A study of crack propagation in Pb-free solder joints

The higher stiffness of Pb-free SAC solders makes Pb-free assemblies more sensitive to drop impact. In order to be able to optimize the drop test performance, it is necessary to have better insight into the crack propagation in the Pb-free solder joints. This study combines crack-front mapping using the dye and pry method and electrical FE simulation to establish a relation between DC electrical resistance and cracked area, and hence monitor the initiation and propagation of cracks in individual solder joints as the PCB assemblies are subjected to JEDEC type mechanical shock and high speed cyclic bending. The carrier in the study is a ball grid array (BGA), a critical component family for drop impact. Combinations of solder alloys and pad finishes, SnPb on organic solderable preservative (OSP), SAC305 on electroless nickel/immersion gold (ENIG), SAC101 on OSP, and SAC101(d) on ENIG are studied regarding the failure mode and crack propagation. This paper demonstrates that, for the large majority of Pb-free solder joints, there is a negligible initiation period; cracks can 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 mΩ per interconnect, which is far from the 100 Ω 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 optimize the drop test performance of a Pb-free BGA assembly is to prolong the crack propagation within the ductile bulk solder material.