New materials highlighted at SEMI-THERM

06/01/2002

MEETING REPORT

SAN JOSE, CALIF. - Many new materials and technologies for addressing thermal management challenges were presented at IEEE's 18th annual Semiconductor Thermal Measurement and Management Symposium and Exhibition.

On the exhibit floor, several companies had new and innovative materials. W.L. Gore had a new thin, thermally conductive interface material targeting gap-fill needs in applications with Z-axis constraints. Dow Corning introduced a new silicone gap-fill material, along with some low modulus, thermally conductive adhesives.

Other new materials included a cemented diamond composite from Skeleton Technologies. The material can be processed with powder metallurgy techniques, so it can be shaped into finned heatsinks or other geometries for thermal management. Its thermal conductivity is 600 W/mK, about 50 percent higher than pure Cu, and it is less brittle than natural diamond.

Graftech had a natural graphite material with a thermal conductivity that is anisotropic but controllable. This allows the formation of some novel heat sink structures, and the low density of the graphite might allow it to replace metal heat sinks in some applications. Another metal alternative was offered by Cool Shield, which showed a line of thermally conductive injection molded polymers with thermal conductivities in the range of 10 to 100 W/mK. The processing allows the material to be used for heat sinks as well as EMI shielding.

Osprey Metals was also on hand to discuss its engineered thermal management materials based on a spray forming method. They are able to control the composition and microstructure of Si-Al alloys to get a low coefficient of thermal expansion that varies little over temperatures of interest, with a density lower than most competing materials and a thermal conductivity above 100 W/mK.

On the test and equipment side, DCI displayed a thermal socket system with a miniature thermoelectric cooler (TEC) integrated into the lid of the socket for more efficient gathering of data. The TEC can establish a stable temperature for the device under test more quickly than typical temperature control methods. Mathis Instruments (New Brunswick, Canada) had a new tool that measures thermal conductivity non-destructively by applying current at a point and measuring the temperature rise. Calibration samples are used to extract the thermal conductivity and other parameters from the results.

The SEMI-THERM technical sessions included both theoretical and real issues. Stephen Montgomery (Intel) presented an analysis of the "fouling" of heat sinks, in which the particles in a computer's ambient can accumulate on a finned heatsink and significantly degrade the performance. For example, a 25-µm thick non-conductive layer on a heatsink can reduce the fin effectiveness by more than 10 percent for a typical high-density heatsink, both by reducing the conductivity of the pins and by reducing the space for airflow.

Stacked packaging earned some attention because of the inherent challenges in thermal management of vertical packaging approaches. Sridhar Krishnan (Tessera) presented a comparison of measurements and modeling that indicated the importance of accurately representing the circuit board in the model. In a stack of four chips in a folded flex package, the analysis showed the benefits of using a heat spreader. This was a bigger factor than properties of the encapsulant or substrate.