 600 MPa. In particularly preferred embodiments, the compliant spacers are comprised of a silicone elastomer. Prior to die attach some or all of the compliant spacers  107  may be in an uncured, partially cured or fully cured state. An adhesive may be disposed on the top surface of such spacers  107 . Commonly assigned U.S. patent application with Ser. No. 08/931,680 and a filing date of Sep. 16, 1997, the disclosure of which is incorporated herein by reference, teaches one method of disposing an adhesive over a compliant spacer or compliant pad. 
     As depicted in FIG. 8C, a plurality of semiconductor chips  108  are then disposed over the top surface  102  of flexible dielectric element  101 . Each chip  108  has a face surface (not shown), a back surface  111  opposite the face surface, and a plurality of electrically conductive contacts (not shown) disposed on the face surface  110 . Each chip  108  is positioned over one set of bond windows  104  and the face surface of each is adhered to flexible dielectric element  101 . If compliant spacers  107  are in an uncured state, a partially cured state, or have an adhesive disposed on the top surfaces of such spacers, chips  108  may be adhered to flexible dielectric element  101  using such spacers  107 . Heat and pressure may be required to achieve a good bond between spacers  107  and chips  108 . 
     As depicted in FIG. 8D, a strip of expander rings  109  is provided. Each expander ring  109  has a first surface  112 , a second surface (not shown) opposite first surface  112 , and four inner side walls  113  which define a central opening  114 . Each expander ring is used to support the solder balls which are attached to the terminals of the fan-out leads and to add structural stability to the package. Various methods of packaging semiconductor chips using expander rings are described in co-pending, commonly assigned U.S. patent application Ser. No. 09/067,310, having a filing date of Apr. 28, 1998, the disclosure of which is hereby w incorporated herein by reference. The expander rings of the &#39;310 application are referred to as unitary support structures. 
     The strip of expander rings  109  may be made of any material which will support the solder balls. The expander rings may be made of a conductive or a non-conductive material. The expander rings may be made of a metal, a plastic, or a paper based material. In preferred embodiments, the expander rings are comprised of a material selected from alloy 42, copper, invar, steel, polypropylene, epoxy or paper phenolic, or alloys thereof, or combinations thereof. In particularly preferred embodiments, the expander rings are comprised of a material selected from copper, copper alloys, steel and combinations thereof. The expander ring may be thicker or thinner than the associated semiconductor chip. In preferred embodiments however, the thickness of the expander ring is less than or equal to the thickness of the semiconductor die. The CTE of the expander ring is preferably intermediate between the CTE of the semiconductor chip and the CTE of the flexible dielectric element. If the package contains a thermal spreader, the CTE of the thermal spreader is preferably low, close to the CTE of the semiconductor chip, and the CTE of the expander ring is preferably intermediate between the CTE of the thermal spreader and the CTE of the flexible dielectric element. One or more capacitors, resistors, and/or transistors, may be embedded in the expander ring and electrically connected, via wire bonds, solder or a conductive adhesive, to one or more terminals on the flexible dielectric element. 
     As depicted in FIG. 8E, the strip of expander rings  109  is disposed over the flexible dielectric element  101  such that a) the second surface of each expander ring  109  confronts the top surface  102  of the flexible dielectric element  101 ; b) the central opening  114  of each expander ring  109  is disposed around one of the semiconductor chips  108 ; and c) for each semiconductor chip  108 , a gap  115  is maintained between each inner side wall  113  and the outer perimeter of the semiconductor chip  108 . In preferred embodiments, 
     W≧{(CTE expander ring −CTE chip )X c }/{CTE encapsulant (1+2p)}; 
     where w is the width of gap  115 ; CTE expander ring  is the coefficient of thermal expansion of the expander ring; CTE chip  is the coefficient of thermal expansion of the semiconductor chip; X c  is the shortest distance between the outer edge of the chip and the center of the chip; CTE encapsulant  is the coefficient of thermal expansion of the encapsulant; and p is the Poisson ratio for the encapsulant which will be disposed within the gap. With some chips, such as, for example rectangular chips, Xc is not constant for all points on the outer edge of the chip. For such chips, w can be calculated for each point on the outer edge of the chip. The gap between the chip and the expander ring, as measured at each such point on the outer edge of the chip should be at least the value of w calculated for that point. In preferred embodiment however, the width of the gap is constant and is selected to be at least as wide as the highest value of w calculated for the chip. 
