FPC surface plating process

1. FPC plating

(1) FPC plating pretreatment

The copper conductor surface exposed by the FPC after the coating process may be contaminated by adhesive or ink, and there may also be oxidation and discoloration due to high-temperature processes. If you want to obtain a dense coating with good adhesion must remove the contamination and oxide layer on the conductor surface to make the conductor surface 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.

Most of the coating adhesives are Epoxy resin and have poor alkali resistance, which will lead to a decrease in bonding strength, although it will not be visible. However, in the FPC plating process, the plating solution may penetrate from the edge of the covering layer, and it will cause the covering layer in severe cases. Peel off. In the final soldering, 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 the flexible printed circuit board FPC, and full attention must be paid to the processing conditions.

(2) The thickness of FPC plating

During plating, the deposition speed of the plated metal is directly related to the electric field intensity. The electric field intensity changes with the shape of the circuit pattern and the position relationship of the electrode. Generally, the thinner the line width of the wire, the terminal of the terminal The sharper, the closer the distance from the electrode, the greater the electric field strength, and the thicker the coating at this part. 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 plating 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). The standards are higher. , 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 plating.

The state of the plating layer that has just been plated, especially the appearance, is not a problem. However, there will be stains, dirt, discoloration, and other phenomena on the surface soon, especially during the factory inspection. What’s wrong, but when the user conducts a reception check, 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. 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 needs to be fully dried. The high-temperature thermal aging test can be used to confirm whether the drift is sufficient.

2. FPC electroless plating

When the line conductor to be plated 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 value. When using this plating process, it is easy for the plating solution to drill under the covering layer, especially if the quality of the covering film lamination process is not strictly controlled 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 plating with this process.

3. FPC hot air leveling

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

Due to the harsh conditions of the hot air leveling process, it is easy for 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 easily absorbs 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, before the FPC hot air leveling process, it must be dried and moisture-proof. Governance.