Heat exchange – Intermediate fluent heat exchange material receiving and... – Liquid fluent heat exchange material
Reexamination Certificate
1999-05-12
2001-10-16
Atkinson, Christopher (Department: 3743)
Heat exchange
Intermediate fluent heat exchange material receiving and...
Liquid fluent heat exchange material
C165S104210, C165S104330
Reexamination Certificate
active
06302192
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to active solid state devices, and more specifically to a heat pipe for cooling an integrated circuit chip, with the heat pipe designed to be held in direct contact with the integrated circuit.
As integrated circuit chips decrease in size and increase in power, the required heat sinks and heat spreaders have grown to be larger than the chips. Heat sinks are most effective when there is a uniform heat flux applied over the entire heat input surface. When a heat sink with a large heat input surface is attached to a heat source of much smaller contact area, there is significant resistance to the flow of heat along the heat input surface of the heat sink to the other portions of the heat sink surface which are not in direct contact with the contact area of the integrated circuit chip. Higher power and smaller heat sources, or heat sources which are off center from the heat sink, increase the resistance to heat flow to the balance of the heat sink. This phenomenon can cause great differences in the effectiveness of heat transfer from various parts of a heat sink. The effect of this unbalanced heat transfer is reduced performance of the integrated circuit chip and decreased reliability due to high operating temperatures.
The brute force approach to overcoming the resistance to heat flow within heat sinks which are larger than the device being cooled is to increase the size of the heat sink, increase the thickness of the heat sink surface which contacts the device to be cooled, increase the air flow which cools the heat sink, or reduce the temperature of the cooling air. However, these approaches increase weight, noise, system complexity, and expense.
It would be a great advantage to have a simple, light weight heat sink for an integrated circuit chip which includes an essentially isothermal surface even though only a part of the surface is in contact with the chip, and also includes a simple means for assuring intimate contact with the integrated circuit chip to provide good heat transfer between the chip and the heat sink.
SUMMARY OF THE INVENTION
The present invention is an inexpensive heat pipe heat spreader for integrated circuit chips which is of simple, light weight construction. It is easily manufactured, requires little additional space, and provides additional surface area for cooling the integrated circuit and for attachment to heat transfer devices for moving the heat away from the integrated circuit chip to a location from which the heat can be more easily disposed of. Furthermore, the heat pipe heat spreader is constructed to assure precise flatness and to maximize heat transfer from the heat source and to the heat sink, and has holes through its body to facilitate mounting.
The heat spreader of the present invention is a heat pipe which requires no significant modification of the circuit board or socket because it is held in intimate contact with the integrated circuit chip by conventional screws attached to the integrated circuit mounting board. This means that the invention uses a very minimum number of simple parts. Furthermore, the same screws which hold the heat spreader against the chip can also be used to clamp a finned heat sink to the opposite surface of the heat spreader.
The internal structure of the heat pipe is an evacuated vapor chamber with a limited amount of liquid and includes a pattern of spacers extending between and contacting the two plates or any other boundary structure forming the vapor chamber. The spacers prevent the plates from bowing inward, and therefore maintain the vital flat surface for contact with the integrated circuit chip. These spacers can be solid columns, embossed depressions formed in one of the plates, or a mixture of the two. Porous capillary wick material also covers the inside surfaces of the heat pipe and has a substantial thickness surrounding the surfaces of the spacers within the heat pipe, thus forming pillars of porous wick surrounding the supporting spacers. The wick material therefore spans the space between the plates in multiple locations.
The spacers thus serve important purposes. They support the flat plates and prevent them from deflecting inward and distorting the plates to deform the flat surfaces which are required for good heat transfer. The spacers also serve as critical support for the portions of the capillary wick which span the internal space between the plates. The capillary wick pillars which span the space between the plates provide a gravity independent characteristic to the heat spreader, and the spacers around which the wick pillars are located assure that the capillary wick is not subjected to destructive compression forces.
The spacers also make it possible to provide holes into and through the vapor chamber, an apparent inconsistency since a heat pipe vacuum chamber is supposed to be vacuum tight. This is accomplished by bonding the spacers, if they are solid, to both plates of the heat pipe, or, if they are embossed in one plate, bonding the portions of the depressions which contact the opposite plate to that opposite plate. With the spacer bonded to one or both plates, a through hole can be formed within the spacer and it has no effect on the vacuum integrity of the heat pipe vapor chamber, from which the hole is completely isolated.
An alternate embodiment of the invention provides the same provision for mounting the heat pipe heat spreader with simple screws even when the heat pipe is constructed without internal spacers. This embodiment forms the through holes in the solid boundary structure around the outside edges of the two plates. This region of the heat pipe is by its basic function already sealed off from the vapor chamber by the bond between the two plates, and the only additional requirement for forming a through hole within it is that the width of the bonded region be larger than the diameter of the hole. Clearly, with the holes located in the peripheral lips, the heat pipe boundary structure can be any shape.
Another alternate embodiment of the invention provides for improved heat transfer between the integrated circuit chip and the heat pipe heat spreader. This is accomplished by using a different capillary wick material within the heat pipe at the location which is directly in contact with the chip. Instead of using the same sintered copper powder wick which is used throughout the rest of the heat pipe, the part of the wick which is on the region of the heat pipe surface which is in contact with the chip is constructed of higher thermal conductivity sintered powder. Such powder can be silver, diamond, or many other materials well known in the art. This provides for significantly better heat transfer in the most critical heat transfer area, right at the integrated circuit chip.
The present invention thereby provides a heat pipe with superior heat transfer characteristics, and the simplest of all mounting devices, just several standard screws.
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Dussinger Peter M.
Myers Thomas L.
Atkinson Christopher
Duane Morris & Heckscher LLP
Thermal Corp.
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