Thermal Materials Solve Power Electronics Challenges

Feb 1, 2006 12:00 PM
By Carl Zweben, Ph.D., Advanced Thermal Materials Consultant, Devon, Pa.

Thermal management deals with problems arising from heat dissipation, thermal stresses and warping. It is critical in the packaging of power semiconductors and other microelectronic and optoelectronic devices, including microprocessors, high-power RF devices, laser diodes and light-emitting diodes (LEDs).^[1-2] Intel's acknowledgement that it has hit a “thermal wall” highlights the severity of the problem.^[2] Laptop heating has increased to the point where, in one case, medical treatment was required.^[3] Further evidence of the thermal problem is Apple's new Computer Power Mac G5 desktop, which has an automotive-like pumped liquid-cooling system. Replacing convection cooling with liquid cooling requires the addition of new manufacturing and servicing infrastructures, and raises significant reliability and cost issues. Heat dissipation is currently the key factor-limiting power levels. It will have to be solved in order to meet the well-publicized heat flux goal of 1000 W/cm² required for future military systems like the all-electric ship.

Thermal stresses and warpage in electronic components arise primarily from different coefficients of thermal expansion (CTEs). The increasing use of lead-free solders, which have much higher processing temperatures than lead-tin types, exacerbates the problem. Note that even when liquid cooling is used, thermal stresses caused by CTE mismatches are still important. Semiconductors and ceramics have CTEs in the range of 2 ppm/K to 7 ppm/K. The CTEs of copper, aluminum and glass fiber-reinforced polymer pc boards are much higher. Decades-old traditional low-CTE materials like copper/tungsten (Cu/W), copper/molybdenum (Cu/Mo), copper-Invar-copper (Cu/I/Cu) and copper-molybdenum-copper (Cu/Mo/Cu) have high densities and thermal conductivities that are little or no better than that of aluminum (Table 1). We call these first-generation thermal management materials.

Table 1 also shows an improved first-generation material, a laminate consisting of Cu-Mo bonded to outer copper layers (Cu/Cu-Mo/Cu). When both weight and thermal conductivity are important, a useful figure of merit is specific thermal conductivity (thermal conductivity divided by density or, in this case, specific gravity, which is dimensionless), as introduced by me and K.A. Schmidt many years ago.^[4] All of the tables in this article include this property, which can provide a good estimate of potential weight reduction (higher is better).

When aluminum and copper are used for heat dissipation, significant design compromises are typically required, which can significantly reduce cooling efficiency. For example, to minimize thermal stresses, it is common to use compliant polymeric thermal interface materials (TIMs) to attach high-CTE heatsinks. It is widely recognized that TIMs increasingly account for most of the system total thermal resistance.^[5]

The high thermal resistance of TIMs can be overcome by direct solder attach, but this can result in high thermal stresses. At present, the key way to work around this issue is to employ “soft” solders, typically Indium based, which have low yield stresses. However, these solders also have poor thermal fatigue and metallurgical characteristics. Use of materials with matching CTEs allows the packaging design engineer to select from a wider range of solders. Low-CTE solders, now under development, will further alleviate the thermal stress problem.

Weight is a key consideration in most portable systems, including notebook computers, cell phones, hybrid automobile electronics and avionics. Even if system weight is not important, low-density materials are needed for components like heatsinks to minimize shock-load stresses during shipping. New high-performance third-generation materials developed in the last few years have ultrahigh thermal conductivities, low CTEs and low densities that can solve key packaging problems, including reducing the CTE and increasing thermal conductivity of pc boards. When the low-CTE solders under development are commercialized, it will be possible to match the CTEs of virtually all packaging materials.

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