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.