A client provided some sample PCBs (1 bare, 1 assembled) that were exhibiting discoloration on the ENIG plated surface. The client asked us to characterize this defect and determine root cause.

This image shows what appears to be a "water spot" on an ENIG pad.
This image shows what appears to be a “water spot” on an ENIG pad.

 

This is a microsection of the same pad through the approximate center of the water spot.
This is a microsection of the same pad through the approximate center of the water spot.

 

This is a high magnification image of the ENIG plating.
This is a high magnification image of the ENIG plating.

The nickel (EN) deposit appears normal except at the interface with the gold (IG layer).  The defects (shown by the yellow arrows) are referred to as “IG spiking”, where the gold plating solution etches the Ni-P grain boundaries.  This creates small voids or pits under the IG plating that trap process chemistry and cause corrosion resulting in the stain on the surface.  Root cause = IG spiking.  A secondary cause is water droplets left on the board surface after cleaning.

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