Cross-interaction study of Cu/Sn/Pd and Ni/Sn/Pd sandwich solder joint structures

Cross-interactions between Cu/Sn/Pd and Ni/Sn/Pd sandwich structures were investigated in this work. For the Cu/Sn/Pd case, the growth behavior and morphology of the interfacial (Pd,Cu)Sn ₄ compound layer was very similar to that of the single Pd/Sn interfacial reaction. This indicates that the growth of the (Pd,Cu)Sn ₄ layer at the Sn/Pd interface would not be affected by the opposite Cu/Sn interfacial reaction. We can conclude that there is no cross-interaction effect between the two interfacial reactions in the Cu/Sn/Pd sandwich structure. For the Ni/Sn/Pd case, we observed that: (1) after 300 s of reflow time, the (Pd,Ni)Sn ₄ compound heterogeneously nucleated on the Ni ₃Sn ₄ compound layer at the Sn/Ni interface; (2) the growth of the interfacial PdSn ₄ compound layer was greatly suppressed by the formation of the (Pd,Ni)Sn ₄ compound at the Sn/Ni interface. We believe that this suppression of PdSn ₄ growth is caused by heterogeneous nucleation of the (Pd,Ni)Sn ₄ compound in the Ni ₃Sn ₄ compound layer, which decreases the free energy of the entire sandwich reaction system. The difference in the chemical potential of Pd in the PdSn ₄ phase at the Pd/Sn interface and in the (Pd,Ni)Sn ₄ phase at the Sn/Ni interface is the driving force for the Pd atomic flux across the molten Sn. The diffusion of Ni into the ternary (Pd,Ni)Sn ₄ compound layer controls the Pd atomic flux across the molten Sn and the growth of the ternary (Pd,Ni)Sn ₄ compound at the Sn/Ni interface.