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Issues Mixing Silicone and Acrylic Conformal Coatings



Issues Mixing Silicone and Acrylic Conformal Coatings
Our repair PWA assemblies have a silicone based conformal coating. During repair some of the coating is removed. Is there be a compatibility problem with the coatings? The Assembly Brothers, Phil Zarrow and Jim Hall, answer these questions.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
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.

Transcript


Phil
And welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting, the Assembly Brothers. Today we are coming to you from the ITM conformal coatings closet.

We are here to talk about electronic assembly, materials, equipment, components, practices, procedures, among other things. Jim, what is today's question.

Jim
Well, today's question comes from B.H. It is about conformal coating.

Phil
I knew there was a reason we were in this room today.

Jim
We do a lot of repair work on assembled PWAs. These assemblies have a silicone based conformal coating. During repair some of the coating is removed.

After repair our tech uses an acrylic base conformal coat to touch up the area. Could there be a compatibility problem, particularly with adhesion, when we mix these two coatings?

Phil
I would say there is more of a fundamental problem than just the physical and chemical dynamics of it. The question is why was silicone used in that application in the first place?

With that in mind, is acrylic sufficient, appropriate, ample based on the application? They are very different materials, for very different applications.

Jim
Well, I don't know if I would go that far. But the fundamental thing is that all conformal coatings are different, and have different properties. And we all know that silicones are typically more expensive and more difficult to apply.

If it came with silicone, it was there for a reason. Somebody spent some extra money to put silicone on there. And the one that jumps out of course, is higher temperature reasons.

This is one of the things that silicones excel at. Although yes, there could be a compatibility issue, I think it is much more fundamental than that. Even if acrylic coating doesn't have adhesion or compatibility problems, is it going to protect that circuit in its expected functioning life?

Temperatures is one thing, there is chemical resistance, and other things that are different between these materials. Particularly when you have a higher priced material, you know that was put on there for a reason. Somebody spent some extra money to put silicone on that board.

Phil
Yeah, it couldn't be just because they liked the salesman. Absolutely check that it is application appropriate. As my brother said, there was a reason they went with the expensive stuff.

Jim
And then your compatibility questions go away.

Phil
Right. So I believe we answered your question. Not exactly what you wanted to hear because we threw a major red flag down.

Jim
But that is important, not all conformal coatings were created equal.

Phil
No, like a lot of other things. So great, you have been listening to Board Talk with Jim Hall and Phil Zarrow of ITM Consulting. Board Talk melts in your mind, not in your ears.

And whatever you are doing underneath that conformal coating, however you have soldered those joints, whatever you do please don't solder like my brother.

Jim
And please, don't solder like my brother.

Comments

I agree with Phil and Jim, there is a reason for the which coating is used. I believe IPC would say this is a call for Customer contract review. I bet you would need permission, (and I would get that in writing) to use an alternative coating.
Patty S., BitFlow, Inc

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