The microstructure and mechanical properties of TLP bonding with (SnBiInZn)100-xGax high entropy alloys

In this experiment, (SnBiInZn)_100-xGa_x (x = 0.5, 1, 2, 4, at.%) high entropy alloys (HEAs) were prepared in a vacuum furnace, and Cu/(SnBiInZn)_100-xGa_x/Cu solder joints for sandwich construction were prepared by transient liquid phase (TLP) bonding at 160 °C. The effects of different Ga content and bonding time on the interface intermetallic compound (IMC) layer and mechanical properties of the joints were investigated. The results showed that when the Ga content was lower than 1 at.%, Ga atoms were almost completely solid-soluted inside the microstructure and did not affect the strength of the joint. Exceeding this critical level led to Ga being firmly incorporated into the IMC layer's surface, impairing the interfacial bonding and triggering an abrupt drop in shear strength. Specifically, when Ga was 0.5 at.%, the IMC layer at the interface thickened progressively with longer bonding durations. After 30 min, the CuZn₅ IMC fully transformed into the stable Cu₅Zn₈ phase, resulting in a shear strength of up to 31.7 MPa. The fracture surface exhibited large honeycomb-like dimples, indicative of a ductile fracture. Successful formation of high-quality joints under low-temperature bonding conditions has been realized. However, when the bonding time was excessively prolonged, the Cu₅Zn₈ IMC layer grew too thick. Due to the significant difference in Cu and Zn diffusion speeds, Kirkendall voids and cracks formed, ultimately degrading the shear strength of the joint. The soldered joints tended to fracture along the Kirkendall voids at the interface, and the fracture mechanism shifted from ductile to brittle.