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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This is a fairly typical example of wear out failure of an aluminum electrolytic capacitor.

AE-Cap-1

This is an optical image of the capacitor that captures the part date code.

AE-Cap-2

This is an image of the top of the device showing the vent, which ruptured and leaked a small amount of electrolyte.

AE-Cap-3

This is an image of the capacitor after dissecting the can. The capacitor roll was charred indicating that the core reached excessive temperature.

AE-Cap-4

This is a plot of capacitance versus age for “passing” capacitors from the same population of capacitors. The trend line is an inverse exponential function indicative of the wear out process.

The analysis results suggest that the capacitor failure was caused by the normal wear out processes that are characteristic of aluminum electrolytic capacitors, i.e. capacitance decreases and electrolyte volume decreases (i.e. capacitor dries out) as the capacitors age and approach end-of-life.

Check out SEM Lab, Inc.  to learn more.