Thermal management of electronics is one of the biggest engineering challenges of this decade, as billions of transistors are put in each microprocessor and the increasing density leads to higher heat generation. Interfaces between the different components of electronics packaging arise during its assembly. Due to imperfections, the contact area of the mating surfaces can be as low as a few percent of the nominal surface area of either surface. Thermal interface materials (TIMs), consisting of high conductivity filler particles dispersed in a polymer matrix, are used to bridge the gap between the different components and enable efficient heat conduction from the microprocessor to the heat sink. Without TIMs, chip performance deteriorates as a result of elevated temperature and this deterioration can ultimately lead to failure of the chip. On an industrial assembly line, TIMs are dispensed on the chip, heat spreader, or the heat sink using a nozzle via an automated process. Various dispense patterns such as dot, line, spiral, serpentine, “X”, and star shapes exist. The TIM is then squeezed to spread over the substrate by using 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 [1] and, after squeezing, the particle spatial distribution may be non-uniform [2]. The flow behavior depends on the TIM dispense pattern, parameters of the squeezing process (e.g., force and squeeze rate), and the TIM composition (e.g., particle shape, size distribution, volume fraction, and matrix viscosity). The velocity and applied pressure during squeezing significantly impact 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. 

While much work has focused on macroscopic analysis of the TIMs from experiments and numerical models, there are open questions regarding the impact of squeezing on the particle arrangements and on thermal conduction within the particle network. In this work, 3D X-Ray micro-Computed Tomography (XRCT) is used to visualize and quantify 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 115 microns) is created by hand-mixing the particles with a UV-curable epoxy (Epoxies, Etc., UV Cure 60-7158). The dispensed and squeezed TIM samples are cured using UV light prior to imaging. The 3D XRCT images are analyzed to extract particle locations and size. Next, 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. We also model constant velocity squeezing of spherical particles using the discrete element method (DEM) to predict particle spatial distribution. The model includes one-way fluid-particle coupling via a drag force acting on particles. Particle information obtained from analyzing XRCT images of the dispensed TIM is used as input in the DEM model. Microstructural features analyzed from the experiment are compared with the model predictions to validate the model. Ultimately, validated models can be used to optimize the dispensing and squeezing of TIMs to achieve uniform particle spatial distribution and high thermal conductivity.