Residue left behind from no-clean assembly is a visually obvious artifact of the manufacturing process that can cause concern to those with RF circuit assemblies. This paper describes a test vehicle and test procedure, independent of product design and function, that allows for a quick and relatively simple examination of the electrical effects of residue frequencies up to 7.15 GHz and the determination of the capacitive parasitic associated with it. Two test vehicles were compared, each comprising a bi-directional coupler. Flux medium was extracted from no-clean paste using centrifuge, applied to the center of the bi-directional coupler and reflowed. It was found empirically that for null frequencies in the range of 7 GHz, one of these bi-directional couplers was more sensitive than the other to parasitic induced by residue. This finding was in accordance with linear circuit simulations performed on the electrical model equivalents of the couplers that were also developed during this project. The flux residues from extracted flux medium of five pastes were examined. The flux residue that induced the most amount of parasitic could be modeled as a capacitor of 25 fF +/- 15 fF. Any change in parasitic as a result of aging of the extracted flux medium to an equivalent of one-year floor life was determined to be negligible.
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