Abstract:
A connection component including a flexible substrate having a top surface and a bottom surface, a layer of a compliant, dielectric material overlying the top surface of the substrate, the compliant material layer having a top surface remote from the substrate, an array of flexible, conductive leads having first ends attached to terminals accessible at the bottom surface of the substrate and second ends adjacent the top surface of the compliant layer, wherein each lead comprises a core of a first conductive material surrounded by a layer of a second conductive material, the second conductive material having a greater yield strength than the first conductive material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. application Ser. No. 09/955,695 filed Sep. 19, 2001 U.S. Pat. No. 6,589,819, which claims benefit of U.S. Provisional Application 60/236,395, filed Sep. 29, 2000, the disclosures of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microelectronic packages having leads or traces and specifically relates to microelectronic packages having arrays of resilient leads and methods of making such microelectronic packages. 
     BACKGROUND OF THE INVENTION 
     Complex microelectronic devices such as semiconductor chips typically require numerous connections to other electronic components. For example, a complex device including a semiconductor chip may require hundreds of electrical connections between the chip and one or more external devices. These electrical connections may be made using several alternative methods, including wire bonding, tape automated bonding and flip-chip bonding. Each of these techniques presents various problems including difficulty in testing the chip after bonding, long lead lengths, large areas occupied by the chip on a microelectronic assembly, and fatigue of the connections due to changes in size of the chip and the substrate during thermal expansion and contraction. 
     In many microelectronic devices, it is desirable to provide an electrical connection between components that can accommodate relative movement between the components. For example, where a semiconductor chip is mounted to a circuit board, thermal expansion and contraction of the chip and circuit board can cause the contacts on the chip to move relative to contacts on the circuit board. This movement can occur during operation of the device and can also occur during manufacturing operations (e.g. when soldering the chip to the circuit board). 
     One structure that has been used to successfully address these problems is commonly referred to as an “interposer” or “chip carrier”, such as that shown in certain preferred embodiments of commonly assigned U.S. Pat. Nos. 5,148,265, 5,148,266 and 5,455,390, the disclosures of which are hereby incorporated by reference herein. Interposers typically include a flexible, sheet-like element having a plurality of terminals disposed thereon, and including flexible leads used to connect the terminals with contacts on a microelectronic element, such as a semiconductor chip or wafer. The flexible leads permit thermal expansion of the various components without inducing stresses in the connection. The terminals of the interposer may then be used to test the assembly, and/or permanently attach the assembly to another microelectronic element. 
     A compliant layer may be disposed between a microelectronic element and the interposer. The compliant layer typically encapsulates the leads connecting the interposer and microelectronic element and facilitates connection of the terminals to a test device and/or to the final electronic assembly by compensating for variations in component flatness and terminal heights. 
     As illustrated in certain preferred embodiments of commonly assigned U.S. Pat. No. 5,518,964 (“the &#39;964 patent”), the disclosure of which is hereby incorporated by reference herein, an array of moveable electrical connections between two microelectronic elements, such as a semiconductor chip and a substrate, can be provided by first connecting leads between the microelectronic elements and then moving the elements away from one another through a predetermined displacement so as to bend the leads. One of the microelectronic elements may be a connection component including a dielectric body having leads extending along a surface of the dielectric body. The leads may have first ends permanently attached to the dielectric body and second ends releasably attached to the dielectric body. The dielectric body, with the leads thereon, may be juxtaposed with a semiconductor chip having contacts and the second releasable ends of the leads may be bonded to the contacts on the chip. Following bonding, the dielectric body and chip are moved away from one another, thereby bending the leads toward a vertically extensive disposition. During or after movement, a curable material such as a liquid composition may be introduced between the elements. The curable material may then be cured, such as by using heat, to form a compliant dielectric layer surrounding the leads. The resulting semiconductor chip package has terminals on the dielectric body or connection component which are electrically connected to the contacts on the chip, but which can move relative to the chip so as to compensate for thermal effects. For example, the semiconductor chip package may be mounted to a circuit board by solder-bonding the terminals to conductive pads on the circuit board. Relative movement between the circuit board and the chip due to thermal effects is allowed by the moveable interconnection provided by the leads and the compliant layer. 
     In other embodiments of the &#39;964 patent, the package-forming process can be conducted on a wafer scale, so that all of the semiconductor chips in a wafer may be connected to connection components in a single step. The resulting wafer package is then severed so as to provide individual units, each including one or more of the chips and a portion of the dielectric body. The above-described leads may be formed on the chip or wafer, rather than on the dielectric body. In further embodiments of the &#39;964 patent, a dielectric body having terminals and leads is connected to terminal structures on a temporary sheet. The temporary sheet and dielectric body are moved away from one another so as to vertically extend the leads, and a curable liquid material is introduced around the leads and cured so as to form a compliant layer between the temporary sheet and the dielectric body. The temporary sheet is then removed, leaving the tip ends of the terminal structures projecting from a surface of the compliant layer. Such a component, commonly referred to as a connection component, may be used between two other components. For example, the terminal structures may be engaged with a semiconductor chip and the terminals engaged with a circuit panel or other microelectronic component. 
