further to early message, we have done a series of trials.

inspect panels after copper plating                           no dull copper

tin plate few panel, etch ,tin strip( from above panels)      dull copper pads

tin plate few panels, tin strip ( from above panels)          dull copper pads 

pass panels after copper plating with photoresist on
through tin strip                                              dull copper pads


so we concluded that tin stripper we use is the culprit

so we did trials with 3 different nitric acid based strippers and
one bifluride based stripper                                    dull copper pads

so worked on ways to remove the dullness or passivation

microetch 5 micron run through electroless nickel line           less nickel on
                                                                 dull pADS

dip in alkaline cleaner  of blackoxide ( to remove anti
tarnish of tin stripper                                            do

dip in alkaline cleaner and brush pu mice clean                     do
(normally we brush before masking)

increase concentration of palladium in activator ( 3times)           do

increase immersion time in activator to 10 minutes
with 3 timespalladium                                            background                                                                   plating but less                                                               nickel on dull pad  

 deburr boards after tin strip at normal deburring                problem seems
pressure, then H2SO4 clean, brush                                 to be less but
                                                                  still exist.


to check whether intermetallic is the cause,
we did black oxide of boards after tin strip                     oxide takes                                                                  place but observed
                                                              shade difference                                                              in dull pads

                                                             after H2SO4 STRIP
                                                            of b.o dull pads                                                                 seen.


gosh, what is this dull pad ?

those who use acid tin and nitric acid based stripper see this dull pads after tin strip ?

the dull pads are seen in boards going through HASL also but do not create any problem there.

is it a elctroless nickel chemistry problem


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                       original message


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 aprreciated.

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