Heated PCB interconnect for cooled IC chip modules

Details Heated PCB interconnect for cooled IC chip modules Heated PCB interconnect for cooled IC chip modules

1999-10-12

2001-06-12

Picard, Leo P. (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C361S702000, C361S699000, C361S704000, C361S719000, C257S713000, C257S715000, C165S104330, C165S104210

Reexamination Certificate

active

06246581

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a cooling assembly for lowering the temperature of integrated circuit (IC) chip modules mounted on a printed circuit board (PCB) substrate, and, more particularly, to a heated interconnect between the cooled IC chip module and the PCB to prevent condensation.
The high circuit densities and operating frequencies in modern integrated circuit devices and multi-chip modules of today's computer systems has resulted in a significant increase in the power dissipated by such chip and module components. No matter how fast one wishes to operate a given electronic circuit chip, there almost always is the potential for running it faster if the chip were to be cooled further and more thermal energy were removed during its operation. This is true of computer processor circuit chips and more particularly of such chips disposed within multi-chip modules that generate significant amounts of heat. Because of the demand to run processor modules at increasingly higher speeds, the clock frequencies at which the devices must operate also increases. Power generation correspondingly rises in proportion to the clock frequency, generating thermal demands in terms of energy which must be removed for faster, safer, and more reliable circuit operation. It is required that cooling arrangements be provided so that the heat generated by the operation of these components be effectively and efficiently removed in order to maintain the temperature of the devices within the limits that will keep the operating parameters of the devices in a predetermined range, and, further, to prevent the damage or destruction of the integrated circuit devices by overheating from the high temperatures generated.
Using refrigeration technology, integrated circuit chips and multi-chip modules readily can be cooled to appropriately low temperatures. In addition, however, to the necessity of cooling to prevent damage from overheating, it is also recognized that cooling offers marked advantages in circuit speed, system throughput, and component reliability. With the advent of CMOS processors for computers, the potential performance improvements obtained by lowering chip temperature are intriguing. It is known that a CMOS circuit is capable of operating at higher clock speeds as the circuit temperature is lowered. Current CMOS chip circuit designs generally perform about two percent faster for each 10° C. the chip temperature is lowered. Accordingly, it would not be unreasonable to achieve a 100° C. reduction in chip temperature with refrigeration techniques as compared to cooling with ambient air, thus achieving a 20% performance improvement. It has been reported that the processor frequency of a CMOS processor has been improved by nearly threefold by cooling the processor to temperatures around −200° C.
Various techniques for the cooling of integrated circuit electronic devices are known and many have been implemented with success. Some practiced techniques involve conventional methods such as by directing ambient air onto the components to be cooled, by sealing the computer cabinet and refrigerating the interior of the cabinet; as well as by immersing components in coolants such as liquid nitrogen. Individual integrated chip or multi-chip module components also have been cooled through specialized devices such as hollow cold plates which are attached to the components to be cooled. Liquid coolants can be circulated through the hollow cold plates to effect cooling of the attached components.
However, in order to take practical advantage of the performance improvements achievable by lowering integrated chip temperatures to levels, for example, in the range of −40° C. to −60° C., many engineering problems must be addressed. In addition to issues involving refrigeration system design, evaporator design, and thermal controls, cooling of the electronic components to a temperature below the ambient environment dew point results in condensation problems in that moisture will condense on the cooled components and on the structures and components to which the cooled components may be attached. This condensation can damage and literally destroy the electronic circuitry associated with the integrated chip or multi-chip module and the circuit board on which it is mounted.
Accordingly, an arrangement is needed for an integrated chip or multi-chip module device which effectively and efficiently can prevent condensation damage to the device or contiguous components or circuitry when the temperature of the device is reduced to below ambient dew point.
SUMMARY OF THE INVENTION
Now, an improved interconnect for an integrated circuit (IC) device mounted on a printed circuit board (PCB) within a computer system has been developed whereby the IC device is effectively cooled while damaging condensation is controlled from forming on the PCB on which the IC device is mounted, as well as other electrical components contiguous to the cooled device. According to the present invention, a cooling assembly is provided which comprises an evaporator unit attached to the top of the hat, or cover, of an integrated chip module and in thermal communication with the hat surface. In order to maintain the reduced temperature of the cooled device and isolate the device from the ambient warm environment, the IC module and attached cooling evaporator unit are housed within an insulated enclosure which fully envelops the device and evaporator and is itself bonded around its bottom peripheral edge to the surface of the printed circuit board around the outer perimeter of the module to board interface. The enclosure includes walls fabricated from thermal insulating material. Such material comprises rigid structural foam such as polyurethane foam, and the like, in order to provide structural integrity as well as a thermal insulative barrier. Typically, such structural foams feature thermal conductivities ranging from about 0.04 to about 0.12 W/m.° K. The preferred insulating material is rigid polyurethane foam. While it is critical to cool the IC module to low temperatures, typically in the range of about −40° C. to about −60° C., cooling to such temperatures below ambient dew point presents potential condensation problems on the cooled component as well as structures and other components attached to and around the cooled component. The described insulation helps to isolate the cooled module from the surrounding environment, but, at the low temperatures to which the module is cooled, the module and the insulated enclosure itself may conduct enough cold to the PCB on which they are mounted to result in condensation on the PCB and adjacent components. Accordingly, in order to prevent condensation at the interface of the cooled module assembly and the printed circuit board, an electrical interconnect member is provided between the module and the PCB which effectively raises the temperature of the interface. The interconnect serves to couple the module to the printed circuit board and can be any suitable sheet material with means to provide electrical connection between the contacts on the base of the module to those on the PCB. Such materials include polycarbonate polymer material available from the General Electric Company under the trademark of Lexan, or polyurethane, and the like. The interconnect may have a thickness from about 0.02 mm to about 1 mm, and may be used in single or multiple layers. Preferably, in order to provide electrical connection between the module and the PCB, the interconnect material includes a pattern of electrical pads, made from copper composites, platinum wire, gold wire, and the like, matching the desired pattern of contacts between the module and PCB. Because the electrical interconnect between the module and PCB also provides a good thermal path between the PCB and module, cooling of the module results in low temperatures on the PCB to which it is connected, causing condensation to form at the module-to-PCB interface, as well as adjacent areas of the PCB

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