SnBiIn

SnBiIn refers to a class of solder alloys based on tin with minor additions of bismuth and indium. These ternary alloys are developed for low-temperature soldering in electronics and for applications where heat-sensitive components must be protected during assembly or rework. The term SnBiIn covers several compositions whose exact ratios vary, but all share tin as the dominant element and rely on Bi and In to tailor melting behavior and wetting.

The melting behavior is a key characteristic: SnBiIn alloys are designed to exhibit low melting points relative to traditional lead-containing solders, with melting ranges or single melting points typically in the vicinity of about 110–180°C depending on composition. Some formulations are near-eutectic, providing sharp melting, while off-eutectic variants melt over a small temperature interval.

In joints, tin-rich SnBiIn alloys can form typical tin-based intermetallics with copper pads, such as Cu6Sn5 and Cu3Sn, which influence reliability. Indium additions generally improve ductility and wetting on certain finishes, while bismuth can reduce melting temperature but may promote embrittlement if present in higher fractions. Oxidation and aging behavior vary with composition and processing environment.

Applications include rework and repair of temperature-sensitive assemblies, low-temperature reflow processes, and some specialty interconnects where conventional lead-free solders would cause thermal stress. They are often used where rapid cooling and controlled heat input are critical.

Limitations include potentially reduced mechanical strength and cycling reliability, sensitivity to oxidation, and cost considerations, particularly for indium-containing alloys. Proper flux choice and temperature profiles are essential for reliable joints.