Board Talk is presented by ITM Consulting
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
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And welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. We are speaking to you from high atop Mount Rialto at ITM Global Headquarters.
Jim, today our question is from P.C. Is it possible to scrub the pads of a PCB to bring it back into compliance? 63-37 solder was mistakenly used to hand solder parts. I assume on a board that is supposed to be lead-free.
We have seen this scenario before. You are doing primary ops, in this case reflow and maybe through-hole soldering with lead-free. Somebody put out the wrong material for the manual soldering operation. We have seen it happen with flux. We have seen it happen with solder.
What the reader is asking here is, is it possible to scrub the pads of the board to bring it back into compliance. Jim, to me scrubbed implies a physical abrasive methodology, but what pops into your mind on that?
That is possible, grind it down to the copper pad underneath. First of all, let's think about the technical things. You have a lead-free board. I assume you are trying to meet the RoHS and other regulations which say that every distinct solder joint and so forth has to be less than 1/10th of 1% lead.
You have just created a joint and a coating on one or more pads on the board that is now approximately 37% lead. We want to be strict. Is it really practical to get that down to 1/10th of a percent lead in that specific joint?
I just don't know, Phil. Maybe if you could grind it down. This is unusual. We are doing forward compatibility. We are trying to get what is now a tin-lead joint into a lead-free board.
For years we have been dealing mostly with the opposite trying to get backward compatibility, getting lead-free parts into a tin-lead assembly. But the traditional things on leads and components and so forth is strip off the solder through a molten immersion dilution process, such as you have the leads on a QFN or the wrong alloy.
You dip those leads several times into a pot of the desired alloy and the undesired alloy dissolves away and you are left with the proper alloy on those leads. I guess you could do that on a pad with heating it up and putting lead-free solder on it then wicking it off and putting some more lead-free solder and wicking it off.
Two things, one it is a dilution effect, so you are never going to get all of the lead off and could you get it down to 1/10th of a percent? I don't know.
The other factor of course is reheating up that joint multiple times is going to build up the intermetallic layer on there. That is not going to lend itself to a good, long-term reliable solder joint.
I also have to wonder the effect of what surface finish is being used. For example, with ENIG we know it is very, very difficult to remove any of the solder from like a gold finish. Boy, this is a tough one.
The answer may be, no, you just can't do it. And that is scary because the implication is that you have to throw the whole board away.
I am trying to imagine what the reader originally suggested, scrubbing. Somebody sitting there with a grinder on a drummel tool and sitting there trying to bring down the copper but not going too far.
I guess the methodology we talked about is worth trying but you may be chasing your tail on this.
Using a soldering iron though, I really question getting down to 1/10th of a percent with a dilution.
I think you really have to grind it down to do that and still leave enough copper on there, not going to be fun. A real challenge one way or another. Good luck P.C.
Yeah, really because it may come down to scrapping or selling the board to some third world country, where they don't care about ROHS. Somebody made a big whoops.
In the future, follow our advice and don't solder like my brother.
Phil And please don't solder like my brother.