Moonman,

IMHO, much better to eliminate the cause than to try and prevent the
ill. By analogy, the transmission of aids can be reduced by the use of
condoms, but these have been known to split. Abstinence eliminates the
need of condoms. Coatings are just the electronic equivalent of condoms.
Of course, if sexually transmitted diseases were able to be totally
eradicated, then condoms would no longer need to be used (birth control
apart). This utopian thought is analogous to the perfect substrate. I
don't like coatings (or silica gel sachets) used to cover up faulty
processes or materials, because you are not attacking the root cause,
the disease.

I have not followed the diatribes here re CAF (there is still a limit to
my powers of concentration after my problem!) but let me tell you my
unique experience with them. PCB: hi volts/hi currents 6 layers, 70 µm
inner layers, 105 µm outer layers, 2 7628 prepregs between each
laminate, 2.4 mm built-up thickness, mixed PTH/SMD. Total Cu thickness
in some places 2 x 105 + 4 x 70 µm = 0.49 mm. Total Cu thickness round
most vias and component holes 2 x 105 mm = 0.21 mm. Difference = 0.28
mm. Total prepreg thickness = 4 x 7628. Result: grave resin starvation
round holes and CAFs after a few months in service. Cure: I insisted
they put pads on all layers for both vias and component holes. This
alone cured the situation as they were now drilling into a much more
compressed structure. I didn't even have to suggest a more resin-rich
monofibre prepreg style (in any case, I was afraid of slippage with more
layers of less robust prepregs with such a massive board, which weighed
over a kg each before stuffing with components).

Pacemaker applications I hold dear to my heart, but there ain't no FR-4
in mine, just ceramics. The non-conformal coating is in titanium!

Best regards,

Brian

Earl Moon wrote:
>
> Thanks Brian,
>
> Our bridges too, especially the Golden Gate, provides "permanent" job
> security for strippers and painters as well - willing to work at such lofty
> heights, in poor environmental conditions, for so long. You've given me
> exactly the answer I was seeking.
>
> Bridges are quite another matter from devices or product that can be
> environmentally "shielded" to some extent. My pacemaker (don't really have
> one yet) is such an example but the situation is better exemplified by
> military hardware, or Class 3 product, having to meet a less critical, at
> times, situations.
>
> I'm just suggesting that if we know a cause of something, we might do more
> to prevent its occurrence. If we better protect a product, until it is
> required for use as an assembly for flight operations, as an example, by
> better "sealing" it before it is integrated into a "sealed" system,
> shouldn't we expect better life performance without serious, life
> threatening possibilities as when a PCB glass fillament shorts because of a
> moisture, or whatever, problem?
>
> I'm just trying to establish that if CAF, for example, testing has revealed
> it really exists, and it it's obvious it does, we should be able to prevent
> it's negative consequences. If we bake, heat, solder, dry, dessicate, seal,
> conformally coat, and keep dry, by whatever means, haven't we solved an
> issue before it becomes a serious problem? By normal course of fabrication
> and assembly, we do all these things as a "natural" matter of fact.
>
> I believe we have discovered a major impediment to reliability and human
> safety. However, it just seems to simple to prevent it from being a problem
> over the life of a product.
>
> Thanks again for your response and thanks for continuing to be,
>
> Earl

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