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September 10, 2018

Mixing Silicone and Acrylic Conformal Coatings

We do a lot of repair work on assembled PWA's. These assemblies have a silicone based conformal coating. During repair some of the coating is removed. After repair our techs use an acrylic based conformal coat to touch-up the area. Could there be a compatibility problem, particularly with adhesion, when we mix these two coatings?

B.H.

Experts Comments

There can be issues with compatibility of silicone coatings with most other generic types. Most coatings don't stick well to the silicone especially if they are not fully cured and can lead to poor adhesion, de-wetting and other defects.   

It also depends on the conformal coating itself since some coatings claim to be silicones but actually have a small % of silicone blended with another generic resin. This type of coating acts very similarly to acrylics and urethanes and would generally have a better adhesion.
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Lee Hitchens
Owner
SCH Technologies
Lee has worked within the conformal coating and electronics industry for over 18 years. His work includes scientific research into long term reliability of electronics, technical sales of conformal coating materials and equipment, owner of SCH Technologies, a conformal coating service in the UK, a member of the Diamond Coating Solutions Group, a global liquid conformal coating and Parylene coating service solutions provider, a founding member of Nexus3c, Conformal Coating Centre and a partner of Thin Film Partners.
Is there a compatibility problem? O yes. Acrylics and silicones do not mix well. It is going to be difficult to fully remove all the silicone material and silicone residues as silicones are very chemically resistant. I would expect you to see dewetting occurring during acrylic application and I would expect poor adhesion in thermal cycling when it returns to service. 

Most repair methods involve overlapping the repair coating onto the existing silicone coating. Acrylics don't like to adhere to silicones, so you have a second issue there. The larger question is whether your customer accepts the patch coating in lieu of the original coating. Presumably, silicone was chosen for a reason. 

Does acrylic work as well as the silicone for the end use environment? In my view, unless authorized, you are not returning the assembly to the condition specified by the engineering drawing.  In my own view, acrylics do not protect the substrate as well as silicones, so acrylics are a step back in my opinion.
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Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
It is not a good idea to mix different conformal coating technologies on the same PCB. Here's why:  

In general, there are four main classes of conformal coatings: silicone, acrylic, urethane, and epoxy. Each type of coating has different strengths and weaknesses, and that coating was chosen when the board was made for a specific reason.

For example, silicones usually are chosen when the PCB will be expected to endure extreme temperatures; silicone has remarkable ability to handle high temperatures. Acrylics are the easiest to remove and rework; urethanes tend to be less expensive while epoxies are the most durable. So by mixing a different coating with the existing one, you probably will not match the underlying performance objective that was desired when the PCB was designed.  

There's another reason not to mix-and-match, as well. These different materials each behave differently. They have different levels of adhesion, different ability to flow into tight spaces (viscosity), different chemical resistance, and different thermal properties. Once you mix-and-match, you are guestimating that your new answer will be "close enough" to not make a difference, and this generally is not a prudent assumption.

For example, suppose you apply an acrylic coating to a PCB that will go back into service controlling a system in a chemical factory. There may be fumes in that factory that would attack the acrylic and eventually eliminate the protection the conformal coating was supposed to provide. Or, if temperature swings are an issue, the acrylic will eventually crack and peel while the silicone would be undamaged.   

There are excellent companies making good benchtop conformal coatings in all four "flavors." Your techs should have the ones they need on the bench so they can very closely match the original performance with the coating after the repair. You also should have the proper cleaning chemistries on the benchtop to remove these  coatings.

For example, its almost impossible to remove a silicone coating with alcohol, but MicroCare has several cleaners that will dissolve silicone coatings almost instantly, saving time and improving the quality of the repair. As is so often the case, getting the right tools will result in a better job in the end.
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Mike Jones
Vice President
Micro Care
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
In a word, "yes." Adhesion of the acrylic to the silicone material, or to any residues left after removal of same, will be poor. Best practice is to use the same material as the original, and where that is not possible, to qualify a touch-up material that is compatible with the original and meets the desired performance specifications. For touch-up of silicone coatings, that would almost certainly need to be another silicone coating, in my opinion.
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Fritz Byle
Process Engineer
Astronautics
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.
The mixing of the two conformal coatings is not ideal and if there is a way to avoid using the acrylic and doing the repair with a silicone based product, I would do that.  A one-part, methoxy-based, silicone that cures with humidity might be sufficient as long as the assembly is allowed 2 to 4 days open to a high humidity environment.

The repair using the acrylic coating will not bond well with the silicone, unless some type of primer is used, but this is simply not good manufacturing technique.  I do not recommend the repair with the acrylic conformal coating.
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Rick Perkins
President
Chem Logic
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.
The main problem is that cc's of different chemistries should not be considered interchangeable. If someone used silicone to begin with most likely was for a reason and not just for personal preference.

Ignoring the coating specificity I would recommend to recoat silicone coatings only with silicone coatings, just as coating acrylics only with acrylics and so on. Otherwise it may be unintentionally helping his repair service since the new repaired parts will require repair soon.
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Wayne Wagner
President
Krayden Inc.
Wayne Wagner has over 25 years in the conformal coating industry and is the president of Krayden Inc., a leading distributor of engineered materials.
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