First of all there should be proper ventilation so that the assembler should not smell the flux. Many fluxes contain acids, amines and resins that can cause irritation to the respiratory system and in some case cause asthma. It is not recommended to breathe any smoke that comes from soldering even in a lead free environment.

There are many good air purification systems that are bench top to eliminate the odor. As far as the solder wire the material with the best SIR, corrosion rating, and that wets is normally the safest for the assembly.

Beyond the product performance of wetting and low residue, the amount of smoke and type of odor released during soldering is a common concern. Although interconnect performance attributes can be measured objectively, the amount of smoke and smell tend to be more subjective.

Some manufacturers claim to address these concerns as well with specially developed flux cored wire. One such product by Indium Corporation of America (CW-501) is a rosin free no-clean flux cored wire specifically designed to offer superior wetting performance while minimizing smoke and odor. In addition, the flux core is colophony free (does not contain natural pine rosin), which reduces the potential for respiratory issues with operating personnel.