What is FPC flexible circuit board surface plating knowledge

Electroplating is a process in which metal or alloy is deposited on the surface of a workpiece by using the principle of electrolysis to form a uniform, dense and well-bonded metal layer. During electroplating, the plated metal is used as the anode, which is oxidized into cations and enters 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 plated layer.

In order to eliminate the interference of other cations and make the coating uniform and firm, a solution containing metal cations in the coating should be used as the electroplating solution to keep the concentration of the metal cations in the coating unchanged. The purpose of electroplating is to coat a metal coating on the substrate to change the surface properties or dimensions of the substrate. Electroplating can enhance the corrosion resistance of metals (the plated metals are mostly corrosion-resistant metals), increase hardness, prevent wear, improve electrical conductivity, lubricity, heat resistance, and surface beauty.

1. Pretreatment of FPC electroplating

The surface of the copper conductor exposed by the FPC coating process of the flexible circuit board may be contaminated with adhesive or ink, as well as oxidation and discoloration caused by the high temperature process. In order to obtain a tight coating with good adhesion, the conductor must be Surface contamination and oxide layers are removed, leaving conductor surfaces clean. However, some of these contaminations are very firmly combined with copper conductors and cannot be completely removed with cleaning agents. Therefore, most of them are often treated with alkaline abrasives with a certain strength and polishing brushes. Most of the coating adhesives are epoxy resins. type, its alkali resistance is poor, which will lead to a decrease in bonding strength. Although it will not be clearly visible, in the FPC electroplating process, the plating solution may infiltrate from the edge of the cover layer, and in severe cases, the cover layer will be peeled off. The phenomenon of solder drilling under the overlay occurs during final soldering. It can be said that the pretreatment cleaning process will have a significant impact on the basic characteristics of the flexible printed board FPC, and full attention must be paid to the processing conditions.

Second, the thickness of FPC plating

During electroplating, the deposition speed of the electroplated metal is directly related to the electric field strength, and the electric field strength varies with the shape of the circuit pattern and the positional relationship of the electrodes. Generally, the thinner the line width of the wire, the sharper the terminal at the terminal, and the distance from the electrode. The closer the electric field strength is, the thicker the coating will be. In applications related to flexible printed boards, there are many cases where the width of many wires in the same circuit is very different, which makes it easier to produce uneven thickness of the coating. 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 that the thickness of the coating on all parts is uniform to the greatest extent. Therefore, efforts must be made in the structure of the electrodes. A compromise solution is proposed here. The standards for the parts with high requirements on the uniformity of the coating thickness are strict, and the standards for other parts are relatively relaxed, such as lead-tin plating for fusion welding, gold-plating for metal wire lap (welding), etc. High, and for general anti-corrosion lead-tin plating, the plating thickness requirements are relatively relaxed.

Third, smudges and dirt from FPC plating

There is no problem with the state of the coating that has just been plated, especially the appearance, but some surfaces will appear smudges, dirt, discoloration and other phenomena soon afterward. , found to have cosmetic problems. This is caused by insufficient drifting and residual plating solution on the surface of the plating layer, which slowly undergoes a chemical reaction over a period of time. Especially the flexible circuit board, because it is soft and not very flat, it is easy for various solutions to accumulate in the concave? Then it will react in this part and change color. In order to prevent this from happening, it is not only necessary to fully float, but also to Fully dry. Whether or not the drift is sufficient can be confirmed by a high-temperature thermal aging test.

Fourth, FPC hot air leveling

Hot air leveling is a technology developed for the coating of lead and tin on rigid printed circuit boards (PCBs). Hot air leveling is to directly immerse the board in the molten lead and tin bath vertically, and the excess solder is blown away 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 taking any measures, the flexible printed board FPC must be sandwiched between the silk 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 fluxed 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 the underside of the cover layer, especially when the bonding strength between the cover layer and the surface of the copper foil 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 moisture will cause the cover layer to foam or even peel off due to rapid thermal evaporation. Therefore, before FPC hot air leveling, it must be dried and moisture-proof manage.

Fifth, 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 solutions used in electroless gold plating have strong chemical effects, and the electroless gold plating process is a typical example. The chemical gold plating solution is an alkaline aqueous solution with a very high pH value, so if the flexible FPC has a gold plating process, a gold-resistant solder resist insulating ink needs to be screen printed. When using this electroplating process, it is easy for the plating solution to penetrate under the cover layer, especially if the quality control of the cover 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 displacement reaction is more prone to the phenomenon that the plating solution penetrates into the cover layer, and it is difficult to obtain ideal plating conditions by this process.