Flux Residues and RF Signal Integrity

A test method for examining the effect of flux residues at frequencies as high as 10 GHz was developed (Ref 40). A bidirectional coupler was used (Fig. 18) and a lumped element model to 

A two-port T-resonator was used (Ref 41) (Fig. 19) to characterize the effect of solder flux residue on the transmission of RF signals up to 10 GHz. Bare copper conductors were used on an FR4 substrate. The change in effective dielectric constant (ASgff) between bare and processed conditions was the parameter used to compare different solder pastes on the basis of residue-attributable RF signal transmission loss. 

A circuit test vehicle with both active and passive elements was designed (Ref 44). The passive circuits consisted of band pass filters (6 to 7 GHz range) and T-resonators (5, 10, and 15 GHz), while the active circuits included a mixer device (4 to 6.5 GHz), a VCO module and a wide-band amplifier circuit. The assemblies were tested after soldering and reflow, and again after flux/paste residues had been removed. Variations in signal transmission for two different solder pastes were observed on the passive circuit elements. For the mixer device, variation between signal transmissions for the measurements with/without paste residues was approximately 1%. 

Effects of Non-ionic Water Soluble Rux Residues, Western Electric Engineer, Issue 1, 1983, p 41 8 

Water Soluble Flux and its Effect on PC Board Insulation Resistance, Electronic Packaging and Production, Vol 21 (No. 7), 1981, p 80-85 

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