     In certain preferred embodiments of commonly assigned U.S. Pat. No. 6,117,694, the disclosure of which is hereby incorporated herein by reference, a microelectronic component, such as a connector or a packaged semiconductor device, is made by connecting multiple leads between a pair of elements and moving the elements away from one another so as to bend the leads toward a vertically extensive disposition. One of the elements may include a temporary support that may be removed after bending the leads 
     After the leads interconnect the microelectronic elements, an encapsulant, such as a flowable, curable dielectric material, may be injected between the microelectronic elements. The encapsulant may be injected between the microelectronic elements immediately after bonding, whereby the force of the pressurized encapsulant acting on the elements separates them and bends the leads, forming a compliant lead configuration. Alternatively, the leads may be formed before injecting the encapsulant by retaining the elements against moveable platens by vacuum, and moving the platens with respect to each other, bending and forming the leads. The encapsulant is then injected while the dielectric sheet and the wafer are in their displaced positions. 
     After the flowable, curable dielectric material has been cured, the microelectronic assembly may be removed from the fixture, trimmed and tested. The fixture may then be reused to perform the above operations on the next microelectronic assembly. 
     Despite these and other advances in the art, still further improvements would be desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with certain preferred embodiments of the present invention, a method of making a microelectronic package having an array of resilient leads includes providing a first element having a plurality of conductive leads on a first surface thereof. The conductive leads preferably have terminals ends permanently attached to the first element and tip ends remote from the terminal ends, the tip ends of the conductive leads being movable relative to the terminal ends. The method preferably includes providing a second element having a plurality of contacts on a first surface thereof and juxtaposing the first surface of the second element with the first surface of the first element. The tip ends of the conductive leads may then be connected with the contacts of the second microelectronic element. The first and second microelectronic elements may then be moved away from one another so as to vertically extend the conductive lead between the first and second microelectronic elements. After the moving step, a layer of a spring-like material may then be formed over the conductive leads. The layer of a spring-like material preferably has greater yield strength than the conductive leads. Although the present invention is not limited by any particular theory of operation, it is believed that providing a layer of a spring-like material having greater yield strength then the yield strength of the conductive leads produces a highly resilient composite lead able to withstand substantial flexing and bending. As used herein, the term “composite lead” means a conductive lead or trace having a core made of a first conductive material that is coated by a shell of a second conductive material. 
     The conductive leads may be made of a material selected from the group consisting of aluminum, gold, copper, tin, and their alloys and combinations thereof. The layer of a spring-like material formed over the conductive leads is preferably selected from the group consisting of nickel, copper, cobalt, iron, gold, silver, platinum, noble metals, semi-noble metals, tungsten, molybdenum, tin, leads, bismuth, indium, their alloys, and combinations thereof. 
     In certain preferred embodiments, the method includes depositing a curable liquid encapsulant between the first and second microelectronic elements and around the vertically extended composite leads. In preferred embodiments, the curable liquid encapsulant is selected from the group consisting of materials that are curable to elastomers and adhesives. The preferred elastomers and adhesives are selected from the group consisting of silicones and epoxies. In highly preferred embodiments, the curable liquid encapsulant is a composition which is curable to a silicone elastomer. After the curable liquid encapsulant is deposited, the encapsulant may be cured to provide a compliant layer between the first and second microelectronic elements and around the vertically extended leads. In preferred embodiments, the terminals are accessible at the second surface of the second microelectronic element. Conductive elements, such as solder balls, may then be attached to the terminals ends of the leads. The conductive elements may be fusible masses of conductive metal, such as tin/lead solder balls. 
     In preferred embodiments, the first element and/or the second element is a microelectronic element. In preferred embodiments, the microelectronic elements are selected from the group consisting of a semiconductor chips, semiconductor wafers, packaged semiconductor chips or wafers, dielectric sheets, flexible substrates, flexible circuitized substrates, printed circuit boards, and sacrificial layers. In more preferred embodiments, the first and second microelectronic elements are selected from the group consisting of semiconductor chips, semiconductor wafers, and flexible substrates. In particularly preferred embodiments, the first microelectronic element is a chip and the second microelectronic element is a flexible substrate. In certain embodiments, at least one of the microelectronic elements may be a sacrificial layer. The sacrificial layer may be removed during one step of an assembly process so as to expose either the terminal ends or the tip ends of the conductive leads. In an alternative embodiment, the sacrificial layer may be conductive and the terminal ends or the tips ends of the conductive leads may be formed and/or exposed by removing a portion of the conductive sacrificial layer. 
