Abstract:
A method is disclosed for making a leadframe package stand having application in semiconductor packaging and microelectronic assembly in which an IC device (e.g., a bare chip IC, a wafer level package, or a chipscale package) is received for electrical connection to a PWB or for vertical package over package stacking. Electrically conductive leadframe traces are arranged in an area array circuit pattern between outer leads at the periphery of the mold body of a leadframe for connection to the PWB to inner leads for connection to the IC device. The inner lead tips terminate at each side of the IC device in groups of parallel aligned rows and columns to facilitate connection to the IC device without using intermediate bonding wires. Prior to molding, the inner leads of the conductive traces are secured by sacrificial tie-bars or adhesive tape to prevent movement of the inner leads and possible short circuits during molding. A cavity is formed in the mold body during molding so as to lie above the inner leads. After molding, the sacrificial tie-bars are separated from the inner leads, and the IC device is located in the cavity to be assembled to the leadframe to complete a leadframe package.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a method for making a leadframe package stand for converting area array CSP type packages into leadframe type packages.  
         [0003]     2. Background Art  
         [0004]     Leadframe type packaging for semiconductor ICs has been used for more than 30 years. The most popular leadframe packages are used with various integrated circuits including ASIC, CPU, memory, microcontroller and DSP. The corresponding packaging formats for such integrated circuits include QFP (quad flat pack), TSOP (thin small outline packages), QFN (quad flat no leadframe), and MLF (microleadframe) packages. Following the trends in semiconductor packaging and assembly, the IC size and signal output (I/O) pad pitches are shrinking with the package size and I/O lead pitch.  
         [0005]     The function of metal, electrically conductive leads in the leadframe is to fan out the original IC bonding pads to a larger area that have wider pitches for the leads such that the leads are more suitable to interconnection to a printed wiring board (PWB). The most commonly used method of electrical interconnection between the individual leads of a leadframe and its IC bonding pads is wire bonding. For wire bonding, the terminal ends of each lead need to be located in proximity to the receiving bonding pad on the IC to receive a short wire loop therebetween. A jumping wire bridges the gap or separation between the IC pad and the lead terminal. In the case where flip chip bumping uses solder or some other conductive materials (e.g., gold stud bump, conductive polymer bump, and the like), the lead terminal must be located directly above or below the specific IC bonding pad to receive the bump for interconnection. Hence, the layout or pattern of the leadframe must account for fanning in the outer leads to the inner lead tips that are connected to the IC bonding pads.  
         [0006]     To provide easy accessibility and manufacturability, the leadframe leads typically stay in parallel aligned directions but generally shrink in width. Typically, the inner lead tips are aligned with one another along a single row with the appropriate pitch to receive the connecting wires. For ICs having perimeter pads along all four side of the IC body, the inner leads are arranged along four single rows, one row for each side of the body, to receive the wires. Sometimes, to relieve the tight pitch, the leads can be “offset” or staggered to form two parallel rows such that the wires with a shorter loop are connected to the first or front row and the wires with a longer loop are connected to the second or back row. In other cases, even two or more layers of leads at different elevations are used in a three-dimensional array for wire interconnections in order to spread out the pitch.  
         [0007]     For ICs that have centerline bonding pads such as those ICs that are made for dynamic random access memory (DRAM), the leadframe inner leads originate from two opposite sides and end in opposing rows that are in parallel alignment to the IC centerline bonding pads. As the rows of individual leads fan in, their widths shrink gradually in size to meet the narrower pitch of the IC pads. This kind of leadframe layout allows for low cost manufacturing by mechanical stamping from thin copper or alloy foils. Chemical etching may also be used to form the desired patterns.  
         [0008]     As the IC sizes shrink and the IC I/O layout becomes more complicated, not all I/O pads will be located in the center or at the perimeter. There are flip chip bumped ICs that have I/O pad layouts in the format of an area array. There are also wafer level packages (WLP) that use a fan out layer to redistribute the I/O pads of the IC from the perimeter or centerline to an area array format. If the area array has a large pitch or a low number of I/Os, it is possible to shape the leadframe metal leads and form a corresponding area array pattern, for example, a 3×3 for a 9-I/O package or a 4×4 for a 16 I/O package. However, for many modern IC memory and ASIC applications, the I/O number is more than 50 and the IC body is reduced to a size of about 10×10 mm or smaller. For such conditions, it is very difficult and time consuming to achieve mass production of a stamped leadframe or even an etched leadframe with fine pitch (less than 200 um) and higher I/O (greater than 50) in an array-area format.  
