Good Morning
Responses included in your note.
--- Forwarded mail from Jim Marsico 516-595-5879 <[log in to unmask]>
>Good afternoon.
>Two questions:
>1) Is "negative etchback" typical for multilayer board fabrication
>these days, or is the "good" kind still available?
Ah yes, those thru holes made with 98% H2SO4 and 50% HF. Tougher than
the current Pick-up truck commercials. One solid, "wicked" hole. Don't
know anyplace which makes them any more. I would try looking to the few
solid old 55110 shops or a ex-military captive PWB house.
Negative Etchback. I perfer not seeing negative etchbach on our
products. I can not site a study, but when we do failure anaylsis on a
board with intermittent problems... usually see negative etchback. Is
it the cause, no. Is it a factor, seems to be from where I sit.
>2) With a .002" minimum internal annular ring requirement
>(MIL-P-55110), and a
>.040" diameter internal pad, what is the maximum drill size that can be
>used for an 8-layer Polyimide board .062" thick?
Well, 40-2*2= 36. 36 is the maximum hole size with perfect registation.
No shop has demonstrated perfect registration to us yet, (one
sales-type claimed it once.) Most shops would want 8 to 16 mil, the
eight mil board might have a 50 % yield, so you are going to pay for it.
8 mil = +/- 4 for all registration and drill issues. So, 20 mil max
for good yields and 28 mil max for a low a yield.
>OK., three questions:
>3) Would the drill size differ for negative etchback than for a
>positive etchback condition?
No, but above I assumed little or no etchback. If I had assumed 3 mil
etch back above, the maximum hole size would have been, 40-2*2-2*3=30.
If I had a customer out there who demanded 55110, I would not allow
negative etchback on that customers boards.
>Thanks for the info...
You're welcome
George Franck
--
George Franck Jr
Raytheon E-Systems
Product Assurance Engineer
Falls Church Va
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