     In other embodiments, a method of making a microelectronic package having a plurality of resilient leads includes providing a first microelectronic element having conductive leads extending along a first surface thereof, the conductive leads having terminal ends permanently attached to the first microelectronic element and tip ends releasably secured to the first microelectronic element. A second microelectronic element having contacts on a first surface thereof, may then be juxtaposed with the first surface of the first microelectronic element and the tip ends of the conductive leads may be connected with the contacts of the second microelectronic element. The first and second microelectronic elements may then be moved away from one another so as to vertically extend the conductive leads between the first and second microelectronic elements. In certain preferred embodiments, the tip ends of the conductive leads may be releasably secured to the first microelectronic element. After the moving step, a layer of a spring-like material may be formed over the conductive leads. The layer of a spring-like material may be formed by plating a conductive metal over the conductive leads. The conductive metal plated over the conductive leads preferably has a higher yield strength than the conductive leads. A curable liquid encapsulant may then be disposed between the first and second microelectronic elements and around the conductive leads. The curable liquid encapsulant may be cured to form a compliant layer between the microelectronic elements and around the leads. 
     In still other preferred embodiments of the present invention, a method of making a microelectronic package includes providing a first microelectronic element having a first surface with a plurality of conductive leads formed thereon, each lead having a first end permanently attached to the first microelectronic element and a second end movable away from the first microelectronic element. A second microelectronic element having conductive pads accessible at a first surface thereof may then be provided and the first surface of the first microelectronic element may be juxtaposed with the first surface of the second microelectronic element. The method may also include attaching the second ends of the conductive leads with the conductive pads or contacts of the second microelectronic element. After the attaching step, the first and second microelectronic elements may be moved away from one another so as to vertically extend the conductive leads. After the moving step, a layer of a conductive metal may be formed over the conductive leads, the layer of a conductive metal having a greater yield strength then the conductive leads. 
     In certain preferred embodiments, the first microelectronic element is a flexible substrate, such as a flexible dielectric sheet, and the second microelectronic element is a semiconductor chip or a semiconductor wafer. In other preferred embodiments, the first microelectronic element is a semiconductor chip or wafer and the second microelectronic element is a flexible substrate such as a flexible dielectric sheet. If the first microelectronic element is a wafer, the method may also include severing the semiconductor wafer and the flexible dielectric sheet to provide a plurality of semiconductor packages, whereby each semiconductor package includes at least one semiconductor chip connected to a portion of the flexible dielectric sheet. 
     In yet other preferred embodiments of the present invention, a method of making semiconductor packages having resilient leads includes providing a semiconductor chip or wafer having a plurality of contacts on a first surface thereof, and providing a flexible dielectric sheet having a plurality of conductive leads over a first surface thereof, whereby each lead has a terminal end permanently attached to the flexible dielectric sheet and a tip end movable away from the first surface of the dielectric sheet. The tip ends of the leads may then be electrically interconnected with the contacts of the wafer. If a wafer is used, then the wafer and the dielectric sheet may then be moved away from one another in a controlled fashion so as to vertically extend the leads. After the wafer and dielectric sheet are moved away from one another, a layer of a spring-like material may be formed over the outer surface of the conductive leads to form composite leads. The spring-like material desirable has a greater yield strength than the yield strength of the conductive leads. A layer of a compliant material may then be disposed between the wafer and the dielectric sheet and around the composite leads. The wafer and the dielectric sheet may then be severed so as to provide a plurality of semiconductor packages, each semiconductor package including at least one semiconductor chip and a portion of the dielectric sheet. 
     In further preferred embodiments of the present invention, a method of making a microelectronic element includes providing a dielectric sheet having a plurality of conductive leads overlying the first surface of the sheet and a plurality of terminals accessible at a second surface of the sheet, whereby each lead has a first end permanently attached to one of the terminals and a second end movable away from the first surface of the dielectric sheet. The method includes providing a fixture having a first surface and a plurality of contacts accessible at the first surface of the fixture, juxtaposing the first surface of the fixture with the first surface of the dielectric sheet and attaching the second ends of leads with the contacts of the fixture. After the attaching step, the fixture and the dielectric sheet may be moved away from one another so as to vertically extend the leads. A layer of a conductive spring-like material may then be formed over the exterior surface of the conductive leads to form composite leads, and a layer of a curable liquid encapsulant may be provided between the fixture and the dielectric sheet and around the composite leads. The encapsulant may then be cured to form a compliant layer. After the curing step, the fixture may be removed so as to expose the contacts at a top surface of the package. 
