What issues should be considered in the surface plating of FPC (Flexible Printed Circuit) boards

What issues should be considered when electroplating the surface of FPC (Flexible Printed Circuit) boards? Let's briefly understand them below:

1、Electroplating of flexible circuit board

After the pre-treatment process of FPC (Flexible Printed Circuit) electroplating, the copper conductor surface exposed after the masking layer coating process may be contaminated with adhesive or ink, and there may also be oxidation and discoloration due to high-temperature processes. To obtain a closely attached plating layer with excellent adhesion, it is necessary to remove the contamination and oxide layer on the conductor surface to keep it clean.

However, some of these contaminants are very stable when fused with copper conductors and cannot be removed with weak scrubbing agents. Therefore, alkaline abrasives with certain strength are often used in combination with polishing brushes for treatment. Most masking layer adhesives are epoxy resins with poor alkali resistance, which can lead to a decrease in bonding strength. Although this may not be visibly noticeable, in the FPC electroplating process, the plating solution may penetrate from the edge of the masking layer, which can cause the masking layer to peel off in severe cases. During soldering, there may be a phenomenon where solder penetrates into the bottom of the masking layer.

2、Electroless plating of flexible circuit boards

When the circuit conductor to be electroplated is isolated and cannot serve as an electrode, electroless plating is the only option. Typically, the plating solutions used in electroless plating exhibit strong chemical reactions, with electroless gold plating being a prime example. The electroless gold plating solution is an alkaline aqueous solution with a relatively high pH value. During the application of this electroplating process, it is easy for the plating solution to penetrate beneath the masking layer, especially if the quality of the masking film lamination process is not strictly controlled and the bonding strength is low.

Due to the characteristics of the plating solution, the phenomenon of the plating solution penetrating into the underlying masking layer is more likely to occur in chemical plating through displacement reaction. It is difficult to achieve the desired plating conditions using this process.

3、Thermal air leveling of flexible circuit board

Hot air leveling, originally a technique developed for coating rigid printed circuit boards (PCB) with lead-tin, has also been applied to flexible printed circuit boards (FPC) due to its simplicity. Hot air leveling involves directly immersing the board being manufactured vertically into a molten lead-tin bath, and using hot air to blow away excess solder. These conditions are quite harsh for FPC. If no measures are taken for FPC, it cannot be immersed in solder. Therefore, it is necessary to sandwich the FPC between titanium steel screens and immerse it in molten solder. Of course, the surface of the FPC must also be cleaned and coated with flux beforehand.