Abstract:
An improved multi-chip module includes a main circuit board having an array of electrical interconnection pads to which are mounted a plurality of IC package units. Each IC package unit includes a pair of IC packages, both of which are mounted on opposite sides of a package carrier. The package units may be mounted on one or both sides of the main circuit board. A first primary embodiment of the invention employs a laminar package carrier having a pair of major planar surfaces. Each planar surface incorporates electrical contact pads. One IC package is surface mounted on each major planar surface, by interconnecting the leads of the package with the contact pads on the planar surface, to form the IC package unit. Several different variations of the chip module are disclosed.

Description:
This application is a continuation of Ser. No. 09/524,324 filed Mar. 13, 2000 now U.S. Pat. No. 6,487,078 invention is related to U.S. patent application Ser. No. 09/285,354, which was filed on Apr. 2, 1999, and which is titled ELECTRONIC MODULE HAVING A THREE DIMENSIONAL ARRAY OF INTEGRATED CIRCUIT PACKAGES. 

   FIELD OF THE INVENTION 
   This invention relates to the production of multi-chip electronic modules, and more particularly to a method and apparatus for attaching multiple integrated circuit packages to printed circuit boards. It also relates to high-density memory modules having three-dimensional arrangements of integrated circuit packages. 
   BACKGROUND OF THE INVENTION 
   Demand for semiconductor memory is highly elastic. On one hand, when such memory is relatively inexpensive compared to the overall cost of a computer system, an almost unsatiable demand results, with computer manufacturers tending to install an amount of main memory in each system that greatly exceeds the amount required for average program use. On the other hand, when it is costly, manufacturers typically install an amount in each system that only marginally fulfills the requirement of the average program. Although the sales prices of computers may, thus, be maintained at low levels, the end user may soon find that he must upgrade his computer&#39;s main memory. 
   The ever increasing demand for large random access computer memories, and the growing demand for increasingly compact computers, coupled with an incentive on the part of the semiconductor manufacturers to reduce the cost per bit, has lead to not only a quadrupling of circuit density approximately every three years, but to increasingly efficient techniques for packaging and mounting the circuit chips. Up until the late 1980&#39;s, semiconductor memory chips were usually packaged as dual in-line pin packages (DIPPs). The pins of these DIPP packages were generally soldered directly within through-holes in a main circuit board (e.g., the motherboard), or they were inserted in sockets which were, in turn, soldered within through-holes in the main circuit board. With the advent of surface mount technology, conventional plated through-holes on printed circuit boards have been replaced with conductive mounting pads. Small Outline J-lead (SOJ) packages have lead to Thin Small Outline Packages (TSOPs). Because the pitch or spacing between centers of adjacent surface mount pins is significantly less than the conventional 0.10-inch spacing for conventional through-hole components, surface mount chips tend to be considerably smaller than corresponding conventional chips, thus taking up less space on a printed circuit board. Additionally, as through holes are no longer needed, surface mount technology lends itself to the mounting of components on both sides of a printed circuit board. Memory modules utilizing surface-mount packages on both sides have become the standard. Both the earlier single in-line memory modules (SIMMs) and the currently used dual in-line memory modules (DIMMs) are inserted into sockets on the motherboard. 
   Packaging density may be increased rather dramatically by fabricating modules in which a plurality of integrated circuit (IC) chips, such as memory chips, are stacked in a three dimensional arrangement. As a general rule, the three-dimensional stacking of chips requires complex, non-standard packaging methods. 
   One example of a vertical stack of IC chips is provided by U.S. Pat. No. 4,956,694 to Floyd Eide, titled INTEGRATED CIRCUIT CHIP STACKING. A plurality of integrated circuits are packaged within package carriers and stacked, one on top of the other, on a printed circuit board. Except for the chip select terminal, all other like terminals on the chips are connected in parallel. 
   Another example of chip stacking is given in U.S. Pat. No. 5,128,831 to Fox, et al. titled HIGH-DENSITY ELECTRONIC PACKAGE COMPRISING STACKED SUB-MODULES WHICH ARE ELECTRICALLY INTERCONNECTED BY SOLDER-FILLED VIAS. The package is assembled from individually testable sub-modules, each of which has a single chip bonded thereto. The sub-modules are interleaved with frame-like spacers. Both the sub-modules and the spacers have alignable vias which provide interconnection between the various sub-modules. 
