In the Joint Industry Standard 004A, the IPC specifies the testing methods used to categorize a flux. The end classification includes a designation for the type of ingredients used, the flux residue activity level, and the amount of halides included in the flux. A sample designation would be ROL1, indicating that the flux is a rosin-based flux with low residue activity, and does contain halides.
Fluxes are specified for their use based on this nomenclature. The correct flux is chosen based on the method of cleaning to be used post-soldering, the method used for reflowing the solder, the method in which the flux is to be applied, the reflow temperature it will be exposed to, and the substrate it is intended to be used on.
The ideal flux for an application will activate during reflow and any residue left at the end of solder processing (including any required cleaning) will be inert. The flux will be deposited in a repeatable way and the flux chosen will be the least active flux possible which can remove oxides from the substrates soldered.
For use with preforms, flux options include liquid fluxes which are water-soluble, rosin-based, or no-clean, Tacfluxes® with similar options, or pre-fluxed preform coatings including no-clean and rosin options.