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Darkened SMT Capacitor or Resistor Terminations After LeadFree Soldering Reflow

Question: Lately some of our terminations on SMT caps and resistors look dark, almost purple, like they have been overheated during the reflow process in a lead free environment. According to everything (e.g.,.Data Sheets, mole profiles, oven temps), we are not overheating any of these components. I am wondering if anyone else has had this particular condition occur?

Answer: I’ve not heard of this condition, but I would suspect the initial plating on the component terminations. If this coating is oxidized by going through the surface mount process, it could change the color of the terminations.

A question, however; is whether or not it is just the component termination turning color or the total solder joint including the component termination and the solder which is turning color. If it is the entire joint, I would look at the activity of the flux and whether or not the fluxes have changed and the vapors from the flux during the reflow operation are tarnishing the metal surfaces.

To fully answer this question, we would need an investigation of the component termination metallization, the type of solder alloy being used, the manufacturer of the solder and the type of flux base material being used in the solder paste. This could provide some information to proceed in the solving of that problem.