Indium Corporation - Choosing a Low-Cost Alternative to SAC Alloys for PCB Assembly

Our thanks to Indium Corporation for allowing us to reprint the following article.
By Brook Sandy and Ronald C. Lasky, PhD, PE., Indium Corporation

Abstract


Developing low-cost alternatives to near-eutectic SAC alloys for Pb-free assembly is crucial to continue producing affordable electronics products. Metals prices, especially silver, have been on the rise, and will likely stay at their near historic high levels. Solder alloys with lower silver content have been considered with trade-offs in performance, but are there alternatives?


There are many reasons to consider alternative Pb-free alloys to SAC305. Several new alloys have been recently introduced, while others, which had little popularity in the past, are showing more potential due to changes in the industry. The question is: how much do subtle variations in alloy composition affect the performance and process requirements of PCB assembly? This paper will compare some of these alloys side-by-side and discuss whether existing processes need to be modified for alternative alloys.


Introduction


Numerous groups studied lead-free solders in the late 1990s to early 2000 timeframe in anticipation of the enactment of RoHS legislation in 2006. It was shown that tin-bismuth-based solders may not be desirable due to potential fillet lifting in wave soldering. In addition, the poor mechanical properties of tin-bismuth solders, when alloyed with even small amounts of lead, were a concern as lead might still be in component leads or PWB pad finishes. Tin-zinc solders were also rejected due to the short shelf life of such solder pastes. Hence, tin silvercopper (SAC) seemed to be the solder alloy of choice...



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