We have a requirement to make a quantity of circuit card assemblies for a 50 Ohm RF application which requires a uniform phase delay over the entire production quantity, to be tested at 32MHz using a network analyser. The circuit tracking has been designed around FR4 substrate, with a nominal dielectric constant of 4.7 and 0.060'' thick. However, the standard range of 4.6 to 4.8, ie 4.7 +/- 2%, is too wide and we are looking for stock with a tolerance of 1% or better. The nominal value does not have to be exactly 4.7 but should be in that range. One way of achieving this is to select the sheets from standard stock by checking capacitance, area and thickness of each sheet and calculating the dielectric constant. However, we are told that a typical sheet is thicker in the middle and thinner around the edges, and we may need to use only a centre portion of each sheet to obtain better uniformity. Does anyone have experience in this area, or suggestions as to how to go about this? [log in to unmask] Lothar Thole Senior Manager - Manufacturing Planning QPSX Communications Ltd 33 Richardson Street West Perth 6005 Western Australia Tel. +61-9-262-2000 Fax. +61-9-324-1642