Low-Temperature Attach Today and Tomorrow

For more than a decade, tens to hundreds of millions of dollars have been invested in research on tin-bismuth-based, low-temperature lead-free solders. At conferences like SMTAI, a considerable fraction of all of the papers have been on this topic. This situation raises the questions: what is the current status of these low-temperature solders, how do they perform in thermal cycle and drop shock experiments, who is using them, and what is their experience? This paper will summarize the research on these low-temperature solders and attempt to predict their future applications and implementations. The paper will also review low-temperature current alternatives to tin-bismuth solders that have performance similar to SAC solder. One solder uses supercooled SAC305 that has the potential of soldering at well below 170°C. Metastable solder pastes present an alternative solution to low-temperature soldering, allowing the ability to create full metal interconnects at much lower temperatures than traditional solders of the same alloy compositions. Lower temperature interconnects enable next-generation electronics where material, geometry, and performance would degrade at higher processing temperatures, such as in additive printed electronics. This paper presents progress-to-date using supercooled solder pastes of core-shell bismuth-tin (BiSn) particles to accomplish solder interconnects below the melting point of the alloy. Reflow of the supercooled BiSn paste is achieved in an inline reflow oven with a peak temperature of 125°C, which is 40°C lower than what is found in a typical reflow profile of the alloy. Cross-sectional images demonstrate sufficient intermetallic compound growth having taken place at 125°C. Comparisons between supercooled BiSn solder pastes and traditional BiSn pastes are discussed and progress towards process development and application guidelines are presented.