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Conformal Coat peeling

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#58510

Conformal Coat peeling | 5 April, 2009

Forum:

Has anyone experience conformal coat peeling after HASS and the failure mechanism being low surface energy attributed to the solder mask?

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#58526

Conformal Coat peeling | 7 April, 2009

Most of the conformal coating manufacturers recommend verifying the compatibility of their coating with the solder mask, so it's no surprise that occassionally there will be such issues.

Are you sure your conformal coat is completely cured?

If your subjecting your boards to HASS exposures prior to the full cure - and that can be as long as 7 days for some conformal coatings - you will cause delamination because the coating has not yet acheived it's full adhesion strength to the mask. This would be particularly evident in situations where the coated surface is characterized by low surface energy.

I'd suggest you verify that you are fully curing per the manufacturers data sheet and that your boards are truly clean and dry (hours in a recirculating air oven) before coating.

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#58527

Conformal Coat peeling | 7 April, 2009

We doubt that HASS has much to do with your conformal coat peeling. Sure, poor wicking [low surface energy] could cause the issue that you see. Here are links to a couple of good [no formulas or differential equations] articles on surface energy: * http://www.visionmark.com/pdfs/adhesion.pdf

* http://www.porex.com/by_function/by_function_wicking/interfacial_surface.cfm

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#58552

Conformal Coat peeling | 9 April, 2009

We aware of the extend time for cure, and in the CA activity we tried plasma etch and expose the sample to the current process, a screen test of 4 cycles -50C to 125C and the CC held in place.

Std IPC tape test passed in current process and modified process. We mention this because it was our std test prior to screen testing.

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#58553

Conformal Coat peeling | 9 April, 2009

We have different Solder mask 'gloss' and 'satin' the gloss has a surface energy =40 dynes. The test evidence to date is strong correlation between surface energy, mask type, HASS failure

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#58562

Conformal Coat peeling | 9 April, 2009

seen it many times before when the resist is not cured at manufacturing. get PCB man to check with K type thermal couple that boards ARE in fact cured. this is normally 155C ON board temperture not oven setting Cheers Greg

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#58563

Conformal Coat peeling | 9 April, 2009

Interesting, A north American Solder resist firm stated the same thing to a degree. They said altering the resist cure profile can effectively change the surface energy thus making the surface more conducive to conformal coat adhesion.

At in coming inspection is there a another method to verify the resist cure besides the the surface energy method?

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#58564

Conformal Coat peeling | 9 April, 2009

Hello, another SME is saying the solder resist cure level is suspect. I have had a industry SME make reference to this too. Adjusting the cure of the resist changes the surface energy

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#58566

Conformal Coat peeling | 10 April, 2009

On cure checking, we have this undated note from Jack Crawford at IPC

* IPC-SM-840C "Qualification and Performance of Permanent Solder Mask" has several Test Methods for different types of solder resist material, but probably the only applicable one TM 2.3.23 addresses the use of methylene choride in addition to other environmentally unfriendly materials. I have some notes pencilled in my working copy that suggests that this TM will be removed in the next revision. The "work-a-round" we have is to suggest consulting the manufacturer of the solder resist you are using and ask them to provide you with the best method to test the cure. (I'm not sure if that is like the fox guarding the hen house or not.) [J Crawford]

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#58596

Conformal Coat peeling | 14 April, 2009

Sorry been away for a while. It is actually due to high boiling solvents or plasticizers leaching from the porous resist that kills the cure as it puddles out and has nothing to adhere to. We do have some wipes to check the cure and are looking to market them in a pen format IF enough companies were interested in actually using them for incoming QC checks. It would be a very simple operation. It is not Methylene Chloride as most resist are designed to withstand this solvent systems now. Cheers Greg BLT

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