Fluxes do not contain metals, such as lead, so all fluxes are lead free. Your question must be asking about flux that is marketed as being specially formulated for use with lead free alloy. The answer is: you will not know unless you try it. Notall lead free alloys work equally well with all fluxes. Flux can be formulated to achieve superior results with aspecific lead free alloy.

When claims such as this are made, I recommend contacting the manufacturer and asking for a FREE sample so you can prove to yourself that their claim is valid. Be sure to evaluate the qualities of the residue when you do your testing. Some "lead free" fluxes just have higher activity levels to compensate for the moredifficult wetting andhigher reflow temperature.

In some cases with no-clean fluxes, a certain minimum reflow time is required to consume their activator so the residue is in fact no-clean.

Lead-free alloys wet slower, have a greater surface tension, and operate at higher temperatures compared to tin/lead.

There are older chemistries which work with lead-free alloys, but they are typically not an optimal choice.

A flux which has been specifically developed for lead-free applications, is by design capable of accommodating the demands these alloys make on the soldering process, thus providing a more optimal process window for the end user.

From the wording of your question, it isn't clear if you are referring to a lead-free flux for a surface mount application (solder paste flux), a wave soldering application (liquid flux) or a hand soldering application (cored wire flux), but in each case, at least to some degree, there is certainly value in utilizing fluxes are known to be lead-free capable.

Although some fluxes that had originally had been designed for leaded applications may work acceptablyinlead-free applications, there are many new fluxes on the market today that were designed specifically for lead-free soldering processes. These fluxes represent improvements over traditional fluxes in terms of solderability.

Since lead-free alloys melt at higher temperatures and don't spread as easily as leaded alloys, lead-free compatible fluxes tend to have higher activity and higher heat stability than traditional fluxes. This gives most users reason enough to make the switch to a newer flux that was designed specifically for lead-free processes.

Yes, due to the higher reflow temperatures and longer soak times if you tried to use eutectic flux in a lead free application the fluxes would activate too soon in the cycle and not be available during the reflow of the solder joint.