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February 2004

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 24 Feb 2004 08:50:18 +0200
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Doesn't sound like a good idea to me. I'm sure that adhesion will be a
BIG problem. RTV, apart from being a silicone, cures on the outside skin
first, working down to the middle of the "blob". The polymerisation is a
condensation reaction, working on humidity being drawn into it. This can
take weeks or months before it is complete. It won't stop because of the
coating, just slowed down, so you will be pumping humidity through your
coating for a long time, just at a place where little or no bonding can
occur between the coating and what is under it. Get the picture?

I appreciate the need to fix the components, but why RTV? If you use a
correctly mixed and cured epoxy, you will have no problems.

I must admit that I'm "allergic" to silicones of any nature being used
for anything in electronics, where they can be avoided. They are never a
good protection and they are generally quite fragile. Their only
usefulness, IMHO, is for high-temperature insulation, provided they are
supported with, e.g., glass fibres.

Brian

Mark Charlton wrote:

> My learned and esteemed colleagues,
>
> One of our customers has requested that we conformal coat over a DOW 3145
> RTV adhesive used to stake some sensitive components.  The specified coating
> material is MS470-N, a urethane product.
>
> Is this a standard practice (to coat over RTV)?  We have always conformal
> coated the PCA and then staked the components using the RTV.  Our CC vendor
> isn't too pleased with this request.
>
> Mark Charlton
>
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