   U.S. Pat. No. 5,313,096, also issued to Floyd Eide and titled IC CHIP PACKAGE HAVING CHIP ATTACHED TO AND WIRE BONDED WITHIN AN OVERLYING SUBSTRATE, is another example. Such a package includes a chip having an upper active surface bonded to the lower surface of a lower substrate layer having conductive traces on its upper surface which terminate in conductive pads on its periphery. Connection between terminals on the active surface and the traces is made with wire bonds through apertures within the lower substrate layer. An upper substrate layer, which is bonded to the lower substrate layer, has apertures which coincide with those of the lower substrate layer and provide space in which the wire bonding may occur. After wire bonding has occurred, the apertures are filled with epoxy to form an individually testable sub-module. Multiple sub-modules can be stacked and interconnected with metal strips attached to their edges. 
   A final example of a stacked-chip module is disclosed in U.S. Pat. No. 5,869,353 to A. U. Levy, et al. titled MODULAR PANEL STACKING PROCESS. A plurality of panels are fabricated having apertures therein, an array of chip-mounting pads at the bottom of the apertures, and interfacing conductive pads. Both the chip-mounting pads and the interfacing conductive pads are coated with solder paste. Plastic-encapsulated surface-mount IC chips are positioned on the paste-covered mounting pads, multiple panels are stacked in a layered arrangement and the stack is heated to solder the chip leads to the mounting pads and the interfacing pads of adjacent panels together. Individual chip package stacks are then separated from the panel stack by a cutting and cleaving operation. 
   As can be seen by the foregoing examples, increased chip density is achieved through the use of complicated packaging and stacking arrangements, which must necessarily be reflected in a higher cost per bit of storage. 
   SUMMARY OF THE INVENTION 
   The present invention provides for increased circuit density on printed circuit boards. The invention is particularly useful for increasing the density of memory chips on memory modules used for computer systems. The invention includes a package carrier that is designed to mount on a printed circuit board (PCB) on top of a first integrated circuit (IC) package that is also mounted on the PCB. The carrier has an upper major surface having a pad array on which a second IC package is mountable. When mounted on top of the first IC package, the carrier may be thought of as a canopy, on top of which the second IC package is mounted. The carrier has a plurality of leads by means of which the carrier is surface mounted to the PCB. Each carrier lead is also electrically connected to a single pad of the pad array on the upper surface. The invention also includes a multi-chip module assembled using at least one PCB, at least one package carrier and at least two IC packages. For multi-chip modules where the IC package beneath the carrier shares all or most connections in common with the IC package mounted thereupon, a single lead of the carrier and a single lead of the package beneath the carrier may share a mounting/connection pad on the PCB. When separate connections must be made by similarly positioned leads on the carrier and the package beneath the carrier, the corresponding pad on the PCB may be split so that each lead has a unique connection. 
   A first embodiment of the carrier includes a body having a first pad array, arranged as two parallel linear rows of pads, and adhered to an upper major surface thereof. The leads of an IC package may be conductively bonded to the pads of the first pad array. The body also has a second pad array, arranged as two parallel linear rows of pads positioned along the longitudinal edges, and adhered to the lower major surface thereof. Pads of the first and second arrays are interconnected with conductively-plated vias, or through holes. The carrier leads are conductively bonded to the pads of the second array. The carrier incorporates a heat sink feature. The end leads on a first side of the carrier are both power leads. These two power leads are interconnected by a first laminar sheet which is continuous with and extends between those two leads, and which may extend the entire length of the carrier. An end portion of the first laminar sheet may be exposed at each end of the carrier to facilitate the transfer of heat to the ambient air. The end leads on a second side of the carrier are both ground leads. These two ground leads are interconnected by a second laminar sheet which is continuous with and extends between these two leads, and which may extend the entire length of the carrier. An end portion of the second laminar sheet may be exposed at each end of the carrier to facilitate the transfer of heat to the ambient air. Each laminar sheet is spaced apart from the intervening leads of the same row. The first and second laminar sheets are spaced apart from one another along the center of the carrier. Each IC package includes a dielectric body, an IC chip embedded within the body, and a plurality of leads, an end of each of which is also embedded within the body and electrically conductively coupled to a connection terminal on the IC chip. For a preferred embodiment of the multi-chip module, an upper surface of the body of the lower IC package is either in intimate contact with both laminar sheets, or thermally coupled thereto via a thermally-conductive compound, or in close proximity thereto to facilitate heat transfer from the package body to the laminar sheets. 
