Effect of Ga addition on interfacial reactions and intermetallic evolution in low-melting-point In–34Bi–15Sn solder alloys on Cu substrates

This study developed a quaternary low-melting-point alloy solder based on the In-34Bi-15Sn- x Ga ( x = 0, 0.25, 0.5, 0.75, 1.0 wt%) system. The experiments focused on investigating the effects of Ga addition on the melting behavior, microstructure, wetting properties, interfacial reaction, and mechanical properties of the InBiSn solder. The experimental results showed that the solder's melting point decreased gradually with increasing Ga content from 0 to 1.0 wt%. Specifically, with 0.25 wt% Ga incorporation, the alloy exhibits a melting point of 57.6 °C. Ga also exerts a pronounced influence on the interfacial microstructure: the coarse rod-like and grayish-white Bi(In,Sn) 2 phase, the gray InSn phase, and the deep black Ga-rich phase. Furthermore, Ga modification reduces the solder's surface tension and substantially improves its wettability. Optimal wetting performance is achieved at 0.25 wt% Ga, yielding a contact angle of 34.6° and a spreading area of 39.4 mm 2 . Ga addition fundamentally alters the evolution pathway of the intermetallic compound (IMC) layer at the solder/Cu interface. The IMC sequence transitions from Cu 3 Sn/Cu 6 (Sn,In) 5 (in the Ga-free alloy) to Cu 3 Sn/Cu 9 Ga 4 and ultimately to Cu 9 Ga 4 /CuGa 2 as Ga content increases. Notably, an appropriate Ga dosage (0.25 wt%) effectively suppresses the formation of the brittle Cu 6 (Sn,In) 5 phase at the interface. Under this condition, the total IMC thickness is minimized at 1.68μm. Among the Ga-added In-34Bi-15Sn solders, the mechanical properties of the joints with 0.25 wt% Ga were significantly superior to those of the other joints. Specifically, their mechanical strength reached 18.2 MPa, representing an increase of approximately 8.33% compared with that of the Ga-free joints.