Investigation of BGA crack issue in normal production line

Abstract

Recently, a serious crack issue for a commercial BGA has taken place frequently in the normal PCBA assembly line, which has lead to abnormal failure rate spikes in the first pass yield trend chart, and caused line down issue already. Both cross section and Dye & Pry has been carried out to find out the fracture mode in the suspected pin of the failed unit. The cross section result shows that both BGA solder separation and PCB prepreg separation occurs there. Dye & Pry reveals that the crack pins mainly gather at corner “D” of this BGA, and 100% dye is presented there. In order to find out the root cause of this case, a series of destructive experiments have been carried out. Firstly, shear test of the raw BGA solder joint are performed. It reveals that there is no obvious difference for the solder strength between the BGA balls at the four corners and those in the center area, which indicates that the BGA raw material is not related to the root cause of this case. Secondly, in order to identify the point at which the ball crack are starting in production line, samples including bare PCB and assembled PCBA have been collected at different work station in both SMT and product build line, which will all act as the experiment specimens for a series of cross section and Dye & Pry of next step. The results show that the FCT (Functional Circuit Test) after SMT line is one of the suspected stations where the solder crack initiation will take place. After that, Strain gauge has been performed to check in both these locations, and the results show that the strain value is out of spec. Finally, some idle probes under BGA in FCT is suggested to be the root cause to trigger solder crack initiation, and both cross section and Dye & Pry result proves that the BGA crack issue can be solved after removing those probes.