A shorted PWB was submitted for microsection analysis. The client reported that there was a known issue involving break out of the board from the panel (i.e. misalignment of the V-score). The client attempted to mitigate the break out damage through application of a bead of epoxy on the edge of the board (see optical image below).
The BSE SEM image below shows the location of the short (dashed oval) and a laminate fracture that propagated from the edge of the board (arrows).
The higher magnification image below of the short site showed carbonized epoxy resin and melted glass bundles.
A region just outside of the short site showed conductive anodic filaments (CAF) in epoxy fractures and de-bonded glass/resin interfaces.
EDS analysis confirmed that the filaments were copper.
So, why didn’t the epoxy bead on the edge of the PWB prevent the CAF short? The likely explanation is either one or both of these reasons, (1) epoxy is non-hermetic so moisture can ingress in spite of an epoxy bead or (2) moisture and perhaps process chemistry were trapped in the capillary spaces created by the break out damage before the epoxy bead was applied.