PCB Technical - Surface procession of FPC

1. FPC plating

(1) Pretreatment of FPC electroplating. The copper conductors exposed by the FPC after the masking layer process may have adhesive or ink contamination, and oxidation and discoloration due to high temperature processes. A tight coating with excellent adhesion is necessary to remove the contamination and oxide layer on the surface of the conductor 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. Most of the masking layer adhesives are Epoxy resin has poor alkali resistance, which will lead to a decrease in bonding strength, which of course will not be noticeable. However, in the FPC electroplating process, the plating solution may penetrate from the edge of the masking layer, which will cover it in severe cases Layer peeling. In the final soldering, the phenomenon that the solder penetrates under the mask layer appears. It can be said that the pre-treatment cleaning process will have a major impact on the fundamental characteristics of the flexible printed circuit board FPC, and it is necessary to pay sufficient attention to the processing conditions.

(2) The 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 of the electrode. Generally, the thinner the line width of the wire, the terminal at the terminal The sharper, the closer the distance from the electrode, the greater the electric field strength, and the thicker the plating layer at that 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 the occurrence of this situation, a shunt cathode pattern can be attached around the circuit., Absorb the uneven current scattered on the electroplating pattern, and ensure the uniform thickness of the plating layer on all parts to the maximum limit. Therefore, it is necessary to work hard on the electrode structure. A compromise is proposed here. The specifications for parts requiring high coating thickness uniformity are strict, while the specifications for other parts are relatively relaxed, such as lead-tin plating for fusion welding, and gold plating for metal wire overlap (welding)., As for the lead-tin plating for general anti-corrosion, the plating thickness requirements are relatively relaxed.

(3) The stains and dirt of FPC electroplating. The condition of the newly electroplated coating, especially the appearance, does not have any title, but soon afterwards, some appearances showed stains, dirt, discoloration, etc., especially when the factory inspection did not find any What's wrong, but when the user checked the acceptance, it was found that there was an appearance title. This is caused by insufficient drifting, and there is residual plating solution on the surface of the coating, which is caused by a slow chemical reaction after a period of time. Especially the flexible printed board, because it is soft and not very flat, it is easy to have various solutions "accumulate?", and then the part will react and change color. In order to avoid the onset of this situation, it is not only necessary to carry out sufficient drifting, but also It also needs to be fully dried. It can be confirmed by high temperature heat aging test whether the drifting is sufficient.

2. FPC electroless plating

When the line conductor to be plated is isolated and cannot be used as an electrode, only electroless plating can be performed. Generally, the plating solution used in electroless plating has intense chemical action, 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 get under the masking layer, especially if the quality management of the masking film lamination process is not strict and the bonding strength is low, this title is more likely to occur. Due to the characteristics of the plating solution, electroless plating with replacement reaction is more prone to the phenomenon of the plating solution drilling under the masking layer. It is difficult to obtain the ideal plating conditions for electroplating with this process.

3. FPC hot air leveling

Hot air leveling was originally a skill developed for the rigid printed board PCB coating with lead and tin. Because this skill is simple, it has also been applied to the flexible printed board FPC. Hot air leveling is to immerse the board directly into the molten lead-tin bath, and blow the excess solder with hot air. This condition is very harsh for the flexible printed board FPC. Assuming that the flexible printed board FPC cannot be immersed in the soldering guess without any measures, it is necessary to clamp the flexible printed board FPC to the titanium steel The center of the screen is then immersed in the fusion welding process. Of course, the surface of the flexible printed circuit FPC must be cleaned and flux coated beforehand. Because of the harsh conditions of the hot air leveling process, it is easy to cause the solder to drill from the end of the masking layer to below the masking layer, especially when the bonding strength between the masking layer and the copper foil surface is low, this phenomenon is more likely to occur frequently. Because the polyimide film is easy to absorb moisture, when the hot air leveling process is selected, the moisture absorbed will cause the masking layer to bubble or even peel off due to the rapid heat transpiration. Therefore, it is necessary to perform a dry treatment and Moisture-proof management.