         [0009]     Therefore, it would be desirable to be able to make a leadframe type package by forming the individual inner leads in an area array format with very fine pitch and very small size to accommodate commercially available IC packages that are characterized by very small size and fine pitch in the area array, such as chip scale packages (CSP) and fine pitch ball grid array (FBGA) packages. It is also desirable to be able to use the leadframe package in known standard formats, such as TSOP, QFP or QFN, for accepting other CSP or WLP packages for vertical stacking of one package format on top of another to increase the electronic assembly density and performance without affecting the existing board level footprint or increasing the board area. Likewise, it would be desirable to be able to connect the flip chip IC directly to the individual inner leads while avoiding the space consuming wire bonds and the inherent disadvantages associated therewith, as in the case where a flip chip IC is used for stacking.  
       SUMMARY OF THE INVENTION  
       [0010]     In general terms, disclosed herein is a method for making a leadframe package stand having an external format that is identical to those of commercially available (e.g., QFP, TSOP, QFN and MLF) semiconductor packages in which a flip chip integrated circuit (IC) or an area array (e.g., CSP or WLP) package is assembled to the stand by a solder interconnect. The leadframe package stand functions as an adapter to allow an area array package to be interconnected to printed wiring boards (PWBs) designed for leadframe type packages having perimeter bonding leads. Furthermore, the stand/adapter can also be used to vertically stack the top area array package over a second package directly beneath it, hence achieving vertical stacking over the footprint used for a single package on the PWB.  
         [0011]     Inside the leadframe adapter, electrically conductive leadframe traces are arranged in an area array grid pattern on the leadframe skeleton to interface their inner leads to an IC or a WLP package. The outer leads of the leadframe stand are interconnected to the bonding pads on a PWB or any other suitable substrate. Conductive traces extend from the outer leads located at the periphery of the leadframe mold body at which to be connected to the PWB or substrate to inner leads at which to be connected to the IC bonding pads. In accordance with a first preferred embodiment, the tips of the inner leads terminate in two groups at opposite sides of the leadframe skeleton to conform to the typical ball-out patterns of DRAM FBGA packages. Each group of inner lead tips is arranged in an area array pattern including a plurality of parallel-aligned rows and columns, instead of a single row as has been customary for stamped leadframes. Bonding pads having a suitably wide (e.g., circular) configuration are connected to respective inner lead tips of each group to facilitate the IC or paackage being connected directly to the leadframe traces without the use of intermediate bonding wires as has also been customary in many leadframe packages.  
         [0012]     In accordance with another preferred embodiment, the inner leads of the electrically conductive traces on the leadframe skeleton are initially linked together prior to molding by means of a set of sacrificial tie-bars. The tie-bars prevent the individual leads from floating or being displaced relative to one another during the mold process so as to avoid undesirable shifts and possible short circuits between adjacent leads. In an alternate embodiment, the tie-bars may be replaced by thin adhesive tape which links the inner leads so as to prevent a displacement thereof during molding.  
