Thiol-based, Thermally-Conductive Adhesives for High Power Electronics

Abstract

High power electronics, such as those enabling the next generation of so-called ‘5G’ devices, evolve significant amounts of heat and require improved mechanisms to dissipate that heat. As an alternative to sintered copper-based nanomaterial composites, recent work in silver nanoparticle pastes has shown great promise to provide high thermal conductivities (278.5 W/mK)[1] approaching that of neat silver (429 W/mK); however, pastes, in general, are not functional as adhesives for chip attachment under repeated thermal cycling without requisite mechanical reinforcement and interfacial stress mitigation. Work at UT Dallas, and Adaptive3D, on thermally conductive systems explores thiol-based resins composites as adhesives with excellent adhesion to metal substrates, interfaces and fillers, with tunable electrical and thermal conductivities. Voit, Lund, et al. have demonstrated thiol-ene polymer systems for flexible electronics and semiconductor packaging with excellent adhesion to metal substrates (due to the thiol-metal interaction) and excellent survivability to thermal cycling despite significant interfacial coefficient of thermal expansion (CTE) mismatch. This advantage is due to the thermoset nature of the polymer matrix, the ability to accommodate strains at the interface between CTE-mismatched materials and the lack of stress concentrators that evolve during polymerization due to the thiol-mediated chain transfer mechanisms during curing [2]. We demonstrate the ability to deposit plasma-enhanced chemical vapor deposited Silicon Nitride (CTE <3) on thiol-based polymers (CTE ~70) and survive thermal cycling to 270°C without interfacial delamination. The same polymerization phenomena that enable this behavior are conducive for loading organic and inorganic filler into thiol-based systems to optimize electrical and thermal conductivity in die attach adhesives, molding compounds and unique semiconductor packaging materials. Aliev, Baughman, et al. have previously utilized multiwalled carbon nanotubes and graphene-based structures to tune thermal conductivities and properties of organic-inorganic composites [3]. Herein we will present a multi-materials approach to enhance both thermal and electrical conductivity through control of phononic resonances in semiconductor packaging adhesives and discuss the impacts of modifying metallic (silver) and carbonaceous (nanotube and graphene) fillers imbedded within a thiol-based polymer matrix. We present early work on the effects of thiol-mediated chain transfer mechanisms during polymerization on the percolation threshold of fillers and resulting electrical and thermal conductivity.