Abstract:
A component incorporating a dielectric element such as a polymeric film with leads and terminals thereon is assembled with a semiconductor chip and bond regions of the leads are connected to contacts of the chip. At least one lead incorporates a plural set of connecting regions connecting the bond region of that lead to a plurality of terminals. One or more of the connecting regions in each such plural set are severed so as to leave less than all of the terminals associated with each such plural set connected to the contacts of the chip.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional of U.S. patent application Ser. No. 09/306,623, filed May 6, 1999 now U.S. Pat. No. 6,603,209, which is a Divisional of U.S. patent application Ser. No. 08/365,749, filed Dec. 29, 1994 now U.S. Pat. No. 5,929,517, the disclosures of which are hereby incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates, generally, to integrated circuit packaging, and more particularly relates to methods and devices for packaging microelectronic devices. 
     Microelectronic devices are typically comprised of one or more silicon die/dice having, at least in material part, a multitude of die bond pads on a front surface, a chip body, an interconnection scheme to connect the pads on the die to a supporting substrate and an encapsulant to ensure that the die is protected from contaminants. The combination of these elements is generally referred to as a chip package. The specific function of this package is to protect the die from mechanical, electrostatic and environmental stresses while at the same time providing a thermal path for the heat dissipated from the at least one die during use. 
     More specifically, chip packages must be able to accommodate for many inherent microelectronic device problems, such as die power dissipation, mismatches in the thermal coefficients of expansion between the chip and its supporting substrate, and increasingly smaller die bond pad pitch, which ultimately allows smaller dies to be used and thus has the potential to produce either smaller packages or more densely packed multi-die packages so long as the interconnection scheme chosen can accommodate the fineness of the pad pitch. 
     Microelectronic devices are typically connected to external circuitry through contacts on a surface of the chip. The device contacts are generally either disposed in regular grid-like patterns, substantially covering the front surface of the chip (commonly referred to as an “area array”) or in elongated rows extending parallel to and adjacent each edge of the chip front surface. The various prior art processes for making the interconnections between such microelectronic devices and their supporting substrates use prefabricated arrays or rows of leads, discrete wires, solder bumps or combinations thereof. For example, in a wirebonding process, the chip may be physically mounted on a supporting substrate. A fine wire is fed through a bonding tool. The tool is brought into engagement with the contact on the chip so as to bond the wire to the contact. The tool is then moved to a connection point of the circuit on the substrate, so that a small piece of wire is dispensed and formed into a lead, and connected to the substrate. This process is repeated for every contact on the chip. The wire bonding process may also be used to connect the die bond pads to lead frame fingers that are then connected to the supporting substrate. 
     In a tape automated bonding (“TAB”) process, a dielectric supporting tape, such as a thin foil of polyimide is provided with a hole slightly larger than the chip. An array of metallic leads is provided on one surface of the dielectric film. These leads extend inwardly from around the hole towards the edges of the hole. Each lead has an innermost end projecting inwardly, beyond the edge of the hole. The innermost ends of the leads are arranged side by side at spacing corresponding to the spacing of the contacts on the chip. The dielectric film is juxtaposed with the chip so that the hole is aligned with the chip and so that the innermost ends of the leads will extend over the front or contact bearing surface on the chip. The innermost ends of the leads are then bonded to the contacts of the chip, typically using ultrasonic or thermocompression bonding. The outer ends of the leads are connected to external circuitry. 
     In a “beam lead” process, the chip is provided with individual leads extending from contacts on the front surface of the chip outwardly beyond the edges of the chip. The chip is positioned on a substrate with the outermost ends of the individual leads protruding over contacts on the substrate. The leads are then engaged with the contacts and bonded thereto so as to connect the contacts on the chip with contacts on the substrate. 
     More recently, flip chip configurations have been used. In flip chip configurations, a solder ball is deposited on top of each of the chip contacts and then abutted against respective substrate contacts. The solder balls are then reflowed to provide an electrical connection between the chip and the substrate. 
     The rapid evolution of semiconductor art in recent years has created a continued demand for progressively greater numbers of contacts and leads in a given amount of space. An individual chip may require hundreds or even thousands of contacts, all within a very small area and many times within the area of the front surface of the chip package. For example, a complex semiconductor chip package in current practice may have a row of contact pads spaced apart from one another at center-to-center distances of 0.15 mm or less and, in some cases, 0.10 mm or less. These distances are expected to decrease progressively with continued progress in the art of semiconductor fabrication. Wire bonding can currently only accommodate a die pad pitch of approximately 100 pm and TAB bonding allows only a pad pitch or about 70-80 pm. If a smaller pad pitch were possible in production, it would allow the die size to be reduced for “pad limited designs where the die perimeter is required to be large enough to fit all of the bond pads. 
     Further, with such closely-spaced contacts, the leads connected to the chip contacts, must be extremely fine structures, typically less than 50 pm wide. Such fine structures are susceptible to damage and deformation. With closely spaced contacts, even minor deviation of a lead from its normal position will result in misalignment of the leads and contacts. Thus, a given lead may be out of alignment with the proper contact on the chip or substrate, or else it may be erroneously aligned with an adjacent contact. Either condition can yield a defective chip assembly. Errors of this nature materially reduce the yield of good devices and introduce defects into the product stream. These problems are particularly acute with those chips having relatively fine contact spacing and small distances between adjacent contacts. 
     Many of the prior art techniques for attachment further run into problems because of the thermal expansion mismatch between the material comprising the microelectronic device and the material comprising the supporting substrate. In other words, when heat is applied to the microelectronic device/substrate combination, they both expand; and when the heat is removed, the device and the substrate both contract The problem that arises is that the device and the substrate expand and contract at different rates and at different times, thereby stressing the interconnections between them. This directly affects the reliability of these connection schemes. 
