Measurement of stray capacitance due to solder flux residue on radio frequency circuit boards

A microstrip directional coupler is used as a test circuit to measure the stray capacitance due to solder flux residue at frequencies up to 4.8 GHz. Comparison of experimental data with simulation results and coupled line theory provides an estimate of 25/spl plusmn/15 femtofarads for the stray capacitance from one flux residue. This measurement technique may be used to experimentally compare the RF effects of different solder fluxes. When combined with information about conductor spacings, residue properties, and the amount of residue, this measurement could validate estimates of the stray capacitance due to flux residue in a variety of circuit geometries.