On the mechanism of Sn tunnelling induced intermetallic formation between Sn-8Zn-3Bi solder alloys and Cu substrates

The interfacial reaction of Sn-8Zn-3Bi-xSb (x = 0, 0.5, 1.0, 1.5) on Cu substrate has been investigated under 150 °C thermal aging up to 720 h and an interfacial reaction mechanism, which governs the aging process, has been proposed. A 1 μm thin layer of scallop-shaped ε-CuZn5 intermetallic compounds (IMCs) and a 4 μm thin layer of planar γ-Cu5Zn8 IMCs were formed at the interface after solder reflow. During initial aging, ε-CuZn5 transformed to γ-Cu5Zn8 and the intermetallic layer (IML) at the interface thickened. As aging prolonged, Cu5Zn8 started to degrade due to the depletion of Zn and voids formed throughout the IML. These voids were rapidly filled by diffusing Sn atoms, inducing a tunnelling effect, allowing Sn to reach the Cu substrate and form Cu6Sn5 IMCs at the interface. The thickening of the IML is controlled by a diffusion mechanism and this is suppressed by introducing Sb as a micro-alloying component, hypothetically though the pinning effect to reduce the diffusion rate of Zn in Sn matrix.