A study of crack propagation in Pb-free solder joints under drop impact

The higher stiffness of Pb-free SAC solders makes Pb-free assemblies more sensitive to drop impact. In order to be able to optimise the drop test performance, it is necessary to have better insight into the crack propagation in the Pb-free solder joints. This study combines on-line resistance measurements of a solder joint during drop testing and high speed bend testing, failure analysis of the assembly with dye-and-pry method and with cross-sections , and electrical FE simulation. The result is a fingerprint of the crack propagation during consecutive loading cycles. The carrier in the study is BGA´s, a critical component family for drop impact. Combinations of solder alloys and pad finishes, SnPb on OSP, SAC305 on ENIG, SAC101 on OSP and SAC101(d) on ENIG are studied regarding the failure mode and crack propagation. This study demonstrates that for the large majority of Pb-free solder joints, there is a negligible initiation period; cracks start forming at the first PCB bending cycle. The presence of large cracks, especially at both sides can increase the compliance of the joint and slow down crack growth. Even if large cracks are present, the resistance increase is less than 1 mOmega per interconnect, which is far from the 100Omega that is often taken as a failure criterion. Brittle joints as found with SAC305 on ENIG have erratic propagation rates while ductile joints are much more predictable. Therefore, the way to optimise the drop test performance of a Pb-free BGA assembly, is to prolong the crack propagation within the ductile solder material.