Effect of Local Grain Distribution and Enhancement on Edgebond Applied Wafer-Level Chip-Scale Package (WLCSP) Thermal Cycling Performance

The demand on reliable Wafer level chip scale packages (WLCSP) are getting higher due to the need of small form factor and cost. As demonstrated in earlier publications the degradation and deformation mechanism show microstructure evolution associated with the thermal cycling induced damage accumulation. The mechanism, leading to crack initiation and propagation during thermal cycling by sub-grain boundary development can be observed as a general damage accumulation mechanism in various solder joints. The correlation between crack propagation and localized recrystallization are compared in a series of cross section analyses on thermal cycled WLCSP components with normal and elevated temperature thermal cycling conditions. Damage accumulated solder joint locations are identified and an attempt of using edge-bond materials to strengthen the localized solder joints demonstrate that the Tg and CTE needs adjustment to have the thermal cycling performance enhancement. Edge-bond material applied components show either a shift of damage accumulation to a more localized region, thus potentially accelerated the degradation, or mitigate the distribution resulting in an enhancement of thermal cycling based on their Tg and CTE properties. The edge-bond applied WLCSPs were thermally cycled from 0oC to 100oC with 10min. dwell time, and to simulate the function temperature environment, components were also tested at elevated temperature cycles for comparison. Using an analysis on localized distribution of recrystallized areas inside the solder joint provide more information on the localized microstructure evolution during thermal cycling. The results show that the crack propagation distribution and recrystallization region correlation can explain the enhancement and potential degradation mechanisms and support the damage accumulation history in a more efficient way.