PCB Technical - FPC surface plating of flexible circuit board

FPC is the abbreviation of Flexible Printed Circuit, also known as flexible circuit board, flexible printed circuit board, flexible circuit board, abbreviated as soft board or FPC, PCB LAYOUT training has the characteristics of high wiring density, light weight and thin thickness. Mainly used in mobile phones, notebook computers, PDAs, digital cameras, LCMs and many other products.

FPC flexible printed circuit is a highly reliable and excellent flexible printed circuit made of polyimide or polyester film as the base material.

FPC has three main characteristics: flexibility, reliability and economy.

1. Electroplating of flexible circuit boards

(1) Pre-treatment of FPC electroplating. The copper conductor surface exposed by 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 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. 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. 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 at the terminal The sharper, the closer the distance to 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 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 plating layer that has just been electroplated, especially the appearance, there is no problem, but soon there will be stains, dirt, discoloration and other phenomena on the surface, 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 also needs to be fully dried. The high temperature thermal aging test can be used to confirm whether the drift is sufficient.

2. Electroless Plating of Flexible Circuit Board

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 for electroplating with this process.

3. 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. This phenomenon is more likely to occur frequently when the bonding strength between the cover layer and the copper foil surface is low. 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 process must be dried and moisture-proof before the FPC hot air leveling process manage.