Abstract:
A transfer mold assembly including a first mold chase; a second mold chase; a first lead frame; at least one first lead frame die mounted on the first lead frame; a second lead frame substantially identical to the first lead frame; at least one second lead frame die mounted on the second lead frame; and wherein the first and second mold chases define a transfer mold cavity and wherein the first and second lead frames are positioned in stacked relationship inside the transfer mold cavity. Also disclosed is a method of integrated circuit packaging.

Description:
BACKGROUND 
       [0001]    In producing integrated circuits, it is often desirable to provide packaged integrated circuits having plastic or resin packages that encapsulate the die and a portion of the lead frame and leads. These packages have been produced a variety of ways. 
         [0002]    Conventional molding techniques take advantage of the physical characteristics of the mold compounds. For integrated circuit package molding applications, these compounds are typically thermoset compounds that include an epoxy novolac resin or similar material combined with a filler, such as alumina, and other materials to make the compound suitable for molding, such as accelerators, curing agents, filters, and mold release agents. 
         [0003]    The transfer molding process as known in the prior art takes advantage of the viscosity characteristics of the molding compound to fill cavity molds containing the die and leadframe assemblies with the mold compound, which then cures around the die and leadframe assemblies to form a hermetic package which is relatively inexpensive and durable, and a good protective package for the integrated circuit. 
         [0004]      FIG. 1  depicts a conventional single plunger transfer mold press  11 . The press includes a plunger or ram  13  that is operated under hydraulic pressure, a top platen  15 , a top mold chase  17 , a bottom platen  19 , and a bottom mold chase  21 . A fixed head  23  supports the plunger and a movable head  18  support the top platen, and allows the top platen to be removed for loading and unloading the mold from the top. Mold heaters  25  provide heat to the mold in both the top and bottom platens. An automated mold controller, although not shown, is usually coupled to the press. The top and bottom platens are usually steel and receive the stresses of the pressing operation; both are heated to provide the temperature needed to perform the transfer molding operation. 
         [0005]      FIG. 2  depicts a typical bottom mold chase. In  FIG. 2 , a top view of bottom mold chase  21  is shown. There are six primary runners  31 , each will support a pair of leadframe strips holding wire bonded dies and lead assemblies over each cavity  33 . The cavities are formed along the runners  31 , which are cylindrical shaped paths that extend from the mold pot  32  and into the rows of cavities. Each cavity is coupled to the runners by a secondary runner  35  which ends in a gate  37 , a small opening that lets the mold compound into the cavity. The size and shape of the gate is critical to the speed and control of the transfer and filling stages of the molding process. 
         [0006]      FIG. 3  is a detailed drawing of a single runner  31  with a single die cavity  33  shown. The secondary runner  35  is shown coupling the primary runner to the gate  37  and to the die cavity  33 . Runner  31  is coupled to the pot  32 . 
         [0007]      FIG. 4  depicts a cross section BB from  FIG. 3 . This cross section is taken across the primary runner  31  and along secondary runner  35 , and depicts the sloped shape of secondary runner  35  up to the gate  37 . The lead frame  51  of a typical bonded part is shown over the bottom mold chase cavity and under the top mold chase cavity  34 . Die  53  is shown with the bond wires  55  coupling it to leadframe  51 . 
         [0008]    The operation of the conventional single pot transfer mold will now be described with reference to  FIGS. 2-4 . To begin a new molding operation, the mold press is opened and the top and bottom mold chases  17  and  21  are separated. The leadframe and die assemblies are loaded into the bottom mold chases. The mold compound is preheated using an R/F heater or other heater before being placed into the heated mold. 
         [0009]    The top and bottom platens are closed, bringing the top and bottom mold chases together. The top and bottom mold chases  17  and  21  are patterned to define a cavity around each die, with the lead frames extending outside the cavity and a space formed around each die. Several leadframe strips each having a row of dies  53 , which are bonded to their respective lead frames  51 , are placed over the cavities  33  in the bottom mold chase  21 . A pellet of resin or similar material mold compound is placed in the mold pot within the top mold chase  17 . After an initial heating stage to put the mold compound into its low viscosity state, the plunger or ram  13  is used to begin the transfer phase of the operation. The plunger  13  is brought down through the top mold chase  17  onto the mold compound pellet at a predetermined rate, forcing the mold compound into the primary runners  31 . As the runners fill with mold compound the compound will begin filling the secondary runners  35 , entering the gates  37  beneath the leadframe and die assemblies  51  and filling the cavities  33 . 
