Les, The problem with TDR is that everyone has a good P/T but can't correlate to anyone else. My search for a non destructive method of checking dielectric spacing is to find a mechanical way to measure impedance. I am only concerned with trace perimeter and dielectric thickness. Dk is more difficult to measure and I hoped I could make it a constant at the nominal impedance value we fix and the construction we specify. I know we are not the PCB experts, but the tolerances are so tight that we need at least to create a consensus from the PCB manufacturers for construction. Is there any hope here or am I just wasting my time? Thanks for all the responses on this. Steve ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################