Al-- We design avionics hardware for the military business, what there is left of it. We route either .006/.008 mil traces on internal layers depending whether we are routing 3 or 4 traces between .040 pads on .100 spacings. We allow +/- .001 for phototooling features (generous considering we supply Gerber data) and +.000/-.002 for process allowances. We usually have impedance requirements for specific layer pairs and allow the supplier to vary trace width and dielectric thickness to ensure that the pwb meets the end product impedance requirements. However, in NO case can the finished internal trace width be less than .004. Over the years, multiple suppliers have meet these requirements with very few cases of a supplier asking for a wavier for a non-compliant board. I will admit, I am not sure of the impact to cost of these requirements, but those are the design parameters required to meet the electrical and mechanical constraints. Hope this wasn't too windy and helped somewhat (at least you know that your not alone). If you would like to discuss this further, please feel free to contact me. Have a good day. Bob Vanech (203) 852-4810