     In yet other preferred embodiments of the present invention, a connection component includes a flexible substrate having a top surface and a bottom surface, and a layer of a compliant dielectric material overlying the top surface of the substrate, the compliant material having a top surface remote from the substrate. The connection component includes an array of flexible, conductive leads having first ends attached to terminals accessible at the second surface of the said substrate and second ends adjacent the top surface of the compliant layer. Each lead preferably includes a core made of a first conductive material that is surrounded by a layer of a second conductive material. Such leads are similar to the composite leads described above. The second conductive material of the lead preferably has a greater yield strength than the first conductive material. The second ends of the leads may be accessible at the tope surface of the compliant layer. The connection component may also include contacts attached to the second ends of the leads, the contacts being accessible at the top surface of the compliant layer. 
     These and other preferred embodiments of the present invention will be set forth in further detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1G  show a method of making a microelectronic package having an array of resilient leads, in accordance with one preferred embodiment of the present invention. 
         FIGS. 2A-2I  show a method of making microelectronic packages having arrays of resilient leads, in accordance with further preferred embodiments of the present invention. 
         FIGS. 3A-3H  show a method of making a compliant connection component having an array of resilient leads, in accordance with still further preferred embodiments of the present invention. 
         FIG. 4  shows leads having tip ends releasably attached to a substrate, in accordance with preferred embodiments of the present invention. 
         FIG. 5  shows leads having tip ends releasable attached to a substrate, in accordance with other preferred embodiments of the present invention. 
         FIG. 6  shows a variety of leads formed atop a substrate, in accordance with still other preferred embodiments of the present invention. 
         FIG. 7  shows leads shown and restraining straps formed atop a substrate, in accordance with further preferred embodiments of the present invention. 
         FIG. 8  shows leads formed atop a substrate, in accordance with still further preferred embodiments of the present invention. 
         FIG. 9  shows leads formed atop a substrate in accordance with yet further preferred embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1A , a substrate  20  includes a first surface  22  and a second surface  24  remote therefrom. Although substrate  20  may be rigid, semi-rigid or flexible, in preferred embodiments substrate  20  is flexible, such as a flexible dielectric sheet. The flexible substrate  20  includes a plurality of flexible conductive leads  26  formed on the first surface  22  thereof. The flexible conductive leads  26  may be made from a wide variety of materials, including gold, aluminum, copper, their alloys, and combinations thereof. Each conductive lead  26  desirable includes a terminal end  28  permanently secured to flexible substrate  20  and a tip end  30  remote from the terminal end. As will be described in more detail below, the tip ends  30  of the leads  26  are preferably releasably attached to and movable away from the top surface  22  of flexible substrate  20 . The terminal end  28  of each lead  26  is preferably aligned with an opening  32  extending between the first and second surfaces  22 ,  24  of flexible substrate  20 . In preferred embodiments, the flexible substrate  20  is comprised of a sheet of a dielectric material, more preferably of a sheet of a polymeric dielectric material. In particularly preferred embodiments, flexible substrate  20  is comprised of a sheet of polyamide. 
     As will be described in more detail below, the flexible substrate  20  is preferably assembled to another microelectronic element. Referring to  FIG. 1B , one such microelectronic element is a semiconductor wafer  34  having a contact bearing face  36  including a plurality of contacts  38  formed on the contact bearing face, and a rear face  40  remote from contact bearing face  36 . The plurality of contacts  38  are preferably positioned in an array over contact bearing face  36  of wafer  34 . When semiconductor wafer  34  is positioned over the first surface  22  of flexible substrate  20 , the contacts  38  are preferably placed in substantial alignment with the tip ends  30  of conductive leads  26 . 
     Referring to  FIG. 1C , the contact bearing face  36  of semiconductor wafer  34  is then juxtaposed with the first surface  22  of flexible substrate  20  so that contacts  38  are in substantial alignment with tip ends  30  of conductive leads  26 . A conductive paste (not shown) may be applied to the tip ends  30  of conductive leads  26  in order to temporarily attach contacts  38  to the tip ends  30 . The leads  26  may be permanently attached to contacts  38  by bonding the tip ends  30  of leads  26  to contacts  38 . 
     The tip ends  30  of the conductive leads  26  are preferably peelable or releasable from the first surface  22  of flexible substrate  20 . Adhesion between the flexible substrate  20  and the tip ends  30  of leads  26  may be reduced by using the methods disclosed in commonly assigned U.S. Pat. No. 5,763,941; and U.S. patent application Ser. Nos. 09/020,750; 09/200,100; 09/225,669; 09/566,273; 09/577,474; 09/317,675; and 09/757,968, the disclosures of which are hereby incorporated by reference herein. For example, prior to forming the conductive leads  26  atop the flexible substrate  20 , an adhesion reducing substance such as silicone may be provided over the first surface  22  of substrate  20  for reducing the level of adhesion between tip ends  30  and substrate  20 . In the particular embodiment shown in  FIGS. 1A-1C , the tip ends  30  of leads  26  are commonly referred to as being releasable and the terminal ends  28  of leads  26  are commonly referred to as being fixed. In embodiments where the substrate is made of a polymeric material, there may be no need to take affirmative steps to enhance peelability between leads  26  and flexible substrate  20  because poor adhesion generally results between leads  26  and polymeric layers. 
