Lead-free flux technology for electronic industry is mainly driven by high soldering temperature, high alloy surface tension, miniaturization, air soldering due to low cost consideration, and environmental concern. Accordingly, the flux features desired included high thermal stability, high resistance against burn-off, high oxidation resistance, high oxygen barrier capability, low surface tension, high fluxing capacity, slow wetting, low moisture pickup, high hot viscosity, and halogen-free. For each of the features listed above, corresponding desired chemical structures can be deduced, and the impact of those structures on flux residue cleanability can be speculated. Overall, lead-free flux technology results in a greater difficulty in cleaning. Cleaner with a better matching solvency for the residue as well as a higher cleaning temperature or agitation are needed. Alkaline and polar cleaner are often needed to deal with the larger quantity of fluxing products. Reactive cleaner is also desired to address the side reaction products such as crosslinked residue.
Key words: lead-free, flux, flux residue, solder, soldering, cleaner, cleaning, SMT