   A second embodiment of the carrier includes modified leads, each of which functions as a heat sink. A center portion of each lead is bonded to a pad of the second pad array on lower surface of the carrier body. An outer portion of each lead is shaped for surface mounting to a mounting/connection pad on a PCB. An inner portion of each lead extends toward the center of the body. For a preferred embodiment of the multi-chip module, an upper surface of the body of the lower IC package is either in intimate contact with the inner portion of each lead, or thermally coupled thereto via a thermally-conductive compound, or in close proximity thereto to facilitate heat transfer from the package body to the leads. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of a first embodiment package carrier; 
       FIG. 2  is an isometric view of the first embodiment package carrier body, showing the underside thereof; 
       FIG. 3  is an isometric view of the carrier leads of the package carrier of  FIG. 1 ; 
       FIG. 4  is an isometric view of the first and second heat sink sheets, which are connected to the ground leads and the power leads, respectively; 
       FIG. 5  is an isometric view of an exploded portion first embodiment electronic module; 
       FIG. 6  is an isometric view of an assembled portion of the first embodiment electronic module; 
       FIG. 7  is an isometric view of a second embodiment package carrier; 
       FIG. 8  is an isometric view of the second embodiment package carrier body, showing the underside thereof; 
       FIG. 9  is an isometric view of the carrier leads of the package carrier of  FIG. 7 ; 
       FIG. 10  is a top plan view of a carrier body of either the first or second embodiment package carrier; 
       FIG. 11  is an isometric view of an exploded portion second embodiment electronic module; and 
       FIG. 12  is an isometric view of an assembled portion of the second embodiment electronic module. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As will be evident from the attached drawing figures, the present invention permits the manufacture of electronic modules having increased circuit density. The invention may be used for a variety of applications. One very obvious usage is in the manufacture of memory modules. As memory modules typically incorporate a printed circuit board having rigidly prescribed dimensions, more efficient use of the board real estate will result in a module having greater total memory capacity. The invention may also be utilized to closely couple related, but dissimilar, IC packages. For example, it may be desirable to mount an IC package containing high-speed cache memory on top of an IC package containing a microprocessor chip. The various embodiments of the improved electronic module will now be described in detail with reference to the accompanying drawings. 
   Referring now to  FIGS. 1 and 2 , a first embodiment package carrier  100  has a dielectric body  101  having upper and lower parallel major planar surfaces  102 U and  102 L, respectively. For a preferred embodiment of the invention, the body is made from the fiberglass-reinforced plastic material commonly used to manufacture printed circuit boards. The dielectric body  101  also has a first mounting pad array  103  affixed to said upper major planar surface  102 U. The mounting pads  104  of the array  103  are individually shaped and collectively arrayed to receive the leads of a first integrated circuit package (not shown in this drawing figure). The dielectric body  101  also includes a second mounting pad array  105  affixed to said lower major planar surface  102 L. Each pad  106  of the second array  105  is coupled to a pad  104  of said first array  103  by means of an internally plated aperture  107  which extends between the upper major planar surface  102 U and the lower major surface  102 L. The package carrier  100  also includes a set of carrier leads  108 , each of which is conductively bonded to a pad  106  of the second mounting pad array  105 . The individual leads  109  of the carrier lead set  108  are spaced and configured for surface mounting on a printed circuit board (not shown in this drawing figure). It will be noted that the body  101  has a cutout  109  at each end thereof. It will also be noted that for this embodiment of a carrier, the spacing between the two rows of pads  104  of the first array  103  is narrower than the spacing between the two rows of pads  106  of the second array. The reason for this difference in spacing is that the package carrier  100  may be thought of as a canopy which overlies and bridges a second integrated circuit package mounted on the printed circuit board. Thus, the carrier leads must be wider spaced so that they mount outside of the leads of the package so covered. The package carrier  100  also includes a pair of capacitor mounting pads  110  at each end thereof. The pads of each pair are sized and spaced to receive a surface mount decoupling capacitor  111 . 