     In preferred embodiments, the encapsulant is elastomeric. In more preferred embodiments, the elastomeric encapsulant has a modulus of 0.5 to 600 MPa. and is comprised oF a silicone gel, a silicone elastomer, a filled silicone elastomer, a urethane, an epoxy, or a flexiblized epoxy. In particularly preferred embodiments, the elastomeric encapsulant is comprised of a silicone elastomer. 
     The strip of expander rings  109  may have one or more fidicuals to aid in the proper alignment of the expander rings on the flexible dielectric element. The sprocket holes  105  may also be used to aid in the alignment of the expander rings. The second surface of each of the expander rings  109  is adhered to the compliant spacers  107 , preferably using heat and/or pressure. In preferred embodiments, the first surface  112  of each expander ring  109  is coplanar with the back surface  111  of each semiconductor chip  108 . The second surface of the expander ring may be coplanar with the face surface of each semiconductor chip  108 . Such heat and pressure can also be used to correct for any lack of coplanarity between each expander ring  109  and the associated semiconductor chip  108 . 
     As depicted in FIG. 8F, a strip of thermal spreaders  116  is provided. The strip of thermal spreaders  116  has an alpha surface (not shown) and a beta surface  117  opposite the alpha surface. The thermal spreader is made from a material having a high thermal conductivity. In preferred embodiments, the CTE of the thermal spreader is close to the CTE of the semiconductor chip. For semiconductor chip packages which will be used in low power applications, the thermal spreader is preferably made from a material selected from the group consisting of copper, copper alloys, nickel plated copper alloys, aluminum, aluminum alloys, anodized aluminum alloys, and steel. For semiconductor chip packages which will be used in medium power applications, the thermal spreader is preferably made from a material selected from the group consisting of copper, copper alloys, alloy 42 and multi-layered laminates containing copper coated invar. The preferred multi-layer laminate is copper-invar-copper. For semiconductor chip packages which will be used in high power applications, the thermal spreader is preferably made of a material selected from the group consisting of aluminum nitride and tungsten copper. 
     The strip of thermal spreaders  116  may have a plurality of elongated slots  119 . Such slots  119  are incorporated in the strip of thermal spreaders  116  to ease the singulation process in which the strip of packaged semiconductor chips are cut into individual packages. The strip of thermal spreaders  116  may have one or more fiducials to aid in the alignment of the thermal spreaders. The strip of thermal spreaders may be aligned with sprocket holes  105  in flexible dielectric element  101  to aid in the positioning of the thermal spreaders. A first adhesive  118  is disposed on the beta surface  117 . Such adhesive may take for example, the form of a pad, a film or a dispensed pattern such as a plurality of dots of adhesive. Adhesive  118  will eventually be used to bond beta surface  117  to the back surfaces of each of semiconductor chips  108 . A second adhesive or ring adhesive  118 ′ may also be disposed on beta surface  117  and be in the form of a pad, a film or a plurality of dots. Second adhesive  118  may be used to accommodate for differences and tolerances between the semiconductor chip and the expander ring. The dots of adhesive  118 ′ will eventually be used to bond beta surface  117  to first surface  112  of each expander ring  109 . If the CTE of the strip of thermal spreaders  116  and the CTE of the semiconductor chips is not matched, then adhesive  118  should be compliant. In preferred embodiments, both adhesives  118  and  118 ′ are compliant. Adhesives  118  and  118 ′ may be comprised of the same or different materials. In preferred embodiments, the first and second adhesives are comprised of a silicone gel, a silicone elastomer, a polyimide siloxane, or a flexiblized epoxy. The first and second adhesive may further comprises one or more fillers. In preferred embodiments, at least one of such fillers has a high thermal conductivity. Such highly thermally conductive fillers may be metallic or non-metallic. In preferred embodiments, the first and second adhesives have a modulus between 0.5 to 600 MPa. and are comprised of a silicone gel, a silicone elastomer, a polyimide siloxane, or a flexiblized epoxy. In particularly preferred embodiments the second adhesive is comprised of a silicone elastomer. For semiconductor chip packages which will be used in low power applications, the preferred first adhesive is selected from the group consisting of filled flexiblized epoxies and filled silicone elastomers. Filled flexiblized epoxies are particularly preferred. For semiconductor chip packages which will be used in medium power applications, the preferred first adhesive is selected from the group consisting of filled flexiblized epoxies, filled polyimide siloxanes and filled silicone elastomers. For semiconductor chip packages which will be used in high power applications, the preferred first adhesive is an epoxy filled with silver/glass, an epoxy filled with gold/geranium alloys, or an epoxy filled with gold/silicon alloys. The dimensions of the dots of adhesives  118  and  118 ′ may be the same or different. 
