PCB Technical - FPC surface electroplating process introduction

Electroplating uses the principle of electrolysis to deposit metal or alloy on the surface of the workpiece to form a uniform, dense, and good bonding metal layer. During electroplating, the plated metal is used as the anode and oxidized into cations to enter the electroplating solution; the metal product to be plated is used as the cathode, and the cations of the plated metal are reduced on the metal surface to form a plating layer. In order to eliminate the interference of other cations and make the coating uniform and firm, it is necessary to use a solution containing the coating metal cations as the electroplating solution to keep the concentration of the coating metal cations unchanged. The purpose of electroplating is to plate a metal coating (deposit) on the substrate to change the surface properties or size of the substrate. Electroplating can enhance the corrosion resistance of the metal (the coating metal is mostly corrosion-resistant metal), increase the hardness, prevent abrasion, and improve Electrical conductivity, lubricity, heat resistance, and beautiful surface. This article mainly introduces some basic knowledge of FPC surface electroplating.

(1) Pretreatment of FPC electroplating: the copper conductor surface exposed by the coating process of the flexible printed board FPCmay 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 conductor surface must be removed 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. 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 the flexible printed circuit board F{C, and full attention must be paid to the processing conditions.

(2) Thickness of FPC electroplating: During electroplating, the deposition speed of the electroplated 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 more The sharper the terminal, the closer the distance to the electrode, the greater the electric field strength, and the thicker the plating layer 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 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) FPC electroplating stains and dirt: the state of the plating layer that has just been electroplated, especially the appearance, there is no problem, but there will be stains, dirt, discoloration, etc. on the surface soon, especially when it is not found during the factory inspection What's wrong, but when the user checks the reception, 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. Especially flexible printed boards, because they are soft and not very flat, various solutions are prone to "accumulate?" in their recesses, and then react and change color in this part. In order to prevent this, not only must be fully drifted, but also It also needs to be fully dried. The high temperature thermal aging test can be used to confirm whether the drift is sufficient.

2. FPC hot air leveling

Hot air leveling was originally developed for the application of lead and tin on rigid printed boards PCB. Because this technology is simple and convenient, it has also been applied to flexible printed boards 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 will cause the cover layer to bubble or even peel off due to the rapid heat evaporation. Therefore, the FPC hot air leveling must be dried and moisture-proof before the FPC hot air leveling process manage.

3. FPC 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. When using this kind of electroplating process, it is easy for the plating solution to drill under the covering layer, especially if the quality management 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 by electroplating with this process.

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