A Testing Apparatus for Thermal Interface Materials

The task of cooling electronic devices is becoming a greater challenge as the performance requirement and power consumption of microprocessors increase. One challenge is the performance of thermal interface materials, or materials that enhance heat transfer from the microprocessor to the cooling device. With new greases, tapes, and phase change materials appearing on the market, it is important to be able to investigate their thermal performance. This paper describes a testing apparatus to characterize thermal interface materials used in electronic packaging. This testing apparatus is capable of measuring apparent (effective) thermal conductivity and thermal resistance across such materials. Apparent thermal conductivity is found to be more applicable to micro-electronic packaging than the usual bulk thermal conductivity. This paper includes a case study of using the testing apparatus to compare nine different manufacturing lots of the same thermal grease. This testing apparatus was demonstrated to be able to monitor the incoming thermal grease quality. In addition, a quick benchmarking of different thermal greases using this testing apparatus was also shown. Regardless of thermal grease tested, the apparent thermal conductivity is always lower than the bulk thermal conductivity at a thin Bond Line Thickness (BLT). Contact resistance is the primary reason for the discrepancy between these two values. Contact resistance can be ignored in bulk thermal conductivity measurements while it is critical to influence the apparent thermal conductivity measurements since contact resistance contributes greatly to the overall thermal resistance. The apparent thermal conductivity approaches the bulk thermal conductivity as the BLT increases.