     Referring to  FIG. 1D , after leads  26  are attached to contacts  38 , the semiconductor wafer  34  and the flexible substrate  20  are moved away from one another through a controlled displacement using platens  40 ,  42  as disclosed in commonly assigned U.S. Pat. No. 5,801,441, the disclosure of which is hereby incorporated by reference herein. A vacuum is preferably applied through platen  40  for firmly holding semiconductor wafer  34  and through platen  42  for firmly holding flexible substrate  20 . One or both of the platens are moved so that semiconductor wafer  34  moves vertically away from flexible substrate  20  in the direction indicated by arrow V 1 . At the same time, platen  40  and semiconductor wafer  34  may be moved horizontally relative to platen  42  and flexible substrate  20  in a horizontal direction indicated H 1 . Stated another way, flexible substrate  20  may also be moved in a horizontal direction such that the horizontal component of motion of the flexible substrate  20  is in a second direction H 2 , opposite the first horizontal direction H 1 . Thus, the semiconductor wafer  34  and the tip ends  30  of the leads  26  move, relative to the flexible substrate  20  and the terminal ends  28  of leads  26 , along the direction indicated by A 1 . The vertical movement typically is about 100-500 microns, and the horizontal movement is typically approximately equal to the vertical movement. During the controlled movement, the tip ends  30  of the leads  26  peel away from the first surface  22  of the flexible substrate  20 . The terminal ends  28  of the lead  26  remain fixed to the flexible substrate  20 . During movement of the semiconductor wafer  34  and the flexible substrate  20  away from one another, the leads  26  deform and/or bend in a vertical direction away from the flexible substrate  20  and the terminal ends  28  thereof. 
     When the wafer  34  is moved in the direction indicated by A 1 , the net effect of the relative movement of the semiconductor wafer  34  and the flexible substrate  20  is to move the tip ends  30  of conductive lead  26  horizontally towards and vertically away from the terminal ends  28  of the same leads, thus forming each flexible lead  26  into a vertically extensive, curved structure as illustrated in FIG.  1 D. Such a lead structure is able to flex and bend so as to compensate for movement of wafer  34  and substrate  20  relative to one another. In other embodiments, the movement of the semiconductor wafer  34  and flexible substrate  20  may not include a horizontal component, but only a vertical component. In these embodiments, the vertical movement will serve to partially straighten the leads  26 . In preferred embodiments, some slack is left in the vertically extended leads  26  so as to allow for subsequent movement of wafer  34  and substrate  20  relative to one another. 
     Referring to  FIG. 1E , after the semiconductor wafer  34  and flexible substrate  20  have been moved away from one another so as to vertically extend leads  26 , a spring-like material preferably is formed over the outer surface of leads  26 . The layer of spring-like material  44  preferably has substantially higher yield strength than the material comprising the flexible lead  26 . In preferred embodiments, the spring-like material  44  is selected from the group consisting of nickel, copper, cobalt, iron, tin, lead, bismuth, indium, gold, silver, platinum, tungsten, molybdenum, semi-noble metals, their alloys, and combinations thereof. The layer of spring-like material may be electroplated or may be formed by sputtering, chemical vapor deposition or combinations of any of the above methods. Although the present invention is not limited by any particular theory of operation, it is believed that the formation a layer of a spring-like material over vertically extended conductive leads  26  will substantially enhance the resiliency of the composite leads  46 . 
     Referring to  FIG. 1F , after forming the layer of a spring-like material  44  around leads  26 , an encapsulant  48  such as a curable liquid material is preferably introduced between semiconductor wafer  34  and flexible substrate  20  and around composite leads  46 . Preferred methods for disposing an encapsulant layer between microelectronic elements are disclosed in certain preferred embodiments of the above-mentioned U.S. Pat. No. 5,801,441. The encapsulant preferably has a low viscosity and is introduced in an uncured state. The encapsulant  48  preferably wets to the semiconductor wafer  34  and flexible substrate  20 , effectively fills a gap therebetween and penetrates between composite leads  46 . The encapsulant may be rigid or compliant. In preferred embodiments, the encapsulant  48  is selected so that it will form a compliant material, such as a gel or an elastomer, upon being cured. Preferred encapsulants include silicones and epoxies, with silicone elastomers and flexiblized epoxies being particularly preferred. In some embodiments, the encapsulant around the composite leads  46  is rigid and the remainder of the encapsulant between semiconductor chip or wafer  34  and flexible substrate  20  is compliant. In still other embodiments, the encapsulant around the composite leads  46  is compliant and the remainder of the encapsulant  48  between semiconductor wafer  34  and flexible substrate  20  is rigid. 
