The Zn accumulation behavior, phase evolution and void formation in Sn-xZn/Cu systems by considering trace Zn: a combined experimental and theoretical study

The strong effects of Zn addition on the reaction in Sn/Cu systems are widely known. Nevertheless, the micro-mechanism for Zn accumulation behavior, phase evolution and void formation in Sn-xZn/Cu systems remain unclear. In this work, the effect of Zn addition on the interface behavior is investigated. Structures of Sn-xZn/Cu are prepared by the reaction between Sn-xZn solders (x = 0, 0.2, 0.5 and 0.8 wt.%) and the electroplated Cu (EP Cu) joints. During the thermal aging of Sn/EP Cu joints, many voids formed inside the Cu3Sn phase, especially close to the Cu3Sn/EP Cu interface. Whereas the voids were greatly suppressed with Zn addition. During the thermal aging, Zn gradually accumulated near the interface of IMCs/EP Cu. As thermal aging time to168 h and Zn content to 0.8 wt.%, Zn participated in the interface reaction, three layers with different contrasts, (Cu, Zn)6Sn5, Cu6(Sn, Zn)5 and (Cu,Sn)3Zn were formed. Theoretical analysis shows that Zn atom accumulated near the interface of IMCs/EP Cu due to lower diffusion energy barriers (Edf) of Zn than Cu atom. The preferential occupation of Zn atoms in Cu6(Zn,Sn)5 and (Cu,Zn)6Sn5 are Sn3 (4e) and Cu3 (4a) site because of the lowest lattice strain. The diffusion path interstice size for Cu atom via the IMCs layer gradually shrink with the increment of charge transfer in Zn-rich layers, corresponding the Edf of Cu atom gradually rise. As a result, the fluxes of Cu and Sn via the IMCs layer were balanced out, which reduced the Kirkendall voids formation.