It's very common to see a bga with one pad out of a thousand that is dull.  This pad will measure low nickel thickness.  As an outerlayer imaging engineer I review a lot of scrap for opens.  I have noticed that when a trace from a bga pad to other circuitry is damaged causing an open,  the remaining bga pad and the part of the etch from the bga pad to the open is often dull.  Fiducials at the corners of a bga are often (but not always dull.)

This causes me to believe the phenomenon is galvanic.  Pads which are electrically connected to a dissimilar metal in a highly conductive plating solution at elevated temperature plate at a different rate than isolated pads.   

Our friend who noticed that a dummy panel plated on one side and not the other didn't mention if the dummy panel had ever seen mask and may have had a contamination that kept one side from plating.
My guess would be that he got a clean, piece of cheap (sic) laminate from stock and put it through his normal, pre-plate clean process but that the two foils were different and a slight difference in alloy caused one side to preferentially plate.

This leads me to another question, has anyone set up a line to plate electroless copper as a strike before plating electroless nickel/immersion gold?  Copper should be less affected by the base alloy and once coated,  the surface should be homogeneous going into the nickel. 

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