Last week, we discussed How do you choose the right soldering flux for PCB assembly. This post continues that discussion by looking at the three main components of flux that are acids, solids, and solvents, and how each affects soldering performance, residue, and reliability.
Understanding these ingredients helps designers and assemblers make more informed decisions. Flux is not a single material; it is a combination of different materials that each play a specific role. Evaluating them separately makes it easier to understand why some fluxes perform better in certain processes than others.
I wanted to start a discussion on how you evaluate these three parts of a flux when selecting one for PCB assembly.
The acids in flux remove oxides so the solder can wet the metal surface. Stronger acids can provide greater deoxidizing ability, but their residues can also increase conductivity, corrosion, and ionic contamination risks. Flux activity classifications such as L, M, and H provide a starting point, but the actual application and reliability requirements also matter.
The solids serve a different purpose. They protect the deoxidized surface from reoxidation until soldering occurs. Rosin, resins, and water-soluble solids can behave differently after soldering, particularly in terms of visible residue and cleaning requirements. Visible residue does not necessarily indicate poor reliability, so how do you decide when residue removal is actually necessary?
The solvent also affects how the flux is applied. Alcohol-based solvents have low surface tension and evaporate quickly, helping flux reach tight areas. Water-based fluxes behave differently because of their higher surface tension, slower evaporation, and greater energy requirement for drying.
When evaluating a flux, how do you balance its acid activity, solids content, and solvent properties with the assembly process and reliability requirements?
For a more detailed explanation of flux composition and how acids, solids, and solvents work, check out this article: Soldering Flux: Acids, Solids, and Solvents.
