Measuring Gold Thickness on Printed Circuit Boards for Control of Production Plating Processes

Beta-backscatter is used to control the thickness of gold plate at a Tektronix, Inc., captive circuit board shop. Gold is deposited from deep tanks on an intermediate deposit of nickel. Routine calibration and standardization procedures resulted in discrepancies of up to 40 percent between beta-backscatter measurements and measurements of cross sections using an optical microscope. Investigation revealed significant differences due to variations in gold density, copper and nickel thickness, and in calibration and measurement procedures. A system of beta-backscatter measurements was implemented which reduced the average discrepancy between beta-backscatter and metallographic values to less than 15 percent. In addition to assuring greater product reliability, an estimated 20 percent savings in gold costs was achieved. The investigations are summarized, the new system and its implementation are described, and current system costs are presented.