Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers Pick and Place, otherwise known as the Consultants of ITM Consulting. And today's question is a repair problem from J.T.

Jim
This question deal with mixed alloys, backward and forward compatibility. J.T. asks, "Can circuit boards assemblies originally built with lead-free solder be reworked successfully with leaded solder? I realize that the assemblies may not comply with RoHS requirements, but will they be reliable?”

Phil
With all due respect to J.T., what are the circumstances – I hate to answer a question with a question, but if you've already assembled it with lead-free and it survives, obviously it's compatible with lead-free thermal excursions, why are you using tin lead? Why does this issue come up? I mean, maybe you got a good deal on the tin-lead solder, you know, kind of surplus or something.

But, putting that aside, let's attempt to answer your question.

Jim
Well certainly you can repair it. The question is reliability. So there's two ways to look at this or two ways to approach it. You can be very scrupulous and carefully remove all of the lead-free solder from the joint or joints that you're going to rework, strip them down as close as you can to just the surface finishes, and then make the repair adding all of the necessary tin-lead solder. So you're finished with a good standard tin-lead joint. So now you have a tin-lead joint.

Now will that be more or less reliable? The answer is maybe. Because, as we know from the work of Jean-Paul Clech, if you're reliability, if the real life of your product induces high stresses on your product, then the tin-lead joint will probably be more reliable. If your – the service life of your product induces low stresses on the joints, then the lead-free – the tin-lead joint will be less reliable than the original lead-free joint.

If you're not as scrupulous in your repair and you add tin-lead solder to some volume of the remaining lead-free solder, you get a mixed alloy. And, in our humble opinion, then you'll have no idea what the reliability will be because you don't know what the mix ratio is and so forth and there's been all kinds of varied outcomes of testing mixed alloys. So that would be the most risky situation in terms of long-term reliability.

Phil
So again, best, safest thing to do is to do the lead-free – the same alloy that it was originally soldered in, but, beyond that, there you go. Well, whatever you're doing up there on the process floor.

Jim
Don't solder like my brother.

Phil
And don't solder like my brother.