Certain semiconductor devices in electrical and electronic circuits, such as large scale integrated circuits, voltage regulators, current switching devices, high current drivers and other similar devices, generate heat that is deleterious to their operation and must be dissipated. An individual semiconductor junction may be subject to thermal runaway current conduction leading to further heating and damage. In large scale digital integrated circuits, operation at or above the maximum rated temperature can result in spurious switching operations and functional failure.
In a highly integrated semiconductor device such as a computer processor, a single semiconductor switching transistor may conduct little concurrent on its own, but is densely mounted with many other transistors. A single integrated device
may generate heat energy of a hundred Watts or more. Supplemental cooling arrangements may be needed in addition to convective cooling by heat driven circulation of ambient air, conduction of heat through circuit lands and the like, for maintaining operational temperatures within design ranges. For this purpose, thermally conductive heat sink devices, normally of cast or sheet metal and having a substantial surface area exposed to the air, are mounted on a base that is clamped to bear physically against the heat generating circuit element.
A large-scale integrated circuit such as a computer processor or similar device typically is mounted removably in a receptacle that is soldered to a printed circuit board. The receptacle has inward-facing resilient contacts for conductively coupling to contacts on the circuit package, which package may be several cm on a side. The receptacle or auxiliary structures associated with the printed circuit board carry spring-clip clamping mechanisms that engage over part of the heat sink, rested atop the circuit package. The clamping mechanism physically presses a heat sink against the circuit package when mounted. The heat sink typically comprises a base block that is relatively thick, integrally cast with an array of thin fins functioning as a heat exchanger to release heat into the ambient air. The array of fins and/or the base has structure to cooperate with the clamping mechanism, and can also provide a point of attachment for a fan for forcing a flow of air over the fins.
Heat energy diffuses from the active circuit elements into the circuit substrate and into the circuit packaging structure, which comprises thermally conductive plastic or ceramic. The heat energy diffuses by conductive contact into the base of the heat sink, and then diffuses through the integral or thermally conductively attached structures of the heat sink to the surfaces at which air contact heat exchange convection carries the heat away. The array of fins typically is cast integrally with the heat sink base, but also can be thermally conductively attached in contact with the base. The function of the fins is to present a relatively large surface area, preferably within a relatively small total volume, for efficient thermal energy release. The electrically powered fan, mounted on the heat exchanger by screws or clamps, forces air over the heat exchanger fins and may improve thermal transfer. However, the fan also dissipates a certain amount of heat into the air. The heat sink spreads out the heat energy from the source, primarily the integrated circuit, into the cabinet or housing volume of the device. Another fan may be provided to circulate air between the housing and the ambient room air.
Integrated circuit devices are available according to more or less demanding temperature specifications. Devices that have a relatively wider temperature range are more expensive. Standard commercial computer processor components, for example, may be rated up to 70° C. (about 160° F.). The most durable military application devices may be rated up 125° C. (about 260° F.). These devices are sometimes required to operate in ambient air temperature conditions ranging from −40 to +55° C. (about −40 to +130° F.).
Movement of thermal energy from an integrated circuit or other localized heat source, toward a remote area or toward a structure that carries the heat away, occurs from one or more of thermal conduction, convection and radiation. Conduction of heat energy requires contact between thermally conductive masses and proceeds at a rate that depends in part on the difference in temperature between the masses. Convection involves conduction between a heated body and adjacent heat transfer fluid (gas or liquid), typically air, involves differences in fluid density due to differences in fluid temperature, and is substantially affected by forced air currents. Radiation also dissipates heat, but its contribution is normally small at the temperature ranges of interest.
Heat transfer arrangements can involve passing a current of cooler air or other heat transfer fluid over a hotter surface to be cooled. A captive heat transfer fluid can be provided in closed volume and arranged to circulate. The fluid is heated by a source of heat energy that is in heat transfer relationship with one part of the closed volume. A heat sink is arranged in heat transfer relationship with another part of the closed volume, releasing heat (provided that the heat exchange medium, such as air, is kept cooler than the heat sink), and cooling the fluid. The heat transfer fluid advantageously undergoes cyclic changes of phase. Each change of phase either stores or releases a quantity of heat energy due to the latent thermal energy involved in the phase change itself.
In this way, a liquid phase change heat transfer fluid can be evaporated (vaporized) into gas at the heat source and condensed again into liquid at the heat sink. Different techniques can be used to return the condensed liquid from the condenser to the evaporator, which need not be powered by outside energy sources. A return path is possible, for example, over a gravity flow path in a thermo-siphon arrangement. In a heat pipe arrangement, a return path for the condensed liquid can be provided by lining the vessel confining the heat transfer fluid with a wicking material that supports capillary flow, such as a sintered particulate or powder lining. The capillary flow is driven substantially by surface tension and can proceed regardless of orientation and gravity.
Assuming that the heat transfer fluid is confined in an integral metal vessel, some thermal conduction from the heat source to the sink can occur through the vessel walls. It is desirable on grounds of efficiency to separate the evaporator and condenser sections by a distance or otherwise to interpose a thermal barrier that permits maintenance of a temperature difference. Nevertheless, phase change heat exchange circuits as described can operate with a very modest temperature difference between the source and the sink and can efficiently move heat energy to assist in heat dissipation.
