Undissolved gold in fine-pitch BGA solder joint under thermal cycling test

Crack propagation has been observed on the bulk solder area of the reworked fine-pitch ball grid array component with the solder joint type SAC305 (Sn97.18 Ag2.0 Cu0.75 Ni0.07) post 550 cycles thermal cycling test (TCT). Four variables were analyzed; control sample PT0, rework sample RT0, and two samples being exposed to the TCT at 550 cycles: sample without rework PT550 and sample with rework RT550. Nondestructive and destructive tests such as 3D X-ray, dye and pull test, cross-section, optical inspection, field emission scanning electron microscopy, and energy dispersive X-ray analysis (EDX) have been complemented with strain-energy density simulation results to further understand the symptoms. EDX analysis results show that up to 12.04 percent, by weight (wt%) gold content is present in the intermetallic compound particles along the crack propagation area in the bulk solder joint’s area due to the undissolved gold. Strain-energy density up to 3.04 Pa to the solder joints causing thermal fatigue crack as observed in the 3D X-ray, dye and pull test, and cross-section sample.