   Referring now to  FIG. 3 , the carrier lead set  108  of the first embodiment package carrier  100  includes a plurality of articulated leads  301 , each of which is individually attached to a pad  106  of the second mounting pad array  105 . The outer portion of each of the leads  301  is essentially C-shaped. The carrier lead set  108  also includes a trio of power leads  302 , which are interconnected via a first laminar sheet  303 , which also serves as a heat sink layer. Also included in the carrier lead set  108  is a trio of ground leads  304 , which are interconnected via a second laminar sheet  305 , which also serves as a heat sink layer. Both the first and second laminar sheets  303  and  305 , respectively, incorporate a pair of extension tabs  306 , which enhance heat dissipation from the laminar sheets. The cutouts  109  expose portions of the first and second laminar sheets  103  and  305 , thereby aiding in heat dissipation to the ambient air. 
     FIG. 4  shows the carrier lead set  108  minus all articulated leads  301 . The three power leads  302  and associated interconnected heat sink layer  303  are on the left, while the three ground leads  304  and associated interconnected heat sink layer  305  are on the right. The extension tabs  306  are also readily visible. 
   Referring now to the exploded view of a portion of an electronic module  500  in  FIG. 5 , a first integrated circuit package  501  having a plurality of leads  502  is shown aligned for surface mounting to the first mounting pad array  103  on the upper major planar surface  102 U of the first embodiment package carrier  100 . A printed circuit board  503  includes a second mounting pad array  504  having individual mounting pads  505  arranged in two parallel rows  506 L and  506 R. A second integrated circuit package  507  having a plurality of leads  508  is shown aligned for surface mounting to the third mounting pad array  504 . The package carrier  100  is also aligned for surface mounting to the third mounting pad array. The package carrier is designed to that its two rows of leads  109  constituting its carrier lead set  108  are spaced wider than the rows of leads  508  on the second integrated circuit package  507 . Such an arrangement permits one carrier lead  109  and one second package lead  508  to share a common mounting pad  505  on the printed circuit board  503 . Where the signals and/or power inputs are common, the pad  505  need not be split. However, where the signals are different (e.g., chip select signals), then the pad  505  may be split so that a different signal or power requirement may be delivered to the proper lead. Pad  505 S is such a split pad. If both the first and second packages  501  and  507 , respectively, are memory chips and the first package  501  is surface mounted to the carrier  100  and the carrier  100  and the second package are surface mounted to the printed circuit board  503 , then each chip may be individually selected by sending a signal to the appropriate half of pad  505 S. An alternative method of routing chip select signals to two identical chips involves utilizing a pad for an unused lead (of which there are typically several on each package) for one of the chip select signals and then rerouting the signal within the carrier body  101  to the pad where the chip select lead will be bonded. It will be noted that the printed circuit board includes a pair of capacitor mounting pads  509  at opposite corners of the third mounting pad array  504 . The pads of each pair are sized and spaced to receive a surface mount decoupling capacitor  111 . The positioning of the decoupling capacitors is generally not a critical issue, and the capacitors might just as easily be mounted on the same side of the carrier  101 . Additionally, more than two capacitors for each chip may be employed. It should be evident that for a pair of identical memory chips, all connections, other than the chip select input, will be vertically superimposed. In such a case, the internally-plated apertures  107  will be used to interconnect a pad  104  of the first mounting pad array  103  with a vertically-aligned pad  106  of the second mounting pad array  105 . When dissimilar first and second integrated circuit packages are employed, rerouting of the connections may be necessary. This may be accomplished in the same manner as used for printed circuit board design. Thus, between the first and second mounting pad arrays which are respectively located on the upper  102 U and lower  102 L surfaces of the carrier body  101 , one or more intervening layers of traces are embedded within the dielectric material of the body  101 . The intervening layers may also be interconnected with internally plated apertures. This technique is so common that it hardly requires discussion in this document. 
