Address,
     
     I consider the PCBs we currently design and assemble as high 
     technology or slightly ahead of PCB industry standards.  Several 
     characteristics of our current technology are:
     
     1.  5/9 channel on .100 centres
     2.  .004 and .005 lines with a tolerance of +-.0016in or usually 
         +-.0016 of the supplied artwork. 
     3.  .012 QFP pitch
     4.  10 layers impedance controlled
     5.  .074
     6.  6:1 aspect ratio
     7.  MINIMUM CONDUCTOR TO CONDUCTOR: .13mm (.0051in)
         MINIMUM CONDUCTOR TO PAD:       .16mm (.0063in)
         MINIMUM PAD TO PAD:             .19mm (.0075in)

     The additional information gives you an idea on how we are using fine 
     line technology in parallel with other technologies.

     The final basis of determining the minimum conductor thickness and 
     width shall be related to the current carring capacity.  IPC-D-275
     entitled, "Design Standard for Rigid PCBs and PBAs", explains it
     in more detail. 

     IPC-RB-276 also explains acceptable limits for conductor width and 
     spacing.


Thomas John Gulley
PCB Project Engineer
email:[log in to unmask]




______________________________ Reply Separator _________________________________
Subject: How do we compare?
Author:  [log in to unmask] at halsic_ccsmtp
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.