Coupling effect in Pt/Sn/Cu sandwich solder joint structures

The interaction between Sn/Cu and Sn/Pt interfacial reactions in Pt/Sn/Cu sandwich joint structures was studied. We found the interfacial Sn/Pt reaction to be greatly influenced by the opposite Sn/Cu reaction. The PtSn₄ interfacial compound formation rate was very sluggish compared with that of the single Sn/Pt reaction case. On the other hand, the growth rate of the Cu₆Sn₅ compound at the Sn/Cu interface was not affected by the opposite Sn/Pt reaction, which has a rate similar to that of the single Sn/Cu reaction case. However, the morphology of the Cu₆Sn₅ grains was different than in the single Sn/Cu reaction case (i.e. it had the conventional scallop-type shape). In the sandwich case, the Cu₆Sn₅ grains had a column-like appearance. The column-like morphology of the Cu₆Sn₅ grains is due to the small interfacial energy, γ_{solder / Cu6 Sn5}, caused by the dissolution of Pt from the molten solder. Also, we found that the Pt dissolution would also cause a reduction in the solubility of Cu in the molten solder. The above two parameter changes lead to a diminishing of the ripening flux among Cu₆Sn₅ grains. Hence, smaller Cu₆Sn₅ grains would not be depleted and the separation distance between Cu₆Sn₅ grains would not be widened.