Surface insulation resistance (SIR) measurements have become necessary to perform, especially due to the introduction of lead free solders. It has been reported that SIR and other test methods have to be used for evaluating the reliability of printed wiring boards. This paper presents an investigation on how the SIR test environment influences the test results. The factors varied were temperature and humidity. The temperature was varied between 40 and 85°C, and the relative humidity was varied between 60 and 85%. Furthermore, the influence of different types of process chemicals on SIR was evaluated. Seven lead free solder pastes, of which five were no-clean and two were water soluble, were compared. The influence of using conformal coating was also studied. These solder pastes were used to verify the developed method. A proper test method for SIR measurements is suggested in the paper, as well as a recommendation for use of conformal coating.