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.
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.
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.
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.
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.
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.
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.
