PCB Tech - Introduction to FPC soft board surface electroplating

Flexible circuit board plating

(1) The pretreatment of FPC soft board electroplating. The copper conductor surface exposed by the FPC after the coating process may be contaminated with adhesive or ink, and there may also be oxidation and discoloration due to high temperature processes. To obtain a dense coating with good adhesion, the contamination and oxide layer on the surface of the conductor must be removed to make the surface of the conductor clean.

However, some of these pollutions are very strong in combination with copper conductors and cannot be completely removed with weak cleaning agents. Therefore, most of them are often treated with a certain strength of alkaline abrasives and brushing. The covering adhesives are mostly ring Oxygen resins have poor alkali resistance, which will lead to a decrease in bonding strength, although it will not be visible, but in the FPC electroplating process, the plating solution may penetrate from the edge of the cover layer, and the cover layer will peel off in severe cases. . In the final welding, the solder penetrates under the covering layer.

It can be said that the pre-treatment cleaning process will have a significant impact on the basic characteristics of flexible printed boards, and full attention must be paid to the processing conditions.

(2) The thickness of FPC electroplating During electroplating, the deposition speed of electroplated metal is directly related to the electric field intensity, and the electric field intensity changes with the shape of the circuit pattern and the position relationship of the electrode. According to the online understanding of the coating, the line width of the general wire is more Thinner, the sharper the terminal at the terminal site, and the closer the distance to the electrode, the greater the electric field strength, and the thicker the plating layer at this site.

In applications related to flexible printed boards, there is a situation where the width of many wires in the same circuit is very different, which makes it easier to produce uneven plating thickness. In order to prevent this from happening, a shunt cathode pattern can be attached around the circuit., Absorb the uneven current distributed on the electroplating pattern, and ensure the uniform thickness of the coating on all parts to the greatest extent. Therefore, efforts must be made in the structure of the electrode. A compromise is proposed here. The standards for parts that require high coating thickness uniformity are strict, while the standards for other parts are relatively relaxed, such as lead-tin plating for fusion welding, and gold plating for metal wire overlap (welding). High, and for the lead-tin plating used for general anti-corrosion, the plating thickness requirements are relatively relaxed.

(3) The stains and dirt of FPC electroplating. The state of the plated layer that has just been electroplated, especially the appearance, is not a problem, but soon afterwards, some of the surfaces appear stains, dirt, discoloration, etc., especially the factory inspection did not find any What is different, but when the user conducts a reception inspection, it is found that there is an appearance problem. This is caused by insufficient drifting, and there is residual plating solution on the surface of the plating layer, which is caused by the slow chemical reaction after a period of time.

In particular, flexible printed boards are not very flat due to their softness, and various solutions are prone to "accumulate?" in the recesses, which will then react and change color in this part. In order to prevent this from happening, not only must be fully drifted, but also It is also necessary to carry out sufficient drying treatment. The high temperature thermal aging test can be used to confirm whether the drift is sufficient.

Flexible circuit board electroless plating

When the line conductor to be electroplated is isolated and cannot be used as an electrode, electroless plating can only be performed. Generally, the plating solution used in electroless plating has a strong chemical effect, and the electroless gold plating process is a typical example. The electroless gold plating solution is an alkaline aqueous solution with a very high pH. According to Coating Online, when using this kind of electroplating process, it is easy for the plating solution to drill under the covering layer, especially if the quality control of the covering film lamination process is not strict and the bonding strength is low, this problem is more likely to occur.

Due to the characteristics of the plating solution, the electroless plating of the displacement reaction is more prone to the phenomenon that the plating solution penetrates under the covering layer. It is difficult to obtain ideal plating conditions for electroplating with this process.

Hot air leveling of flexible circuit boards

Hot air leveling was originally a technology developed for the rigid printed board PCB coating with lead and tin. Because this technology is simple, it has also been applied to the flexible printed board FPC. Hot air leveling is to immerse the board in a molten lead-tin bath directly and vertically, and blow off the excess solder with hot air. This condition is very harsh for the flexible printed board FPC. If the flexible printed board FPC cannot be immersed in the solder without any measures, the flexible printed board FPC must be clamped between the screen made of titanium steel, And then immersed in the molten solder, of course, the surface of the flexible printed board FPC must be cleaned and coated with flux in advance.

Due to the harsh conditions of the hot air leveling process, it is easy to cause the solder to drill from the end of the cover layer to under the cover layer, especially when the bonding strength of the cover layer and the copper foil surface is low, this phenomenon is more likely to occur frequently. Since the polyimide film is easy to absorb moisture, when the hot air leveling process is used, the moisture absorbed by the hot air will evaporate rapidly and cause the cover layer to bubble or even peel off. Therefore, it is recommended that the coating line must be carried out before the FPC hot air leveling process. Drying treatment and moisture-proof management.