     In its liquid state, the encapsulant  48  may be injected under pressure. The encapsulant may also be injected without external pressure and allowed to fill the gap between semiconductor wafer  34  and flexible substrate  20  only by capillary action. After being disposed between semiconductor wafer  34  and flexible substrate  20  and around composite leads  46 , the encapsulant is cured in placed. Depending upon the formulation of the encapsulant, such curing may take place spontaneously at room temperature or else may require exposure to energy, such as heat or radiant energy. 
     Referring to  FIG. 1G , after encapsulant layer  48  has been cured to provide a compliant or resilient layer between semiconductor wafer  34  and flexible substrate  20 , conductive elements  50  may by attached to the terminal ends  28  of composite leads  46 . The conductive elements  50  are preferably tin/lead solder balls that extend through the openings  32  in the flexible substrate  20 . The conductive elements  50  may be reflowed so as to permanently attach the conductive elements  50  to terminal ends  28  of composite leads  46 . Upon being reflowed, the conductive elements  50  preferably form an intermetallic bond with the terminal ends  28  of the leads  26 . Surface tension may also result in the reflowed conductive elements  50  having a substantially spherical shape. In other preferred embodiments, the conductive elements  50  may include material such as gold and platinum. 
     Referring to  FIG. 2A , in accordance with further preferred embodiments of the present invention a first microelectronic component  134 , such as a semiconductor wafer, has a first surface  136  and a second surface  140  remote therefrom. The first surface  136  of semiconductor wafer  134  has a plurality of conductive traces or leads  126  formed thereon. Each conductive lead  126  includes a first end  130  releasably attached to first face  136  and a second end  128  permanently attached to wafer  134 . 
     Referring to  FIG. 2B , the front face  136  of wafer  134  is preferably juxtaposed with a flexible substrate  120 . In a particular preferred embodiment shown in  FIG. 2B , the flexible substrate  120  is a two-metal tape having a first surface  122  and a second surface  124  remote therefrom. The flexible tape  120  includes a series of vias  132  extending between the first and second surfaces  122 ,  124  thereof. Each via  132  preferably has a layer of a conductive metal  152  deposited therein. Each layer of conductive material  152  deposited in vias  132  preferably includes a flange region  154  that extends outwardly from the via  132  along the second surface  124  of substrate  120 . 
     Referring to  FIG. 2C , the first face  136  of semiconductor wafer  134  is juxtaposed with top surface  122  of flexible tape  120 . The releasable first ends  130  of conductive leads  126  are preferably placed in substantial alignment with the conductive metal  152  deposited in the vias  132 . A portion  154  of metal layer  152  is preferably accessible at the top surface  122  of flexible tape  120 . The wafer  134  is moved toward the top surface  122  of flexible tape  120  until the conductive leads  126  contact the deposited metal  152  accessible at the first surface  122  of flexible tape  120 . Immediately before the first ends  130  of leads  126  contact the metal portion  154 , a conductive paste or adhesive  156  may be applied to the releasable ends  130  of leads  126 . The conductive adhesive allows the leads to be attached to the metal portion  154 .  FIG. 2D , shows the releasable ends  130  of leads  126  attached to metal portion  154  of the metalized vias  132 . 
     Referring to  FIG. 2E , semiconductor wafer  134  and flexible tape  120  are then moved away from one another in a controlled manner using platens  140  and  142  as described above in reference to FIG.  1 D. As semiconductor wafer  134  and flexible tape  120  move away from one another, conductive leads  126  are vertically extended. 
     Referring to  FIG. 2F , a layer of a spring-like material, such as nickel, is then formed over the exterior surface of each conductive lead  126 . As mentioned above, the layer of spring-like material  144  preferably has a relatively higher yield strength than the yield strength of the conductive leads  126 . Together, the conductive leads  126  with the layer of a spring-like material formed thereon comprise composite leads  146 . 
     Referring to  FIG. 2G , after composite leads  146  have been formed, a curable encapsulant may then be disposed between the front face  136  of semiconductor wafer  134  and the first surface  122  of flexible tape  120 . As mentioned above, the curable encapsulant is preferably disposed between the wafer and tape while the curable encapsulant is in a liquid form. The encapsulant may then be cured in situ by applying energy or exposing the encapsulant to atmosphere. The cured encapsulant layer is preferably compliant so as to compensate for thermal expansion and contraction of the wafer  134  and substrate  120  during assembly and operation of the microelectronic package. 