         [0010]    At the end of the transfer stage the mold compound should fill each cavity  33 , preferably at the same time and before the mold compound begins to cure. The rate of the downward force brought by the plunger  13  is varied during the transfer phase to help control the transfer process. Experimental use of the press  11  with a particular mold and compound combination will provide the best combination of pressure and transfer speed which can then be programmed into the automatic press controls to uniformly repeat the process. 
         [0011]    After the transfer stage, the packaged parts are cured. Curing the molded parts typically takes 1 to 3 minutes of sitting in the heated mold without disturbance. The compound cure is fairly rapid and may be enhanced by adding curing agents to the compound. At the end of the curing cycle the press is opened and the molded parts and the mold compound sprue or flash in the runners and pot are ejected. This is done by having ejection pins extending through the bottom mold chase  21  and bottom platen  19  push upward under pressure at the same instant, popping the molded parts and sprue out of the bottom mold chase  21 . The packaged parts are then removed to other areas where they are separated and trim and form operations performed on the parts. 
         [0012]      FIGS. 1-4  depict a transfer mold operation in which each mold cavity is adapted to receive a lead frame  51  having a single die  53  mounted thereon and in which both sides of the lead frame are to be encapsulated with mold compound. In some transfer molding operations only a single side of a leadframe is encapsulated. In such single side encapsulation operations, multiple dies may be mounted on a portion of a lead frame that is positioned within a single cavity formed by a single chase. Such an operation is depicted in  FIGS. 5 and 6 . 
         [0013]      FIGS. 5 and 6  are schematic cross section views of a transfer mold press  78  in a first and second operating state, respectively. The press has a top mold chase  80  that has no cavity therein. The top mold chase  80  has a flat bottom surface  81 . A bottom mold chase  82  has a cavity  84  that is adapted to receive a leadframe  90  having a first side  91  and an opposite second side  93 ,  FIG. 5 . Multiple dies  100  are mounted on the first side  91  of the leadframe  90 . Each die  100  has bond wires  102 ,  104  electrically connecting it to leadframe  90 . A release film  106  is positioned between the second side  91  of the leadframe  90  and the flat bottom surface  81  of the top mold chase  80 . The release film  106  is used to facilitate removal of the leadframe  90  from the mold  78  at the end of the molding operation. 
         [0014]    A mold pot, shown schematically at  112 , is in fluid communication with the bottom mold cavity  84  through a gate  114 ,  FIGS. 5 and 6 . The mold pot  112  has a plunger  116  reciprocally mounted therein. Mold compound  120  may be placed in the mold pot,  FIG. 5 . Plunger  116  may be moved in direction  118 ,  FIG. 5 , to cause molten mold compound to flow from the mold pot  112  through gate  114  into cavity  84  as illustrated in  FIG. 6 . Vents (not shown) in fluid communication with cavity  84  enable air to escape from cavity  84  as the mold compound enters. The mold compound fills cavity  84  encapsulating the dies  100 . After the mold compound cools, an encapsulation block  130 , thus formed and attached to lead frame  90 , is removed from the mold  78  and singulated, i.e. cut into individual, typically rectangular packages, each containing a portion of the lead frame  90  and an attached, epoxy encapsulated die  82 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a front view of a conventional single plunger mold press; 
           [0016]      FIG. 2  is a schematic top view of a bottom mold chase used with the conventional mold press of  FIG. 1 ; 
           [0017]      FIG. 3  is a detail view of a portion of the bottom mold chase of  FIG. 2 ; 
