Low melting point solders based on Sn, Bi, and In elements

In the big data era, Si chips are integrated more and more to satisfy the fast growing demand from customers. In addition, in the post-COVID-19 virus era, the trend of distance teaching and home office has increased greatly the need of advanced consumer electronic products. To provide more processing tolerance and to build a wider temperature window in manufacturing, it becomes necessary to develop low melting temperature solders because a hierarchy of solders is needed in the packaging technology. Much research has been on Pb-free and Sn-based solders with a melting point from 180 to 230 °C. In this review, we will concentrate on low melting point solder alloys with a melting point lower than 180 °C and even below 100 °C. We review eutectic SnBi, eutectic SnIn, and the alloys with trace addition of a third element to them. Eutectic Sn-Bi solder is too brittle, and Sn-In solder is too soft. However, third element addition can only improve the solder properties partially; thus, we will further review the properties of Sn-Bi-In ternary alloys and the effect on the addition of a fourth element to them. This approach, when we include the Cu substrate, becomes a 5-element system, which leads us to consider high entropy alloys for soldering. With these Sn-, Bi-, and In-based alloys, we hope to develop new ways to design industrial applicable low melting point solders.