   Referring now to  FIG. 6 , a second integrated circuit package  507  is surface mounted to a third mounting pad array  504  on a printed circuit board  503 , a first embodiment package carrier  100  is also surface mounted to the third mounting pad array  504 , and a first integrated circuit package  501  is surface mounted to the first mounting pad array  103  of the package carrier  100 . The assembly also includes four decoupling capacitors  111  which are surface mounted to capacitor mounting pads  110  and  509 . 
     FIGS. 7 ,  8  and  9  show a second embodiment package carrier  700  in both assembled form ( FIG. 7 ) and component form (FIGS.  8  and  9 ). The principal difference between the first embodiment carrier  100  and the second embodiment carrier  700  is the shape of the leads  701 . It will be noted that each lead has an elongated portion which functions as a heat sink. There are no laminar sheets coupled to either the power and ground leads, as is the case for the first embodiment carrier  100 .  FIG. 8  shows the under side of the dielectric carrier body  101  which, in this case, is identical to that of the first embodiment carrier  100 . 
   Referring now to  FIG. 10 , a top view of the body of either the first or second chip carrier shows one configuration for the routing of traces for the decoupling capacitor mounting pads  110  and  509 . Trace  1001  couples pad  110 A/ 509 A to a power mounting pad  104 P of the first mounting pad array  103 , while trace  1002  couples pad  110 B/ 509 B to a ground mounting pad  104 G of the first mounting pad array  103 . Likewise, trace  1003  couples pad  110 C/ 509 C to a ground mounting pad  104 G of the first mounting pad array  103 , while trace  1004  couples pad  110 D/ 509 D to a power mounting pad  104 P of the first mounting pad array  103 . 
   Referring now to the exploded view of  FIG. 11 , a first integrated circuit package  501  having a plurality of leads  502  is shown aligned for surface mounting to the first mounting pad array  103  on the upper major planar surface  102 U of the second embodiment package carrier  700 . A printed circuit board  503  includes a third mounting pad array  504  having individual mounting pads  505  arranged in two parallel rows  506 L and  506 R. A second integrated circuit package  507  having a plurality of leads  508  is shown aligned for surface mounting to the third mounting pad array  504 . The second embodiment package carrier  700  is also aligned for surface mounting to the third mounting pad array. 
   Referring now to the assembled second embodiment electronic module  1200  of  FIG. 12 , a second integrated circuit package  507  is surface mounted to a third mounting pad array  504  on a printed circuit board  503 , a second embodiment package carrier  700  is also surface mounted to the third mounting pad array  504 , and a first integrated circuit package  501  is surface mounted to the first mounting pad array  103  of the package carrier  100 . The assembly also includes four decoupling capacitors  111  which are surface mounted to capacitor mounting pads  110  and  509 . 
   Although only several single embodiments of the invention have been heretofore described, it will be obvious to those having ordinary skill in the art that changes and modifications may be made thereto without departing from the scope and the spirit of the invention as hereinafter claimed. For example, many variations of two basic embodiments are possible. For example, the leads of surface mount IC packages may vary. In addition, the shape of the outer portions of the carrier leads may also vary from the “C” shape disclosed herein. At the present time, two types of leads are most commonly used for surface mount components. One lead is “J”-shaped; the other is “S”-shaped. The “S”, or gull-wing-shaped, leads are becoming increasingly widespread. Other types of leads for surface-mount components may also be developed. The invention should not be considered limited by the type of leads which are utilized on any of the constituent components or on the chip carrier  101 . Lead types may also be mixed between components comprising a module. Thus, assemblies having a number of different lead combinations are possible. At one end of the spectrum, both packages and the carrier may utilize “C”-shaped or “J”-shaped leads. At the other end, all components will use “S”-shaped leads. Between those two extremes, each of the components may utilize either of the three leads currently available for surface-mount components, as well as leads which might be developed. In addition, the surface mounting of components typically involves a solder reflow process, where leads and/or mounting pads are coated with a solder emulsion. The components are then assembled and the assembly is subjected to a reflow step in an oven. The leads are thus conductively bonded to the mounting pads. There are other known techniques for bonding leads to mounting pads. Placing a metal ball (usually gold) on each of the mounting pads, placing a lead on top of each ball, and using ultrasonic energy to fuse each ball to both its associated pad and lead is another surface mount option.