Contaminant Particles in Electronic Solder Fluxes

The filterable particles found in electronic solder fluxes vary considerably in both concentration and chemistry. Four fluxes from three manufacturers were examined, including both rosin fluxes and mildly activated resin fluxes. Individual particles were examined by optical light microscopy (OLM) and scanning electron microscopy/energy dispersive X‐ray spectroscopy (SEM/EDX). Finally, an automated SEM/EDX system was used to collect and summarise information about the size and chemistry of a hundred or more particles from each flux. The number of particles per microgram of flux was found to vary by two orders of magnitude (0.004 to 0.4 per μg). The particle diameters ranged from 0.2–20 μm with averages of 1–3 μm. A large fraction of the particles (33–75% by number) were organic substances not soluble in the flux. The bulk of the inorganic particles were composed of sulphates, silicates and metal oxides. Thus, some solder fluxes may be introducing several contaminant particles into each solder contact. These contaminants may affect the quality of the solder joint depending on particle size and composition.