Rainbow bridge

Have any experience that Sn/Pb plated integrated circuit products will appear some rainbowbridge-like substance after exposing at conditions of 121degC, 100%RH and 15psi for 48hours or above? The rainbow bridge substance was analyzed by EDAX and found a significant amount of Sn which should only be existed on the lead plating but not on the mold compound surface of the ICs. Is there any theory to support that Sn migration from plating to its surfaces around? The rainbow bridge is conductive so that it can cause leakage current or even short circuit between IC leads and causing "invalid" failure.

reply »