Soldering Defects on Immersion Tin and the Impact of the Tin-Oxide Layer

Immersion tin is a final finish highly accepted in the automotive industry and finds increasingly application in the package substrate industry. As due to the immersion reaction the plating thickness of tin is limited, there are concerns that due to the formation of the intermetallic phase during the reflow process, the solderability of the tin layer might be affected. During the assembly process and the formation of the intermetallic phase the free tin is consumed and only islands of free tin remain after the second reflow cycle. Additionally, an oxide layer is formed, and the homogeneity and thickness of the oxide layer is impacted by the reflow conditions and the overall process cleanliness. The interaction of intermetallic phase formation, the amount of remaining free tin and the tin oxide layer will define the soldering performance of the tin layer and the risk of the occurrence of soldering defects. In this paper different failure modes and connected failure mechanisms will be explained. The tin oxide layer is identified to play a crucial role in this interaction and factors are investigated, which impact the oxide layer formation and homogeneity. Various methods are presented to determine the oxide layer thickness and different approaches are studied to modify it. Results are presented that show that with a dedicated solution to modify the oxide layer the risk for solderability defects on immersion tin can be reduced.