PCB Tech - FPC surface plating knowledge of flexible circuit board

FPC surface plating knowledge of flexible circuit board

1. FPC electroplating on flexible circuit boards

(1) Pre-treatment of FPC electroplating. The exposed copper conductor surface of FPC after the masking layer process may be contaminated with adhesive or ink, and there may also be oxidation and discoloration due to high temperature process. If you want to obtain A dense coating with good adhesion is necessary to remove the contamination and oxide layer on the surface of the conductor, so that the surface of the conductor is clean.

However, some of these pollutions are very firmly combined with copper conductors and cannot be completely removed with weak cleaning agents. Therefore, most of them are often treated with alkaline abrasives and brushes with a certain strength. Most of the masking layer adhesives are 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 enter from the edge of the masking layer, and the masking layer will peel off in severe cases. . In the final welding, the phenomenon that the solder penetrates under the masking layer appears.

It can be said that the pre-treatment cleaning process will have a significant impact on the fundamental characteristics of flexible printed circuit boards FPC, and it is necessary to pay sufficient attention 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 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. This is 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 coating on all parts to the greatest extent. Therefore, it is necessary to work hard on the structure of the electrode.

A compromise plan is proposed here. The specifications for parts with high plating 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). High, and the lead-tin plating for general anti-corrosion, its plating thickness requirements are relatively relaxed.

(3) The stains and dusts of FPC electroplating The state of the plating layer that has just been electroplated, especially the appearance, there is no problem, but soon some of the appearances showed stains, dust, discoloration, etc., especially when the factory inspection did not find any What's wrong, but when the user received and checked, it was found that there was an appearance problem.

This is caused by inadequate drift and residual plating solution on the surface of the coating, which is caused by a slow chemical reaction over a period of time.

Especially the flexible printed circuit board, because it is soft and not very flat, it is easy to have various solutions "accumulate?" Then it will react and change color in this part. In order to avoid the onset of this situation, it is not only necessary to carry out sufficient drifting, but also Adequate and boring treatment is required. High temperature heat aging test can be used to confirm whether there is sufficient drifting.

2. FPC electroless plating on flexible circuit boards When the circuit 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 very easy to cause the plating solution to penetrate 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 problem is more likely to occur.

Due to the characteristics of the electroplating solution, electroless plating with replacement reaction is more likely to cause the phenomenon of the plating solution burrowing under the masking layer. It is difficult to obtain the ideal electroplating conditions for electroplating with this process.

3. Hot air leveling of flexible circuit board FPC Hot air leveling is a skill developed for the rigid printed board PCB with lead and tin. Because this skill is simple and convenient, 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 remaining 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, it is necessary to clamp the flexible printed board FPC to a screen made of titanium steel The center is then immersed in molten solder. Of course, the surface of the FPC must be cleaned and flux-coated in advance.

Because of the harsh conditions of the hot air leveling process, the phenomenon that the solder drills from the end of the masking layer to below the masking layer can easily occur, especially when the bonding strength between the masking layer and the copper foil surface is low, this phenomenon occurs more easily and frequently.

Because the polyimide film simply absorbs 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.
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