Evaluation of Thermal Interface Materials and the Laser Flash Method

Title of Document: EVALUATION OF THERMAL INTERFACE MATERIALS AND THE LASER FLASH METHOD Vinh Khuu, Doctor of Philosophy, 2009 Directed By: Chair Professor, Michael Pecht, Department of Mechanical Engineering Thermal interface materials (TIMs) are used to reduce the interfacial thermal resistance between the chip and the heat sink, which has become a bottleneck to heat removal in a variety of electronic applications. Degradation in thermal performance of the TIM can contribute to unacceptably high chip temperatures, which can significantly impact device or system performance during operation. While progress has been made in recent years in the development of tools to measure beginning-oflife thermal performance, characterizing the long-term performance of the TIM can be crucial from a life cycle stand point since TIMs may experience harsh operating conditions, including high temperature and high humidity, for extended periods of time in typical applications. The laser flash method is one approach for measuring thermal conductivity that has an advantage over more commonly used techniques because of the noncontact nature of the measurement. This technique was applied to 3-layer structures to investigate the effects of thermal cycling and elevated temperature/humidity on the thermal performance of select polymer TIMs in pad form, as well as an adhesive and a gel. While most samples showed little change (less than 10% in thermal resistance) or slight improvement in the thermal performance, one thermal putty material showed degradation due to temperature cycling resulting from bulk material changes near the glass transition temperature. Scanning acoustic microscope images revealed delamination in one group of gap pad samples and cracking in some putty samples due to temperature cycling. Finite element simulations and laser flash measurements performed to validate the laser flash data indicated that sample holder plate heating, an effect previously unexamined in the literature, can lead to inaccurately high TIM thermal conductivity values due to suppression of the sample temperature rise during the laser flash measurement. This study proposed a semi-empirical methodology to correct for these effects. Simulated laser flash test specimens had bondlines that showed little thickness variation (usually within the measurement error) due to clamping by the sample holder plates. Future work was proposed to refine the laser flash sample holder design and perform additional validation studies using thermal test vehicles based on nonfunctional packages. EVALUATION OF THERMAL INTERFACE MATERIALS AND THE LASER FLASH METHOD