Solder Matrix Fiber Composite Thermal Interface Materials

Licentiate thesis, 2018

Overheating has been a problem for microelectronics devices for decades, and the problem is exacerbated by the continued trend of miniaturization of features and the corresponding increase in power density. Thermal interface materials (TIMs) target one of the main bottlenecks in heat transfer: the interface between two materials, such as between a heat-generating microchip and a heatsink. By filling out microgaps caused by the roughness of the mating surfaces, TIMs improve the heat transfer over the interface by orders of magnitude. Nonetheless, even with a TIM the interface can be a limiting factor for the overall cooling. Thus, the development of new and improved TIMs is a big challenge for the electronics field.

This thesis thoroughly reviews the overall status of the field of TIM research, and identifies three main tracks for novel research. First, particle laden polymers, which utilizes thermally conductive particles inside a polymer matrix which can conform to surfaces. Second, continuous metal phase TIM, which forms metallurgical bonds to both surfaces, and utilizes the inherently high thermal conductivity of metals. Third, carbon nanotube (CNT) array TIMs, which utilize the incredible thermal conductivity of CNTs. Here, an array of vertically aligned CNTs is used as nanosprings to connect the two surfaces together. In addition to these main tracks, various novel ideas based on polymers, metal and carbonaceous materials are explored.

From the reviewed categories, continuous metal phase TIM in the form of solder is already widely used in industry, but comes with severe drawbacks in terms of mechanical properties and handling issues. Solder matrix fiber composites (SMFCs) have been shown to address these challenges, but have so far required complicated procedures and components. In this thesis, we present the fabrication of a new SMFCs based on commercially available polymer and carbon fiber networks infiltrated with Sn-Ag-Cu alloy (SAC) or Indium using equipment for large volume production. The composite material exhibits similar thermal properties compared to pure solder, and mechanical properties that can be tailored towards specific applications. We also show that the handling properties of the SMFC allows it to be used in process flows where multiple reflow cycles are required, and can achieve a well-defined bond line thickness and good bonding using fluxless reflow under pressure.