Nanoscale Conformable Thermal Interface Materials with Electronically Enhanced Heat Conduction

Carbon nanotubes (CNTs), single and multi-walled, have very high thermal conductivity and are natural choices to increase teffective thermal conductivity of thermal interface materials (TIMs). However, they have high interfacial thermal resistances of CNT at the base (seed) and at the heat sink end. Broadly, two approaches have been taken to reduce interfacial thermal resistance of CNT on heatsink side. The first method involves increasing the interfacial pressure and the material above (Fischer and co-workers). In the second approach a layer of solder is introduced between CNT and the heat sink. The later approach reduced the interfacial thermal resistance by an order of magnitude. No attempts have been made to reduce the interfacial resistance on the seed side of CNT. In this proposal, a metallic conformal coatings have been proposed. They consist of a flash of Pd, followed by Cu, followed by solder. Such a conformal layer ensures complete, area coverage and is tolerant to variations in manufacturing variable such as statistical variation in CNT heights. On the seed side, it is proposed to etch out the seed followed by polishing/CMP is proposed to eliminate that interface. We propose to investigate indium and a lead-free solder. Attempts will be made to understand the interfaces through modeling. We also propose to simulate thermal performance of a package, to be chosen by Air Force, with proposed changes to TIM. BENEFIT: The proposed conformal coatings can significantly decrease the overall thermal resistance of a TIM with CNT compared to a CNT-solder interface. They can accommodate statistical variations common to CNT manufacturing process. The conformal coatings are compliant and can better withstand cyclic stresses induced due to coefficient of thermal expansion mismatch during temperature cycling. Additionally, we are the first ones to decrease (eliminate) the interfacial stress between CNT and its seed. The impact of significantly decreasing TIM thermal interfacial resistance is beneficial across all product lines where TIMs are used.