An investigation into curing behavior and kinetics of underfills for flip chip packages

Flip-chip technology is becoming increasingly attractive for high performance ICs. However, flip chip package solder joints suffer thermomechanical fatigue and creep failures due to the CTE mismatch between silicon die and substrate. Flip chip underfills are used to redistribute CTE mismatch stresses over the entire device/substrate interfaces to improve solder joint reliability. However, underfill materials require a long cure time, and the final physical and mechanical properties of underfill materials depend greatly on the degree of curing. It is thus of great importance to study the curing behavior and kinetics of underfills. Two conventional capillary underfills and one no-flow underfill were used in this study. Both isothermal and dynamic differential scanning calorimetry (DSC) were used to characterize the curing processes. An auto-catalyzed kinetic model was applied to describe the underfill curing for different temperatures and times. Good agreement was found between the kinetic model and experimental results. From DSC analysis, underfill curing reaction orders and constants could be calculated using software developed in-house. As the reaction constant and curing temperature followed the Arrhenius relationship, the activation energy of the curing process could also be determined. Flux application is necessary in a flip chip process and flux residues may interact with the underfill during the flow and curing processes. In this study, two types of no-clean flux (a low viscosity liquid flux and a high viscosity tacky flux) were applied. Results showed the effects of fluxes on the underfill transition temperatures and CTEs