     It has been proposed to provide a pressure clamped TAB structure where the outer leads have bumps which can be pressure clamped to respective contacts on the supporting substrate. A compliant pad is then placed over the TAB leads to help hold each of the bumps into electrical contact with corresponding lead contacts on the substrate. However, the compliant pad will eventually take a permanent set, thereby reducing the reliability of the contact force over time. An alternate TAB solution put forth involves replacing the outer lead bond pads of the TAB chip carrier, which connects to the substrate, with an area array of solder balls. The die is then connected to the carrier by means of solder bumps, wire bonds, or TAB inner lead bond pads. The problem here is that the solder balls undergo mechanical stress due to differential thermal expansion of the TAB chip carrier relative to the supporting substrate thereby causing cracking of the solder balls reducing their reliability. 
     Thermal mismatch issues will be more significant as multiple chip modules grow in popularity. Typically, as more dice are packaged together, more heat will be dissipated by each package which, in turn, means the package will expand to a greater extent thereby further stressing the interconnections. Effective package heat dissipation schemes have thus become increasing important. Typical package cooling schemes include heat sinks and small air fans which are applied or affixed to the back side of the chip body, which is further typically made of ceramic or plastic. One problem with these solutions is that the back layer of the chip package body, to varying degrees, acts as a thermal barrier between the die and the thermal cooling device inhibiting good thermal conduction to the exterior surface of the package. 
     Further, impedance, inductance and capacitance problems begin to seriously degrade a chip package&#39;s performance as the pad pitch becomes finer and the clock speed of a chip is increased. Factors such as the length of interconnection wires and the crosstalk between the chip&#39;s interconnections also need to be addressed when a chip package is being designed for the same reason. 
     To be commercially viable, the aforementioned problems must be solved in a manner which respects the small package size, multichip constraints, fine die bond pad pitch, thermal problems, compliancy problems, electrical problems and in addition must be a cost effective IC package. 
     Thus, despite the substantial time and effort devoted heretofore to the problems associated with mounting and connecting of microelectronic devices, there are still been unmet needs for improvements in such processes and in the equipment and components used to practice them. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for providing a integrated circuit package while substantially obviating thermal, compliancy and interconnection problems. 
     More specifically, one embodiment of the present invention provides a method of fabricating a compliant microelectronic device package and an associated apparatus, wherein a flexible, dielectric layer having on a first surface a plurality of conductive leads which are each electrically coupled at a first end to a conductive pad on the first surface of the dielectric layer. A second end of each conductive lead is further coupled to a first surface of a removable film across a bonding gap. A first surface of a compliant layer is coupled to a second surface of the dielectric layer and the removable film is supported atop a first surface of an IC die. Each conductive lead is then detached and bonded to a respective juxtaposed die bond pad. At this point the removable film is no longer attached by the conductive leads and may be removed from the first surface of the die creating a die window. A second surface of the die and a second surface of the compliant layer is next attached to an interior surface of a protective structure, such as a heat spreading enclosure. A liquid encapsulant is then introduced between the die and the dielectric layer and is cured at a suitable temperature. 
     In an alternate embodiment, the removable film from the preceding embodiment, comprising a second flexible dielectric layer, can be coupled to the first surface of the die using a second compliant layer. The second dielectric layer further having a plurality of the second conductive leads each electrically coupled at a first end to at least one second conductive pad also coupled to the first surface of the second dielectric layer. A second end of each of the second conductive leads is then coupled to the first surface of the first dielectric layer across the bonding gap. Each of the conductive leads may then be detached within or near the bonding gap and bonded to respective juxtaposed die bond pads on to the first surface of the die. This embodiment thus provides a compliant microelectronic package having perimeter and center conductive pads. 
     These embodiments can also make use of a specific point of detachment on each of the conductive leads within or near the perimeter of the bonding gap to better determine the point of detachment of the lead. This embodiment may further have a conductive layer coupled to the second surface of the dielectric layer, between the dielectric layer and the compliant layer. This conductive layer can be used as a ground layer or a voltage reference layer and can be selectively coupled to any of the conductive pads through a conductive via through the thickness dimension of the dielectric layer. The conductive layer further helps to shield electrical transients between the contact pads when the device is in use. A solder mask, coupled to the first surface of the first and second dielectric layer, may also be used to electrically shield the conductive leads and cover the die window, but not shield the conductive pads. A small hole in the solder mask, aligned with the bonding gap, can further be used to introduce the liquid encapsulant between the die and the solder mask. The solder mask also performs the function of preventing the liquid encapsulant from overflowing onto the conductive pads. 
     A further embodiment of the present invention includes a method of fabricating a compliant microelectronic device package and an associated apparatus having its conductive leads and pads on alternate surfaces of a dielectric layer. More specifically, this embodiment includes providing a first and a second flexible dielectric layer lying in a common plane with a space between them defining a bonding gap. The first and second dielectric layers respectively having on a first surface a first and second conductive pad and on a second surface a first and second conductive lead. The first and second conductive pads are coupled respectively to the first and a second conductive leads through a conductive via in the first and second dielectric layers. A first surface of a third and fourth flexible, dielectric layer are coupled respectively to the second surface of the first and second dielectric layers, wherein an end of the second conductive lead is attached between the first and third dielectric layers such that the second conductive lead bridges the bonding gap. A third conductive lead is then coupled to a second surface of the third dielectric layer and coupled to the first conductive lead through a conductive via extending from the first to the second surface of the third dielectric layer. An end of the third conductive lead is further coupled between the second and fourth dielectric layers such that the third conductive lead bridges the bonding gap. A first and second compliant layer, each having a first and second surface, wherein the first surface of the first and second compliant layer are coupled to a respective second surface of the third and fourth dielectric layers. A die having a first and second surface and a plurality of die bond pads is next coupled on its first surface to a second surface of the second compliant layer, and the second surface of the first compliant layer and the second surface of the die are attached to an interior surface of a protective structure. The second and third conductive leads are then detached and bonded to a respective, juxtaposed die bond pad, and a liquid encapsulant is introduced between the die and the dielectric layer and cured. 
