Brent
Yes, I agree with you. All RTVs are not equal, but none of them are
brilliant, compared with some other materials, in terms of allowing
moisture to pass. Unfortunately, where I am today I cannot give you
exact facts and figures, but 20-odd years ago I did a series of tests
whereby I cast a number of conformal coating materials and used the
resulting diaphragm as a barrier between a moist atmospheric environment
and a dry one. The silicone ones passed moisture, if I remember
correctly, about 20 times faster that acrylic, 15 times epoxy and 10
times 2-component polyurethane (the figures are in my book). These
figures are quite significant and no-one has ever contradicted them, up
to now. It MAY be that more modern silicones are better than 20 years
ago, but there has been relatively little progress in this field.
I also agree that all conformal coatings will let moisture cross their
barrier if there are any hydrophilic molecules on the other side, but at
different rates. This is why I consider cleaning essential before
applying a coating, because all fluxes leave hydrophilic residues. I
won't argue the rare possible exceptions here, but even these lead to a
loss of electrical qualities.
I also agree that all coatings have their pros and cons, even within the
individual categories.
Many years ago, when I made SIR testers, I did a series of tests on
different coatings to allow compensated measurements up to 1E14 ohms to
be made, with FR-4 multilayer substrates. I found the acrylics were, by
far, the most stable and, just to give you an idea, I recalibrated an
instrument which had been stored in an unheated garage (in Switzerland)
for over 3 years. There were some small differences noticeable above
1E12 ohms on first switching on. After 4 hours, even these had
disappeared and the calibration was within +/-2% up to 5E13 ohms without
the smallest tweaking. I ascribe this, to some extent, to the protection
afforded by the acrylic coating. I would consider the same performance
very unlikely with a silicone-based coating of either the RTV or
reactive types.
I think the nec plus ultra of coatings is the para-xylylene
(vacuum-deposited) types (which I didn't test in the above-mentioned
series), but too expensive for many uses.
Brian
Brent Alcorn wrote:
>
> Hi Folks,
>
> I am interested in this exchange and I would like to drop my two
> cents. I am not an expert on this but I am under the following
> impressions:
>
> - Not ALL RTVs are equal,
> - ALL conformal Coatings absorb moisture at some rate,
> - There are PROs and CONs for all conformal coatings (ie. Rework, TC,
> resistance to Solvents - acrlyics poor, RTV good), therefore each
> application is different.
>
> I am interested to know what the moisture absorption rate for Acrylic
> is?
>
> Regards,
> Brent
>
> >>>>>>>>>>>>>>>>>> Original Message <<<<<<<<<<<<<<<<<<
>
> On 5/26/2000, 3:58:26 AM, Brian Ellis <[log in to unmask]> wrote
> regarding Re: [TN] Solder Mask:
>
> > I'm sorry, Jerry, any coating that easily permits the passage of
> > moisture cannot have, by definition, good dielectric properties. The
> > dielectric constant will increase dramatically, as will the loss
> factor,
> > if there is moisture absorbed into the coating. Many RTVs even depend
> on
> > their moisture absorption to polymerise! If you want the best
> dielectric
> > qualities, some of the acrylics are unbeatable.
>
> > Brian
>
> > Jerry Mosur wrote:
> > >
> > > Actually we use RTV for its good dielectric properties, and not for
> its
> > > moisture blocking capabiliteis.
> > > The root of this problem lies in insuficient space for a given
> electrical
> > > characteristics of a circuit. If one can not open up spacing, one must
> > > resort to other techniques like routing the board in-between pads and
> > > encapsulating.
> > > As far as the conventional coatings go, we have found that in order to
> > > prevent delamination we have to plasma etch boards as well. With
> rework
> > > being easier with RTVs, we decided to use them instead.
> > >
> > > Graham, I will e-mail you for more info on solventless or water based
> > > coatins. How easy is the rework?
> > >
> > > Jerry Mosur
> > >
> > > Spellman High Voltage
> > >
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> > --
> > Brian Ellis
> > Protonique SA
> > PO Box 78
> > CH-1032 Romanel-sur-Lausanne, Switzerland
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--
Brian Ellis
Protonique SA
PO Box 78
CH-1032 Romanel-sur-Lausanne, Switzerland
Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
E-mail: [log in to unmask]
URL: Technical and consultancy divisions:
http://www.protonique.com
Web services division:
http://www.protonique.com/webserv
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