Is the current surface insulation resistance (SIR) methodology appropriate to today's manufacturing technology?

Abstract

Corrosive contaminants left on a circuit from assembly and manufacturing processes present reliability problems. Contemporary SIR measurement procedures consist of daily resistance measurements across a comb pattern on samples that age in environmental chambers. Yet these tests lack information on the corrosiveness of the contaminants and often exhibit inconsistency as quantitative measures. A DC continuous measurement method is used here to study the fundamental science behind these measurements for ionic contaminants on a printed circuit board. For ionic contaminants, such as those left from low-solids-fluxes (LSF), the SIR values exhibited continuous and irreversible changes during the test. The DC voltage causes mobile ions to migrate towards the electrodes and are thus being depleted from the insulating surface. Hence, only the initial measurement on a virgin sample gives the true quantitative measure of these contaminants. The subsequent rise in SIR value should not be interpreted as an improvement in reliability but rather be indicative of the presence of mobile ions which might be corrosive. Once the board is depleted of ions, reversing the applied voltage polarity cannot restore the initially low SIR value, but only gives a very slow drop followed by a very slow rise in the SIR value. These changes are responsible for many measurement anomaly commonly observed under various DC biasing schemes where the SIR values are recorded only once daily. An alternate SIR methodology using AC measurement is shown to give more consistent results. The AC voltage causes no net ion migration but may still cause voltage-accelerated aging. While this work uses printed circuit board as a test vehicle, the fundamental science is applicable to Hibrid IC and to other processes where cleanliness is needed for reliability.