Kester - Tin-copper based solder options for lead-free assembly

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By Peter Biocca, Senior Market Development Engineer, Kester

As the transition to lead-free progresses a substantial percentage of assemblers have either implemented less costly solder alloys or are investigating them. Tin-copper solder by itself without dopants has limitations however the addition of certain elements helps out in the deficiencies normally seen with tin-copper. This paper discusses several options and the advantages they offer when compared with SAC based solders. It compares tin-copper based solders with SAC305 and describes results being obtained by large assemblers.

Tin-silver-copper has received much publicity in recent years as the lead-free solder of choice. The IPC Solder Value Product Council SAC305 (Sn96.5 Ag3.0 Cu0.5) was endorsed by as the preferred option for SMT assembly, and most assemblers have transitioned to this alloy for their solder paste requirements. However, due to the 3.0% silver content, the SAC305 is expensive when compared to traditional Sn63Pb37. For this reason, many wave assemblers are opting for less costly options, such as tin-copper based solders, for their wave, selective and dip tinning operations.

In recent years, tin-copper based solders with a variety of elemental dopants have emerged and have improved the overall properties and performance of tin-copper solders. Tin-copper solder without the incremental additions of certain elements is rarely used, but the addition of nickel or nickel and bismuth (as found K100 and K100LD, respectively) do offer improvements in wetting, joint cosmetics and in some cases solder joint reliability. K100LD is a Kester patent pending alloy. Figure 1. Comparative solder button showing different surface finishes.

Cost advantage of SnCu based solders
The approximate cost of solder alloys is indicated in Table 1, showing clearly that filling wave solder pots is less expe...

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