1、 Definition of pink circle

After oxidation, a fluff layer (copper oxide and cuprous oxide) is formed on the surface of the plate. In essence, the villi will be eroded by acid or reducing solution, making the original black or reddish brown villi appear red copper color;

After the board is pressed and drilled, there is a copper color ring around the hole, which is called pink circle.

In the manufacturing process of multilayer printed circuit board, the pink bare copper surface produced after the oxide on the surface of copper foil of inner layer board is dissolved

The pink circle.

2、 The cause of formation of pink circle

1. This kind of black circle can not prevent the formation of black circle.

2. Press fit - due to poor press (pressure, heating rate, glue flow rate, etc.), the adhesion between the resin and the oxide layer is insufficient, resulting in the formation of acid solution invading the pore path.

3. Drilling: in the borehole, due to the stress and high heat, the resin and oxide layer are stratified or cracked, resulting in acid intrusion and dissolution.

4. Chemical copper --- acid solution exists in the process of through hole, which makes villi dissolve.

3、 The influence of pink circle

1. Appearance --- in the trend of small holes, can no longer effectively cover up, resulting in poor appearance.

2. Quality --- pink circle represents the local part of the layer, more hole broken.

3. In the process --- in the increasingly high precision demand, the appearance of pink circle represents the instability of the process.

4. Cost --- the number of drilling pieces on the drilling machine, the use of film substrate with different glue content is affected by it.

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4、 How to improve the occurrence of pink circle

1. Improve the thickness and shape of oxidized fluff

2. Storage, use and stacking of base materials

3. Improvement of pressing process conditions

4. Improvement of drilling conditions

5. Improvement of wet process

The application practice shows that the following methods can achieve good results

1. The oxidation surface of inner layer copper foil was reduced by alkaline solution with dimethylborane as the main component. The reduced metal copper can enhance the acid resistance and adhesion;

2. The whiskers on the surface of copper were treated with sodium thiosulfate solution with pH value of 3-3.5. After acid leaching and passivation, the composite coating layer of copper and cuprous oxide was formed by ESCA;

3. The copper surface was treated with a mixture of 1-2% hydrogen peroxide, 9-20% inorganic acid, 0.5-2.5% tetraamino cationic surfactant, 0.1-1% corrosion inhibitor and 0.05-1% hydrogen peroxide stabilizer before laminating;

4. The electroless tin plating process was used as the covering layer on the surface of copper foil.