Accelerated thermal cycling: is it different for lead-free solder?

Accelerated test parameters such as extreme temperatures, temperature range, temperature ramps and dwell times have a major influence on deformation mechanisms in solder joints. With the increasing push for the introduction of lead-free solders, there is a need to revisit the accelerated thermal cycling regimes, as the solder composition can also influence the deformation mechanisms in solder joints. Time independent plastic deformation due to dislocations leading to slip bands and time dependent creep deformation due to diffusional flow of vacancies leading to grain boundary sliding are the two primary deformation mechanisms in solder joints. It is recognized, for example, that the lead-free solders creep 10 to 100 times slower than tin-lead solders under a given stress state. This paper examines the inelastic deformation mechanisms in lead and lead-free solders in BGA packages. Accelerated thermal cycling guidelines have been developed keeping in perspective the field-use conditions. The accelerated thermal cycles developed mimic the solder deformation mechanisms as in the field-use conditions and also reduce the time and the cost associated with accelerated testing. The finite-element models developed in this work are validated using experimental thermal cycling data and Moire interferometry data.