Dehumidified cooling assembly for IC chip modules

Details Dehumidified cooling assembly for IC chip modules Dehumidified cooling assembly for IC chip modules

1999-10-12

2001-05-22

Doerrler, William (Department: 3744)

Refrigeration

Structural installation

With electrical component cooling

C062S003400, C062S003200, C062S092000

Reexamination Certificate

active

06233959

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to cooling technology for electronic packaging for computer systems, and, more particularly, relates to a cooling assembly for lowering the temperature of integrated circuit (IC) chip modules mounted on a printed circuit board (PCB) substrate.
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 is 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, a cooling system is needed for an integrated chip or multi-chip module device which effectively and efficiently can reduce the temperature of the device to below ambient dew point without causing condensation damage to the device or contiguous components or circuitry.
SUMMARY OF THE INVENTION
Now, an improved assembly for cooling an integrated circuit (IC) device mounted on a printed circuit board (PCB) within a computer system has been developed whereby cooling of the IC device is efficiently and effectively accomplished while damaging condensation is controlled from forming on the cooled devices as well as the PCB on which the IC device is mounted and 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 enclosed within an insulated housing 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 housing includes walls fabricated from thermal insulating material. Such material comprises rigid structural foam such as polyurethane foam, expanded expanded polystyrene, 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 outer surface of the insulated enclosure may still get cool enough to result in condensation. Accordingly, in order to reduce the dew point around the insulated enclosure and thus avoid condensation on the cooled module housing and the PCB which it is attached, pursuant to the present invention, a low humidity atmosphere is provided around the housing. This low humidity atmosphere is provided by surrounding the module housing with an enclosure cover, through which a flow or atmosphere of dehumidified air is maintained. This most readily can be accomplished by introducing a low air flowrate stream through an inlet conduit in flow communication with the interior airspace of the enclosure, and providing an exhaust outlet at a remote position on the enclosure and in flow communication with the interior of the enclosure, so that the flow of air can exit the airspace within the enclosure (a flow rate of about 0.05 to 0.1 m/s through exhaust paths greater than 3 mm long has been shown to be effective). The incoming air can be provided by any air moving device, such as a fan or a blower. Depending on the flowrate of the introduced dry air and the integrity of the enclosure seal around the module housing, provision of a designated exhaust outlet might not be required, as existing leakage through the cover seals might suffice as an exhaust outlet. A feature of the present invention is that it can be designed to utilize existing components that

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