Degradation of Pb/Sn solder balls on electrolytic Ni/Au substrates as a result of post assembly heat treatments

The electrolytic Ni/Au surface finish is widely used in the electronic industry to provide both a highly solderable and wire bondable surface for BGA packages. We found that the robustness of the solder ball/substrate joint could be compromised by exposure to post attachment thermal aging such as dry-bake processing after assembly. The degree of degradation is dependent upon both the substrate Au thickness and the post assembly exposure time. We were able to identify using cross sectional SEM analysis coupled with EDX that the degradation of the solder joint was resultant from the formation of brittle Au-Sn IMC at the solder to Ni interface. This occurred even though the Au concentration in the solder joint was less than 0.7wt.%, significantly less than the 3-5wt% needed for brittle fracture in bulk materials. The brittle Au-Sn IMC was not present in the as assembled solder joint, however it began to form after only 2 hours of exposure to 125°C. The brittle IMC effectively reduces the fracture toughness of the joint. Additional thermal exposure of the package lead to further degradation of the solder toughness.