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?



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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