Analysis on Electroplating Technology of High Aspect Ratio Multilayer Board (2)

EIR＝IL2/2KD
During electroplating, according to the characteristics of the multilayer board, in order to make the current evenly distributed on the inner surface of the hole on the entire board, in addition to adding auxiliary cathodes around the board surface, the process parameters must be adjusted to ensure the uniform distribution of the coating in the hole. From the analysis of the electroplating principle, to scientifically adjust the existing process parameters, it is necessary to understand the relationship between the current and the thickness to be plated. The thickness drop of the coating (also called “potential difference EIR”) is determined by the following formula:

EIR=IL2/2KD

In the formula, I-DK, L-plate thickness, K-conductivity, D-aperture.

It can be seen from the above formula that reducing the cathode current density, increasing the conductivity of the solution or reducing the potential difference makes deep hole plating possible. However, if a small current density is adopted, it can only increase the electroplating time, which will inevitably reduce the production efficiency. More importantly, it cannot ensure the integrity of the deep-hole electroplating copper layer. It is possible that the current has not yet reached the center of the hole, and part of the conductive layer has been dissolved by acid. However, it is also feasible to appropriately reduce the current density, because it can increase the dispersion ability of the plating solution to a certain extent, and improve the ability of the plating solution for deep-hole electroplating. Under the premise of not affecting the etching quality, the process method of full-board electroplating + pattern electroplating is adopted to solve the quality problem of deep-hole electroplating. The specific method is to increase the original full plate electroplating time from 15-25 minutes to 40 minutes. The original pattern electroplating time is reduced from the original 60 minutes to 35-40 minutes. The reason is very clear, that is, to improve the conductivity of both sides, so that the deep holes that form the two sides of the conductive can also increase the uniform distribution of the current on the surface, and also achieve the uniform distribution of the metal on the surface of the hole wall.

According to the electrolyte solution theory, conductivity represents the conductance of a cubic centimeter of solution with a side length of 1 cm, and conductance is a physical quantity that expresses the electrical conductivity of a conductor. It is known from the experiment that the conductivity K value in the electroplating copper solution is greatly affected by the sulfuric acid content. That is, the higher the sulfuric acid content, the greater the K value, and the smaller the EIR; when the sulfuric acid reaches 200 grams per liter, the K value tends to be stable. The deep hole theory holds that the real effect of deep hole electroplating is determined by the ratio of sulfuric acid to copper sulfate. The larger the ratio, the stronger the deep plating ability of the plating solution. According to this theory, the concentration of sulfuric acid is 210 grams per liter, the concentration of copper sulfate is 55 grams per liter, the ratio is 4, and the ratio of acid to copper is 16. In addition, to increase the temperature of the electroplating solution, reduce the degree of hydration ions, reduce the radius of hydration ions, and also reduce the viscosity of the solution, strengthen the fluidity of the solution, and speed up the movement of ions, which is beneficial to the conductance rate increasing. However, it is necessary to properly increase the solution temperature, because if the temperature is too high, the current density can be increased, but the consumption of additives will also increase, it is easy to lose balance, and the leveling and brightness of the coating will decrease.

3. Conclusions and Recommendations

Adopt pulsed horizontal copper electroplating process. According to the inspiration of the pulse electroplating process method used in electroforming copper, in order to solve the problem of copper electroplating in deep holes or deep blind holes, the “timed counter current or timed counter pulse” power supply method is proposed to try to solve the problems faced in the production process of printed circuit boards‘ Technical difficulties of deep hole plating. Of course, pulse electroplating is not a new process in the printed circuit board manufacturing industry. The pulse power supply is different from the DC power supply. It switches the rectifier on/off at a speed of (s through a switching element, and provides a pulse signal to the cathode. When the rectifier is in the closed state, it is more effective than DC to replenish copper to the boundary layer in the hole, so that the thickness of the copper plating layer on the inner wall of the high aspect ratio hole can be more uniform.

Solve the problem of electroplating of deep holes or deep blind holes (aspect ratio of 1:1 or more – for blind holes) on multi-layer boards and laminates. It is compared with the conventional power supply method electroplating copper, and its data list is as follows:

Sample plate Hole length (plate thickness) (mm) Aperture (mm) Aspect ratio Current density ASD
Pulse copper electroplating DC electroplating copper current ratio (%) positive and negative time ratio (ms) distributed force (%) full time (min) distributed force (%) full time (min)
A 2.4 0.3 8:1 3.3 310 20/1.0 92 58 75 113
B 3.2 0.3 10.7:1 3.0 250 20/1.0 78 45 70-75 70
C 3.2 0.35 9.2:1 3.0 250 20/1.0 85 45 80-85 70
D 1.5 0.40 4:1 3.0 250 15/0.5 112 60 103 120
Plate thickness = 3.2mm
Aperture = 0.8mm
Current density = 3.2A/dm2
Copper plating time = 50 minutes

4. Conclusion

In the conventional electroplating process, in order to solve the problem of deep-hole electroplating of multilayer boards, according to the electroplating theory and practical experience, the electroplating process parameters and the ratio of sulfuric acid and copper sulfate in the plating solution are adjusted appropriately, so as to improve the high-aspect-ratio deep-hole electroplating of multilayer boards. The qualified rate of quality will be greatly improved, but when the aspect ratio is very high, it is recommended to use the pulsed horizontal plating process.