     The attached ends of the second and third may alternately be sandwiched between the respective dielectric layers or the leads may be coupled to one of the opposing dielectric layers, either way holding them in place over the bond gap. 
     A conductive layer may be added to the first surface of the first and second dielectric layers and selectively coupled to the first and second conductive pads, thereby providing for a ground of reference voltage plane. A solder mask may further be affixed atop the conductive layer so that the leads are protected from electrically shorting but the contact pads are exposed so they may be connected to the contacts on a supporting substrate. A hole may also be provided from the exposed surface of such a solder mask to the second surface of the conductive layer and aligned over the bonding gap so that the liquid encapsulant may be injected between the die and the conductive layer. 
     The foregoing and other objects and advantages of the present invention will be better understood from the following Detailed Description of a Preferred Embodiment, taken together with the attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a side view of one embodiment according to the present invention. 
         FIG. 2  shows a side view of a flex circuit according to the present invention. 
         FIG. 3  shows a chart comparing various package characteristics according to the present invention. 
         FIG. 4  shows a before bonding side view of a flex circuit and die according to the present invention. 
         FIG. 5  shows a before bonding top view of a flex circuit and die according to the present invention. 
         FIG. 6  shows a magnified before bonding top view of a flex circuit and die according to the present invention. 
         FIG. 7  shows a before bonding perspective view of a bonding tool and unbonded leads according to the present invention. 
         FIG. 8  shows a perspective view of the leads after bonding according to the present invention. 
         FIG. 9  shows an encapsulation step according to the present invention. 
         FIGS. 10A-B  show a top view of an alternate embodiment having programmable discretionary wiring according to the present invention. 
         FIG. 11  shows a top view of a further alternate multiple die embodiment according to the present invention. 
         FIG. 12  shows a top view of a still further alternate embodiment according to the present invention. 
         FIG. 13  shows a magnified top view of a still further alternate embodiment having both fan in and fan out leads according to the present invention. 
         FIG. 14  shows a perspective view of a still further alternate embodiment having a ground layer according to the present invention. 
         FIG. 15  shows a side view of the embodiment shown in  FIG. 12  according to the present invention. 
         FIG. 16  shows a side view of an still further alternate embodiment having multiple wiring levels coupled to the die bond pads according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a side view of one embodiment of the present invention. A flexible, but substantially inextensible, film circuit element (or “flex circuit)  100 , preferably formed from a polymeric material such as Kapton™, of an approximate thickness between 25 and 100 microns is used as a flexible, intermediate substrate having lithographically pre-formed flexible, conductive leads  110  and bumps  120  (which may be comprised of plated bumps, solder balls, etc.) on a first surface. The conductive leads  110 , which are typically between 20 to 30 microns thick, connect a plurality of bumps  120  to at least one respective die bond pad, as is discussed in greater detail hereinafter. The flex circuit  100  further has a bond window  130  through which the conductive leads  110  are attached to the die bond pads (not shown) on the front surface of the at least one die  140 . A back surface of the die  140  is coupled to an interior surface of a protective structure  150  generally using a snap cure adhesive  160  or another suitable method for attaching the die  140  to the protective structure  150 . The protective structure  150  further has a shelf section  155  encircling the die to provide support for the bumps  120 . 
     A compliant layer  170 , typically an elastomeric pad, is placed between and coupled to a second side of the flex circuit  100  and the front side of the shelf section  155  of the protective structure  150  to accommodate for the temperature coefficient of expansion mismatch which occurs after the bumps are attached to a supporting substrate, such as a printed wiring board, and the die  140  begins to expand at a different rate than the supporting substrate due to the die&#39;s dissipation of heat during operation. The open area in the interior of the protective structure  150  is filled with an elastomeric encapsulant  180  to protect the die  140  from contamination due to dust, moisture or the like, as is disclosed in more detail in the commonly owned U.S. patent application Ser. No. 08/246,113, filed May 19, 1994, now U.S. Pat. No. 5,663,106. A flexible protective dielectric layer  190  is placed over the first side of the flex circuit  100  electrically isolating the conductive leads  110 , but leaving the bumps  120   50  that they may be electrically coupled to a supporting substrate. Dielectric layer  190  further extends across the bond window  130 . 
     Referring now to  FIG. 2 , the process begins by lithographically forming the flexible, conductive leads  110  on a first surface of the flex circuit  100  thereby coupling the leads  110  between the bumps  120  on a first end of the lead  110  and a removable flex film structure  210  on a second end of the lead  110 . The bumps  120  are preferably comprised of nickel or copper of approximately 90 microns in height and 300 microns in diameter. Typically, a one micron thick gold (or gold cobalt alloy) coating will further be flash plated on the surface of each of the bumps  120  to protect the bumps from oxidation and to enhance the bonding of the bumps to a supporting substrate. 
     The flex film  210  is similar to flex circuit  100  in that it is typically comprised of a flexible, substantially inextensible film circuit element formed from a polymeric material such as Kapton™, and is attached to a second end of lead  110 . In effect, the flex film  210  is an island that is attached to and completely supported by the leads  110  which are generally placed on all four sides of the flex film  210 . This arrangement creates a bonding gap  260  between the flex circuit  100  and the flex film  210 . In practice, the flex film  210 /bonding window  130  structure may be created by laser ablating or punching a “picture frame” portion from flex circuit  100 . 
     The compliant layer  170 , which is typically made of an elastomer material such as the Dow Corning silicone elastomer 577 known as “Silgard®,” is next adhered to the second surface of the flex circuit  100  and the flex film  210  typically by conventional stencil printing techniques. The silicone elastomer used in the preferred embodiment is filled with about 5-10% of fumed silica in order to obtain a stiff consistency that allows the stenciled layer to retain its shape after the stencil is removed. The fumed silica also improves the thermal conductivity of the elastomer and further reduces the thermal coefficient of expansion of the elastomer. The silicone is then cured at a suitable temperature. 
     Preferably, there is a sufficient number of bumps  120  so that at least one bump  120  is provided for each die bond pad  200 . If a pad limited die is assumed, Table I provides typical bump configuration information. The standard bump array size may thus be chosen from Table I so that a large number of applications may be supported with a minimum number of size and tool configurations. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Grid 
                 Bonding 
                 Rows 
                 No. of 
                 % Bump 
                 Bonding Window size 
                 Die Size 
               
