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Why Uneven Conformal Coating?

Why Uneven Conformal Coating?
We are experiencing an issue with our conformal coating. On matte finish boards the coating is vibrating away from some others and building up in others. The Assembly Brothers, Phil Zarrow and Jim Hall, reflect on this and share their own advice.
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.


And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers, who by day go as ITM Consulting. We are here at Board Talk to help you solve your various process problems and dilemmas concerning components, materials, equipment, procedures, and all sorts of things.

Jim, today's question is with regard to conformal coating. R.H. writes, we are experiencing an issue with our conformal coating process. We chemically wash prior to coating and spray the material in two coats.

On matte finish boards the conformal coating is vibrating away from some others and building up in others. We are getting coverage, but this causing our boards to look like a topographical map with hills and valleys.

We don't have an issue with adhesion just uneven finish. Any help on this would be greatly appreciated.

Well, fortunately my brother is a world-known expert in conformal coating. What do you think Phil?

No, I'm not.

I think there are a lot of unknowns in this particular case with regard to materials. It could be a number of things. It could be the way your equipment is set-up, the way you are spraying it.

I think most likely it is the material itself, the way you are prepping the material. If it has been mixed properly, if it is stored properly, if it is within expiration date. What kind of humidity has it been exposed to? If it has been titrated properly.

So there is a lot of gray area.

Phil, they say we chemically wash prior to coating. Could there be some residues from the washing process?

Yeah, depending what kind of chemicals you are using, there could be residues that way. I am assuming the boards are dry.

Really the best place to defer this question is to the supplier of your material. It is their materials that is acting kind of funny.

I would advise you to contact your conformal coating manufacturer and your applications people and explain what is going on. Perhaps they can give you some insight.

Better than us?

Yeah, so called expert.

We hope that helps R.H., sends you in the right direction. Definitely talk to the manufacturer of the conformal coating. Who knows, maybe you have a bad batch.

You didn't say where you are doing this operation, and I am not just talking facility wise with humidity. That is obviously going to affect it.

This sounds obtuse, but we have seen it happen, make sure you have the real material. There are certain countries abroad where we have seen cases of counterfeit materials. They were packaged under the names of well-known, recognized, validated conformal coating manufacturers.

Sometimes when they mix them up, with the open air, things can get contaminated, overexposed to humidity and things like that. Make sure you have good, pure material. Counterfeiting does extend to materials, as well as components. Been there, done that, and own the stained t-shirt.

I hope that sets R.H. in the right direction.

Remember, whether your board is going to be conformally coated or not when you go to solder, please don't solder like my brother.

And don't solder like my brother.


Assuming that the coating is not adulterated, what you may be seeing is dewetting. Areas of the board do not take coating evenly if the surface energy is variable. This can be due to a residue or it can be how the solder mask is cured. I recommend checking the surface energy using dyne pens. If you have surface energies greater than 40 dynes/cm, then you should not see dewetting issues.
Doug Pauls, Rockwell Collins
I encountered this problem in the past. Using a one part epoxy with an oven cure. There was insufficient ventilation in the oven. Epoxy vapors would build up and when additional cold, freshly coated boards were placed in the oven, the excess vapors would condense and wash the coating off of the tops of the components leaving a dull finish on the component tops and excess epoxy below. Additional oven ventalation cured the problem.
Thomas Mealey, Virtual Industries, Inc.
It sounds to be like what you are witnessing is capillary flow, which is material being sucked beneath components hence why you get a build up around components and thin spots where the material is robbed from.

It mentions that the users is applying the material in 2 coating layers, which would normally be the recommendation. It might be worth experimenting by applying a thin 'dusting' as the first coat, and then building up the thickness in 2 or 3 subsequent coating layers, allowing the material to reach a tack-free stage between each coat, to avoid solvent entrapment. As the brothers sugegst, call your supplier, they should be able to help, but it may be that the design of your board will require some compromise - i.e. to get coverage you might need to accept a more uneven finish. Coverage is key to performance.
Phil Kinner, Electrolube Ltd

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