Novel TIM Evaluation Method With in Situ Electrical Monitoring

Abstract

Thermal interface materials (TIMs) transfer heat between two surfaces in electronic assemblies. Specifically, a TIM 2 is a TIM used to transfer heat from a component’s lid to a heat sink. During use, TIM 2s experience thermal aging and mechanical stress (caused by board/component/heat sink expansion mismatches and warpage during heating/cooling). Understanding the response of TIMs to these stresses is critical for understanding TIM and product reliability. However, the responses of TIMs are complex and difficult to predict or model. TIMs are often polymeric materials heavily loaded with thermally conductive fillers and have time, temperature, and rate-dependent properties. Therefore, experimental studies are needed to address TIM reliability. However, monitoring TIM stability or performance while in situ is difficult. Electrical capacitance has been used to monitor the stability of dielectric TIM in situ, allowing for monitoring TIM stability and, indirectly, the TIM thermal performance, but electrically conductive TIMs cannot be used with capacitance measurements, so a new technique is needed. Monitoring a conductive TIM’s electrical resistance could give insight into a TIM’s stability, as both thermal and electrical resistances are affected by a TIM’s contact with surfaces, thickness, and area. This study examines four electrically conductive graphite pad TIMs on a test board, representing a large printed circuit board with a large shared heat sink, in an accelerated thermal cycle from 0 °C to 100 °C. The electrical resistance of the TIMs was monitored in situ, and after testing, postmortem failure analysis of the TIMs was done. The efficacy of electrical resistance monitoring for evaluating TIM stability was examined, and correlations with failure analysis results were made. Electrical resistance monitoring provided insight into the TIMs’ stability, failure time, and extent of failure.