             
          
           
               
                 Array 
                 Window 
                 Deep 
                 Bumps 
                 Coverage 
                 1.0 
                 mm 
                 1.27 
                 mm 
                 100 
                 μm 
                 80 
                 μm 
               
               
                   
               
             
          
           
               
                 12 × 12 
                 6 × 6 
                 3 
                 108 
                 75 
                 6 
                 mm 
                 7.6 
                 mm 
                 2.7 
                 mm 
                 2.2 
                 mm 
               
             
          
           
               
                 13 × 13 
                 7 × 7 
                 3 
                 120 
                 71 
                 7 
                 8.9 
                 3.0 
                 2.4 
               
               
                 14 × 14 
                 8 × 8 
                 3 
                 132 
                 67 
                 8 
                 10.2 
                 3.3 
                 2.6 
               
               
                 15 × 15 
                 9 × 9 
                 3 
                 144 
                 67 
                 9 
                 11.4 
                 3.6 
                 2.9 
               
               
                 16 × 16 
                 8 × 8 
                 4 
                 192 
                 75 
                 8 
                 10.2 
                 4.8 
                 3.8 
               
               
                 17 × 17 
                 9 × 9 
                 4 
                 208 
                 72 
                 9 
                 11.4 
                 5.2 
                 4.2 
               
               
                 18 × 18 
                 10 × 10 
                 4 
                 224 
                 69 
                 10 
                 12.7 
                 5.6 
                 4.5 
               
               
                 19 × 19 
                 11 × 11 
                 4 
                 240 
                 66 
                 11 
                 14 
                 7.0 
                 5.6 
               
               
                 20 × 20 
                 10 × 10 
                 5 
                 300 
                 75 
                 10 
                 12.7 
                 7.5 
                 6.0 
               
               
                 22 × 22 
                 12 × 12 
                 5 
                 340 
                 80 
                 12 
                 15.2 
                 9.6 
                 7.7 
               
               
                 24 × 24 
                 14 × 14 
                 5 
                 380 
                 63.7 
                 14 
                 17.8 
                 10.8 
                 8.6 
               
               
                 26 × 26 
                 14 × 14 
                 6 
                 480 
                 71 
                 14 
                 17.8 
                 12.8 
                 9.6 
               
               
                 28 × 28 
                 14 × 14 
                 6 
                 528 
                 67 
                 16 
                 20.3 
                 14.7 
                 11.8 
               
               
                 30 × 30 
                 18 × 18 
                 6 
                 576 
                 67 
                 18 
                 22.9 
                 16.1 
                 12.9 
               
               
                   
               
             
          
         
       
     
