Definition and cause of pink circle on circuit board PCB

Definition of pink circle

After the board surface is oxidized, a fluff layer (copper oxide and cuprous oxide) is formed.In essence, the villi will be eroded by acid or reducing solution, making the originally black or red-brown villi appear red copper.

After the board is subjected to subsequent processes such as press-drilling and drilling, a copper ring with a contrasting dissolution color appears on the fluff around the hole, which is called a pink ring.

The technical approach to the generation and solution of the "pink circle" In the multilayer printed circuit board manufacturing process, the pink bare copper surface produced by dissolving the oxide on the copper foil surface of the inner layer board is commonly referred to as the "pink circle".



Causes of the Pink Circle

1. Blackening--The shape and thickness of the blackened fluff can cause pink circles to varying degrees, but this blackened fluff cannot effectively prevent the occurrence of pink circles.

2. Lamination-due to insufficient lamination (pressure, heating rate, glue flow, etc.) caused by the insufficient bond between the resin and the oxide layer, resulting in the formation of acid invasion into the void path.

3. Drilling--Due to the stress and high heat in the drilling, the resin layer and the oxide layer delaminate or crack, which will cause the acid to invade and dissolve.

4, chemical copper --- the presence of acid in the process of the via hole, so that the fluff corrosion.

Impact of the pink circle

1. Appearance --- Under the trend of small holes, it can no longer effectively cover up and cause poor appearance.

2. In terms of quality, the pink circle represents partial delamination, which is more likely to be broken.

3. In the process --- the appearance of pink circles under the increasing demand for high precision represents the instability of the process.

4. In terms of cost --- the number of holes drilled on the drilling machine, the use of different film substrates with different rubber content components is affected by it.



Ways to improve pink circles

1.Improve the thickness and shape of oxidized fluff

2. Storage and use of substrates and stacking

3.Improvement of lamination process conditions

4.Improvement of drilling conditions

5.Improvement of wet process

Methods to solve such problems, application practice has proved that the following methods can achieve good results:

1. Reducing the oxidized surface of the inner layer copper foil with an alkaline solution containing dimethylborane as the main component. The reduced metallic copper can enhance acid resistance and improve adhesion;

2. Treat the copper surface whiskers with a sodium thiosulfate reducing solution with a PH value of 3--3.5. After acid leaching and passivation, ESCA generates a copper and cuprous oxide mixture coating layer;

3. Treat the inner layer board with a mixture of 1-2% hydrogen peroxide, 9-20% inorganic acid, 0.5-2.5% tetraamine cationic surfactant, 0.1-1% corrosion inhibitor and 0.05-1% hydrogen peroxide stabilizer. Laminating process after copper surface;

4. The chemical tin plating process is adopted as the covering layer on the surface of the copper foil of the inner layer.