Fabrication and characterization of a carbon fiber solder composite thermal interface material

One of the significant bottlenecks in thermal management is to develop thermal interface materials (TIMs) with lower thermal interface resistance while retaining good reliability, mechanical properties and handling capabilities. Recently, the combination of electrospun polymer fibers and solder matrix has gathered interest, combining the excellent thermal properties of metal with the mechanical properties of polymers. Carbon fibers are increasingly common as reinforcement in composites, owing to their high strength and thermal conductivity. Utilizing carbon fibers in a similar composite could allow for the fabrication of TIMs with so far unexplored properties such as increased thermal conductivity, strength and tunable CTE. In this work, we have fabricated and characterized a TIM based on a carbon fiber network infiltrated by an alloy of Sn-Ag-Cu, (CF-TIM). Commercially available carbon fibers are coated with a thin layer of Ag and infiltrated by molten alloy under high pressure. The result is a preform TIM, easy to handle and compatible with standard SMT assembly. A thermal interface resistance lower than 2 Kmm2/W between two ENIG coated Cu substrates was measured with laser flash. Comparing total thermal interface resistance to bond line thickness indicates a very low contact resistance consistent with good metallurgical bonding and a bulk thermal conductivity of 24 W/mK for the TIM. X-ray inspection and SEM of cross section of the assembled structure indicates good adhesion between fiber and matrix, and a very low degree of voiding. To demonstrate the handling capabilities of CF-TIM, a variety of reflow conditions were investigated. A consistent bond line thickness (BLT) of 45±5µm was achieved independent on applied pressure during reflow, and decreased less than 20% after 10 additional reflow cycles, without additional material leakage. This demonstrates the possibility of CF-TIM use in assembly line processes requiring additional reflow steps. Solder preforms are common in industry, and due to similar handling characteristics of the CF-TIM, it should be easily integrated into existing electronics assembly lines. The usage of commercial fibers not reliant on slow and expensive processes such as electrospinning further opens up the potential for mass production.