A client provided several PCBAs for failure analysis of out-of-tolerance MELF resistors.
This is an optical image of a typical failed resistor showing localized delamination of the protective coating from the Tantalum thick film resistor material and Sn-Pb particles trapped at the interface.
This is a high magnification optical image of a Sn-Pb particle as-viewed through the outer coating that bridged the laser cut causing a short.
This elemental spectrum shows that the short is a Sn-Pb particle (sample particle as in previous image).
Sn-Pb particles were found in microsections sitting on top of the Tantalum thick film resistor material under ~ 40 microns of outer coating.
The root cause of the failures was tramp metal (Sn-Pb) contamination under the coating suggesting inattention to cleanliness in the manufacturing environment. I am left wondering why the manufacturing operation didn’t use AOI techniques (or human inspection of samples) to prevent this from getting out of the factory. The loss of reputation seems to be a cost greater than the investment in AOI.