we find less or no nickel plating in few pads after eletroless nickel - immersion gold process. this can be identified by inspecting the panels at an angle. on good pads , we get 4 to 5 micron nickle and 0.06micron and these bad ones as low as 0.4 micron nickel but we find gold thickness higher always above o.1 micron. 

we found dullcopper pads after soldermasking and went back and found same dullpads after tin stripping.

we use acid tin as etch resist and nitric acid based stripper for tin stripping.

on a qfp ,a typical dull pad is as follows

                    -          -  bright pad 
                    -          -  bright pad
                    -          -  dull pad after tin stripping
                    -          -  bright pad
                    -          -  bright pad

the dull pads occurr at random in a qfp.at times pads connected through a connector are dull.at times some portion of connecting track is also dull.  since just one pad is dull and adjacent pads are bright indicate that it is unlikely to be copper plating problem.

we did hot air solder levelling of boards found after tin stripping with dull copper pads and found them to solder well.

has any onefound similar problem?
what is the likely cause ? intermetallics? if so how to identify presence of it and removal.

any help will be much appreciated.

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