The Effect of Cooling Rate on Grain Orientation and Misorientation Microstructure of SAC105 Solder Joints Before and After Impact Drop Tests

The effect of different cooling rates on as-assembled Sn1.0Ag0.5Cu (wt.%) solder joints was investigated by using orientation imaging microscopy to characterize evolution of the microstructure and orientation distribution on test samples before and after shock testing. Evolution of the microstructure of joints located near the corners after shock testing differed substantially for samples cooled at different rates after fabrication. After shock and impact testing, much recrystallization was observed for the rapidly cooled samples; this led to polycrystalline microstructures that were effective in absorbing impact energy, by incorporating a growing crack into the recrystallized tin microstructure rather than the lower-energy intermetallic interface, and thus prolonging life. The slowly cooled samples contained large amounts of (301)[(1) over bar 03] mechanical twins, which also led to an increased number of interfaces that were effective in absorbing energy. The smallest amount of new interface generation after shock testing occurred in the normal cooling rate microstructures, which had the shortest life. Analysis of the crack-propagation paths associated with different cooling rates indicates that development of interfaces from either twinning or polycrystalline microstructure favors crack propagation through the solder rather than the intermetallic layer interface, which toughens the joint.