Using in situ capacitance measurements to monitor the stability of thermal interface materials in complex PCB assemblies

Abstract

A thermal solution for an array of voltage transformer modules which are cooled by a large area, common aluminum heat spreader for a high end server was evaluated using an in situ, capacitive bond line thermal measurement technique. The method measures the capacitance of a non-electrically conducting thermal interface material (TIM) between the electronic module and heat spreader to quantify the TIM bond line effective thickness during assembly and operation. The thermal resistance of the TIM has the same geometric dependence as the inverse of capacitance, therefore, the capacitive technique also provided a monitor of the thermal performance of the interface. This technique was applied to measure the bond line in real time during the assembly of the heat spreader to an array of 37 modules mounted on a printed circuit board. The results showed that the target bond lines were not achieved by application of a constant force alone on the heat spreader, and guided an improved assembly process. The mechanical motion of the TIM was monitored in situ during thermal cycling and found to fluctuate systematically from the hot to cold portions of the thermal cycle, either compressing or stretching the TIM respectively. The capacitive bond line trend showed thermal interface degradation vs. cycle count for several modules which was confirmed by disassembly and visual inspection. Areas of depleted TIM ranged as high as 25% of the module area. Several design and material changes were shown to improve the TIM stability. Power cycling tests were run in parallel to the thermal cycle tests to help relate the results to field performance. The capacitance technique enabled the development and verification of a thermal solution for a complex mechanical system early in the development cycle. Copyright© (2010) by IMAPS - International Microelectronics & Packaging Society.