While increasing heat generation will make liquid cooling an attractive option for some electronic systems in coming years, cooling system engineers likely will spend most of their time on air-cooled designs built around thermally conductive interface materials and heat sinks. The good news is that engineers have a growing range of materials from which to choose; the inevitable downside is that all material choices involve trade-offs. By understanding the range of available possibilities, designers can ensure that they make the right choice for the cost and performance goals of their particular project.
Device and Heat Sink InterfaceThe fundamental principle of cooling systems under consideration is their ability to move and spread heat through conduction - the transfer of heat within a material due to intimate contact between molecules. Properties of various heat sink materials, including their conductivity, cost and weight, are unimportant if there is a lack of intimate contact throughout the cooling path. A make-or-break point is the interface between the hot device and the heat sink.
Surfaces of real-world semiconductor packages and heat sinks are imperfect, and establishing a robust, void-free physical connection between them requires use of an interface material. Choices of interface material range from thermal grease to thermally applied compounds.
Ease of manufacturing is an important concern in the selection of an interface material. Some, including traditional thermal grease and epoxy- or acrylic-thermal adhesives, are relatively inexpensive but can be messy to apply and difficult to control under production line conditions. Others, particularly tape-based materials, are easy to apply but carry the trade-off of poorer thermal conductivity and possible weak mechanical adhesion over time.
Phase-change materials combine several qualities: lower thermal resistance than greases, good filling properties and ability to be applied in solid form at room temperature. A subsequent heating step causes a phase change to semiliquid, allowing the material to fill gaps in the interface.
Thermally applied compounds, which are applied in a free-flowing liquid state and then cured to a rubber-like film, also may be an attractive option. One caveat: they often require that the heat sink be preheated prior to assembly, which may complicate the manufacturing process.
Whatever material is selected, designers and manufacturing managers must ensure that proper application procedures are specified and followed. Even the best interface materials cannot do their job if they are filled with voids, have poor thickness uniformity or insufficient contact pressure. In some cases, additional mechanical connectors such as screws, clips or spring-loaded pins may provide an important supplement to the interface material.
Heat Sink MaterialsConductive cooling is the transfer of heat within a material. Conductivity of a material is measured in watts per meter times degrees Kelvin (W/mo°K). This conductivity factor quantifies the amount of heat that transfers through a given size and thickness of the material based on a temperature difference from one side to the other. Pure aluminum, with a conductivity of 230 W/mo°K, is traditionally the benchmark material for heat sinks. Despite its comparatively high conductivity, light weight and ease of manufacture, aluminum's ability to conduct heat in thin sections is not sufficient to cool many of the high heat flux components commonly found in current systems.
Many alternative materials and techniques are being explored. Each of these materials or systems has potentially higher conductivity and thus will offer increased heat spreading and lower temperature increases in limited cross-sections as compared to aluminum extrusion. However, all will be more costly and heavier than aluminum.
Copper provides a good conductive path - 80% better than aluminum in thin cross-sections - but it also poses a number of drawbacks. For instance, it weighs about three times as much as aluminum but costs about the same per kilogram. Copper is far more difficult to shape and machine, resulting in higher manufacturing costs than aluminum. Moreover, because of its high melt temperature and poor flow characteristics, copper is not suited to making extrusions of the same fine detail as aluminum. However, when cost and weight are less important than performance and size, copper is a commonly used alternative.
Improved Conductivity MaterialsScientists are developing new materials at a remarkable rate. Among them are high conductivity polymers, carbon-based composites, sintered metallic powders, phase-change compounds, synthetic diamonds and strand-oriented graphite materials. But, the quest for the ideal heat sink material is a difficult one - the perfect choice would offer controlled levels of conductivity, high machinability, low weight, a coefficient of expansion that closely matches that of silicon, low toxicity and a cost as low or lower than aluminum. To date, no alternative material has put together a combination that can knock aluminum off the top of the hill. Many of the best materials do offer some physical properties that improve on aluminum, but typically they do so at a higher cost. However, they are worth consideration in some specialized applications.
The new material that has found the most acceptance in the marketplace is aluminum silicon carbide (AlSiC). Used in cast form in conjunction with various aluminum alloys, it can be tailored to meet a number of the most desirable characteristics for electronic cooling systems. These include controlled thermal expansion, high conductivity and enough strength to form relatively thin cross-sections. Many available compounds offer conductivities similar to aluminum at equal weight density. For some high demand applications, where cost is a low priority, AlSiC can be used to form the entire heat sink. When cost is an issue, AlSiC can be used as a base section with a conventional extruded aluminum heat sink, or as a substrate for direct die attachment in applications such as power module bases.
Heat Pipes Are an AlternativeAn alternative to advanced materials, heat pipes are a closed-loop, phase-change system that transfers heat inside a sealed vessel using evaporation and condensation. A typical heat pipe for electronics applications is made of copper and uses water or alcohol as the cooling fluid.
With this system, heat from the device to be cooled causes the fluid to change from liquid to vapor state. The vapor flows along the pipe until it is cooled enough to condense back into a liquid. It then is returned to the heated area by either a wicking system (often made of sintered metal, which provides high heat flux density control and some ability to return the cooled liquid against gravity) or through a separate liquid return channel inside the pipe. Designers can draw on a variety of heat pipe configurations, including round, square and rectangular cross-sections.
Thermal siphons, or loop heat pipes, are similar devices, but they return the cooled liquid in a separate path outside the vessel. They generally are less flexible than heat pipes as they must be installed to allow heat removal at the top of the system.
Heat pipes are not a material per se, and they are capable only of moving heat from one point to another, not dissipating it. But, they offer very high conductivity (up to 1,000 W/mo°K) and can be used in conjunction with heat spreaders made of aluminum or other materials. In this setup, the heat pipe's conductivity averages with that of the heat spreader base material; this approach can boost the conductivity of an aluminum spreader from 200 to 400 W/mo°K or more with a minimum of weight increase. This type of design typically can increase thermal performance by 20 to 30%, based on decreasing junction temperature. A great deal of research is being done on phase-change materials. Therefore, it seems likely that low cost, high efficiency substitutes will be seen in the near future.
Vapor ChambersAttention currently is being given to development of vapor chambers, or flat-plate heat pipes. They operate in the same two-phase manner as a conventional heat pipe, but this design provides a uniform temperature across the heat dissipation surface when used to cool a spot load on the opposite side. Thus, they can be used as heat dissipation fins when attached directly to a flat mounting surface. This has good potential for a number of air-cooled applications such as microprocessors, radio frequency transmitters, thermoelectric devices and power modules.
Developments in low cost wick structures and new working fluids will enable advances in heat pipe and vapor chamber technology. Currently, the use of water as the working fluid provides the best characteristics for heat transport and latent heat of vaporization.
Aluminum has been the workhorse heat sink material in the electronic industry for many decades, and it will continue to be the standard by which other materials are judged. But, a wide range of new interface materials is available to meet more demanding applications.
Phase-change materials and thermally applied compounds offer notable cost, performance and ease of manufacture combinations. For heat sinks, copper provides improved conductivity but with trade-offs in cost, weight and ease of use. Aluminum silicon carbide is emerging as a viable alternative for high performance situations as well as power module base plates. Heat pipes and vapor chambers can be important elements in developing highly effective multicomponent cooling systems. Their performance is likely to improve substantially in the near future as new working fluids and methods of manufacture become available.