Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-12-02
2001-09-11
Thompson, Gregory (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C165S080200, C165S185000, C174S016300, C257S706000, C257S713000, C257S720000, C361S718000, C361S722000, C361S710000
Reexamination Certificate
active
06288900
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to flip-chip, ball grid array (BGA) packages whose improved thermal and mechanical design improves their reliability.
BACKGROUND OF THE INVENTION
In the manufacture of plastic ball grid array (PBGA) packages, a chip is electrically and mechanically attached to a laminated, organic substrate. The thermal expansion of the material of the chip is not usually compatible with that of the organic laminate material. The mismatch in coefficients of thermal expansion (CTE) and contraction causes the module to warp, and leads to fatigue failure of the BGA.
In order to maintain the flatness of a PBGA package, especially those with a flip-chip, the chip may be overlaid with a heat spreading cap, which has been designed to balance the CTE and stiffness of the substrate that is disposed on the opposite side of the chip. However, the heat spreading cap has been only partially successful in maintaining flatness because it correctly balances the substrate only directly above the chip.
Another source of package warping has been determined to be the epoxy adhesive that cements the chip and cap to the substrate. The epoxy adhesive, which extends beyond the periphery of the cap, may not match the CTE of the substrate. Also, it has been observed that the epoxy tends to absorb and desorb moisture, causing additional expansion and contraction and consequent warpage of the module.
The present invention represents means by which the combined thermal mismatches between the chip, cap, adhesive, and laminated substrate can be overcome to maintain flatness over the operating temperature range, thereby increasing the fatigue life of the BGA.
The current inventors seek to vary the thickness, shape, or materials of the cap where it extends beyond the periphery of the chip. This variation in thickness, material, or shape resolves the mismatch problem by changing either the stiffness, the thermal expansion, or both, in the affected regions. The objective can also be achieved by the introduction of holes, grooves, and multiple materials in the cap(s).
Holes, fins, or separating borders may be fabricated by drilling, punching, shaping, etching, etc. to vary the stiffness.
The methods of the invention have been found to significantly reduce module warpage and to increase the fatigue life of the BGA while maintaining good heat dissipation, thus providing a structurally reliable module.
DISCUSSION OF RELATED ART
In U.S. Pat. No. 5,287,248, issued to Montesano, on Feb. 15, 1994 for METAL MATRIX COMPOSITE HEAT TRANSFER DEVICE AND METHOD, a construction is shown that conducts heat from a heat source to a heat sink. Thermal conduction is improved for mounting boards that support heat generating components.
In U.S. Pat. No. 5,247,426, issued on Sep. 21, 1993 to Hamburgen et al for SEMICONDUCTOR HEAT REMOVAL APPARATUS WITH NON-UNIFORM CONDUCTANCE, an apparatus for removing heat from a semiconductor is illustrated. The apparatus features different regions of high and low thermal conductance.
In U.S. Pat. No. 5,537,342, issued to Gainey on Jul. 16, 1996 for ENCAPSULATION OF ELECTRONIC COMPONENTS, an electronic device is shown for reducing stress. The device features an encapsulating member comprising a sandwich of two premolded parts.
In U.S. Pat. No. 5,736,785, issued on Apr. 7, 1998 to Chiang et al for SEMICONDUCTOR PACKAGE FOR IMPROVING THE CAPABILITY OF SPREADING HEAT, a heat spreading device is shown that is attached to a die. The die is supported by a substrate comprising a conductive pad for connection to a solder ball.
In U.S. Pat. No. 5,777,847, issued on Jul. 7, 1998 to Tokuno et al for MULTICHIP MODULE HAVING A COVER WITH SUPPORT PILLAR, a multichip is shown that comprises a substrate supporting a plurality of circuit chips. A cover plate covers semiconductor chips that are attached to a substrate.
In U.S. Pat. No. 5,831,826, issued to Van Ryswyk on Nov. 3, 1998 for HEAT TRANSFER APPARATUS SUITABLE FOR USE IN A CIRCUIT BOARD ASSEMBLY, a heat transfer apparatus is illustrated. The heat transfer apparatus is carried upon a substrate supporting a plurality of pads that connect to electronic components.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a module having a substrate upon which at least one chip or integrated circuit is mounted. A heat spreading cap is disposed over each chip or integrated circuit. The cap is shaped or sized to provide distinct combinations of stiffness and CTE in at least two areas. The configuration of the cap changes as it extends beyond the periphery of the chip.
It is an object of this invent ion to provide an electronic module with longer BGA fatigue life and less warpage.
It is another object of the invention to provide an electronic module having relatively low thermal resistance.
REFERENCES:
patent: 5247426 (1993-09-01), Hamburgen et al.
patent: 5287248 (1994-02-01), Montesano
patent: 5537342 (1996-07-01), Gainey
patent: 5675474 (1997-10-01), Nagase et al.
patent: 5726494 (1998-03-01), Nashimoto et al.
patent: 5736785 (1998-04-01), Chiang et al.
patent: 5777847 (1998-07-01), Tokuno et al.
patent: 5831826 (1998-11-01), Van Ryswyk
“Process Considerations in the Fabrication of Teflon Printed Circuit Boards”, by Light et al, 1994 Proceedings, 44 Electronic Components & Technology Conference, 5/94.
“High Performance Carrier Technology: Materials and Fabrication”, by Light et al, 1993 International Electronics Packaging Conference, San Diego, California, vol. One.
“High Performance Carrier Technology”, by Heck et al, 1993 International Electronics Packaging Conference, San Diego, California, vol. One.
Johnson Eric A.
Park Seung-bae
Fraley Lawrence R.
International Business Machines - Corporation
Salzman & Levy
Thompson Gregory
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