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.