In my opinion, the decision to use formic acid simply to avoid problems with a non-optimized flux process is usually based on a misunderstanding of flux and an unwillingness to optimize the process. If flux residue is the issue, all modern fluxes can be cleaned with DI water or commercially available cleaners depending on chemistry. I do believe formic acid has a good niche in assembly at lower or higher temperatures than flux can handle (<100degC or >350degC). I would only trust it in a system that is designed and proven to contain the formic acid, it does sound dangerous if it escapes.
“Formic acid is readily metabolized and eliminated by the body. Nonetheless, it has specific toxic effects; the formic acid and formaldehyde produced as metabolites of methanol are responsible for the optic nerve damage causing blindness seen in methanol poisoning. Some chronic effects of formic acid exposure have been documented. Some animal experiments have demonstrated it to be a mutagen, and chronic exposure may cause liver or kidney damage. Another affect of chronic exposure is development of a skin allergy that manifests upon re-exposure to the chemical.”
To be fair, they say this about flux:
“Acid flux types (not used in electronics) may contain zinc chloride or ammonium chloride, both of which are harmful to humans. Therefore, flux must be handled with gloves and goggles, and used with adequate ventilation.”
The modern fluxes used in electronics are designed to be benign at room temperature. I am surely somewhat biased due to my experience with flux, so my open question is: am I missing something? Please comment if you feel formic acid is safer than I have been lead to believe, or if you have had experience with this material that you would like to share.