     The strip of thermal spreaders  116  is disposed over semiconductor chips  108  and expander rings  109  such that the beta surface  117  of the strip of thermal spreaders  116  confronts the back surfaces  111  of semiconductor chips  108  and the first surfaces  112  of each expander ring  109 . The strip of thermal spreaders  116  is adhered to such back surfaces and first surfaces with the adhesives  118  and  118 ′. Once this is complete, the alpha surface  120  of the strip of expander rings  109  is visible from a top plan view, as depicted in FIG.  8 G. 
     FIG. 8H is a view of the bottom surface  103  of the flexible dielectric element  101  prior to the processing step in which the leads are formed. A portion of the face surface  110  of each chip  108  is visible in FIG.  8 H through bond windows  104 . FIG. 8H also depicts a plurality of electrically conductive traces  121  disposed on the bottom surface  103  of flexible dielectric element  101 . FIG. 8I is an exploded view of a portion of FIG. 8H, depicting more details of traces  121 . Each trace  121  has a terminal  122  and a contact-end  123 . Some of the traces  121  have a terminal  122  that is disposed on a portion of flexible dielectric element  101  which lies underneath the face surface  110  of semiconductor chip  108 . The directional descriptor “underneath”, as used to describe FIG. 8H (which is a bottom plan view), should be read to mean “below when viewed from a top plan view” and not with reference to any gravitational frame of reference. Some traces will eventually be formed into “fan-in” leads. Some of the traces (such as trace  121 ′) have a terminal  122 ′ that is disposed on a portion of the flexible dielectric element which lies underneath the second surface of expander ring  109 . Such traces  121 ′ will eventually be formed into “fan-out” leads. The package depicted in FIG. 8I has a total of 26 traces. In preferred embodiments, the package will have 40 or more leads, more preferably 40 to 1000 leads. In preferred embodiments, terminals  122  and  122 ′ are disposed in ordered rows or an area array having a consistent pitch. In preferred embodiments, the fan-in and fan-out leads are comprised of gold, copper or alloys thereof or combinations thereof. 
     FIG. 8J depicts the flexible dielectric element  101  and the plurality of chips  108  after the fan-in and fan-out leads have been formed. FIG. 8K is an exploded view of a portion of FIG.  8 J. As depicted in FIG. 8K, the contact-end  123  of each trace  121  is bonded to an electrically conductive contact on the face surface  110  of semiconductor chip  108  to form a fan-in lead  124  which electrically interconnects the chip  108  to the flexible dielectric element  101 . The contact-end  123 ′ of each trace  121 ′ is bonded to an electrically conductive contact on the face surface  110  of semiconductor chip  108  to form a fan-out lead  124 ′. The fan-in and fan-out leads may be formed by any method, including the methods disclosed in commonly assigned U.S. Pat. Nos. 5,390,844; 5,398,863; 5,489,749; 5,491,302; and 5,536,909, the disclosures of which are incorporated herein by reference. In an alternative embodiment, the fan-in and fan-out leads may be formed by wire bonding each contact to the respective terminal. 
     Next, the bond windows  104  and any other apertures or holes in flexible dielectric element  101  are sealed using a coverlay, such as coverlay  125  which is depicted in FIG.  8 L. 
     The coverlay may be a temporary coverlay or a permanent coverlay. The coverlay material must be capable of being bonded, at least temporarily, to the flexible dielectric element and of sealing any apertures or holes in such element. The coverlay is preferably ½ mil to 10 mils thick, more preferably ½ mil to 5 mils thick, most preferably less than 2.5 mils thick. The coverlay material is preferably comprised of polypropylene, polyester, polyimide or combinations thereof, with polyimide being particularly preferred for use as a permanent coverlay and polypropylene being particularly preferred for applications using a temporary coverlay. Materials which are commonly used as solder masks, such as solder masks sold under Dupont&#39;s brand name Pyralux® may also be used as a coverlay. Dupont&#39;s Pyralux® solder masks are generally photoimageable, dry film solder mask which are based on acrylic, urethane and -imide based materials. The coverlay may also comprise an adhesive layer. The adhesive layer is preferably comprised of an acrylic, epoxy or silicone adhesive, with acrylic adhesives being particularly preferred. Prior to the step in which the coverlay is laminated to the flexible dielectric element, the adhesive layer must be tacky or must be in an activatable form, such as a heat and/or pressure activated from. In preferred embodiments, the coverlay used in the present invention is a permanent coverlay. The coverlay may have a plurality of apertures. If the coverlay is comprised of a photoimageable material, the apertures may be formed in the coverlay after it is attached to the flexible dielectric element. 
