Many years ago, when I was designing the Insulohmeter-2, where we had to
have IRs of the order of 1E14 ohms on some tracks, one of my English
colleagues with little technical knowledge decided that the boards
should be polyimide. When I asked why, he went into a long spiel which
boiled down to "it was more expensive, therefore better". I told him,
quite rudely, to leave the technical side to those who knew what they
were talking about, i.e., me! Anyway, I designed the boards in FR-4 with
all the critical tracks on inner layers, with 6 mm clearance round them.
They came to the surface only when they connected to a component. We
obtained our 1E14 ohms, with 1/2 an order of magnitude to spare. After
soldering and cleaning, we sprayed with acrylic conformal coating. I'm
quite sure that if we had used PI, we would not have achieved our
target. PI is useful for high temp applications, but it is not very good
where there is any humidity. Liken it to a sponge.
The instrument never got beyond prototype stage, mainly because of
multiple disagreements of a similar nature. However, the prototype did
work beautifully in my lab but, when it was taken to England, it refused
because of non-respect of my specs (wrong computer hardware, wrong
printer etc.).
Brian
Russell Burdick wrote:
> Here's a question for all you electrical fans...( ha, ha, ha, not those
> type
> of shorts)
>
> We are getting high resistance shorting on multilayer polyimide only
> boards.
> Inner layers test out with no problem. Immediately after outer layer etch
> there are the shorts. Omega meter testing shows no ionic contamination at
> this stage. Baking prior to ET reduces the number of shorts, although after
> a few days the shorts are back. Construction of board is foil copper at
> lamination, not cap-core.
>
> As usual, any and all help is greatly appreciated.
>
> Russ
>
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