Characterization and Prediction of Material Response of Micro And Nano-Underfills for Flip Chip Devices

Abstract

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 [1]. In this research, property prediction models based on representative volume element (RVE) and modified random spatial adsortion have been developed. A size distribution also applied to the spherical particles to generate random diameter of the filler particles. The models can be used for development of nano-silica underfills with desirable thermo-mechanical properties. Elastic Modulus, coefficient of thermal expansion, bulk modulus, Poisson’s ratio and viscoelastic properties have been predicted. Temperature dependent thermo-mechanical properties of nano-silica underfills and micro-silica underfill have been evaluated and correlated with the model prediction in a temperature range of –175°C to +150 °C. Properties investigated include temperature dependent stress-strain, creep and stress relaxation behavior. The trade-offs between using nano-fillers instead of micron-fillers on thermo-mechanical properties and reliability has been benchmarked.