An innovative high-entropy alloy solder for high-reliability low-temperature bonding in 3D electronic packaging–based on nano InSnBiZnAg particles

To address the demand for multilevel package, a low-melting-point high entropy alloy (HEA) solder was developed. It consists of 43In28Sn14Bi9Zn6Ag micro- and nanoparticles with a melting point of about 62.80 °C. We obtained 43In28Sn14Bi9Zn6Ag nanoparticles by combining ultrasonic heating interaction with a "hot shock" process. Upon reflowing at 115 °C for 5 min, a Cu/Zn/Ag ternary intermetallic compound (IMC) formed at the interface between the solder and the copper substrate, leading to a shear strength of 40 MPa. The microstructure of this joint exhibits a complex composition with at least three phases, including Bi3In5 phase, Ag5Zn8 phase, and In0.2Sn0.8 solid solution. The stable Ag5Zn8 phase not only enhances the overall strength of the alloy matrix but also impedes the diffusion of In and Bi atoms, thereby preventing the segregation of the Bi3In5 phase. The high mixing entropy and pronounced lattice distortion in this high entropy alloy hinder the diffusion of elements within the solder matrix, mitigating the issue of excessive interfacial IMC growth. Consequently, the solder joints exhibit little degradation even after a high-temperature aging test. This high-entropy alloy solder has promising potential for multi-level chip stacking in 3D packaging.