     Taking the first row of entries in Table I as an example, a 12×12 grid array provides 12 rows of 12 bumps each for a total of 144 bumps. Since the flex circuit has a bonding window for the die, the number of bumps which would have been included had the bonding window not been provided must be subtracted from the total number of bumps, i.e., for a bonding window which will take up an area that would have provided a 6×6 bump grid array, 36 bumps must be subtracted from the 144 bump total given above. Thus, in this example, 108 bumps in three rows encircle a centrally located bonding window. This corresponds to a 75% coverage of the bump grid array on the flex circuit. If the bumps, in this example, are on a 1.0 mm pitch, the bonding window may be approximately 6 mm on each side, assuming a square bonding window. Further assuming a 100˜tm die bond pad pitch on a single square die, the die may be approximately 2.7 mm on a side allowing for a good amount of room to bond to the die bond pads given the difference between the bonding window size and the die size. Given this die geometry, 27 die bond pads per side will result, further resulting in a total of 108 die bond pads for the square die in this example. This provides one bump per die pad. While the number of allowable pins on the periphery of a die increases fairly linearly with die size, the corresponding bonding window size and number of rows of bumps relative to the number of pins on the die is not linear, as shown in FIG.  3 . As stated earlier, the values shown in Table I are exemplary only and should not be construed as a limitation on the present invention. 
     Referring now to  FIGS. 4-8 , the assembly shown in  FIG. 2  is next mounted to the die  140 . In  FIG. 4 , the die  140  is first placed with the die bonding pads  200  pointing upward in an aligning device  250 , such as a vacuum platen/support post combination, which positions and aligns the die  140  in the x, y, z and 0 directions. The assembly shown in  FIG. 2  is then placed over the top of the aligning device  250  so that projections  270  on the aligning device  250  may provide support for the flex circuit  100  and allow the flex film  210  to rest atop the center of the die  140 . The compliant layer  170  further supports the flex circuit  100  and the flex film  210  and maintains the conductive leads  110  at a fixed height above the die&#39;s surface. Each die bond pad  200  is thus aligned beneath a respective conductive lead  110 . 
       FIG. 5  shows a bottom view of the embodiment shown in  FIG. 4  before the conductive leads  110  are bonded to the die bond pads  200 . As described above, each conductive lead  110  is fixedly held in place on either side of the bonding gap  260  between the flex circuit  100  and the flex film  210  such that a portion of each lead  110  is suspended above a respective die bond pad  200  in the bonding gap. Alternately, each of the conductive leads  110  may be connected to a common center structure coupled to the first surface of the flex film  210  to provide added adherence of the leads  110  to the flex film  210  which, in turn, aids in bonding the leads  110  to the die bonding pads  200 , described in greater detail below. A preferred embodiment of the invention also includes the holding straps  215  which provide a mechanical means to better secure the flex film  210  in place when the leads  110  are being bonded to the die bond pads  200 . The holding straps  215  may be secured to any portion of the flex circuit  100  and removable film  210  that will provide the added bonding support without electrically shorting the leads  110 . Further, the holding straps  215  may be comprised of a conductive material, similar to how the leads  110  are formed, or the straps  215  may be formed of a dielectric material, which helps ensure the straps  215  do not electrically short the leads  110 . After the leads have been bonded (as described in greater detail below), the holding straps  215  may be removed. 
       FIG. 6  shows a magnified view of FIG.  5 . Each conductive lead  110  has a highly conductive joining layer (not shown), such as a 2.5 to 5 micron thick layer of 99.99% gold or gold plated on nickel, which may be disposed on the side of the lead  110  facing the die bonding pad  200  or may extend completely around the conductive lead  110  within the bonding gap between the flex circuit  100  and the flex film  210 . Alternately, the entire conductive leads  110  can be comprised of gold or a gold alloy. As shown in  FIG. 3 , each conductive lead  110  further has a detachment point  230 , typically positioned within the bonding gap, which facilitates fracture of the lead. This detachment point  230  can also be located just inside of the perimeter of the flex film  210 . Although the detachment point  230  is shown in  FIG. 4  as a notched element, the detachment point  230  may also be accurately thought of as simply a “weak” point in the conductive lead  110  which allows for the fracture of the lead at the weak point The detachment point  230  can be created by any suitable means, such as selectively plating or etching the lead, scoring the lead, creating a ‘thin area in the lead either in width or depth or not coupling the highly conductive layer to a small portion of the conductive lead within the bonding gap. This detachable lead feature is described in greater detail in commonly owned U.S. patent application Ser. No. 07/919,772, filed on Jul. 24, 1992. 
     Referring now to the perspective drawing in  FIG. 7 , each flexible, conductive lead  110  is separated at its detachment point  230  and bent towards the die bonding pads  200  until the surface of the highly conductive joining layer contacts the die bonding pads  200  of die  140 . Thus, the bonding gap  260  must be sufficiently large to allow for the thickness of the compliant layer. As stated above, the portion of each conductive lead  110  within the bonding gap  260  is supported during the bonding phase on one side by the flex circuit  110 , which in turn is supported by the compliant layer  170  and by the projections  270  of the aligning device  250 . Each lead is supported on the other side of the bonding gap  260  by the flex film  210 , the compliant layer  170  and the die  140 . Typically, the bonding action is accomplished by using a bonding tool  240  having an elongated groove in its bottom surface which is positioned above each contact so that the groove extends in a pre-selected groove direction and extends across the top of a contact The connection sections of the leads extend generally parallel to the groove direction, so that when the bonding tool is advanced downwardly to engage the lead  110 , the connection section of each lead is seated in the groove. If the lead  110  is slightly out of alignment with the groove, the lead  110  will be moved in lateral directions, transverse to the groove, until it seats in the groove and thus becomes aligned with the die bonding pads  200 . The bonding tool described herein is more fully disclosed in commonly owned U.S. patent application Ser. No. 08/096,700, filed Jul. 23, 1993, now U.S. Pat. No. 5,390,844. 
