Heat exchange – With retainer for removable article – Electrical component
Reexamination Certificate
2003-04-30
2004-08-24
McKinnon, Terrell (Department: 3743)
Heat exchange
With retainer for removable article
Electrical component
C165S185000, C361S697000, C361S704000, C174S016300, C257S706000, C257S722000
Reexamination Certificate
active
06779593
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a cooling device including a heat spreader for removing heat from a component connected with the cooling device. More specifically, the present invention relates to a cooling device that includes a heat spreader made from a dissimilar material with a higher thermal conductivity than a material of a heat mass of the cooling device so that heat in a component connected with the cooling device is efficiently removed.
BACKGROUND OF THE INVENTION
It is well known in the electronics art to place a heat sink in contact with an electronic device so that waste heat generated by operation of the electronic device is thermally transferred to the heat sink thereby cooling the electronic device. With the advent of high clock speed electronic devices such as microprocessors (&mgr;P), digital signal processors (DSP), and application specific integrated circuits (ASIC), the amount of waste heat generated by those electronic devices and the operating temperature of those electronic devices are directly proportional to clock speed. Therefore, higher clock speeds result in increased waste heat generation which in turn increases the operating temperature of the electronic device. However, efficient operation of the electronic device requires that waste heat be continuously and effectively removed.
Heat sink devices have become commonplace as a means for dissipating waste heat from electronic devices such as the types described above. In a typical application, a component to be cooled is carried by a connector that is mounted on a PC board. A heat sink is mounted on the component by attaching the heat sink to the connector using a clip or fasteners, for example. Alternatively, the heat sink is mounted to a PC board that carries the electronic device and fasteners or the like are used to connect the heat sink to the PC board via holes that are drilled in the PC board.
The need to drill holes can be one disadvantage to using fasteners because the fasteners or other mounting hardware used for connecting the heat sink to the PC board are usually electrically conductive and there is a risk of an electrical short due to contact between a PC board trace and the mounting hardware. Moreover, to avoid electrical shorts, the PC board traces can be routed around the hole, but that routing requires keep out zones that can complicate the routing of the traces.
Typically, a heat sink used in conjunction with a modem high clock speed electronic device will use an electrical fan mounted on top of the heat sink or within a cavity formed by cooling fins/vanes of the heat sink. The cooling fins increase the surface area of the heat sink and maximize heat transfer from the heat sink to ambient air that surrounds the heat sink. The fan causes air to circulate over and around the cooling fins thereby transferring heat from the cooling fins into the ambient air.
As mentioned previously, with continuing increases in clock speed, the amount of waste heat generated by electronic devices has also increased. Accordingly, to adequately cool those electronic devices, larger heat sinks and/or larger capacity fans (i.e. in CFM) are required. Increasing the size of the heat sink results in a greater thermal mass and a greater surface area from which the heat can be dissipated. Increases in fan capacity provide for more air flow through the cooling fins.
There are disadvantages to increased fan and heat sink size. First, if the size of the heat sink is increased in a vertical direction (i.e. in a direction transverse to the PC board), then the heat sink is tall and may not fit within a vertical space in many applications, such as the chassis of a desktop computer. Second, if the PC board has a vertical orientation, then a heavy and tall heat sink can mechanically stress the PC board and/or the electronic device resulting in a device or PC board failure.
Third, a tall heat sink will require additional vertical clearance between the heat sink and a chassis the heat sink is contained in to allow for adequate air flow into or out of the fan. Fourth, if the heat sinks size is increased in a horizontal direction, then the amount of area available on the PC board for mounting other electronic devices is limited. Fifth, when the heat sink has a cylindrical shape formed by the fins it is often not possible to mount several such heat sinks in close proximity to each other because air flow into and out of the fins is blocked by adjacent heat sinks with a resulting decrease in cooling efficiency.
Finally, increases in fan size to increase cooling capacity often result in increased noise generation by the fan. In many applications such as the desktop computer or a portable computer, it is highly desirable to minimize noise generation. In portable applications that depend on a battery to supply power, the increased power drain of a larger capacity fan is not an acceptable solution for removing waste heat.
In the above mentioned heat sink with cooling fins there are additional disadvantages to mounting the fan within a cavity formed by the fins. First, a substantial portion of a heat mass of the heat sink is partially blocked by the fan because the fan is mounted directly on the heat mass and therefore blocks a potential path for heat dissipation from the heat mass because air from the fan does not circulate over the blocked portion of the heat mass.
Second, without the fan, a depth of the fins could extend all the way to a center of the heat mass; however, the depth and surface area of the fins is reduced by a diameter of the fan because the fan is mounted in a cavity having a diameter that is slightly larger than the fans diameter to provide clearance for the fan blades. Consequently, the heat mass of the heat sink must be made broader to compensate for the reduced surface area of the fins. The broader heat mass increases the size, cost, and weight of the heat sink.
Third, the reduced depth of the fins makes it easier for the fins to be bent if damaged. One possible consequence of a bent fin is that it will contact and damage the fan blades and/or cause the fan to stall thereby damaging the fan or causing the fan to fail. Fourth, because the fan is mounted in the cavity formed by the fins, power leads for the fan must be routed through a space between the fins. Sharp edges on the fins can cut the power leads or cause an electrical short. In either case, the result is that the fan will fail. Fifth, glue is typically used to mount the fan to the heat sink and the glue can get into the fan and cause the fan to fail. Any of the above mentioned fan failure modes can lead to a failure of the electronic device the heat sink was designed to cool because air circulation generated by the fan is essential to effectively dissipate waste heat from the electronic device.
Some prior heat sinks attempt to overcome the aforementioned disadvantages of a larger heat mass or increased fin area by using a metal heat spreader that is joined with the heat sink. Typical processes for joining the heat spreader with the heat sink include brazing, soldering, heat shrink fitting, vacuum brazing, micro forging and shrink fitting.
One disadvantage of those processes is that they are very expensive resulting in a higher cost for the heat sink. Another disadvantage is that the above processes are limited in their ability to join dissimilar metals for the heat sink and the heat spreader, particularly if the metal for the heat spreader has superior thermal conductivity properties over the metal for the heat sink. A disadvantage of the shrink fitting process is that a thermal resistance between the heat spreader and the heat sink is high due to micro gaps between the heat spreader and the heat sink. Finally, depending on the application, another potential disadvantage to the above processes is that once joined, the heat sink and the heat spreader cannot be easily removed for servicing or for a field upgrade to a better material for the heat spreader.
Consequently, there is a need for a cooling device with a heat
Denny III Trueman H.
Hewlett--Packard Development Company, L.P.
McKinnon Terrell
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