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1995

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Wed, 25 Oct 95 11:18:07 EST
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        These numbers you mention are fairly common for high volume 
     production quantities. Tighter tolerances are achievable if the
     defining image (plate for outerlayers) and etch processes are 
     fairly tightly controlled and repeatable. Some fabrication shops
     are capable of holding a 1:1 process. (This means for 1/2 oz base 
     copper they only remove .0005" and for 1 oz base copper .001"...etc.)
     You're absolutely correct that a spacing greater than .004" would 
     increase your yields. (Or really the fabricators ability to strip 
     and/or etch cleanly the .004" spacing without potential of shorts.)
     You have to really check closely what your fabricators abilities
     are. Each etcher has variation and the amount of panels will 
     contribute to what the etched line resolution will be able to hold.
     
     Groovy   (Hi Joe F)
______________________________ Reply Separator _________________________________
Subject: How do we compare?
Author:  [log in to unmask] at SMTPLINK-HADCO
Date:    10/24/95 2:35 PM


I would just like to get a feel as to how the rest of the industry is currently 
designing fine line PC boards.
     
My company produces military cockpit displays.
     
We route with .007 mil lines, and call for a .003 Max. processing allowance. 
This would mean that the trace could etch down to .004 mils.
     
Is this a normal allowance? or is it being very generous?
Could I use .006 mil traces with a .002 mil processing allowance? 
Would this method make the boards cost more?
Are there any other military contractors out there using simular numbers?
     
     
We've had our share of mis-etched boards. If we would be better off reducing 
our trace widths to .006 mils and expect the fab shops to hold to a .002 mil 
etch allowance, then we could expect to see fewer scraped boards and higher 
yields for the fab shops.
     
Also we route using .004 mils clearence. A slightly smaller trace would also 
help increase producibility.
     
     



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