How To Strip Tin-Lead Solder From SMT Pads for RoHS

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How To Strip Tin-Lead Solder From SMT Pads for RoHS

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.
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
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
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.

Transcript

Phil

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?

Jim

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.

Phil

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.

Jim

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.

Phil

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.

Jim

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.

Phil

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.

Jim

In the future, follow our advice and don't solder like my brother.

Phil

And please don't solder like my brother.

Comments

Good read. I agree, dilution is very effective, and perhaps under few circumstances, it is possible to get down to 1/10th percent of lead. But the discussion becomes worthless in complex circuit boards or RF boards.

Pankaj Shrotre, Sierra

Assuming that we are only talking about a few hand-soldered components, I have some very good news for you! It is entirely possible and practical to bring the boards into full compliance! Dennis Cote is absolutely correct, dilution and removal using only Pb-free solder is extremely effective, and usually it only needs to be done but once, if wicked off very thoroughly.

It depends on the geometry of the solder joints, some larger PTH SJs may require you do it twice, but you can check this by taking just ONE CCA, remove the leaded solder from all of the hand-soldered parts using UNFLUXED copper solder braid (also called solder wick), add a tacky (not liquid) flux of the chemistry called out on the BOM for the assembly (no-clean, RMA, or Water soluble) to the wick first, then wick as much solder from both the board pad or PTH AND as much from the component leads as possible.

Now here is some more good news; you can leave the parts right where they are, you do not need to remove them, at least not on the trial CCA. Instead, using a different solder iron or at least a different tip, flux and solder the parts with the correct solder. Clean the CCA thoroughly, blow dry, and bake for 20 minutes at 105C. Then perform XrF testing on as many of the solder joints as is practical, using a sample rate of at least 25% of the solder joints that were reworked.

If the Pb content in the reworked SJs is less than .1% per the XrF, you have a process! Use tacky flux as it will work much better for this than liquid flux will, is easier to control, and cleans much more completely than liquid. Liquid flux tends to crystallize and or seep under pads, under the SMT parts, etc. and does not stick around long enough to get all of the solder wicked off.

In this particular scenario, never, ever attempt to mechanically scrape or use a Dremel (not Drummal) tool, because in this scenario all of your ESD-sensitive components are connected directly or indirectly to the hand soldered pads/holes, and the ESD charges will quickly destroy or damage many of the remaining parts on the CCA. As for the common fallacy that the IMF will "grow thicker" with each solder reflow, that is false. IMF growth with any given alloy is a very self-limiting process. Once formed, any additional soldering will only increase the IMF in the order of fractions of a nanometer.

I have seen parts reworked on pads and PTHs as many as three times, and justification for doing so on avionics, on nuclear systems, on Space Shuttle and satellite CCAs by qualification using shock, vibration, thermal cycling, pull testing, etc, has proven over and over again there is very little effect on long-term reliability, provided there is sufficient copper thickness (quarter ounce or better) to begin with. After XrF on your sample CCA(s), run them through qualification testing along with a good (never reworked) CCA as proof.

Richard Stadem, Analog Technologies Corp.

Dilution is very effective. If you wick the solder off the pad you have very little solder volume left, at 37% lead. If you then add new lead free solder to the pad, and the volume of the new solder is 10 times the volume on the cleaned pad, then the percent of lead drops by a factor 10 to 3.7%. Repeat this sequence again and it drops to 0.37%, and one more time gets you to 0.037%, well below 0.1%. Furhtermore, the ratio of the volume of solder on the cleaned pad to a pad covered with new lead free solder is likely greater than 10 to 1, so the dilution will probably happen faster, and three clean and resolder steps should reduce the lead content well below the 0.1% limit.

Dennis Cote, Harding Instruments

Because of the build up on intermetallic when using the dilution solution. This will make the removal of lead from a 37/63 joint almost impossible using that method.

Using a quick part removal reheat and then grinding the metal (as long as the grinding does not generate heat) will allow for the coating, intermetallic and copper to be removed, and as long as there is still a thin layer of copper left, copper deposition can be used to rebuild the pure copper thickness on the scrubbed PWB. This would be a rework by the PWB manufacturer as they have the copper deposition capability and not something that an OEM or PWB assembly plant could do.

Tony, Schlumberger