I second Jonathans analysis that the stain could be a result of staining on
the initial copper substrate. Examine the work before the ENIG line. If you
see uneven reflectivity, then optomize the cleaner (temp and dwell time) and
increase the total microinches(uins) of copper removed (to >50uins) in the
micro-etch step.
George Milad
Shipley Ronal Inc

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