       FIG. 8  shows a perspective drawing of each conductive lead  110  after it has been separated at its detachment point  230  and bent toward the die bonding pad  200 . The leads  110  are then attached to the die bonding pads  200  by any suitable means, such as ultrasonic, thermosonic or compression bonding. The actions of detaching, bending and attaching the leads  110  are all typically performed with the bonding tool  240 , shown in FIG.  7 . After each of the conductive leads have been separated from the flex film  210  at their detachment points and bonded to their respective die bonding pad  200  on die  140 , the flex film  210  is no longer attached and may simply be removed. If the holding straps  215  are used, they will be removed at the same time by breaking or peeling each strap off near the edge of the bonding gap  260  nearest flex circuit  110 . At this point, the die  140  is attached to the flex circuit by each of the bonded conductive leads  110 . 
     The flexible, protective dielectric cover layer (or “solder mask”’)  190  in  FIG. 1  is coupled to the first surface of the flex circuit and is typically between 25-50 pm thick. The solder mask  190  is further typically composed of a polyimide, acrylic or epoxy sheet having preformed holes to allow the bumps  120  to extend therethrough. Preferably, the solder mask is vacuum laminated to the top layer of the semiconductor chip assembly and covers the entire first surface of the flex circuit including the bonding window  130 , except for the bumps  120 . Alternately, a solder mask  190  may be coupled to the entire circuit area of the flex circuit  100 . Holes corresponding to the bumps  120  may then be created by lithographically exposing and developing the solder mask such that the bumps  120  extend therethrough. Preferably, an encapsulation hole is made in the solder mask  190  so that elastomeric encapsulant may be disposed within the open indentation area of the protective structure, as described below. The elastomeric encapsulant is typically cured prior to any step of exposing the solder mask to reveal the bumps  120  so that the bumps do not become contaminated by the encapsulant. 
     A protective structure  150  is next placed between the back surface of the die  140  and the aligning device  250  and the die is coupled to the interior surface of the protective structure  150  typically using a snap cure, thermally conductive die attach adhesive, as described above in reference to FIG.  1 . It should be noted that the protective structure may also be coupled to the structure shown in  FIG. 2  prior to the bonding step. The protective structure  150  performs three functions. First, it protects the die and the flex circuit Second, the protective structure  150  is used to conduct heat from the back of the die  140  to the surrounding environment; and third, the shelf section  155  provides support for the bump grid array when it is attached to a supporting substrate. For thermal transfer purposes, the protective structure  150  optimally is comprised of a highly conductive material, such as copper, copper-tungsten, aluminum or aluminum nitride among others. Further, the protective structure  150  is directly attached to the back surface of the die  140  to aid in the conduction of heat from the die through the protective structure  150 . Because the die  140  will heat up more quickly than the protective structure  150 , the preferred embodiment of the invention uses a protective structure  150  which has a thermal coefficient of expansion (TCE) as closely matched to the TCE of the die  140  as possible while still retaining the structure&#39;s  150  thermal transfer properties. Because the protective structure  150  is directly attached to the back surface of the die  140 , matching the TCE characteristics allows the protective structure  150  to expand and contract as the die expands and contracts. Alternately, a conductive grease could be substituted for the snap cure adhesive to add compliancy between the protective structure  150  and the die  140  while still maintaining a good thermal path to dissipate the die&#39;s heat 
     The shelf section  155  of the protective structure  150  is coupled to the compliant layer  170  and must further have sufficient rigidity to provide the needed support for the bump grid array. The shelf section may be coupled using a snap cure adhesive or a tacky elastomer film, of typically the same material the complaint layer is composed of, may be provided on the top surface of the compliant layer  170  and cured after the shelf section  155  is attached, so as to bond the shelf section  155  to the compliant layer  170 . 
     Referring now to  FIG. 9 , the open area, defined by the protective structure  150 , the compliant layer  170  and the solder mask  190 , provides a bounded encapsulation area. The encapsulant  180  performs the function of protecting the die  140  from contamination due to dust, moisture or the like, as is discussed above in reference to FIG.  1 . Typically, a liquid encapsulant  180 , such as the complaint elastomeric material used for the complaint layer  170 , is dispensed into the open area by an encapsulant filled injection head  220  through the hole in the protective layer. Solder mask  190  substantially prevents the encapsulant from contacting or affecting the conductivity of the bumps  120 . The vacuum platen  250  holding the protective structure  150  may be heated to between 1600° C. and 1800° C. in order to cure the encapsulant  180  sufficiently to prevent its running out of the hole in the solder mask  190 . Preferably, the hot platen  250  also cures the adhesive coupling the die  140  to the protective structure  150  and the compliant layer  170  to the shelf section  155  at the same time as it cures the encapsulant  180 . At this point, the microelectronic device is complete and may be removed from the vacuum platen  250 . 
