Kinetic modeling of no-flow underfill cure and its relationship to solder wetting and voiding

The cure kinetics of an epoxy-based no-flow, fluxing underfill were determined using isothermal differential scanning calorimetry (DSC), and then fit to a differential Avrami equation. Integration of this equation permits a determination of the state of underfill cure at any point in a dynamic reflow process. By relating the calculated time interval ▵t between solder melting and polymer gelation to actual solder wetting, we can define acceptable process hunts for high assembly yields. In an extension of the model, expressions were also derived which relate underfill volatility to the degree of underfill cure and the vapor pressure of the pure underfill monomeric components. From this, a value is obtained for the total volatilization for any given underfill and reflow profile. Volatile out-gassing has been implicated as a significant source of underfill voiding. Combining models for solder melting, underfill cure and underfill volatilization allows for the a priori optimization of reflow profiles, to provide maximum assembly yields with a minimum of underfill voiding.