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Mixing conformal coating and RTV

jdumont

#33826

Mixing conformal coating and RTV | 21 April, 2005

Morning all. So today I have been tasked with conformally coating a few of our first run PCBs for a new product line. The problem is that they have already had all the larger caps potted with RTV. Are there any problems with spraying the CC over the RTV? In the future we will be RTV'ing as the last step....

Thanks in advance JD

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#33828

Mixing conformal coating and RTV | 21 April, 2005

We understand the problem of trying to coat over RTV. We have seen the same problem with acrylic and urethane coatings not wanting to stick to RTV. Some acrylics seem to be worse than urethane.

Since this is a new product, may be you could pot your caps with something other than RTV. * RTV is about the worst kind of coating there is. Its chemistry is highly ionic and it requires atmospheric humidity absorbed to "complete" its polymerization. * Many types depend even on strong acetic acid to work. * Also, ammonia + water = ammonium hydroxide, an aggressive alkali. * Furthermore, although it is water repellent, it is the most humidity-porous of all coatings, some 20 times worse than epoxy coatings. So, if you have any hydroscopic contaminant under the RTV, it will get nicely damp and stay damp, just ideal for creating corrosion, especially if it were ionic in nature, as could be with a stoichiometric or incompletely cured RTV [B Ellis].

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jdumont

#33831

Mixing conformal coating and RTV | 21 April, 2005

Well that doesnt sound very promising. What im going to do is make sure these two boards I just coated (with the RTV already cured) will be used in house only. In the future we will do any conformal coating operations before potting with whatever material we end up using. What would you recommend using for capacitor potting other than RTV?

Thanks a lot!

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#33839

Mixing conformal coating and RTV | 21 April, 2005

We prefer epoxy.

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#33849

Mixing conformal coating and RTV | 22 April, 2005

Common RTV does out-gas causing corrosion. NASA allows use of non/low-out-gassing RTV. Try the following companies: * Ablestik Electronic: Materials, 20021 Susana Road, Rancho Dominguez, California 90221 U.S.A., 310-764.4600, F 310-764-2545 * Arlon Inc., Silicone Technologies Division, 1100 Governor Lea Road Bear, Delaware 19701, 800-635-9333 F 302-834-4021 * General Electric Company, 260 Hudson River Road, Waterford, NY 12188, Technical Assistance, 800-255-8886, Customer Service, 800-332-3390 * 3M Company, Industrial Materials, Department, 3M Center, Saint Paul, MN 55144, Gen. 800 364-3577, Tech 612-736-0315 * TRA-CON, Inc., 45 Wiggins Ave, Bedford, MA 01730 USA, 781-275-6363, f 781-275-9249

Regardless of outgassing or not, RTV is silicone rubber and is unlikely to accept being conformally coated.

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#33959

Mixing conformal coating and RTV | 28 April, 2005

If you apply the RTV after coating, what happens to bond strength since you are now bonding to the coating and not the board? Will the caps still survive vibration and shock? I specify applying RTV before coating for this reason. Am I correct?

I also like the idea of getting away from RTV for this application, but it is difficult since we have been using it "forever" and nobody can think of any bad experiences.

As for the comment about acidic properties of RTV during cure, you should only be using electronic grade RTV, like Dow Corning 3145. It is safe for electronic components. If the RTV you are using has a strong vinegar smell, you have the wrong type.

Bob

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