With no standard in sight, emerging alloys require unique fluxes and processes.
Low-temperature soldering is a subject of considerable interest and development. Several forces are driving implementation of solders with lower peak reflow temperatures than SAC 305 and its variants. The most technically significant is reduced warping of component and substrates. Chip suppliers are particularly interested in lower reflow temperatures, as thinner components are needed to meet dimensional limitations of thinner, smaller and faster devices. When a component deforms during reflow, the solder interconnect may be compromised, resulting in non-wet opens (NWO). NWO defects are difficult to detect and may not manifest until after a product is in the field. 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.
As a practical matter, SnBi alloys are the only elements available to reduce peak reflow temperatures. Unfortunately, high-bismuth alloys have a number of disadvantages compared with the tin/silver/copper alloys currently in use. Bismuth alloys exhibit poorer mechanical and thermal fatigue performance than SAC-based materials. 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. 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.
Low-temperature alloys usually refer to alloys with peak reflow requirements lower than 190°C, with typical SnBi-based materials having peak reflow requirements of 170° to 190°C. The brittleness imparted by bismuth can be reduced by increasing the ratio of tin to bismuth from the eutectic Sn42Bi58. However, reducing bismuth content significantly increases the pasty/plastic range of the SnBi alloy, potentially impacting both process capability and product reliability. 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. Historically, silver has been used with SnBi to improve strength and is a common SnBi addition. Other elements incorporated are copper, which slightly reduces the melting temperature and improves mechanical performance. Antimony will also improve strength but can significantly increase melting temperature, while nickel will suppress brittle intermetallic formation at the joint interface. These additives also impact alloy ductility (reduced brittleness), depending on the amount incorporated. In addition to these alloy challenges, entirely new flux systems need to be developed to address the unique properties of bismuth-bearing alloys. Not only do these alloys have different mechanical and thermal properties, their soldering characteristics and requirements can be quite different from the SAC alloys they replace.
In addition to the alloy element variables, the new alloys must be compatible with other materials on the PCB. The effects of surface finish, component tinning and other soldered surfaces are yet to be clearly defined. Solder suppliers are promoting many low-temperature alloy options, and a “standard” has yet to emerge. With all these input variables, constituent elements, additive elements, and their amount and effect on solder alloy performance, a single low-temperature alloy is unlikely to meet all application requirements.
SAC alloys are successful substitutes – albeit with considerable expense and disruption – for most electronics applications. Now, with nearly 20 years of history, most industries are comfortable with the SAC alloy system performance and requirements. As the electronics world continues to evolve, lower temperature alloys are the next frontier of solder interconnect materials. Some industries can adopt low-temperature systems relatively easily with minimal risk, with consumer and disposable applications likely to be early adopters due to cost advantages. Other industries will watch these developments carefully to understand the hidden risks and requirements. The question for most users is can or should I use low temperature on my assemblies? The answer is complicated by the number of available options, the risk/reward equation and the resources required in investigating and developing a viable low-temperature assembly process. We say it often, but let your solder supplier help guide you.