Ask the Experts
January 7, 2019
Cleanliness After Coating Removal
We use polyurethane coating. Rework on acoated assembly happens often. I know how to remove the coating and clean therework site but how do you verify cleanliness of the rework site prior to re-coating?
Expert Panel Responses
There are different waysto remove polyurethane coatings as you know.
The chemical methodrequires rinsing to remove the stripping chemistryPowder abrasion willleave dry powder particles on the board which need to be blown of with air orthe board rinsed.
I believe that there aresystems available that can measure contamination on specific parts of a circuitboard but this will generally only measure NaCl contamination as per the IPCcleanliness spec.This would not tell youif there were still stripping chemistry present.I think the answer is rinse well with a knownclean rinse solution recommended for the specific stripping chemistry.
- Use of specific stripping chemicals.
- Dry powder micro abrasion.
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
This is a situation whereyou have to prove to yourself (and to your customer) that your rework cleaningprocess is "qualified and reproducible". The reason isthat you are not going to be able to quantitatively verify the cleaningof the reworked site before re-coating.
So, you'll need to spend some time with a number of test boards, where you willcoat a small section, then strip that small section completely, then"repair them" with the same solder that is used in your reworkprocess, then follow a prescribed, documented, cleaning process that can bedeemed "reproducible", on that test assembly.
Afterwards,perform on that test assembly whatever normal quantitative process you performon your normal "uncoated" boards. You should do this on asampling, 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 followthat cleaning process and not short cut it. Buen suerte!
Rick Perkins is a chemical engineer with more than 33 years of Materials & Processes experience. He has worked with Honeywell Aerospace in high-reliability manufacturing, as well as with several oil-field manufacturing companies. He also has a good understanding of environmental, health, and safety regulations.
Cleanlinesshere is a big concern, since urethane coatings normally require highly alkalinestrippers to remove. The residues from these strippers are difficult to cleaneffectively. Rework of small areas followed by bulk cleanliness testing (e.g.ROSE) is of course undesirable, since the contamination in the small area isaveraged over the entire assembly area. So you are left with "spot" cleanlinessassessment techniques. I won't mention any by name, but there is at least onepiece of equipment that will do an ion chromatograpy-based analysis on a smallarea.
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
Typically, depending on the type of CCA and the type of coating,after the coating is removed for rework and the rework is completed, anisolated or localized cleaning process can be used to flood and vacuum thereworked area using an ESD-safe vacuum. First mask off the area, strip thecoating, flux and rework, then flood the reworked area with hot de-ionizedwater (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 vacuumingthe area to remove all of the visible moisture. Then the CCA can be baked foran hour, typically at 105 deg. C, to remove any leftover entrapped moisture.The CCA should be inspected under 20X magnification minimum for any visibleflux residues. If none are seen, the coating can then be touched up andre-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 typicallycannot put each reworked, coated CCA into the ionograph or omegameter to checkthe cleanliness, as the IPA in these ROSE testers may affect the coating on therest of the CCA.
The answer to this is that you can qualify the localizedcleaning process by implication using same or similarscrap CCAs that are coated. Look at the history of the rework you have had toperform on scrap CCAs, and identify the two or three most typical reworks.Perform the stripping, rework, and localized cleaning on components on thescrap CCAs just as you would on your documented rework processes which I amsure you have in place ;-), and then you can perform either SIR testing (usingSIR test coupons with component attached), Ion Chromatography or the ROSEtesting (or more than one of these, or all) on these samples to prove that yourlocalized wash process will remove all of the flux residues.
After that, it isa good idea to perform this test on a single rework site using a solder couponor scrap CCA perhaps every 6 months or so with different operators, to validatethat your localized wash process is still getting the CCA clean. Data provingyou are getting the scrap CCAs clean indicates your production CCAs are cleanalso, by implication. It also satisfies the requirement listed within J-STD-001section 12.3 and 8.3. Read it, and good luck.
Richard D. Stadem
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
My first question to you will be what are you using to removethe polyurethane and second, what flux are you using on the rework and how areyou cleaning the area now. Barring all these questions at the moment and eventhough you are concentrating on rework the criteria defined in IPC-A-610 shouldstill apply. Cleanliness acceptability requirements defined in IPC-A-610address the following subjects;
Also stated is "Every production facility should have astandard based on how much of each type of contaminate can be tolerated."IPC-A-610 goes on further to say, "Testing a contaminant forfunctional effects is to be performed under conditions of the expected workingenvironment for the equipment." In your situation this last statement canapply to your conformal coat process. A properly cleaned sight of flux residues is important to theconformal coating process.
Flux related contamination issues on conformal coating can leadto;
- Flux Residues
- Particulate Matter
- Chlorides, Carbonates and White Residues
- Surface Appearance
Back to referencing the importance of my questions above I will bestanding by to review and assist you with your current cleaning process. If youwish to discuss this further please do not hesitate to contact me off line.
- Activators that are insoluble remain as localized pockets ofconducting material in the coating layer poisoning the coating's curemechanism.
- Some flux residues can have an adverse effect on the curedcoating properties.
- Investigation of compatibility between coating and flux(paste, liquid flux, wire) is very important.
Technical Expert Sales Support
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
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.
Eric has been in the electronics industry for over 14 years and manages the C3 technical user group, Failure Analysis project management, Rescue Cleaning Division and is one of three Lead Investigators at Foresite.