Microstructure and mechanical properties of Cu/In–Zn–Sn–Bi/Cu joints bonded with high-entropy alloy solder at ultra-low temperature

To meet the requirements of hierarchical packaging, this paper investigated a low-melting-point high-entropy alloy (HEA) solder. The solder was synthesized by melting pure metal blocks of In, Zn, Sn, and Bi in equimolar proportions to form HEA ingots, which were then deformed into solder foils with a thickness of 100 μm. The solder consists of Zn-rich, Bi-In, Sn-rich, and Bi-rich phases, with a melting point of approximately 82 ℃. Using this solder, Cu/HEA solder/Cu joints were fabricated. The reflow parameters were: temperature of 90–160 ℃, pressure of 0–0.08 MPa, and time of 1–80 min. Microstructural observation and elemental analyses revealed the formation of a dual-layer interface comprising pure Cu5Zn8 and mixed-Cu5Zn8 intermetallic compounds (IMC) at the solder/Cu interface, with an IMC reaction activation energy of 35.67 ± 3.4 kJ/mol. Effective bonding was achieved even at 90 ℃, with a shear strength of up to 19.6 MPa. Cracks initiated at the IMC/solder interface and propagated along grain boundaries within the solder. Moreover, significant lattice distortion was observed at the IMC/solder interface, which inhibited IMC growth and enhanced interface stability. This low-temperature HEA solder provides promising potential for applications in three-dimensional hierarchical packaging.