Advancements in thermal interface materials have reached the resolution limit of typical ASTM D5470 based testers, which is around ±1×10−6 m2K/W. As the interfacial resistance is reduced, the temperature difference at the interface decreases and ultimately becomes difficult to measure. Standard ASTM testers utilize precise temperature sensors and knowledge of the thermal conductivity of the heat meter bar to resolve the temperature difference at the interface. It is difficult to resolve interface resistances on the order of 1×10−6 m2K/W, even when precision RTDs with a resolution of ±0.001°C are utilized, as the location uncertainty of the sensor can become important. Increasing the temperature difference across the interface is necessary for further improvement in the resolution.
This work presents a miniature ASTM type tester that was developed to address the resolution limits of standard testers by reducing the heat meter bar thickness, using a chip resistor as the heater element, and using an IR camera to measure the temperature gradient along the meter bar. Reducing the length of the heat meter bars reduces the overall resistance, and increases the resistance of the interface relative to that of the meter bars. Because of the reduced size scale of the miniature tester, measurement of the temperature profile using the typical ASTM approach of embedding temperature probes along the length of the meter bar was not feasible but instead was achieved using a relatively inexpensive uncooled long wavelength infrared camera with a microscope attachment in order to focus down to ∼100 microns. Although the IR camera increases the uncertainty of the measured temperatures, this method is shown to measure a thermal interfacial resistance of 1.45×10−6 m2K/W with an uncertainty of ±1.1×10−7 m2K/W, where the sample interface was a 2 mil AuSn preform soldered interface.