If there is immersion tin under the mask then that alone may be reason for
the cracked and falling off mask. The tin will flow during your process
causing havoc to the covering mask. It really should be bare copper under
the mask unless the product will only be hand soldered.

Applying the immersion tin before solder mask could also be the cause of
discoloration. The volatile gasses escaping from the epoxy during cure could
be causing the discoloration as could the solder mask developer but most
critical would be the final cure/bake cycle.

Furthermore, the heat from curing the mask is beginning to change the
intermetallic layers and corrosion (including appearance of) as well as
solderability issues will present themselves.

Immersion tin is a good finish, if you limit the thermal cycles before the
assembly processes

Franklin

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