Impact of Squeezing on the Microstructure of Thermal Interface Materials

Thermal interface materials (TIMs), consisting of high conductivity filler particles dispersed in a polymer matrix, are used in thermal management of electronics to bridge the gap between the heat generating components and the heat spreader or heat sink. Without TIMs, there is imperfect contact at the interface, resulting in detrimental chip performance and elevated temperatures that can ultimately lead to failure of the chip. In commercial applications, TIMs are dispensed on the chip, heat spreader, or the heat sink using a nozzle via an automated process. The TIM is then squeezed into a thin layer over the substrate by the alternate component (i.e., the device, heat spreader, or the heat sink), often followed by curing (e.g., at elevated temperature) to form a rigid bond. During squeezing, the particle-laden TIM generally exhibits non-Newtonian behavior and, after squeezing, the particle spatial distribution may be non-uniform. The flow behavior depends on dispense pattern (e.g., dots, lines, star patterns), parameters of the squeezing process (e.g., force and squeezing rate), and the TIM composition (e.g., particle shape, size distribution, volume fraction, and matrix composition). The velocity and applied pressure during squeezing significantly impacts the achievable bond line thickness (BLT) and the particle spatial distribution, which can cause the thermal performance of the TIM to deviate from the vendor-specified thermal characteristics. In practice, the maximum allowable squeeze pressure, which impacts the final BLT, is limited by potential mechanical failure of packaged electronics. There are open questions regarding the effect of squeezing on particle rearrangement and its effect on thermal conduction within the particle network. In this work, X-ray micro-computed tomography (XRCT) is used to measure the spatial distribution of particles in the TIM after (a) dispensing and (b) squeezing processes. A mock TIM with a target of 30 vol% copper microspheres (median diameter 114 μm) is created by handmixing the particles with a UV-curable epoxy. Microstructural features such as the average particle volume fraction, coordination number and radial distribution function (RDF) are computed to gain insights into the particle spatial arrangement in the TIM.