The Correlation between Short‐ and Long‐term SIR Testing

Surface insulation resistance (SIR) testing is mainly used as a qualification procedure to determine the‘best’ combination of materials and processes for a given application. The usual tests are destructive and last generally from ten to 56 days, 28 days being very common. It is clear that such tests are unsuitable for production quality control. With cleaned boards, ionic contamination testing is current and is specified in many standards. Even so, the presence of non‐ionic hygroscopic contaminants, such as residues from many fluxes and solder pastes, remains undetected. Their presence may cause functional failures, especially if there is some ionic contamination, even within acceptable limits, as well. When using ‘no‐clean’ fluxes and pastes, ionic contamination testing is sometimes used to determine whether a process is constant, although it may be extremely difficult or even impossible to interpret the results into a function of reliability. SIR testing is the only easy way of forecasting a loss of reliability. A method is described of non‐destructive SIR testing in a few hours and correlating the results to reliability as determined by long‐term qualification testing. This method is valid for PCB assemblies which have or have not been cleaned.