Investigation of cure kinetics and its effect on adhesion strength of nonconductive adhesives used in flip chip assembly

The reaction kinetics of a commercial fast cure nonconductive adhesive has been systematically investigated using differential scanning calorimetry. Samples were isothermally cured at temperatures from 120 to 160 °C and dynamically cured at ramp rates between 5 and 20 °C/min. A good agreement between the autocatalytic kinetic model prediction and experimental results was demonstrated. Deviation occurred at high degrees of cure for curing below 140 °C due to the occurrence of vitrification. Additionally, by comparing the dynamic cure prediction with the isothermal experiment, good agreements and equivalence were demonstrated. As such, it is possible to predict the isothermal reaction behavior of fast cure materials at high temperature provided that the variation between the actual temperature of the heating system and the setting temperature is not large. Furthermore, the effect of curing process on the adhesion strength has been demonstrated by testing the shear strength of lap joint specimens. It was found that the evolution of adhesion strength was largely dependent on the buildup of mechanical properties during the curing process. At low and medium degrees of cure, cohesive and adhesive failures were respectively observed, while at high degrees of cure, adhesion strength surpassing the shear strength of the solder mask was observed. The sharp increase in adhesion strength was observed to coincide with the gelation point marked by the crossover between the storage and loss modulii, thus suggesting that the contributors to adhesion strength include mechanical interlocking as well as chemical bonding, as evidenced by buildup of storage modulus and mechanical strength of the adhesive.