This is the updated version (ver.2) of the popular document "Fundamentals of Flux Cleaning." In this revision, in addition to the existing content (how to choose cleaning agents and cleaning methods), we have added the basics of "Residue Evaluation (Analysis)." This document is recommended for those who are just starting with flux cleaning and for those who want to understand the entire process from cleaning to analysis. In addition to the purpose of cleaning and the fields where cleaning is necessary, we also explain the necessity and methods of chemical analysis, so please take a moment to read it.