The role of elastic and plastic anisotropy of Sn on microstructure and damage evolution in lead-free solder joints

The elastic, thermal expansion, and plastic anisotropy of Sn is examined to assess how anisotropy affects the microstructural evolution and damage nucleation processes in SAC305 solder joints. Examination of all joints in a package indicates that upon solidification, crystal orientations are nearly randomly distributed. Initial studies of cracked joints after thermal cycling showed that orientations with the c-axis parallel to the joint interface (red orientations) are more likely to crack arising from tensile stresses during the hot part of the cycle. Subsequent studies show that package design has a large influence on how the microstructure evolves; higher strain designs stimulate recrystallization at earlier times. Recrystallization appears to be strongly correlated with crack nucleation and propagation processes, as red orientations often develop and lead to crack nucleation and propagation. The details of the recrystallization process depend strongly on the plastic slip and recovery processes arising from the specific crystal orientation/temperature/strain history that makes microstructural evolution of each joint unique. The unique history for each joint implies that worst case scenarios need to be identified and models developed that can predict microstructural evolution that leads to worst case scenarios.