I don't know. We never had craziness like this before lead-free.
You should have the yellowish-metal analyzed to be sure.
Guessing: You're seeing this color on the terminations of components, not on the bulk of the SnX solder connection. If that's the case, here's what's going on, probably: * Bulk solder and component termination are blistering hot * High soldering temperature has melted the metal solderability protection on the component termination * Surface tension of the bulk solder has pulled the component termination solderability protection into the bulk solder, leaving behind the exposed stainless steel or nickel barrier * Thin nickel oxide films form on the component terminations that change color according to temperature or exposure time during soldering
BR ... davef
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