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Issues With BGA Rework Residue



Issues With BGA Rework Residue
After BGA rework there is an outline of residue on around the pads. Is it necessary to remove this residue to reinstall a new BGA? The Assembly Brothers, Phil Zarrow and Jim Hall, reflect on this question and share their own experiences.
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
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
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
Welcome to Board Talk with Phil Zarrow and Jim Hall by GM Consulting by Day but today with you, the assembly brothers. We're coming to you from ITM headquarters high atop Mount Realto.

We're here to talk about electronic assembly, materials, equipment, components, practices, procedures, mis-practices, mis-procedures and lots of other things.

Jim, what's today's question?

Jim
One of our favorite topics, BGA rework. The question comes from J.C.

After a BGA has been removed for rework, there is an outline of residue on the laminate around the pads. It does not come off easily but has to be carefully scraped away with a knife.

Is it necessary to be removing this residue to reinstall a BGA, or is it fine to leave it?

No, it is probably not fine, or at least risky, to leave that residue and think about what that residue is. It came from the original solder paste when the BGA was reflowed onto the board the first time and that residue has gone through whatever thermal cycle was required to remove that BGA.   

So those residues are baked. Perhaps polymerized depending on the temperature and how carefully the rework was controlled.

Scraping with a knife on any PCB is not fun and it's risky. Removing residues with a cleaner and a solvent and some sort of chemistry is a much better way.  

Phil
And we emphasize using the right solvent, the right chemistry. Don't go necessarily reaching for that bottle of IPA. It may not be the appropriate one.

You may be making more of a mess than you intend. You want to find out to the best of your knowledge what that original residue is or as our cleaning friends call the soil, and you want to find the right solvent chemistry to adequately treat and remove that particular residue. 

If you have trouble identifying what that original flux was, like for example it wasn't done by your process or something along those lines, consult some of your chemistry companies like Kyzen and Zestron and they can probably help you out finding the right solvent.

The other thing of course is the question selective cleaning. In other words are you going to clean in that area only or are you going to attempt to clean the whole board.

There are lot of people out there and I certainly follow the logic, that don't believe in selective cleaning because you're disturbing other things. So you might want to consider using the proper solvent.   

You could look at cleaning the entire board assuming the assembly can go through a cleaning cycle. So it does get a little bit more complicated.

I know, now you're thinking about going back and using that knife and scrape it. But you know we're here to tell you what we think best practices are based on what you described to us.  

Jim
If you know what the original paste formulation was and it truly was a no-clean, then you might have some more confidence in leaving that residue on there.

But if you don't, now you're going to introduce perhaps a different flux chemistry in your rework process, either a paste flux or another form of paste and now you get the residue from the first paste chemistry mixing with the soldering process and the ultimate residue from the second repair operation again, could be quite risky.  

Phil
So we hope we've suggested a better alternative than scraping it off.

Again as Jim said, depending on what that residue is you might be okay leaving it on, but our experience and logic dictates that you probably do want to remove it and as we're emphasizing, remove it properly.

So beyond that you've been listening to Board Talk with Phil and Jim.

Jim
And don't solder like my brother.

Comments

Any cleaning process that is not controlled is unpredictable, and not repeatable. (In other words, risky.) IPA is often an effective solvent for no-clean flux residues. But no-clean flux residues are designed to remain on the board, forming a crystalline structure that is impervious to moisture, and encapsulates (traps) any remaining, potentially reactive remnants and locks them away in a stable, amber prison. When you break down these residues with a solvent, you are unleashing chemical miscreants into solution. Then, if you don't fully contain the IPA, it will transport your partially dissolved residues all over the board.

Under your low-clearance parts, and in between fine pitch component leads. And there they will stay, waiting for rogue moisture and electric potential, when they can begin to cause corrosion, dendrites, current leakage, signal noise, or worse. Unless you can follow up with a robust rinse process, or carefully control the application of IPA (or whatever solvent you may use), you're asking for trouble. You might try putting a kim wipe on the area to be cleaned, and then applying the IPA through the wipe onto your area to be cleaned. With a little care, the excess solvent will wick / soak into the kim wipe, and not go rogue on your board.
Alan Couchman, Process Sciences, Inc.
In the past, I have had luck softening residues by heating the board to 100C. Of course make sure you are doing no harm to anything if you chose to try this.
Alan Woodford, Zentech Manufacturing
This is very good advice from Jim & Phil. Proper identification of the original solder paste chemistry is the way to go with defining a proper removal system for any residues. Likes clean likes, as I have been taught by folks much smarter than me. So, if it is an IPA-based no clean solder paste, IPA probably is the correct solvent. But most BGAs with higher I/O count required a more robust flux. So, care should be taken, and research on the original chemistry is part of that care. There are solvents for rosin-based no clean solder pastes, for example. And synthetic resin-based no cleans also could be found, requiring still another type of solvent.

If it is impossible to learn the chemistry of the original paste, I would recommend experimenting with very small amounts of solvent on the residues to get a better understanding of what dissolves them. If one is found to put the residues in solution, then you are well on the way to a chemical solution versus mechanical scraping.
Russell Claybrook, MicroCare, LLC

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