|Ask the Experts|
February 5, 2020
Cleanliness After Coating Removal
We use polyurethane coating. Rework on a coated assembly happens often. I know how to remove the coating and clean the rework site but how do you verify cleanliness of the rework site prior to re-coating?
|Expert Panel Responses|
There are different ways to remove polyurethane coatings as you know.
I believe that there are systems available that can measure contamination on specific parts of a circuit board but this will generally only measure NaCl contamination as per the IPC cleanliness spec. This would not tell you if there were still stripping chemistry present.I think the answer is rinse well with a known clean rinse solution recommended for the specific stripping chemistry.
This is a situation where you have to prove to yourself (and to your customer) that your rework cleaning process is "qualified and reproducible". The reason is that you are not going to be able to quantitatively verify the cleaning of the reworked site before re-coating.
So, you'll need to spend some time with a number of test boards, where you will coat a small section, then strip that small section completely, then"repair them" with the same solder that is used in your rework process, then follow a prescribed, documented, cleaning process that can be deemed "reproducible", on that test assembly.
Afterwards,perform on that test assembly whatever normal quantitative process you perform on your normal "uncoated" boards. You should do this on a sampling, say 10 test boards, and take the average of the result as your"qualified cleaning process".
The hard part: ensuring that your rework assemblers will follow that cleaning process and not short cut it. Buen suerte!
Cleanliness here is a big concern, since urethane coatings normally require highly alkaline strippers to remove. The residues from these strippers are difficult to clean effectively.
Rework of small areas followed by bulk cleanliness testing (e.g.ROSE) is of course undesirable, since the contamination in the small area is averaged over the entire assembly area. So you are left with "spot" cleanliness assessment techniques. I won't mention any by name, but there is at least one piece of equipment that will do an ion chromatograpy-based analysis on a small area.
Typically, depending on the type of CCA and the type of coating,after the coating is removed for rework and the rework is completed, an isolated or localized cleaning process can be used to flood and vacuum the reworked area using an ESD-safe vacuum.
First mask off the area, strip the coating, flux and rework, then flood the reworked area with hot de-ionized water (or a compatible solvent if no-clean or RMA flux is used), brush clean,and vacuum the water/solvent away followed by a clean rinse, again vacuuming the area to remove all of the visible moisture.
Then the CCA can be baked for an hour, typically at 105 deg. C, to remove any leftover entrapped moisture.The CCA should be inspected under 20X magnification minimum for any visible flux residues. If none are seen, the coating can then be touched up and re-cured, followed by a final inspection.
How do you know this method will clean the CCA well enough notto leave you with any reliability or coating adhesion issues? You typically cannot put each reworked, coated CCA into the ionograph or omegameter to check the cleanliness, as the IPA in these ROSE testers may affect the coating on the rest of the CCA.
The answer to this is that you can qualify the localized cleaning process by implication using same or similar scrap CCAs that are coated. Look at the history of the rework you have had to perform on scrap CCAs, and identify the two or three most typical reworks.
Perform the stripping, rework, and localized cleaning on components on the scrap CCAs just as you would on your documented rework processes which I am sure you have in place ;-), and then you can perform either SIR testing (using SIR test coupons with component attached), Ion Chromatography or the ROSE testing (or more than one of these, or all) on these samples to prove that your localized wash process will remove all of the flux residues.
After that, it is a good idea to perform this test on a single rework site using a solder coupon or scrap CCA perhaps every 6 months or so with different operators, to validate that your localized wash process is still getting the CCA clean. Data proving you are getting the scrap CCAs clean indicates your production CCAs are clean also, by implication. It also satisfies the requirement listed within J-STD-001 section 12.3 and 8.3. Read it, and good luck.
My first question to you will be what are you using to remove the polyurethane and second, what flux are you using on the rework and how are you cleaning the area now. Barring all these questions at the moment and even though you are concentrating on rework the criteria defined in IPC-A-610 should still apply.
Cleanliness acceptability requirements defined in IPC-A-610 address the following subjects;
Flux related contamination issues on conformal coating can lead to;
Technical Expert Sales Support
The best way to determine cleanliness after a specific area rework process as you have described is with a localized extraction followed by ion chromatography testing to fully determine type and amount of residual ionics.
There was a presentation done at the High Reliability Cleaning and Coating Conference that describes a number of methods for localized extractions that may be applicable to your need.
Most of my exposure to conformal coating removal prior to rework; and the reapplication thereof, is from the avionics industry; where CC was of the "Type UV" spec.Tim Hufnell, H. F. Engineering Systems of PA Llc.
The 'removal' often leaves a residue which never totally dries; this creates a perimeter void which allows ingress of contaminants. This, in my opinion, is the most disturbing aspect of the R+R process of the repair. Even agents with no methylene chloride are sometimes subject to this phenomenon.
Cleansing with an approved surfactant, and DeIonized H20 (rinse until acceptable ion neutral rinse-water measurement is attained, and 75° 'drying bake" while time consuming, is the gold standard for components under CC rework, imho.
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