Issues Mixing Silicone and Acrylic Conformal Coatings

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.

In fact there is different type of conformal coating base on silicon. Elastomeric silicone RTV1, copolymere silicone alkid(resin) or copolymere silicon acryl(resin). if it was applied first an elastomeric one, no chance to apply an acrylic one, you will not have adhesion. if it was applied first copolymer silicon alkid, in this case depending your acrylic conformal coating, you can have an adhesion, to be checked if it was applied first a copolymer silicon acryl, in this case you can apply an acrylic one.

In conclusion it really depend of the compatibility between the first one and the second, only in one case you can be sure to have the good solution. However it not resolv the temperature resistance which was maybe requested.

Silicones are primarily used for thermal resistance, flexibility, and a T sub g that is higher so that it can be used on assemblies used in higher temps. Also, silicone provides shock and vibration resistance which is critical for space and defense applications. I doubt the ability of the acrylic to bond to the silicone sufficiently. Contact me if you’d like more info; I have decades as a nonmetallic materials engineer.

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.