           [0018]      FIG. 4  is a cross sectional view of the bottom mold chase shown in  FIG. 3  and a top mold chase; 
           [0019]      FIG. 5  is a schematic cross sectional view of a transfer mold press in a first operating state; 
           [0020]      FIG. 6  is a schematic cross sectional view of a transfer mold press in a second operating state; 
           [0021]      FIG. 7  is a schematic cross sectional view of a transfer mold press in a first operating state; 
           [0022]      FIG. 8  is a schematic cross sectional view of a transfer mold press in a second operating state; 
           [0023]      FIG. 9  is a top plan view of a transfer mold lower chase with stacked first and second lead frames positioned over the lower mold cavity; 
           [0024]      FIG. 10  is a perspective view of two mechanically joined encapsulation blocks and leadframes; 
           [0025]      FIG. 11  is a top plan view of transfer mold lower chase with another embodiment of stacked first and second lead frames positioned over the lower mold cavity; 
           [0026]      FIG. 12  is a flow chart of a method of integrated circuit packaging. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIGS. 7-12  disclose a transfer mold press  278 , the construction and operation of one embodiment of the transfer mold press will now be described generally with reference to  FIGS. 7 and 8 . The transfer mold press has a bottom mold chase  280  and a top mold chase  286 . The bottom mold chase has a bottom mold cavity  284  and the top mold chase has a top mold cavity  288 . The bottom and top mold cavities  284 ,  288  together define the mold cavity of the transfer mold press  278 . The bottom mold cavity  284  is adapted to receive a first substrate  290 . The first substrate  290  has a first side  291  and an opposite second side  293 . At least one first substrate die  300  is mounted on the first substrate first side  291 . The top mold cavity  288  is adapted to receive a second substrate  290 . The second substrate  290  has a first side  295  and an opposite second side  297 . At least one second substrate die  301  is mounted on the top substrate first side  295 . 
         [0028]    The bottom and top mold chases are constructed and arranged such that the bottom mold cavity  284  is positioned directly opposite the top cavity  288  when the transfer mold press  278  is in a closed position as shown in  FIGS. 7 and 8 . In this closed position, the first and second substrates  294 ,  298  are positioned in the bottom and top mold cavities  284 ,  288 , with the second sides  293 ,  297  of the substrates positioned one below the other in adjacent relationship. Molten mold compound  320  from a mold pot  312  is forced into both the bottom and top mold cavities  284 ,  288 . The mold compound forced into the bottom cavity  284  encapsulates the die(s)  300  mounted on the first substrate  290  forming a first encapsulate block  330 . The mold compound forced into the top cavity  288  encapsulates the die(s)  301  mounted on the second substrate  294  forming a second encapsulate block  332 . When the chases are separated the two encapsulate blocks are removed and separated. In embodiments where only one die  300  or  301  is mounted on each substrate  290 ,  294  each encapsulate block forms a single integrated circuit (IC) package, i.e. an encapsulated die/substrate assembly. When multiple dies  300  or  301  are mounted on each substrate, the blocks  330 ,  332  are singulated into multiple IC packages. 
         [0029]    An advantage of this method of IC packaging is that twice as many IC packages can be produce in a single transfer mold press operation as compared to a conventional transfer mold press, without increasing the “footprint” of the transfer mold press. In other words, the output per mold press operating cycle is doubled without increasing the area occupied by the transfer mold press in the horizontal (x,y) plane. 
         [0030]    Having thus described an embodiment of a transfer mold press  278  generally, various embodiments of a transfer mold press will now be described in further detail. 