         [0013]     During the plastic transfer molding process, a cavity is formed in the mold body by preventing the mold resin from flowing into an area lying above the inner leads and the tie-bars or adhesive tape interconnected therebetween. The cavity is sized to accept a bare chip IC or a wafer level package therewithin. The mold material of the mold body surrounds the inner leads, although the top surfaces of the leads are exposed and lie flush with the bottom of the cavity. Following molding, the sacrificial tie-bars exposed in the cavity are detached from the conductive traces by way of patterned etching or laser ablation. An IC or a wafer level package can then be assembled directly to the exposed inner leads of the conductive traces at the (e.g., circular) bonding pads connected to the inner lead tips. Flip chip bumps and solder balls may, for example, be used to assemble the IC and the package to the leadframe. The IC can then be covered over by an underfill or also an encapsulating material. The resulting leadframe package is now ready for connection to a PWB substrate, motherboard, memory card, cell phone/PDA board or for use in a vertical package over package stacking arrangement.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  shows a conventional lead-on-chip (LOC) stamped leadframe with a standard TSOP type  2  fan out;  
         [0015]      FIG. 2  shows the top of a leadframe having an area array circuit pattern that is formed in accordance with a preferred embodiment of this invention;  
         [0016]      FIG. 3  illustrates the area array circuit pattern shown in  FIG. 2  with the inner leads linked together by sacrificial tie-bars located in a cavity that is formed in the mold body;  
         [0017]      FIG. 4  illustrates the area array circuit pattern of  FIG. 3  with the tie-bars removed;  
         [0018]      FIG. 5  shows a cross-section of a leadframe package stand according to one preferred embodiment for receipt of a bare chip IC by means of flip chip bumps connected to the exposed inner leads through the cavity in the mold body;  
         [0019]      FIG. 6  shows an alternate form factor package with 4-sided perimeter leads and a center cavity; and  
         [0020]      FIGS. 7-9  show a cross-section of a leadframe package stand according to another preferred embodiment for receipt of a wafer level package by means of solder balls connected to the exposed inner leads through the cavity in the mold body.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]      FIG. 1  of the drawings is illustrative of a conventional lead-on-chip (LOC) leadframe  10  wherein metallic inner leads  12  are aligned with one another so as to terminate along a single row on each side of an IC  14  that is mounted on the underside of leadframe  10 . A row of bonding pads  15  extends longitudinally along IC  14  so that wire bonds can be connected between the inner leads  12  and the bonding pads  15 . The inner leads  12  extend outwardly by way of conductive traces  16  that terminate at outer leads  18  along two sides of the mold body  19 . The outer leads  18  are exposed outside the mold body  19  in order to be bent downwardly in a gull-wing or similar fashion to form external terminations.  
         [0022]     In the LOC leadframe configuration of  FIG. 1 , the inner leads  12  line up parallel to each other with minimum bending or change in direction. This parallel, side-by-side alignment facilitates the manufacture of the inner leads  12  from a foil by means of stamping or chemical etching. A minimum gap must be maintained between the inner leads  12  in order to avoid electrical shorts. The tips of inner leads  12  may be coated with thin layers of surface metals to facilitate wire bonding to gold or aluminum wires (not shown) that interconnect to the IC  14  at bonding pads  15 .  
         [0023]     In accordance with a preferred embodiment and in order to avoid the wire bonds that are common to leadframes like that shown in  FIG. 1 , an improved leadframe  20  is shown in  FIG. 2  of the drawings having a circuit pattern wherein the outer leads  22  thereof extend outside the mold body  24 . The mold body  24  is typically an electronic chemical molding compound (EMC) that is formed by a transfer mold disposed over the base leadframe skeleton. The outer leads  22  are characterized by a standard spacing and width common to conventional leadframe packages, such as a TSOP type  2  format. Electrically conductive fan out traces  26  connect the outer leads  22  to the inner leads  27 . The traces  26  on the leadframe  20  of  FIG. 2  are bent so that the tips of the inner leads  27  terminate in two groups at opposite sides of the leadframe  20 . Each group of inner lead tips is arranged in a plurality of parallel aligned rows and columns of an area array pattern, instead of a single row like that shown for the leadframe  10  of  FIG. 1 .  
         [0024]     Because the inner leads  27  can now be more widely spaced from one another throughout the area array pattern on both sides of the leadframe  20  of  FIG. 2 , the tips of the inner leads  27  may now be respectively connected to relatively wide (e.g., circular) pads  28  for bonding directly to flip chip bumps from either an IC (best shown in  FIG. 5 ) or another package having solder ball bumps (best shown in  FIG. 9 ). Accordingly, the inner leads  27  may now be arranged with very fine pitch and small size to be connected to today&#39;s very small size (e.g., less than 10 mm) ICs that are characterized by a fine pitch (e.g., less than 100 μm). In this same regard, the fan out traces  26  can be bent and rounted (as opposed to being straight in the manner illustrated in  FIG. 1 ) so as to establish a relatively high density routing pattern between the inner and outer leads  27  and  22 .  
