Influence of composition variations on microstructure and properties of SnBiInAl low-melting-point medium-entropy solder alloys

With the rapid development of three-dimensional integrated packaging technology, higher requirements have been put forward for the performance and reliability of low-temperature lead-free solders. Based on the concept of medium-entropy alloy, four SnBiInAl low-melting-point medium-entropy solder alloys with different compositions were designed and prepared. The results show that the peak melting temperatures of Sn₃₄Bi₃₀In₃₁Al₅ (at.%) and Sn₄₂Bi₂₂In₃₁Al₅ solder alloys are relatively low, reaching 90.5 ℃ and 87.5 ℃, respectively. The peak melting temperatures of Sn₅₅Bi₃₃In₉Al₃ and Sn₅₆Bi₃₀In₉Al₅ solder alloys are relatively higher, being 132.0 ℃ and 128.6 ℃, respectively. Their wetting angles at 180 ℃ are 42.5° and 41.2°, respectively, which meet the requirements of wettability for solders. The Sn₄₂Bi₂₂In₃₁Al₅ solder alloy is composed of Sn-rich phase and Bi₃In₅ phase, while the other three solder alloys are all composed of Sn-rich phase, Bi-rich phase, and InBi phase. Among the four SnBiInAl medium-entropy solders, the Sn₅₆Bi₃₀In₉Al₅ solder alloy exhibits the highest ultimate tensile strength of 67 MPa, but its total elongation after fracture is relatively low, which is about 24%. This is associated with the presence of the large Bi-rich phases in the alloy and the solid solution of a considerable amount of Al elements in each constituent phase. The Sn₅₅Bi₃₃In₉Al₃ solder alloy possesses both relatively high ultimate tensile strength (60 MPa) and total elongation after fracture (40%).