         [0031]      FIGS. 7 and 8  disclose a transfer mold press  278 . The press  278  includes a bottom mold chase  280  having a bottom mold cavity  284  and a top mold chase  286  having a top mold cavity  288 . The top and bottom mold cavities  284 ,  288  collectively define a mold cavity. It is to be understood that this mold cavity may be the single mold cavity of the transferable press  278  or it may be one of many cavities such as described for the transfer mold press  78  of  FIGS. 1-5 . The mold cavity defined by bottom and top mold cavities  284 ,  288  is adapted to receive and support two substrates therein that are positioned in a stacked relationship. Substrate, as used herein, means an organic or other substrate including a leadframe. The two substrates that are stacked together within the mold cavity include a first substrate  290  and a second substrate  294 . The first substrate  290  has a first side  291  and a second side  295 . The second substrate  294  has a first side  295  and a second side  297 . At least one first substrate die  300  is mounted on the first side  291  of the first substrate and at least one second substrate die  301  is mounted on the first side  295  of the second substrate  294 . Each die  300 ,  301  may comprise one or more bond wires  302  which are electrically connected to the associated substrate. Each substrate  290 ,  294  has a generally flat plate shape and may support a single die, a single row of dies or multiple rows and columns of dies which would typically be arranged in a rectangular grid. The illustration of  FIG. 7  has four dies,  300 ,  301  visible on each substrate  290 ,  294 , but it may include further columns of dies that are not visible in this cross sectional view. 
         [0032]    The substrates  290 ,  294  are mounted within the mold cavity  284 / 288  in a stacked relationship in which the first side  291  of the first substrate  290  is positioned adjacent to the first side  295  of the second substrate  294 . “Adjacent” or “abutting” as used herein to describe the relationship of first sides  291 ,  295  means that the two sides  291 ,  295  are positioned close to one another and may or may not be touching one another. In the embodiment shown in  FIG. 7 , a release film  306  is positioned between the two substrates  290 ,  294  and thus the substrates each physically touch the release film  306  without touching the other substrate. In the embodiment shown in  FIG. 7 , each substrate die assembly  290 / 300 ,  294 / 301  may be identical to the other. In the embodiment illustrated in  FIG. 7 , the bottom mold chase  280  includes two recessed portions  281 ,  283  which are positioned at either end of the bottom mold cavity  284 . Similarly, the top mold chase  286  may have recessed portions  287 ,  289 . In the embodiment illustrated in  FIG. 7 , end portions of the first substrate  290  are received and supported in recessed portions  281 ,  283 . In the embodiment illustrated in  FIG. 7 , the end portions of the second substrate  294  are positioned within the recessed portions  287 ,  289  when the mold is in the closed operating position. In some embodiments, recessed portions  281 ,  283  may be made sufficiently deep to receive both substrates  290 ,  294  in which case recesses  287 ,  289  are eliminated. 
         [0033]    Flow of molten mold compound  320  into the bottom mold cavity  284  and top mold cavity  288  will now be described. The transfer mold press  278  comprises a mold pot  312  which may be a conventional mold pot  312  having a plunger  316  therein which may be moved in direction  318  to move molten mold compound  320  from the mold pot  312  into the bottom and top mold cavities  284 ,  288 . In the embodiment illustrated in  FIG. 7 , a fluid passageway  313  in fluid communication with the mold pot  312  is connected to lower cavity gate  314  and upper cavity gate  315 . Thus, as illustrated in  FIG. 8 , molten mold compound  320  flows from the mold pot  312  through passageway  313  and lower cavity gate  314  into the bottom mold cavity  284  and through fluid passageway  313  and upper cavity gate  315  into top mold cavity  288 . As the molten mold compound  320  enters the mold cavities, there is discharge from the mold cavities through vents (not shown) in the cavities. 
         [0034]    When the mold compound cools and solidifies, a first encapsulant block  330  is formed in the bottom mold cavity  284  and a second encapsulant block  332  is formed in the top mold cavity  288 . These encapsulant blocks  330 ,  332  each encapsulate all of the dies located on the first side  291 ,  295  of each substrate  290 ,  294 . The bottom and top mold chases  280 ,  286  are then separated and the two encapsulant blocks  330 ,  332  are then removed from the bottom and top mold cavities and separated. In an embodiment in which a single die  300 ,  301  are mounted on each of the first and second substrates  290 ,  294  respectively, each block represents an integrated circuit package including a substrate,  290  or  294 , and a die,  300  or  301 , mounted thereon and covered with encapsulate. In embodiments in which multiple dies are mounted on each substrate, the encapsulate blocks  330 ,  332  are singulated into multiple integrated circuit packages. 
