2017 Reports

Using Electrical Capacitance and Mechanically Representative Hardware to Evaluate the Thermal Mechanical Stability of Thermal Interface Materials

Authors: Michael A. Gaynes, Lauren Boston and Andrew Yu
    Experimental study of material performance is a necessary complement to virtual qualification efforts that use modeling and simulation to streamline development of new electronic assemblies. The durability of thermal interface materials in large complex printed circuit board (PCB) assemblies is difficult to model and therefore, experimental study and verification is needed. The thermal mechanical stability of dielectric thermal interface materials is monitored using electrical capacitance, the inverse of which has the same geometric dependence on bond line and area as thermal resistance. Electrical capacitance is fast, easy and accurate to measure and can be used with mechanically representative hardware very early in the development cycle. Four thermal interface materials (TIMs) classified as pre-cured thermal putties are evaluated on hardware that is easily prototyped and which is mechanically representative of large complex PCBs. Electrical capacitance of the TIMs was measured in situ during thermal cycle testing and could distinguish stability as well as degradation. Analysis of the TIMs after the test confirmed structural damage in the form of cracks, fissures and material movement. Electrical capacitance has broad application in evaluating the stability of dielectric TIMs in application specific designs very early in development by using easily procured mechanically representative hardware.

    With ever increasing power density in the electronic packages, thermal management has become critical to ensure device operation within specification. Efforts are made to transfer the heat efficiently between the heat source and the heat sink by reducing the thermal resistance. Thermal interface materials (TIMs) improve the efficiency of heat transfer by filling the asperities/air gaps between these two non-ideal mating surfaces.
    In this study, two silicone and one non-silicone thermal interface putty materials are evaluated. Characterization included measurement of unit area thermal resistance, thermal conductivity and dielectric constant, the latter of which is used in conjunction with electrical capacitance to calculate bond line thickness. The thermal interface materials were used between a thermal test vehicle and an aluminum heat sink. The target bond line was 0.5 mm and was achieved by pressing the heat sink down to 0.5 mm shims inserted at the corners between the heat sink and thermal test vehicle. Thermal resistance was measured at time-zero and at various points during thermal cycling between -40°C and 125°C: 250, 500, 750 and 1000 cycles. Increases in thermal resistance did occur. After thermal cycling the thermal test vehicle and heat sink were separated to observe the integrity of the thermal interface material. There was evidence of material degradation in the form of cracks and fissures. Accelerated thermal cycle testing was found to be very useful in assessing the impact from differential thermal expansion in electronic assemblies.