         [0025]      FIG. 3  of the drawings illustrates an intermediate step used to achieve the desired leadframe circuit pattern shown in  FIG. 2 . Instead of the circuit pattern on the leadframe  30  of  FIG. 3  being completely encapsulated, as is customary in conventional packaging arrangements, a cavity or window  34  is formed in the plastic mold body  32  during the plastic transfer mold process. The cavity  34  may be formed in the mold body  32  during molding by preventing the mold resin from flowing into an area lying above the inner leads  36 . By way of particular example, the cavity  34  may be initially covered by a removable film, sheet or insert that is later removed after molding to expose the top surfaces of the inner leads  36  located at the bottom of the cavity  34 .  
         [0026]     The mold body  32  protects and insulates the individual lead traces once the thermosetting EMC has cured during molding. However, because intricate circuit patterns cannot be easily held in place for molding, there is no certainty that the leads  36  will remain at the same pitch, spacing and location during and after molding. Therefore, to overcome this problem, the lead terminals (in the shape of circular pads  38 ) for the inner leads  36  are initially linked together during formation of the leadframe circuit pattern by sacrificial tie-bars  40  that are located within the interior of the cavity  34 . The tie-bars  40  ensure that the individual leads  36  will not float so that one lead trace does not become undesirably attached to another lead trace. Should the inner leads  36  be permitted to float or move during the mold process because of the force created by the molten resin, the result could be a wide position shift and, consequently, contacts (i.e., shorts) between adjacent leads.  
         [0027]     It is preferable that the tie-bars  40  for holding the inner leads  36  in place be manufactured from the same material as that used to make the conductive traces which define the circuit pattern. By way of example, tie-bars  40  may be simply incorporated in the photomask design that is used to lay out the circuit pattern. A chemical etch step can be employed to produce the final circuit pattern including the coextensive tie-bars  40  for linking the inner leads  36  of leadframe  30 .  
         [0028]     Because a majority of the mold body  32  and the leadframe traces will be encapsulated by a plastic insulating material  42 , the inner leads  36  and the contact pads  38  to which the inner leads are connected inside the cavity  34  will not be able to move during molding. The plastic material  42  of mold body  32  should surround the individual inner leads  36  that are located at the interior of cavity  34  so as to fill in the gaps between them, but without completely covering the leads. That is to say, the top surfaces of the leads  36  will remain exposed within cavity  34  and lie flush with the mold material at the bottom of the cavity  34 . The outer terminals  44  that are connected to respective inner leads  36  of leadframe  30  are also exposed so that they can be bent and trimmed to form the final external lead terminals that are used in typical TSOP and other types of leadframe packages.  
         [0029]     Turning now to  FIG. 4  of the drawings, the sacrificial tie-bars  40  shown in  FIG. 3  are preferably removed by means of a second etch step. In this case, grooves (not shown) will remain in the mold material  42  at the bottom of the cavity  34  in place of the tie-bars  40  that have been removed therefrom. Alternatively, since the inner leads  36  inside cavity  34  are accessible, the tie-bars  40  can be simply cut off by means of a fine focus laser beam and thereby disconnected from individual inner lead contact pads  38 . In this case, short portions of tie-bars (also not shown) will remain embedded in the mold material  42  at the bottom of the cavity  34 , but will become non-functional, since they are now separated and isolated from the inner lead contact pads  38 . The area array inner lead format is now ready to be assembled directly to a bare chip IC by means of flip chip bumps (of  FIG. 5 ), a wafer level package (of  FIG. 9 ), or any other suitable chip scale package.  
