In general, most solder alloy suppliers specify that the solder contains less than 0.08wt% Cu. However, solder pots and the baths in wave soldering machines can reach higher levels of Cu, through partial dissolution of Cu-containing components.
The concern with Pb/Sn solders which have more than 0.2-0.3wt% Cu is the formation of intermetallics (Cu3Sn and Cu6Sn5). These intermetallics can cause solder bridging (sticking) on insulation materials. Also Cu will increase the melting range (liquidus) of the solder, thus making it 'sluggish'. The intermetallics will impart a gritty appearance on the solid solder surface.
Further these intermetallics are brittle, and so may be detrimental to the mechanical properties of the solder. The addition of Cu is known to accelerate tin pest, decrease the wetting ability of the solder and lower the resistivity of the solder. It is therefore recommended to keep the content of Cu below 0.2wt%.
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