While inspecting incoming PCBs we detected around all plated-through-holes (only there and all in the same place) some halfmoonshaped light rings that match obviously only with the pink ring symptom shown in the IPC-A-600 chapter 2.5.2.. That would mean the PCBs are acceptable but we are not quite sure about our judgement. Can anybody tell me what a Pink Ring actually is and what the cause of it is or if we face maybe another more serious effect we have to worry about ? The PCB-vendor isn't much help for he always says it's OK and I'm rather more in DFM (CE is what I dream of) and SMT-processes than in the material stuff.

Thanks in advance