Flux – Solder Relationships
1. When solder flows before the flux
Imagine your solder is melting before the flux forms a liquid glaze, unable to connect through to the piece, even balling up. You will have unwanted cadmium losses plus oxidation on the solder, while the solder still can’t sink through the pasty cover. This will lead you to overheat the solder until the glazing is liquid enough to finally let the solder sink through and connect to the metal. Overheating solder causes shorter running distances, porosity; lesser stability and often unevenness of the solder line and unwanted clean ups.
2. When flux gave up before the solder runs
The other variation is that low temperature flux gives up before your high temperature solders come to run, which will leave your solder and surfaces unprotected, oxidizing. Once the solder liquefies, oxidation which had meanwhile developed in the seam as well will limit or stop your solder from running. The outcome for your work pieces shows similar results like the above, porosity, poor stability, eating into the surface when pushing the heat and lots of unwanted clean ups will make for the major issues.
Generally you need to remember, Fluxes based on white pasty Borax mixes are used for the lower work temperatures below (730 Celsius or 1350 Fahrenheit) whereas the green Fluorite flux is used for temperatures higher than these numbers up to 820 C or 1510 F.