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Could you elaborate on the nature of flux residue after soldering? I'm curious to understand if it remains on the soldered joint or if it evaporates completely. In my experience, some fluxes leave behind a residue that can affect the electrical conductivity or durability of the solder joint. However, with advances in soldering materials, are there now fluxes that do not leave behind any significant residue? What are the implications of flux residue, if any, and how should it be managed to ensure optimal solder joint performance? Thank you for your insight.