         [0035]      FIG. 9  represents one alternative structure for causing molten mold compound  320  to flow into both the bottom and top mold cavities  284 A,  286 A (not shown).  FIG. 9  is a top plan view of a bottom mold chase  280 A having a bottom mold cavity  284 A with a rectangular periphery and having a fluid passageway  313 A extending from the mold pot (not shown) into the cavity. A pair of stacked substrates  290 A,  294 A is positioned over the bottom mold cavity  284 A. The second substrate  294 A is positioned below the first substrate  290 A. In this embodiment each substrate  290 A,  294 A may comprise a portion of a continuous substrate strip which is trimmed into individual substrates after the molding process is completed. In the embodiment of  FIG. 9 , the second substrate  294 A has  12  dies  301 A mounted thereon in a three by four grid. In this embodiment, the first and second substrates  290 A,  294 A each have aligned peripheral edges including aligned lateral side portions  336 ,  338 . These lateral side portions  336 ,  338  are positioned inwardly of lateral side walls  340 ,  342  of the bottom mold cavity  284 A. In this embodiment, there is no upper cavity gate  315  in the top mold chase (not shown) but fluid flow into the top mold cavity occurs because the molten mold compound  320  flows from the lower mold cavity  284  up into the top mold cavity  288  through the gaps between the lateral side walls  340 ,  342  of the bottom mold cavity  284 A and the lateral side portions  336 ,  338  of the substrates  290 ,  294 . As a result of this flow around the lateral side portions of the substrates, the two blocks of encapsulant formed in the bottom and top mold cavities  284 A,  288 A are mechanically joined together at lateral sides portions  362 ,  364  thereof to form a single encapsulate block  360 , as illustrated in  FIG. 10 . The substrates  290 A,  294 A and a release film  306 A positioned therebetween are visible projecting from the ends of block  360  in  FIG. 10 . In this embodiment the lateral outside portions  362 ,  364  must be trimmed from block  360 , as with a conventional singulation saw, in order to allow separation of the block  360  into upper and lower blocks. The upper and lower blocks may then each be singulated into  12  integrated circuit packages. 
         [0036]    Another structure for enabling flow of molten mold compound  320  into both the bottom and top mold cavities is illustrated in  FIG. 11  in which the mold has a bottom mold chase  280 B with a bottom mold cavity  284 B. A first substrate  290 B and second substrate  294 B having dies  301  B are positioned over the bottom mold cavity  284 B. In this embodiment two columns of dies  301  B are provided on the second substrate  294 . In this embodiment both substrates and any intermediate release film that may be positioned therebetween, have circular holes  370  extending therethrough to provide at least one fluid passageway from the bottom mold cavity  284 B to the top mold cavity. In this embodiment, as in the embodiment described with respect to  FIGS. 9 and 10 , the upper and lower encapsulant blocks formed in the upper and lower cavities will be mechanically joined. In this embodiment, such mechanical coupling will be caused by the mold compound that extends through holes  370 . Thus in this embodiment, a central portion  332  of the block will need to be trimmed away once the block is removed from the mold cavities. After removal of this section  372 , each lateral half of the block will then need to be split into upper and lower blocks and singulated if there is more than one die  301   b  present. Thus in the embodiment illustrated in  FIG. 11 , sixteen integrated circuit packages would be provided after the trimming and singulation operation. Although three different techniques for causing mold compound to flow into bottom and top mold cavities, it will be appreciated by those skilled in the art that any single one or any combination of these techniques could be used for this purpose. 
         [0037]      FIG. 12  is a flow chart that illustrates a method of integrated circuit packaging. The method includes, as shown in block  400 , providing a first substrate having a first side with at least one first substrate die mounted thereon and an opposite second side and a second substrate having a first side with at least one second substrate die mounted thereon and an opposite second side. The method also includes as shown at block  402  positioning the first and second substrates in stacked relationship in a transfer mold cavity. 
         [0038]    Although embodiments of certain methods and devices are expressly described herein, it will be obvious to those skilled in the art after reading this disclosure that the methods and devices disclosed herein may be otherwise embodied. The claims attached hereto are to be construed broadly to cover such alternative embodiments, except as limited by the prior art.