     The coverlay depicted in FIG. 8L is photoimageable and has been exposed in a pattern corresponding to the pattern of terminals on the flexible dielectric element. As depicted in FIG. 8M, coverlay  125  is laminated to the bottom surface  103  of flexible dielectric element  101 . The coverlay may be vacuum laminated, pressure laminated, vacuum-pressure laminated or otherwise laminated onto the bottom surface  103  of the flexible dielectric element  101 . FIG. 8M depicts the bottom surface  103  after a transparent coverlay  125  has been laminated to it. 
     A protective film  127  is provided, as depicted in FIG.  8 N. The protective film of the present invention can be any of the materials listed above for the coverlay. In preferred embodiments, however, the protective film used in the present invention is a temporary coverlay which is removed after use. The protective film may be removed by, for example, using heat, peeling the film from the strip of thermal spreaders, or immersing the protective film in a caustic solution. Protective film  127  is used to seal the elongated slots and any other apertures in thermal spreader  116  while a liquid composition is injected into the assembly to encapsulated it. Protective film  127  should be capable of being bonded to the alpha surface of thermal spreader  116 . Since protective film  127  may be removed after the encapsulation process, in preferred embodiments, protective film  127  forms only a temporary bond to the alpha surface of the strip of thermal spreaders  116 . As depicted in FIG. 8O, film  127  is adhered to the thermal spreader  116  to seal the elongated slots  119 . 
     After coverlay  125  has been laminated to flexible dielectric element  101  and after protective film  127  has been adhered to the alpha surface of the strip of thermal spreaders  116 , the assembly can be encapsulated using a liquid composition which is curable to an encapsulant In preferred embodiments the encapsulant is elastomeric. The elastomeric encapsulant increases the reliability of the assembly by compensating for the mismatch in CTE between the semiconductor chip package and an external circuit. The liquid composition is disposed between the top surface  102  of the flexible dielectric element  101  and the thermal spreader  116 . The liquid composition fills the open spaces between any of the expander ring, the thermal spreader, the semiconductor chip, the flexible dielectric element, the compliant adhesive, and the compliant spacers. The liquid composition also fills in gap  115  (see FIG. 8E) between the expander ring  109  and the semiconductor chip  108 . The assembly may be encapsulated with the liquid composition via a dispensing operation, a dispensing operation followed by subjecting the assembly to vacuum and or pressure, a dispensing operation preformed while the assembly is under vacuum, or by a pressurized injection operation. Various methods of encapsulating the assembly are disclosed, for example, in commonly assigned U.S. patent application Ser. No. 09/067,698 filed on Apr. 28, 1998. 
     FIG. 8P depicts the assembly of the present invention after the strip has been vacuum impregnated with liquid composition  126 . Terminals  122  and  122 ′ and a portion of each lead  124  and  124 ′ are visible in FIG.  8 P. The coverlay  125  seals against the bottom surface  103  of the flexible dielectric element  101  to prevent the liquid encapsulant  126  from contaminating terminals  122  and  122 ′. After being impregnated into the assembly, liquid composition  126  is cured or at least partially cured. Protective film  127  may then removed from thermal spreader  116 . Holes are formed in coverlay  125  by exposing the photimageable coverlay to a developer, such as potassium carbonate. The holes are formed in a pattern corresponding to the pattern of terminals  122  and  122 ′ on flexible dielectric element  101 . Flux is then applied on the terminals and, as depicted in FIG. 8Q, solder balls  128  are disposed within the holes in coverlay  125 . The solder balls are reflowed. The plurality of semiconductor chips  108  are then singulated as depicted in FIG.  8 R and FIG. 8S to form a plurality of packaged semiconductor chip assemblies  129 . 
     The method described with reference to FIGS. 8A-8S employs various process steps which are conducted on components in strip format. The method of the present invention may also be practiced with components that are supplied in a reel to reel format.