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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