Chip components such as MLCCs, high value (~ 1Mohm) chip resistors, and transorbs that fail due to excessive electrical leakage are often not component failures at all, but rather are SMT design/process induced failures.  For example, the chip component illustrated below exceeded the component level specification of < 1 uA leakage.

In this case, the excessive leakage condition in-circuit was attributed to tin residue on the board surface under the chip component.  The suspect component was mechanically removed and the board surface was examined in a SEM.  The BSE SEM image (top) showed a residue on the board surface between the two terminals of higher average atomic number than the solder mask.  An elemental map for tin (bottom) showed that the tin residue was essentially continuous across the isolation space.

Three possible factors that can contribute to these residues are (1) design – the capillary gap, terminal spacing, and applied voltage are factors, (2) process – the cleaning process and/or solder flux selection may be inadequate, or (3) electromigration – the bias voltage, ionic residues, and humidity cause electromigration of tin across the isolation gap with time. The root cause is often a combination of these factors. 

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