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HIGH DENSITY INTEGRATED CIRCUIT MODULE
CROSS-REFERENCE TO RELATED APPLICATION
This application is a divisional of application Ser. No. 09/222,263, filed Dec. 28, 1998 noW U.S. Pat. No. 6,205, 654, Which is a continuation-in-part of application Ser. No. 08/774,699, filed Dec. 26, 1996, abandoned, Which is a continuation of 08/497,565, filed Jun. 30, 1995, noW issued as U.S. Pat. No. 5,631,193, Which is a continuation of application Ser. No. 07/990,334, filed Dec. 11, 1992, noW issued as U.S. Pat. No. 5,484,959.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high density, integrated circuit module, Which includes a plurality of vertically or horizontally stacked individual surface mount or ball-grid-array integrated circuit packages.
2. Brief Description of the Related Technology
An example of a fabrication method and apparatus for high density lead-on-package modules by laminating one or more lead frames to standard integrated circuit packages is disclosed in U.S. Pat. No. 5,484,959, assigned to the common assignee of the present invention and incorporated herein by reference. Other methods for providing high density, stacked modules are disclosed in U.S. Pat. Nos. 5,279,029, 5,367,766, 5,455,740, 5,450,959 and 5,592,364, all of Which are assigned to the common assignee of the present invention and incorporated herein by reference. The general methods and apparatus disclosed in the referenced patents can be applied to the fabrication of stacked configurations comprised of individual ball-grid-array or surface mount packages. HoWever, the characteristic lead orientation, lead shape and lead content of ball-grid-array or surface mount packages impose a different set of parameters not adequately provided for by prior methods and assemblies.
The present invention provides a novel method and apparatus for manufacturing three-dimensional, high density, integrated circuit modules from standard ball-grid-array or other surface mount integrated circuit packages Which provides improved space efficiency and heat dissipation. One Way to increase space efficiency is to stack individual packages. Generally speaking, higher density generates more localized heat and thus increases the need for efficient heat dissipation. Improving the thermal transfer characteristics of the individual integrated circuit packages results in better heat dissipation for the module, and improves reliability and durability.
The present invention provides a novel method of fabricating a three-dimensional module formed of stacked and aligned surface mount or ball-grid-array packages. BallGrid-Array (BGA) integrated circuit packages typically have leads that extend from the bottom surface of a rectangular solid resin casing in a tWo-dimensional grid pattern.
The external portion of each lead finished With a ball of solder. Package leads provide electrical and thermal coupling to one or more integrated circuit dies that are embedded Within the protective casing. Typically, the protective casing completely surrounds the embedded die but, in some BGA packages, the protective casing does not cover the inactive top surface of the die. Near-chip scale packages provide 1.0 mm center-to-center lead spacing. Chip scale packaging such as MICRO_BGATM have center-to-center lead spacing of 0.5 mm. Chip scale packaging offers excellent electrical characteristics including loW capacitance and thermal design.
Connectivity to the leads of individual packages in a module is provided by thin substantially planar lead carriers located betWeen adjacent packages. Lead carriers are adhered to adjacent packages With a thermally conductive but electrically insulating adhesive. A lead carrier is comprised of elongated electrically and thermally conductive elements formed in one or more thin planes of conductive material that are separated by high-dielectric material. Typically, each conductive element has at least one aperture, adapted to receive and electrically couple to an individual package ball and at least one interconnect lead that extends aWay from the module to provide external circuit connectivity to package leads. Preferably, the lead carriers are formed from custom flexible circuits commercially available from 3MTM or other manufacturers. These Well knoWn flexible circuits are typically comprised of one or more thin layers of conductive material that are die cut and drilled to form ground planes, signal traces, pads and apertures. The conductive layers are typically embedded in and betWeen electrically-insulating, high-dielectric material such as polyamide, polyester or teflon Which results in circuits that are flexible, have dense trace, and provide accurate impedance control.
The present invention utilizes standard manufactured packages to form the multi-package module. Such packages typically have ball irregularities or inconsistencies, particularly ball length and solder coating variations. These variations make automated assembly problematic since the tolerances necessary to accommodate variation in ball length and excess solder, for example, do not permit the packages to be assembled Within the more stringent requirements for automated assembly of the module. According to one aspect of the present invention, the leads of the ball-grid-array packages are scythed prior to assembly or as an automated step during the assembly. Scything is a method Where a hot razor knife skims off a layer from the distal end of all the leads of a ball-grid-array package, reducing random excess lead length and providing a uniform, closely tolerant lead length. The step of scything alloWs multiple packages to be added to the module prior to a final heating step Where the solder for all the packages is floWed. This method also has the advantage of increasing the minimal tolerances for positioning of ball-grid-array package on the lead carrier. An alternative method that may also be used to compensate for excess solder from the leads is to provide channels formed in the Walls or edges of each aperture of the lead carrier that receives the ball so the excess solder, When heated, floWs into the channels A channel is a void area in a conductive element Which merges into the void area of an aperture. An