A Novel SnBiInZn Medium-Entropy Alloy Solder with Low-Melting Point for 3D Electronic Packaging

To meet the demands of 3D electronic packaging, a novel Sn₃₂Bi₃₂In₃₁Zn₅ (at.%) medium-entropy alloy (MEA) solder with a melting point of 85.88°C was developed. Its wetting angle on Cu was about 48.6° and 41.5° after reflow at 130°C and 160°C for 90 s, respectively. The ultimate tensile strength and total elongation of SnBiInZn MEA solder were 35.08 MPa and 107.90%, respectively, revealing its excellent ductility. The SnBiInZn solder consisted of needle-shaped Zn-rich phases and irregularly shaped Sn- and Bi-rich phases embedded in the InBi matrix. A very thin layer of Cu₅Zn₈ intermetallic compounds (IMCs) with 0.38 μm thickness was generated at the SnBiInZn/Cu interface after reflow at 160°C for 5 min. The interfacial IMC growth was controlled by interfacial reaction with a relatively low activation energy (13.12 kJ/mol). However, the IMC growth rate was very slow, which could be attributed to the high configuration entropy of SnBiInZn MEA solder. The SnBiInZn MEA solder has great application prospects in the low-temperature soldering process.