Authored By:
David Hillman, Ross Wilcoxon, Tim Pearson, Paul McKenna
Rockwell Collins
IA, USA
Summary
The 2005 Restriction of Hazardous Substances (RoHS) European Union Directive has significantly reduced the use of tin/lead surface finishes for component terminations. A reexamination of solder joint wetting of lead-free solders with the new component terminations is key to establishing thermal process profiles that ensure acceptable solder joint integrity. One example of how the interactions between solder and surface finish can affect solder joint integrity is the potential formation of brittle gold/tin intermetallic compounds (IMCs) when the solder gold content exceeds 5% by weight. Controlling the component gold plating thickness as well as the soldering process temperature and time can prevent gold embrittlement.
For decades, the electronics industry has relied on the results published by W.G. Bader in 1969 to relate process temperature and time to surface finish dissolution rates in defining acceptable soldering process profiles for tin/lead surface finishes. Because comparable data for lead-free solders are not available, testing was conducted to update Bader’s results to address the needs for today’s lead-free soldering applications. A series of soldering dissolution experiments, modeled on Bader’s, were conducted using SAC305 solder with gold, silver, palladium, platinum, copper and nickel samples. This paper describes the test approach for measuring the solder dissolution of these materials and reports dissolution rates that can be used to optimize soldering process profiles for electronic assemblies.
Conclusions
The dissolution rates of gold, silver, copper, palladium, nickel and platinum for a SAC305 solder alloy were determined over a range of temperatures. These values can be used by solder process engineers to create adequate, consistent, solder process temperatures and dwell times that do not impact the integrity of component or printed circuit board surface finishes. The element dissolution rates may also be useful in the avoidance of creating IMC phases such as gold/tin or palladium/tin which would impact solder joint integrity.
Initially Published in the SMTA Proceedings
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