What is the best method for cleaning bare PCBs with small amounts of oxidation? I know that proper storage and handling may help. Jim Hall and Phil Zarrow, The Assembly Brothers, share their suggestions for this scenario. 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 50 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
Welcome to Board Talk with Phil Zarrow and Jim Hall and Joe Belmonte of ITM Consulting coming to you today from high atop Mount Rialto. Today we have a board question.
Jim
This question comes from H.M. What would be the best method or product for cleaning bare PCBs that have a small amount of oxidation? I know that proper storage and handling may solve the problem, but what would you suggest?
Jim
The first thing is are you sure we're talking about oxidation? Because oxidation's one thing. If it's contamination, dirt, soils or something else, then any good cleaning product, aqueous-based or solvent-based should remove it. If it's truly oxidation, then we have to talk about the particular surface finish. If it's OSP, it can be stripped off, the copper re-cleaned and you can put new OSP down. So that's a possibility. What about other surface finishes, such as immersion silver, ENIG or HASL?
Phil
With ENIG, I don't think you have to worry too much about oxidation. It's one of the beauties of paying for gold, but the others including immersion silver are different. I remember years ago a number of the solder companies used to have chemicals for exactly that, for cleaning up the boards prior to assembly.
Nowadays, besides consulting your solder company, I would also talk to one of the companies that makes cleaning agents, for example, Kyzen or Zestron. But, as our brother, Joe, will point out, we obviously can't recommend a specific product name.
Joe
I'd also recommended you contact your printed circuit board supplier to get their recommendations. Although it says in the question that they understand the importance of handling and storage, you have to find the root cause for this contamination. As we've mentioned in other sessions, finding the root cause of any problem is where you're major effort should be.
One of the things we recommend to all customers is that in handling, whoever handles these boards, once they're out of the package, has finger protection, whether it's a glove or finger cots. That's a world-class practice. When you go to Asia, you see it all the time. Other regions of the world, not so much, but again, a big source of contamination on any board is from human handling. So you want to minimize it, or make sure that hands are protected.
Jim
The implication of the statement here is that these boards may not have been handled correctly. If that's true, the other question you should be asking is what about moisture absorption within these boards.
Should they be baked and if you have an oxidation problem, is that going to make it even worse, should you bake the pcbs in nitrogen? If you've have boards that have been stored improperly for too long, you have to look at the total construction before you use them.
Joe
And one last comment. If they're immersion tin or immersion silver, hot air solder level, whatever, you want to make sure the board was processed properly from your supplier. Is the right thickness of material on those boards? How did the supplier store them and handle them prior to shipping to you? Do you need them packaged individually?
You have to look at all these questions because this is one of the things that should not be a problem in any facility. It is not a common problem. Is there something specific in your supply chain up and to the point the boards are being assembled that's causing this issue? Spend your efforts to find out what that is and correct it.
Phil
So I think we've handled this one. And if you'd like us to mention your product on the air, please write the name of your product on the back of a $100 dollar bill and mail it to Board Talk. Beyond that, this is Phil Zarrow, Jim Hall and Joe Belmonte saying ...
Jim
Don't solder like my brother.
Phil
And don't solder like my brother.
Comments
One of the best ways to remove slight oxidation from HASL-finished bare PWBs and slightly tarnished immersion silver finished PWBs is to use a good-quality plasma cleaner. It can be used to not only remove oxidation of the traces and pads, but it can also be set up to clean the surface of the PWB substrate also. If set up properly, a good plasma cleaning process can improve solder wetting as well as improve subsequent conformal coating adhesion. This is especially true of PWBs that have a lead-free HASL application, which can oxidize much more rapidly than leaded HASL.
Plasma cleaning is also especially effective for removing tarnish on immersion silver, and does not harm the finish itself. Look at it this way; plasma cleaning of wire bonding pads is pretty much a standard process today, therefore why not use it to improve the wetting of conventional SMT and PTH components as well? My experience has been that plasma cleaning is a great tool to have, but you need to qualify the various applications very carefully and very thoroughly.
Richard Stadem, Analog Technologies Corp.
Just went thru this with a supplier in regards to potassium contamination with an ENIG finish. The potassium in the presence of oxygen was corroding the Copper and Nickle edges and porous areas causing a green white powdery corrosion. We ended up using baking soda with a soft cotton tack rag to remove it. Finished with a DI rinse, bake, gold XRF measurement and ionic contamination testing.
Schuyler "Craig" Williams, Lockheed Martin
I would assume that H.M.'s question is not merely theoretical, but was prompted by a visual observation, perhaps some discoloration, that they interpreted as "a small amount of oxidation." As Joe pointed out, root cause failure analysis is indicated if they want to prevent recurrence, and to gauge the seriousness of the problem. H.M. might want to consider ROSE testing (or C3 for localized extraction) if they suspect there may be surface contamination. Alternatively, EDS analysis of a discolored area can tell them what's present elementally, and may reveal whether they're dealing with a plating or surface finish problem, or if there's some environmental contributor.
Alan Couchman, Process Sciences, Inc.
There are two reasons why an OSP coated copper will oxidize.
1)It has exceeded its shelf life
2)OSP coating film may be too thin.
Copper metal is practically insoluble in sulfuric Acid, Copper Oxide is. Re-microetch cleaning may hurt PTH, so cleaning them in Dil. Sulfuric will remove the oxides and then the panel may be re-coated with OSP. Same with immersion silver. If the Ag is too thin it can be re-plated with proper steps. Only thing is that Ag thickness may exceed max thickness spec. Shelf Life and proper storage are of course very important. How hot does your product get and how long does it stay in the truck while it is being shipped to a tropical climate destination?
Lee Desai, Aurora Circuits Inc., USA
Hot Air Level (HAL) solder can be re-coated if the PCB form factor permits re-entry into the HAL machine. Immersion Tin can easily be stripped then re-coated. Ditto for organic coatings (OSP). Arguably, Immersion Silver cannot be easily stripped/re-coated. And ENIG should never see oxidation (unless gold has lost it's status as a noble metal).
