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Issues With BGA Rework Residue
Board Talk
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TranscriptPhil 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. |
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