![]() In addition to these alloy challenges, entirely new flux systems need to be developed to address the unique properties of bismuth-bearing alloys. These additives also impact alloy ductility (reduced brittleness), depending on the amount incorporated. Antimony will also improve strength but can significantly increase melting temperature, while nickel will suppress brittle intermetallic formation at the joint interface. Other elements incorporated are copper, which slightly reduces the melting temperature and improves mechanical performance. Historically, silver has been used with SnBi to improve strength and is a common SnBi addition. Incorporating additional elements into a SnBi system can improve mechanical and thermal performance but can increase melting temperature, thus negating the primary reason for adopting low-temperature materials, or even adversely impacting processing characteristics. However, reducing bismuth content significantly increases the pasty/plastic range of the SnBi alloy, potentially impacting both process capability and product reliability. The brittleness imparted by bismuth can be reduced by increasing the ratio of tin to bismuth from the eutectic Sn42Bi58. Low-temperature alloys usually refer to alloys with peak reflow requirements lower than 190☌, with typical SnBi-based materials having peak reflow requirements of 170° to 190☌. Even with these limitations, SnBi alloys can be adopted for use in SMT and through-hole, but the main benefits are derived in surface-mount assemblies. ![]() Minor element additions and micro-alloy elements can improve the performance of SnBi alloys, but, in general, they will retain the properties of their main constituents and lack the reliability and performance of their SAC-based relatives. Bismuth alloys exhibit poorer mechanical and thermal fatigue performance than SAC-based materials. Unfortunately, high-bismuth alloys have a number of disadvantages compared with the tin/silver/copper alloys currently in use. Other advantages of low-temperature soldering include the incorporation of lower-cost plastics, component and laminate materials, and reduced energy consumption and related environmental benefits.Īs a practical matter, SnBi alloys are the only elements available to reduce peak reflow temperatures. NWO defects are difficult to detect and may not manifest until after a product is in the field. When a component deforms during reflow, the solder interconnect may be compromised, resulting in non-wet opens (NWO). Chip suppliers are particularly interested in lower reflow temperatures, as thinner components are needed to meet dimensional limitations of thinner, smaller and faster devices. The most technically significant is reduced warping of component and substrates. Several forces are driving implementation of solders with lower peak reflow temperatures than SAC 305 and its variants. LOW-TEMPERATURE SOLDERING is a subject of considerable interest and development. With no standard in sight, emerging alloys require unique fluxes and processes.
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