PCB pink circle

First, the definition of pink circle

After oxidation, the surface of the board forms a layer of fluff (copper oxide and cuprous oxide). In essence, the fluff will be eroded by acid or reducing solution, so that the original black or reddish brown fluff is reddish;

After the board is pressed and drilled and other subsequent processes, the copper color ring with obvious contrast color contrast is formed around the hole, which is called pink circle.

The "Pink Circle" is the "pink circle" of the pink bare copper surface produced by the dissolution of the surface layer copper foil oxide in the multilayer printed circuit board manufacturing process.

Second, the cause of the pink circle

1. Blackening---Because of the shape of the blackened fluff and the thickness of the fluff, it will cause different degrees of pink circles, but this blackening fluff cannot effectively prevent the pink circle from happening.

2. Press----The lack of bonding between the resin and the oxide layer due to poor press-fit (pressure, heating rate, flow rate, etc.) causes the formation of acid intrusion void path.

3. Drilling---In the borehole, due to stress and high heat, the resin and the oxide layer are delaminated or broken, so that the acid solution invades and dissolves.

4. Chemical copper---The acid exists in the process of the through-hole, and the villi are dissolved.

Third, the 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, quality - pink circles represent partial stratification, more holes are broken.

3, the process - the emergence of the pink circle under the increasingly high precision demand represents the instability of the process.

4. Cost---The number of drilled holes on the drilling machine, and the combination of different film substrates containing the rubber content is affected by it.

Fourth, the method to improve the occurrence of pink circles

1. Improve the thickness and shape of oxidized fluff

2. Storage and stacking of substrates

3. Improvement of the pressing process conditions

4. Improvement of drilling conditions

5, the improvement of the wet process

The method to solve such problems, the application practice proves that the following methods can achieve good results:

1. The oxidized surface of the inner layer copper foil is reduced by an alkaline liquid containing dimethylborane as a main component, and the reduced metallic copper can enhance acid resistance and improve adhesion;

2. Treating copper surface whiskers with a sodium thiosulfate reducing solution with a pH of 3-3.5, acid leaching and passivation, and forming a coating layer of copper and cuprous oxide mixture via ESCA;

3. Treat the inner layer 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. The copper surface is then subjected to a lamination process;

4. The electroless tin plating process is used as a coating layer on the surface of the inner layer copper foil.