Your first consideration when determining how much flux to use in your solder wire is whether you are doing hand soldering or robotic soldering. Robotic soldering requires a greater flux percent than hand soldering. Flux percent is measured by weight so you need to calculate the alloy density as well as the diameter of the wire when determining the ideal flux percent. For instance, Sn63 is denser than SAC305 so the volume of flux in a Sn63 wire will be more than in a SAC305 wire.
Here is an example: Our CORE 230-RC Flux-Cored Wire with SAC305 at 0.032” diameter solder wire would ideally have about 4.3%-4.7% for robotic soldering. For hand soldering, 2.1%-2.5% would be suitable. Now, if you were using Sn63 for this same flux-cored wire, the flux percentages would be 3.8%-4.2% for robotic soldering and 1.6%-2.0% for hand soldering.
If you need help determining how much flux you need for your solder wire, you can use our flux percent calculator on our website or email me at kflanagan@indium.com. You can also send your questions to our Technical Support team at askus@indium.com.