Pink ring is caused by chemicals seeping along the copper inner layer 
from the drilled hole during board processing, removing the oxide 
coating out past the land diameter so it can be seen from the outer 
surface.  The term pink ring is used as the oxide coating has been 
removed from the inner layer exposing the inner layer copper surface. 
 Looking from the outside of the board you can see the inner layer 
copper surface around the lands as a copper or pink color.

Pink ring is acceptable, providing it is not too severe.  In extreme 
cases, you may even see wedge voids in the plated through hole.   
Remember, when the oxide coating was removed, you have no bond in 
that area and the inner layer is actually delaminated in the pink 
ring area.

There are several reasons that cause pink ring during processing.  
Drilling, chemical processes and lamination generally.  Fortunatly, 
the chemical suppliers have done a good job of developing a process 
to reduce or eliminate pink ring.  Very few vendors have pink ring 
problems today, though it was a severe problem only a few years ago.  
If your vendors are still having pink ring problems, their chemical 
supplier can usually help solve the problem very quickly.

Regards, 

Norm Einarson
Printed Circuit Technology  

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