Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Utility Patent
1999-11-01
2001-01-02
Picard, Leo P. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S710000, C361S784000, C165S104260, C165S104330, C257S718000, C257S724000, C174S016300
Utility Patent
active
06169660
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention deals generally with the cooling of integrated circuit packages, and more specifically with a cooler designed to prevent stress on an integrated circuit package caused by the weight of the cooler or the torque of a cooler separated from the integrated circuit package by a long lever arm.
One of the more common high speed methods for mounting integrated circuit chips into electronic systems uses what is called the “ball grid array”. In this approach, the integrated circuit package is furnished with tiny balls of solder temporarily attached to its bottom with flux, with one solder ball located at each of its electrical contacts. In order to install the integrated circuit package, it is placed upon a printed circuit wiring board such as a graphics card which has a pattern of contacts that matches the pattern of the solder balls, and the entire assembly is heated to the melting point of the solder. The solder then melts, wets the contacts on both the integrated circuit package and the circuit board, and bonds the two sets of contacts together.
As the solder melts, the balls shrink and cause the integrated circuit package to settle somewhat onto the circuit board, but this settling movement is unpredictable. Therefore, the final clearance between the integrated circuit package and the circuit board can vary between integrated circuit packages and even from corner to corner on a single package. This variation in the clearance between the integrated circuit package and the circuit board causes a similar variation in the location of the top of the integrated circuit package, and that causes a problem in the use of cooling devices with the integrated circuit packages.
Since a basic requirement of cooling is that there be continuous intimate contact between the surface of the cooler and the surface of the device being cooled, the unpredictable variation in the height of the integrated circuit package requires the use of a cooling arrangement that can accommodate to the height variation. However, other requirements in the typical cooling arrangement make the accommodation to the height variation a significant problem. For instance, when an assembly of fins is used as the heat disposal device, these fins, sometimes with an associated fan, have a weight which the integrated circuit package itself can not support. The fins must then be rigidly supported from some other structure. The connection to the fins must also somehow adjust to the integrated circuit package height variation, again, without placing excessive force on the weight sensitive integrated circuit package.
It would be very advantageous to have a cooler which would permit intimate contact between the integrated circuit package and a cooling device, but not place significant mechanical stress on the integrated circuit package.
SUMMARY OF THE INVENTION
The present invention provides an integrated circuit cooler in which the cooling fin structure, or the cooling fin and fan assembly, is mounted on the circuit board, but the attachment to the integrated circuit package is separated from the fin structure by one or more heat pipes. In the prior art such an arrangement actually increased the risk of damage to the integrated circuit package and loss of thermal contact because the force to which the attached heat spreader was subjected was then applied as a torque because of the length of the heat pipe. However, in the present invention the heat pipe is constructed in such a manner so as to isolated the integrated circuit package and the attached heat spreader from the force originating at the cooling fin structure.
While the heat pipe is not actually flexible, its casing has an annealed section between the cooling fin structure and the integrated circuit package so that minor misalignments caused by variations in the height or tilt of the integrated circuit package are easily accommodated during installation by bending of the heat pipe at the annealed section. The annealed section of the casing of the heat pipe is actually located between two end sections.
One end section is attached to a heat spreader which is held in thermal contact with the integrated circuit package. The heat spreader is typically a solid copper sheet with one or more holes or channels formed parallel to its surface. At least one heat pipe, often more, is bonded into a hole or channel in the heat spreader. This attachment can be made by soldering, brazing, or the use of a heat conductive epoxy or adhesive. Once a heat pipe is installed into such a close fitting hole or channel, it really does not matter whether that part of its casing is annealed or not. The solid heat spreader block makes the heat pipe casing quite rigid.
In order to assure that the heat spreader is held in intimate contact with the integrated circuit package regardless of the specific height or tilt of the integrated circuit package, the heat spreader is held down against the integrated circuit package by springs. These springs are installed upon posts which are attached to the circuit board to which the integrated circuit package is attached. The posts typically penetrate through loose clearance holes in extensions of the heat spreader. The combination of the springs which permit adjustment perpendicular to the plane of the circuit board and the loose clearance holes, which permit minor adjustment in a plane parallel to the circuit board, allows the heat spreader and its attached heat pipe to accommodate to variations in both the height and the tilt of the integrated circuit package.
However, such adjustment capability would be of no benefit if the heat pipes attached to the heat spreader were themselves very rigid, as are typical prior art heat pipes. Rigid heat pipes would prevent the heat spreader from moving vertically or tilting, and proper contact between the heat spreader and the integrated circuit package could not be maintained when, as with the use of “ball grid array” soldering, very tight tolerances are not maintained.
In the present invention, the heat pipe or heat pipes used are processed so that the portions of their casings extending between the low mass heat spreader and a high mass cooling fin assembly are annealed to be much softer and more pliable than prior art heat pipes. The casing annealing is restricted only by the limit imposed by the strength required to prevent the casing from deforming from the pressure differentials to which it is subjected during operation. This pressure differential is typically 15 psi.
The end of the heat pipe opposite from the end at the integrated circuit package is attached to an assembly of cooling fins. In the preferred embodiment of the invention, each fin is formed with enough holes to match the number of heat pipes being used, and the sets of holes in all the fins have the same location on the fins. Small cups are formed into the fins at each hole as the holes are being made, and the sides of each cup are slitted. This forms a simple spring clamp at each hole in a fin to hold tightly onto the heat pipe. Thus, the individual fins can be assembled onto the heat pipes and they remain attached and provide a good thermal interface without any other bonding material.
The present invention also includes a feature which provides a simple and inexpensive means for attaching a fan to the cooling fins to provide cooling capability beyond that available with only natural convection. To attach a “pancake” type fan, a fan that is characterized by its small height and is usually constructed with a close fitting frame surrounding it, the fan frame is located in direct contact with the edges of the fins of the cooling fin assembly. To hold the fan in this location the end fins of the assembly are made larger than the other fins, and they extend above the sides of the installed fan frame, or tabs or strips attached to the end fins extend higher than the other fins and above the sides of the frame of the fan. The extra length of the end fins, or the tabs on the end fins, which extend over the sides of the fan ar
Gernert Nelson J.
Sarraf David B.
Toth Jerome E.
Datskovsky Michael
Fruitman Martin
Picard Leo P.
Thermal Corp.
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