Indium solder as a thermal interface material using fluxless bonding technology

The capability and diversity of high performance microprocessors is increasing with each process technology generation in order to meet increasing application demand. The cooling designs for these electronic chips have to deal with larger temperature gradients across the die than previously. The key to thermal management is to dissipate the thermal energy from a heat-generating device to a heat sink via conduction through a thermal interface material (TIM). The TIM must also relieve the mechanical stress and absorb strain caused by coefficient of thermal expansion (CTE) mismatch between heat spreader lid and silicon die during field operation. Low modulus TIM is excellent at strain absorption and relieves stress from CTE mismatch of different materials. In this study we explore the fluxless bonding of indium as a candidate TIM for high performance microprocessors due to its high thermal conductivity, low melting temperature and low tensile strength and its dasiagreenpsilaness (non-hazardous material, minimal waste and ease of product reworkability). Challenges in the development process include: controlling bond line thickness, fillet extent and controlling voids in the TIM assembly at reflow temperatures. This paper aims to investigate these key challenges and provide some general recommendations.