There are a number of design considerations for thermal transfer arrangements such as heat pipes. In addition to the ability to handle the necessary flow of thermal energy to keep the heat source within desired temperature limits, the evaporator and the condenser should have a good heat transfer coupling with the heat source and sink, respectively. The thermal transfer characteristics of the heat pipe structures, the various dimensions and quantities, etc. need to operate over the range of expected temperatures. Preferably the device is compact and does not interfere unduly with necessary access to structures associated with the heat source and sink.
A number of heat pipe arrangements according to the foregoing general description are available from Thermacore International, Inc., Lancaster, Pa., and are disclosed in US patents assigned to their licensor, Thermal Corp., Georgetown, Del. In a heat pipe, the liquid and vapor phases of the heat transfer medium reach equilibrium in the absence of temperature differences and remain substantially stagnant. When heat energy is added at the evaporator, a temperature difference arises. Vaporization of the heat transfer medium at the evaporator leads to increased local vapor pressure in that area. The vapor diffuses through the envelope of the heat pipe, and a portion arrives at the condenser. The condenser is at a slightly lower temperature. As the vapor is cooled and condenses, releasing the latent heat energy of vaporization at the area of the condenser, heat energy is transferred from the heat transfer medium to the heat pipe envelope, where air heat exchange fins remove the heat energy.
The condensed liquid phase heat transfer medium flows back to the evaporator due to capillary forces developed in the wick structure, and the cycle can repeat. Where there is a positive temperature difference between the evaporator (e.g., warmed by an electrical circuit element) and the condenser (e.g., cooled by convection, forced air, contact with a thermal sink, etc.) the cycle can continue indefinitely, moving heat energy. The technique is operative at low thermal gradients. The operation is passive in that it can be driven wholly by the heat energy that it transfers.
U.S. Pat. No. 6,381,135—Prasher; U.S. Pat. No. 6,389,696—Heil; and U.S. Pat. No. 6,382,309—Kroliczek teach additional heat dissipation apparatus intended for cooling integrated circuit devices and the like, as described. These references are hereby incorporated for their teachings of heat pipe or thermal siphon devices.
A stacked-fin heat sink device for a large scale integrated circuit or processor chip package is disclosed in U.S. Pat. No. 6,061,235—Cromwell et al. In that device, a mounting fixture is attached to the motherboard or other circuit card to surround the processor, and the fixture receives a spring biased mounting that presses a thermally conductive base plate into mechanical and thermally conductive contact with the processor package. A heat pipe is contained in a cylindrical vessel disposed centrally on and longitudinally extending perpendicular to the thermally conductive plate. A plurality of heat transfer fins are disposed parallel to one another and perpendicular to the extension of the cylindrical vessel. In this patent, which is hereby incorporated in this disclosure, the thermally conductive plate at the bottom end of the heat pipe vessel can function as the evaporator, having a slightly higher temperature than the finned sidewalls of the vessel remote from the bottom, which maintain a lower temperature and can function as the condenser. In the standing configuration shown, gravity can power the return path. In other orientations, a wicking material can be provided so that capillary action drives the return path.
The spaced air-contact fins in Cromwell need to be assembled with the heat pipe tube. Whereas the fins are rectangular and the heat pipe is a cylinder, there are issues respecting vertical, horizontal and rotational alignment of the plates to the one another, and attachment to the cylinder in good thermally conductive contact. These problems appear to have been addressed by affixing the fins to opposed side plates, thus requiring additional parts and assembly while affecting the extent of available air circulation. Air circulation characteristics and heat transfer characteristics are also affected by the relative size of the heat pipe and the fins. It would be advantageous if the structure of such a heat sink could be minimized, preferably such that the heat pipe provide substantially all the structural support needed for the fins.
A mounting base plate arrangement has certain potentially useful aspects in connection with a heat transfer device. A plate is useful to present a large surface area for contact with a heat source having a planar surface, such as a processor or VLSI circuit. The rate of heat transfer by conduction is partly a function of the area and intimacy of contact. The plate can have a reasonably substantial thickness, which provides a thermal storage capacity and leads to rapid heat transfer throughout the material of the plate. Apart from these benefits, the drawbacks include the complications associated with mounting the plate to the heat source, and the need to mount the heat pipe vessel to the base plate and to mount the fins. These needs are met in part by providing structures on the base plate. The structures can include a clasp part that is complementary with a spring clip for affixing the base plate to the heat sources. The structures can also include structures that are complementary with the external structure of the heat pipe vessel, which is capable of various shapes. The structure of the base plate can even provide one or more walls that are assembled to close the heat pipe vessel.
However there is a strong interest in controlling the complication and expense of heat sinks. Heat sinks are preferably made in a manner that maximizes thermal efficiency by providing good contact between the heat source, the heat pipe and the fins. The optimal structure should have very inexpensive parts, mounted by very inexpensive assembly steps, but should provide good thermal efficiency.
It would be advantageous to reduce the complexity of a heat sink containing a heat pipe, to the minimum necessary to achieve the objectives of efficient heat transfer and the structural connections that are involved.