Nano-Underfills for High-Reliability Applications in Extreme Environments

Silica particles are used as a filler material in electronic underfills to reduce coefficient of thermal expansion of the underfill-epoxy matrix. In traditional underfills, the size of silica particles is in the micrometer range. Reduction in particle sizes into the nanometer range has the potential of attaining higher volume fraction particle loading in the underfills and greater control over underfill properties for higher reliability applications. Presently, no-flow underfills have very low or no filler content because micron-size filler particles hinder solder joint formation. Nano-silica underfills have the potential of attaining higher filler loading in no-flow underfills without hindering solder interconnect formation. In this paper, property prediction models based on representative volume element (RVE) and modified random spatial adsortion have been developed. The models can be used for development of nano-silica underfills with desirable thermo-mechanical properties. Temperature dependent thermo-mechanical properties of nano-underfills have been evaluated and correlated with models in a temperature range of -175degC to 150degC. Properties investigated include, temperature dependent stress-strain, creep and stress relaxation behavior. Nano-underfills on 63Sn37Pb eutectic and 95.5Sn3.5Ag1.0Cu leadfree flip-chip devices have been subjected to thermal shock tests in the range of -55degC to 125degC and -55degC to 150degC, respectively. The trade-offs between using nano-fillers instead of micron-fillers on thermo-mechanical properties and reliability has been benchmarked.