     Referring to  FIG. 2H , conductive elements  150  such as solder balls may be then attached to the metalized vias  132 . The conductive elements are then preferably reflowed to permanent attach the conductive elements to the metalized vias. During reflow, surface tension preferably reshapes the outer surface of the conductive elements so that the conductive elements have a substantially spherical shape as shown in FIG.  2 H. After conductive elements  150  have been attached, the microelectronic package of  2 H may be electrically interconnected with another element via the conductive elements  150 . 
     Referring to  FIG. 2I , the microelectronic assembly of  FIG. 2H  may be severed to provide a plurality of microelectronic packages having an array of resilient leads. As shown in  FIG. 2I , semiconductor wafer  134 , encapsulant layer  148  and flexible tape  120  are severed to provide microelectronic packages  160 A and  160 B. Although only two microelectronic packages are shown in  FIG. 2I , the wafer  134  may be severed to provide a plurality of microelectronic packages (e.g., 100-200 chip packages or more). Each microelectronic package desirable includes at least one semiconductor chip  162 , a portion of flexible tape  120  and an array of resilient leads  146  that electrically interconnect chip  162  with conductive elements  150 . As such, the microelectronic packages  160 A,  160 B may be electrically interconnected with other elements such as a test socket, a circuitized substrate or a printed circuit board. During operation of the microelectronic packages  160 A and  160 B, the various components will typically heat up. As the components heat up, the components may expand at different rates due to differences in coefficients of thermal expansion. However, the resilient nature of composite leads  146 , encapsulant layer  148 , and flexible tape  120  will allow the semiconductor chip  162  move relative to substrate  120  so as to remain electrically interconnected with conductive elements  150 . 
       FIGS. 3A-3H  show yet another preferred embodiment of a method of making a microelectronic package having an array of resilient leads. Referring to  FIG. 3A , a substrate  220 , such as a two metal flexible tape, has a first surface  222  and a second surface  224  remote therefrom. The two metal tape  220  includes a plurality of conductive leads  226  formed thereon. Each conductive leads  226  has a first end  230  releasable secured to the first surface  222  of two metal tape  220  and a second or terminal end  228  permanently fixed to two metal tape  220 . The terminal end  228  of conductive leads  226  overlie through vias  232 , then through vias  232  extending between the first and second surfaces  222 ,  224  of two metal tape  220 . 
     Referring to  FIG. 3B , a fixture such as a sacrificial layer may then be juxtaposed with two metal tape  220 . Fixture  234  includes contact bearing surface  236  having a plurality of contacts  238  formed thereon and a back surface  240  remote therefrom. Referring to  FIG. 3C , fixture  234  may be juxtaposed with two metal tape  220  so that contacts  238  are in substantial alignment with the releasable ends  230  of leads  226 . Contacts  230  are preferably permanently attached to releasable tip ends  230  of conductive leads  226 , such as by using a bonding process or a conductive adhesive. 
     Referring to  FIG. 3D , in order to move fixture  234  and tape  220  away from one another, platens  240  and  242  are preferably abutted against fixture  234  and two metal tape  220 , respectively. As described above, platens  240 ,  242  are used to controllably move fixture  234  and two metal tape  220  away from one another in a vertical direction. Fixture  234  and substrate  220  may also be moved relative to one another in a horizontal direction. As fixture  234  and two metal tape  220  move away from one another, conductive leads  226  are extended in a substantially vertical direction. 
     Referring to  FIG. 3E , a layer of a spring-like material  244  may then be deposited over the exterior surface of conductive leads  226  to form composite leads  246 . As mentioned above, the formation of a layer of a spring-like material  244  over conductive leads  226  improves the overall resilience of the final structure, i.e., composite lead  246 . This improved resiliency enhances the ability of the lead to maintain an electrical interconnection between microelectronic elements during thermal cycling. 
     Referring to  FIG. 3F , a layer of a curable liquid material  248  is then preferably deposited between fixture  234  and two metal tape  220  and around composite leads  246 . In preferred embodiments, the layer of curable material  248  may then be cured to provide a compliant material that enables the composite leads  246  to flex and bend during thermal cycling. 
     Referring to  FIG. 3G , the fixture  234  may then be removed to transform the subassembly into a connection component. In certain embodiments the fixture  234  is completely removed, such as by exposing the subassembly to a chemical etchant. In other embodiments, the fixture may be comprised of a conductive material and may be provided without contacts  238 . Portions of the conductive fixture may then be removed. The remaining portions form contacts in the tip ends of the leads. After fixture  234  has been removed, contacts  238  are exposed at a top surface of encapsulant layer  248 . As mentioned above, the subassembly shown in  FIG. 3G  may be used as a compliant connection component  292  that can electrical interconnect two or more microelectronic elements. In certain embodiments, the contacts of a first microelectronic element may be connected with the contacts  238  exposed at a top surface of encapsulant layer  248 . In turn, contacts of a second microelectronic element may be permanently or temporarily attached to terminals exposed at the second surface  224  of two metal tape  220 . 
