I'm not an expert but I would have thought the exact opposite - that using lead free solder on a leaded component would be OK, but that using lead solder on a lead free component might be a problem.

My reasoning is that lead free solder melts at a higher temperature than lead solder. Therefore the lead that is used as a coating on the leaded components will definitely melt and "wet" the joint but if lead solder is used, then the lead free coating used on lead free components might not "melt" and a poor joint might result. We've done some testing using lead free solder paste on leaded surface mount parts and didn't see any problem (other than having to use a higher reflow temperature) but these were not exhaustive tests - just some eperimenting to check the new reflow oven temperature profile. Did your experts tell you why soldering leaded parts with lead free solder was a disaster?

Our company is not taking any chances and switching to 100% lead free in June - both the parts and the solder. Not because we want to but because we have to in order to continue to ship into Europe. Whether this is actually better for the environment or worse is still being hotly debated in the industry. The military is not switching until the reliability issue is proven and I would rather fly on airplanes using components and assembly processes using a proven technology by a company with years of experience.

Regards,

Bert