Can We Rework Lead-Free Boards with Leaded Solder?

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 board 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 the real life of your product induces high stresses on your product, then the tin-lead joint will probably be more reliable. If the service life of your product induces low stresses on the joints, then 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. 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, safest thing to do use the same alloy as the original. 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.

I remove the lead free solder then resolder with 60/40 leaded.the lower temp can be soldered faster thus not damaging ICs.

Since the ROHS directives about disposing of lead containing scrap into landfill sites I have noticed the product life of unleaded electronics using BGA technology has shortened. This is especially evident in notebooks etc and is due to the physical differences between tin and lead. Lead based solder is more " forgiving " to expansion and contraction due to temperature changes than tin. Simply put tin is more brittle than lead and lead will last longer in heat cycling situations.

In your "board talk" concerning repairing lead-free joints with tin-lead, I believe you said, in-part, "If the board is used in a high stress environment then the tin-lead is more reliable but if its a low-stress environment then the lead free repair is more reliable".

If I heard you correctly, could you educate me briefly as to why? I would have thought the repair best suited for a high-stress environment would always be more reliable in any situation. What am I missing here?

Situation: In house repair facility for boards from various vendors. A board may bear a ROHS label, but the lead free solder alloy is not known.

What is the best rework procedure? Clean off original solder residue first, or just use any lead free alloy and not worry about the resulting mix?