     In  FIG. 3I , a test fixture  270  having conductive elements  272  at a top surface thereof, is utilized to test the subassembly shown in FIG.  3 G. The conductive elements  272  of the test fixture are preferably provided in a spaced array, the conductive elements  272  matching the alignment of terminals  290  of connection component  292 . After connection component  292  has been positioned atop test fixture  270 , a microelectronic element or other electronic element having contacts may be juxtaposed with the contacts  238  at the top of compliant layer  248 , thereby allowing the connection component to be tested and evaluated. Alternatively or additionally, connection component  292  may be used to permanently connect two microelectronic elements. 
     Referring to  FIGS. 4 and 5 , the leads shown and described above may be arranged in many different ways on wafers, flexible substrates, flexible tapes and other microelectronic elements. For example, referring to  FIG. 4 , each lead  326  its initial undeformed state, may include an S-shaped strip  380  extending between the terminal ends  328  and tip ends  330  thereof. The S-shaped lead structures may be nested as shown in  FIG. 4  with the terminal ends  328  deposed in rows and the tip ends  330  deposed in similar but offset rows. Referring to  FIG. 5 , the leads  426  may also be substantially U-shaped structures having a single bight between the terminal end  428  and tip end  430  of each lead. Structures with plural bights can also be employed. Such leads are shown and described in certain preferred embodiments of commonly assigned U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein. 
     The conductive leads may also have the various configurations shown in FIG.  6  and disclosed in the above-mentioned &#39;964 patent, as well as in commonly assigned U.S. Pat. Nos. 5,859,472 and 6,191,368, the disclosures of which are hereby incorporated by reference herein. As a result, any gap  586  surrounding the conductive leads may have correspondingly varied shapes. In each case, the gaps extend alongside the flexible, conductive leads. Lead  526  is in the form of a closed loop  588  connecting the second end  530  of flexible lead with the first end  528  thereof. The closed loop section  588  of lead  526  encircles a central region  590 . 
     Referring to  FIG. 7 , in still other preferred embodiments, restraining straps  692 , which are shorter and stronger than conductive leads  626 , are connected between two microelectronic elements. Restraining straps  692  may be formed during the same process steps used to make the conductive leads. Such restraining straps are disclosed in commonly assigned U.S. Pat. No. 5,976,913, the disclosure of which is hereby incorporated by reference herein. After leads  626  electrically interconnect two or more microelectronic elements, restraining straps  692  limit movement of the microelectronic elements away from one another so that sufficient slack remains in the flexible, conductive leads  626 . 
     Referring to  FIG. 8 , in yet further preferred embodiments, the tip end  730  of each lead  726  is connected through a frangible element  794  to the terminal end  728  of the next adjacent lead. The frangible element  794  thus retains each tip end  730  in position, adjacent a surface of a substrate  720  or semiconductor wafer. Frangible element  794  may be formed as a continuation of a strip constituting the lead itself, with V-shaped notches extending in the strip from opposite sides thereof. During the assembly process, the tip ends  730  are bonded to the contacts of a chip or other microelectronic element in the same manner as discussed above. After bonding, the microelectronic element is moved relative to the connector body or dielectric sheet in the same manner as discussed above, so that the tip end  730  of each lead  726  moves vertically away from the body and away from the terminal ends  728 , and so that the tip end  720  also moves toward the associated terminal end  728 . This action breaks the frangible element  794  and hence, releases each tip end from its connection to the next terminal end. Such leads are disclosed are certain preferred embodiments of the &#39;964 patent. 
     Referring to  FIG. 9 , in still other preferred embodiments of the present invention, the tip ends  830  of each lead  826  is not provided with a bulge, but instead constitutes a continuation, of lead  826 . The tip end  830  of the lead is connected to the terminal end  828  of the next adjacent lead by a frangible section  894 . In this component, the dielectric sheet or connector body  820  has holes  832  aligned with the terminal ends  828  of the leads  826 . After connector body  820  and the leads  826  thereon are in alignment with contacts on a microelectronic element or chip, a tool (not shown) is advanced through holes  832  for engaging the tip ends  830  of each lead  826  in succession so as to bond a tip ends  830  to contact. After such bonding, the microelectronic elements or chip may be moved relative to the connector body in the same manner as discussed above. Once again, this movement breaks the frangible section  894  between the tip end of each lead and terminal end  828  of the adjacent lead, thus releasing the tip ends  830  and allowing the leads to bend away from the connector body. Before or after the movement step, holes  832  may be closed by application of a further film or sheet on the top surface of the dielectric layer. 
     Although the present invention has been described with reference to particular preferred embodiments, it is to be understood that the embodiments are merely illustrative of the principle and application of the present invention. It is therefor to be understood that numerous modifications may be made to the preferred embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the claims.