Low-temperature alloys remain essential for practical applications, particularly within the semiconductor industry, where the demand for high-performance materials continues to grow. Bismuth-based solder alloys emerge as promising alternatives to traditional lead-based solders, which are subject to RoHS regulations due to their toxicity. These bismuth-based solders not only present environmental advantages but also exhibit excellent thermal and electrical conductivity, low toxicity, and improved mechanical properties, making them suitable for high-temperature applications, such as SiC and GaN-based power electronics that operate between 200 and 250 °C. This study explores the thermomechanical reliability of the 95Bi-5Sn solder alloy on Cu-OSP and ENIG under-bump metallurgy, with a comparative analysis against the eutectic 58Bi-42Sn solder. Microstructural characterization is conducted to assess intermetallic composition and thickness resulting from solder reflow on Cu-OSP and ENIG substrates. Additionally, hardness and modulus measurements are performed on both bulk solder and interfacial layers before and after high-temperature storage tests at 125 °C for 500 hours. Despite the favorable properties of bismuth-based solders, challenges related to wettability—especially with common substrate materials like copper and nickel—pose significant concerns. Poor wetting can lead to insufficient adhesion and compromised joint integrity, resulting in unreliable solder connections and increased failure rates. This research aims to enhance understanding of the thermomechanical behaviors of these alloys, paving the way for their wider adoption in the electronics industry through tailored property modifications and ongoing research and development efforts.
Wiegand et al. (Fri,) studied this question.