Nanothermal Interface Materials: Technology Review and Recent Results

Thermal interface materials (TIMs) play a critical role in conventionally packaged electronic systems and often represent the highest thermal resistance and/or least reliable element in the heat flow path from the chip to the external ambient. In defense applications, the need to accommodate large differences in the coefficients of thermal expansion (CTE) among the packaging materials, provide for in-field reworkability, and assure physical integrity as well as long-term reliability further exacerbates this situation. Epoxy-based thermoplastic TIMs are compliant and reworkable at low temperature, but their low thermal conductivities pose a significant barrier to the thermal packaging of high-power devices. Alternatively, while solder TIMs offer low thermal interface resistances, their mechanical stiffness and high melting points make them inappropriate for many of these applications. Consequently, Defense Advanced Research Projects Agency (DARPA) initiated a series of studies exploring the potential of nanomaterials and nanostructures to create TIMs with solderlike thermal resistance and thermoplasticlike compliance and reworkability. This paper describes the nano-TIM approaches taken and results obtained by four teams responding to the DARPA challenge of pursuing the development of low thermal resistance of 1 mm² K/W and high compliance and reliability TIMs. These approaches include the use of metal nanosprings (GE), laminated solder and flexible graphite films (Teledyne), multiwalled carbon nanotubes (CNTs) with layered metallic bonding materials (Raytheon), and open-ended CNTs (Georgia Tech (GT)). Following a detailed description of the specific nano-TIM approaches taken and of the metrology developed and used to measure the very low thermal resistivities, the thermal performance achieved by these nano-TIMs, with constant thermal load, as well as under temperature cycling and in extended life testing (aging), will be presented. It has been found that the nano-TIMs developed by all four teams can provide thermal interface resistivities well below 10 mm² K/W and that GE's copper nanospring TIMs can consistently achieve thermal interface resistances in the range of 1 mm² K/W. This paper also introduces efforts undertaken for next generation TIMs to reach thermal interface resistance of just 0.1 mm² K/W.