         [0030]      FIG. 5  of the drawings shows a cross-section of a finished leadframe package stand or adapter  50 , with the outer leads  52  thereof bent in a gull-wing fashion for assembly to a PWB or other suitable substrate (not shown). Molding  54  covers the lead traces and the area array lead pattern  56  that is located at the bottom of the open cavity  58 . A bare chip IC  60  can now be assembled directly to the leadframe package stand  50  inside the cavity  58  by such conventional means as flip chip bumps  62  and flip chip bonding or solder reflow. After assembly, the IC  60  may be underfilled with a liquid underfill material, or the IC  60  may be covered by a liquid encapsulating material for mechanical protection. The assembly may be optionally molded again by filling up the remaining area of cavity  58  once the IC  60  has been assembled therein as previously described. However, it should be appreciated that the ability to assemble the IC  60  directly to the area array lead pattern  56  that is exposed within cavity  58  advantageously avoids the space consumption and increased manufacturing complexity that is associated with intermediate wire bonding techniques that have heretofore been associated with leadframes like that shown in  FIG. 1 .  
         [0031]     The finished leadframe assembly is ready to be surface mounted to a PWB motherboard or substrate. Alternatively, the top IC or IC package and the lower leadframe stand can be assembled in a continuous sequence of steps to a PWB or a substrate, such as a motherboard, a memory card, a cell phone/PDA motherboard, or the like.  
         [0032]     It should be recognized that the leadframe array area pattern (shown in  FIG. 3 ) can be made to fit the envelope footprint used for various other leadframe package types and sizes, such as QFP, QFN, or microleadframe packages. Referring to  FIG. 6  of the drawings, there is shown one example of a cavity leadframe stand  65  with the envelope of a QFP. The inside cavity  66  contains the exposed area array leadframe bonding pads  68 . Surrounding the cavity  66  is mold material  70  that covers and fixes the location of individual lead traces. External leads  72  may be exposed along all four of the perimeter edges of the mold body to be trimmed and formed as either gull-wing or J-lead terminals of the type commonly used in leadframe packages.  
         [0033]     Instead of receiving a bare chip IC  60  as shown in  FIG. 5 ,  FIGS. 7-9  of the drawings show a leadframe package stand or adapter  50 - 1  now being used to receive a wafer level package  80  of the type which carries an integral IC. The package  80  (best shown in  FIGS. 8 and 9 ) may be a well known BGA or CSP package, or the like. The package stand/adapter  50 - 1  of  FIGS. 7-9  is substantially identical to the package stand/adapter  50  of  FIG. 5  and, therefore, identical reference numbers have been used to designate identical elements. However, as an important distinction, the stand  50 - 1  of  FIGS. 7-9  includes a thin layer of adhesive (e.g., tape)  82  that is laid between the area array lead pattern  56  that is exposed within the cavity  58  and the molding material  54  along the bottom of the leadframe package stand  50 - 1  which lies below cavity  58 .  
         [0034]     In this case, the adhesive layer  82  replaces the sacrificial tie-bars  40  that were described while referring to  FIGS. 3 and 4 . That is to say, the adhesive layer  82  is used to prevent the area array lead pattern  56  from moving during the molding of lealdframe package stand  50 - 1  by means of linking the inner leads to one another within the cavity  58  in order to avoid the possibility of inadvertent contact and short circuits therebetween. The adhesive layer  82  is preferably applied after the leadframe is formed (e.g., by means of chemical etching or stamping), but before molding.  
         [0035]     Since the adhesive layer  82  is electrically non-conductive, it does not have to be removed from the area array lead pattern  56 , but may be left in place below the inner leads as the wafer level package  80  is moved through cavity  58  in the manner shown in  FIG. 8 . To this end, solder balls  84  are located at the bottom of package  80 . The package  80  can be assembled to the leadframe package stand  50 - 1  by solder interconnections between solder balls  84  and the contact pads (designated  38  in  FIG. 4 ) of the inner leads  56  which are exposed inside cavity  58 .  
         [0036]     The leadframe fabrication method disclosed herein enables ICs to be assembled to leadframe packages with flip chip bumps (as shown in  FIG. 5 ) or to wafer level packages with area array bumps (as shown in  FIGS. 7-9 ). Such packages will be particularly useful as an interposer during the assembly of vertically stacked packages or a 3-dimensional package-on-package or package-over-package configuration. This 3-dimensional stacking is advantageous for increasing the assembly package density and performance without increasing the assembly footprint or the consumption of board area.