Abstract: A preflux for a printed circuit board is provided, and the preflux has an excellent heat resistance adapted to a method of surface mounting electronic parts on the printed circuit board, and it is highly operable and reliable. The preflux is comprised mainly of a 2-alkylbenzimidazole derivative having the general formula of ##STR1## wherein R.sup.1 and R.sup.2 may be identical with or different from each other and are hydrogen atom, a lower alkyl group or a halogen atom, and R.sup.3 is an alkyl group of three or more carbon atoms, and contains a chelating agent reacting with copper ions added thereto. Addition of a proper amount of the chelating agent to the preflux makes it easy to form a highly heat-resistant film of the preflux only on copper or copper alloy patterns without masking the printed contacts and joint land.