       FIG. 10A  shows a bottom view of an alternate embodiment of the present invention in which the conductive leads  300  may connect more than one terminal  310  to the same die bond pad  320 . Thus, a multiply-connected lead  300 ′ has a bond region  333  which is aligned with a bond window  301  in the dielectric element  337 . Bond region  333  is adapted for connection to the chip contact, also referred to as a die bond pad,  320 . The bond region  333  is connected to two terminals  310   a  and  310   b  through two connecting regions  335   a  and  335   b , shown in magnified view in FIG.  10 B. These two connecting regions  335   a  and  335   b  constitute a plural set of connecting regions. The dielectric film  337  has a disconnection window  331 , and the connecting regions  335   a  and  335   b  extend across this disconnection window. Thus, the connection regions  335   a  and  335   b  can be severed selectively by advancing a tool  340  so as to engage the connection region which is to be disconnected and by forcing the tool into the disconnection window. As shown in the drawing, the connection regions may have weak points  341  to facilitate such severance. Alternatively, the connection regions may be selectively disconnected using etching or scribing techniques generally known in the art to electrically disconnect one of the bumps from the die bond pad by creating a non-conductive region. 
       FIG. 11  shows a bottom view of a multi-die embodiment of the present invention in which a plurality of dies ( 350 / 360 / 370 ) may be mounted to the same flex circuit  380  and combined into a single package. In this embodiment, the flex circuit  380  has a discrete bonding window ( 355 / 365 / 375 ) for each die and each die is coupled to the conductive leads  390  in the same detachable lead/bonding tool manner as described above in reference to  FIGS. 4-8 . This embodiment further shows that the conductive leads  390  may be used to interconnect die bonding pads on the same die or between multiple dies. 
     Referring now to the alternate embodiment of  FIG. 12 , the assembly includes a first flex circuit  450 , similar to the flex circuit  100  discussed above, and a second flex circuit  400 . Flex circuit  400  is similar to the removable flex film structure  210  shown in  FIG. 2  in that it is disposed within the central aperture creating a bonding gap  430  between the two flex circuits ( 400 / 450 ). A plurality of plated or solder bumps  410 / 490  are positioned on each of the flex circuits and are attached to the die bond pads  460  through the bonding gap  430  through the use of flexible leads  440 , described earlier. The addition of the second flex circuit  400  replaces at least a portion if not all of the bumps removed by the bonding window shown in FIG.  1  and described in conjunction with the second column of Table I. This embodiment allows a greater number of bumps  410  on the front surface of the chip package; and thus, a greater number of die bond pads  460  along the periphery of the die  470 . This embodiment also provides further bump  410  to die bond pad  460  selectability. 
     As can be better seen in FIG.  13 &#39;s magnified view of  FIG. 12 , the same method of attaching the first flex circuit to the die bond pads, described in reference to  FIGS. 4-8 , can be used to attach the bumps  410  on the second flex circuit  400  to the die bond pads  460 . The conductive leads  440  are coupled on either side of the bonding gap  430  to the first surface of the first and second flex circuits ( 450 / 400 ) such that a portion of the lead  440  is suspended over the bonding gap  430 . The detachment point  480  is placed on the far side of the bonding gap  430  relative to the bump that is to be electrically connected to its respective die bond pad. Alternately and expanding upon the concept shown in FIGS. IOA and  108 , a conductive lead  440  having dual detachment points  485 A/ 4858  can be provided across the bonding gap  430  so that the bonding tool, described above, may selectively couple one of the bumps ( 410 / 490 ) to the die bond pad  460  while simultaneously ensuring that the other bump ( 410 / 490 ) will not be electrically connected to the same die bond pad  460 . The localized stress placed on one of the dual detachment points, for example point  485 A, by the bonding tool will cause it to separate the detachment point connection  485 A before the lead&#39;s other detachment point  4858  is substantially affected. It should be noted that the second flex circuit feature shown in  FIGS. 12-13  can also be used with the multi-die embodiment shown in FIG.  11  and described above. 
       FIG. 12  further shows an alternate protective structure  500  having a substantially flat back surface across the entire chip package. This embodiment gives added rigidity, and thus added support, for the opposingly positioned bumps  490 . This embodiment further better spreads the heat dissipated from the die throughout the entire chip package; however, the structure  500  will also retain that heat longer than the protective structure  150  shown in  FIG. 1  because of the structure&#39;s  500  greater mass. For this reason, a conductive grease may be used in some applications to mate the back surface of the die  470  to the protective structure  500  to give the connection added compliancy. The back surface of the die  470  directly attaches to the interior surface of the protective structure  500  to aid in the conduction of heat from the back surface of the die  470  through the structure  500 . Likewise, the protective structure  500  is comprised of a highly conductive material and further has a TCE which is matched as closely to the die&#39;s TCE as possible to also aid in the heat conduction away from the die  470  and to limit the problems encountered when a die expands more quickly than its attached protective structure. This embodiment&#39;s substantially flat back surface also allows a larger, conventional heat sink to be attached than does the protective structure  150  shown in FIG.  1 . In another alternate embodiment, the protective structure can be in the form of a substantially flat ring surrounding the die and supporting the bumps without enclosing the back surface of the die (similar to having just the shelf section  155  of the protective structure  150  shown in FIG.  1 ). This embodiment would allow a heat sink/spreader to be attached directly to the back surface of the die thereby improving the transfer of heat from the die to the heat sink. 
