Pink Ring of PCB Boards

The hole ring on the inner plate of the multilayer board is connected with the hole wall of the plated through hole, and the black oxide or brown oxide layer on the surface of the hole ring is affected by various fabrication processes of drilling and plating, so if the hole is eroded by liquid, then it will form into the original color on the copper surface, known as the pink ring.

What causes pink ring?
1. Blackening --- Different shape or thickness of the villi will cause varying degrees of pink ring.
2. Pressing --- Poor pressing (pressure, heating rate, flow of plastic, etc.) caused by the lack of adhesion between the resin and oxide layer leaves the space for acid to create a new path.
3. Drilling --- During the drilling process, delaminated or broken layers between resin and oxide layer caused by pressure and high heat let the acid penetrated into PCB boards. 
4. Electroless copper --- Acid is existed in the process of conducting holes which will dissolve the villi.

The Influence of Pink Ring
1. Appearance --- Small holes are preferentially designed, so pink ring can no longer effectively cover which will cause a poor appearance.
2. Quality --- Pink ring means delamination on some certain part with more risks of hole break.
3. Process --- With the requirement of high precision, the emergence of pink ring means the destabilization of the PCB fabrication process.

How to reduce the pink ring?
1. Improve the shape and thickness of oxidized villi;
2. Improve the storage of substrates;
3. Improve the pressing process conditions;
4. Improve the drilling process.