Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
2002-12-27
2004-08-10
Clark, Jasmine (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S700000, C257S707000, C257S713000, C257S720000, C257S730000
Reexamination Certificate
active
06774482
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to cooling within integrated circuit (IC) packaging structures. More particularly, the present invention is directed to cooling of integrated circuit chips using a thermally conductive conformable material.
BACKGROUND OF THE INVENTION
As heat is generated during the functioning of integrated circuit chips (ICs), the thermal resistance to the heat sink must be as small as possible so that the operating temperature of the chip is low enough to assure the continued reliable operation of the device. The problem of heat removal becomes ever more difficult as chip geometries are scaled down and operating speeds are increased, resulting in increased power density. The ability to adequately cool the chips is therefore a limiting factor in the further increase of system performance. Multiple terms are used in the art to describe the elements of the package which are used to remove heat, including heat sink, heat spreader, cooling plate, and heat transfer surface. In an array of ICs mounted on a substrate such as a mutichip module (MCM), special cooling difficulties are presented. In an MCM, the chips may be mounted very close together and nearly cover the whole top surface of the MCM. With such an arrangement, it may not be possible to use a heat spreader bonded directly to the back surface of the chips, as is sometimes used for isolated chips to reduce the heat flux (power/unit area, i.e. W/cm
2
).
A common technique for removing heat from high-power ICs makes use of a cooling plate or heat sink which is thermally attached to the chip using a thermally conductive material such as a thermal paste or thermal grease. Heat is removed from the cooling plate or heat sink by methods such as forced air cooling or circulating liquid coolants. The term cooling plate will be used generically herein to refer to either a heat sink or a cooling plate. In heat removal techniques, it is critical to minimize the thermal resistance between the chip and the cooling plate or heat sink. The present invention is directed to reducing this thermal resistance.
Various approaches are set forth in the art to achieve cooling of ICs mounted on substrates. For example in U.S. Pat. Nos. 5,239,200 issued Aug. 24, 1993 to Messina et al., in U.S. Pat. No. 5,177,667 issued Jan. 5, 1993 to Graham and Moran, and in U.S. Pat. No. 4,500,945 issued Feb. 19, 1985 to Lipschutz, each of which is assigned to the present assignee, the use of circulating liquid or gas coolant is described. U.S. Pat. No. 5,023,695 issued Jun. 11, 1991 to Umezawa et al., describes the use of a circulating cooling fluid in conjunction with a cooling plate having cut cavities. Circulating fluid coolants or forced air cooling are required to remove heat from the surface of the external module, which is also referred to as a heat sink or cooling plate. The present invention is directed to reducing the thermal resistance between the chip within the package, whether the package is an MCM or an SCM, and the external module surface.
A thermally conductive paste or similar conformable compliant thermally conductive material is typically placed between the IC chip and the cooling plate or heat sink. Thermally conductive paste typically comprises thermally conductive particles having a distribution of sizes dispersed within a binder material or matrix, such as the paste described in U.S. Pat. No. 5,098,609 issued Mar. 24, 1992 to Iruvanti et al. In the '609 patent, paste is applied between the top of the IC mounted on the substrate and the lower flat surface of a cooling plate facing the substrate. The type of paste described in the '609 patent can be used in the present invention, as can other thermally conductive pastes used in the art or other compliant particle-based conformable materials. Typical particle-based materials include those having a wax matrix, commonly known as phase-change materials, those having a silicone-based matrix, and dry particle lubricants such as graphite and metal powders.
When applying a thermal paste between the back of a chip which is electrically attached to a substrate and the lower surface of a cooling plate, the paste layer must be made as thin as possible in order to reduce the thermal resistance through the paste layer. The paste must also be compliant, or flexible, maintain its integrity, surface adhesion and chip coverage despite the expansion and contraction of the packaging structure caused by power and temperature cycling. U.S. Pat. No. 6,091,603 issued Jul. 18, 2000 to Daves and Edwards and assigned to the present assignee, describes the use of a customized deformable lid understructure which permits a reduction in the amount of thermally conductive material in the primary heat dissipation path.
FIG. 1
, taken from the prior art '603 patent, shows a multichip module in which chip
600
, mounted on chip-carrying substrate
500
by solder bumps
650
, is thermally connected to deformable lid understructure
103
, with which it lies in parallel using thermally conformable material
200
. Deformable lid understructure
103
and chip
600
lack the microstructure of the present invention, employing instead deformable lid understructure
103
to reduce the thermally conductive paste thickness, resulting in improved heat dissipation.
Whenever a particle-filled paste is used between a flat cooling plate and a flat chip substrate, the thickness of the thermal paste layer is limited by the size of the largest particle present in the paste. Applying pressure to try to reduce the thickness of the paste layer risks cracking of the chip due to the concentration of pressure falling on the largest particles in the paste. However, it is desirable to have a range of particle size in the paste in order to improve the solid packing density. Separating the largest particles from the paste by sieving is theoretically possible, but impractical because as the particle size is further reduced, it becomes increasingly difficult and expensive to sieve out the particles which exceed the desired size range.
An additional difficulty observed with the use of thermal paste is the migration of the paste from behind the chip and the formation of voids due to differential thermal expansion of the various parts of the package during thermal cycling. Such paste migration can greatly increase the thermal resistance between the chip and the cooling plate during the lifetime of the electronic package, possibly causing catastrophic heating and destruction of the chip.
Several approaches in the art describe providing an altered surface of the cooling plate which is in contact with thermal paste. In U.S. Pat. No. 5,825,087 issued Oct. 20, 1998 to Invanti et al. and assigned to the present assignee, the cooling plate used in conjunction with a thermal paste or a thermal adhesive has been roughened by grit blasting or provided with a plurality of crisscrossing channels in order to improve the adhesion of the thermal medium and inhibit its flow during operation of the electronic module.
FIG. 2
, taken from the prior art '087 patent, shows in detail the channels
18
and corresponding protrusions
17
of roughened. area
16
of cooling plate
14
in which heat generated on chip
13
is removed by transfer through paste
15
. The purpose of roughening the surface of plate
14
is to inhibit the movement of paste
15
. U.S. Pat. No. 5,345,107, issued Sep. 6, 1994 to Daikoku et al. describes the use of a grooved solid body in conjunction with thermally conductive fluid or thermally conductive grease and a low-pressure spring to hold the solid body in close contact with the electronic device. In
FIG. 3
, taken from the prior art '107 patent, the excess capacity of grooves
40
and
41
on heat transfer surface
100
of solid thermal conductor
33
enables closer contact of the thermal conductive grease
11
(not shown in this figure) between heat transfer surface
100
and the heat transfer surface of the chip carrier
101
(not shown in this figure) when the pressure of a spring
34
Colgan Evan George
Magerlein John Harold
Wisnieff Robert Luke
Zitz Jeffrey Allen
Clark Jasmine
International Business Machines - Corporation
Olsen, Esq. Judith D.
Trepp, Esq. Robert M.
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