       FIG. 12  further provides an alternate compliant layer. The complaint layer shown in  FIG. 12  is comprised of elastomeric pads  510  which may be stenciled and cured on the second surface of the flex circuits and may be comprised of a silicone elastomer such as “Silgard®.” The pads  510  are positioned beneath or around each of the bumps ( 410 / 490 ) to provide adequate support for the bumps when they are mated to the contacts on a supporting substrate, such as a printed wiring board. However, the preferred embodiment also provides sufficient support at the edge of the bonding gap to provide support during the lead detachment/bonding steps. The area between these elastomeric pads may be filled with liquid encapsulant that is cured and controlled as described above in reference to FIG.  9 . Alternately, the entire compliant/encapsulation layer between the two flex circuits and the die/protective structure may be formed through the injection molding process described above in connection with FIG.  9 . 
     Referring now to  FIG. 14 , an alternate embodiment of the present invention further includes a ground plane  600  overlying and coupled to the first surface of the flex circuit  620 . The ground plane  600  is used to electrically ground selective bumps  630  which are coupled to the conductive pads  640  and further has the combined effect of reducing the interconnection impedance between the bumps and a supporting substrate and substantially eliminating much of the electrical interference, due to capacitive and inductive coupling, between adjacent bumps. The bumps  630  are plated to the conductive pads  640 / 645 . However, alternately, solder balls or solid core solder balls may be coupled to the pads  650 / 655 . If it is desired to have a particular bump  630  electrically grounded, at least one conductive region  650  will be provided to couple the pad  640  to the ground layer  600 . If it is not desired to have a particular bump  630  electrically grounded to the ground layer  600 , the bump pad  645  will be electrically isolated from the ground layer  600  by an isolation gap  660 . Each pad  640 / 645  is then electrically coupled to a conductive lead on the second side of the flex circuit  620  by a conductive through hole or a blind via”  690 , discussed in greater detail below. The compliant layer  610  is substantially identical to the compliant layer  170  of FIG.  1  and is coupled to the second surface of the flex circuit  620  isolating the conductive leads  670  which are lithographically formed on the second surface of the flex circuit  620  prior to coupling the compliant layer  610 . Thus the conductive leads  670  are isolated from any nonintended electrical connections. The flex circuit  620  has been partially removed in  FIG. 14  so that a portion of lead  670  may be viewed. A solder mask  680  is also applied to the exposed surface of the ground layer  600  so that the bumps  630  may be soldered/connected to respective contacts on a supporting substrate without causing an electrical short between the bumps  630 . 
       FIG. 15  shows a side view of a similar embodiment to that shown in  FIG. 14  before the bumps are plated or soldered to the pads  640 / 645 . As can be seen, pad  640  is electrically connected to the ground plane  600 , while pad  645  is electrically isolated from the ground plane  600  by the combination of the isolation gap  660  and the flexible, dielectric solder mask  680  which covers the ground plane  600  and fills the isolation gap  660 . In the embodiment shown in  FIG. 15 , the identical function of conductive through hole  690  of  FIG. 14  is accomplished by using a conductive via or well on the pads  640 / 645  so that the back side of the well is in electrical contact with the lithographically formed first conductive leads  670 A-B on the first flex circuit  720  and the second flex circuit  730 . The second conductive leads  740 / 750  are coupled to the first conductive leads and are initially suspended within the bonding gap  770 . The second conductive leads  740 / 750  are then separated at their respective detachment points and bonded to the die bond pads  760 , as described in reference to  FIGS. 4-8 . A first side of the compliant layer  610  is adhered to the second surface of the first and second flex circuits  720 / 730  and further adhered to the protective structure  710  and the die  700  on its second surface. 
       FIG. 16  shows a side view of a multiple circuit level embodiment of the embodiment shown in  FIG. 15  before the bumps are plated or soldered to the pads  800 / 805 . As in  FIG. 15 , the ground layer  810  is electrically coupled to pad  805 , but is electrically isolated from pad  800 . Pad  800  is further electrically coupled to multiple circuit layers comprised of conductive leads  820 / 830  which have been coupled, typically through a lithographic process, to either surface of flex circuit  840 . However, multiple single sided flex circuits could also be used or the lead  820  could be formed on the second surface of flex circuit  860 . Flex circuit  840  is laminated to flex circuit  860  by an adhesive  870 . An electrical connection may be from the lead  820  on the first surface of flex circuit  840  to lead  830  on the second surface of the flex circuit  840  by a conductive through hole or by a via solution, as described above. Conductive leads  830 / 880  are initially suspended within the bonding gap  890 . The leads  830 / 880  are then separated at their respective detachment points and bonded to the die bond pads  900 , as described in reference to  FIGS. 4-8 . 
     Having fully described several embodiments of the present invention, it will be apparent to those of ordinary skill in the art that numerous alternatives and equivalents exist that do not depart from the invention set forth above. It is therefore to be understood that the present invention is not to be limited by the foregoing description, but only by the appended claims.