Two different types of fluxes - which are used mainly in wave soldering - were analyzed especially considering how they affect the reliability of electrical circuits. Flux residues mainly have effects on surface insulation resistance (SIR) and electrochemical migration (ECM). If we use fluxes without cleaning process after soldering, the residual organic and inorganic materials absorbed by the surface can extremely decrease the resistance between close running wires. This increases the probability of shortage and sparking. The fluxes were analyzed in freshly opened and aged state. The SIR was significantly larger when aged fluxes were used than in the case of newly opened specimens were applied. The electrochemical migration is one of the most important failure mechanisms in the field of printed wiring boards (PWB). The ECM is one of the most important physical-chemical processes, which limits the realization of fine pitch structures. We have carried out experiments on PWBs covered by different kind of no-clean fluxes in order to compare the mean time to failure of the migration process. The most important characteristics were examined by water drop tests. The experiments were executed by the aid of data acquisition device taking care of reproducibility. Test results demonstrate that the residual organic (or inorganic) materials can cause ionic induced migration and the failure rate can increase.