Knowledge of FPC Surface Plating

This Gold Phoenix PCB Knowledge Center article explains knowledge of FPC (Flexible Printed Circuit) surface plating.

FPC means PCB made of flexible material. It meets the need of high density, high reliability, miniaturization and lightweight development of electronic products, and also meets the strict economic requirements and the need of market and technology competition.

1. FPC Electroplating

(1) Pre-Processing Before FPC Electroplating

After cladding coating, the exposed surface of the copper conductor may have adhesive or ink pollution, and there will also be oxidation and discoloration generated due to the high temperature process. To obtain good tight adhesion coating, the pollution and oxide layer on the surface of the conductor must be removed to achieve a clean conductor surface.

But some of these pollutants combine with copper conductor very strongly, and weak detergent cannot completely remove them. Therefore, most often a certain intensity of alkaline abrasives and brush are used for processing. Most layer adhesives are a kind of epoxy resin with poor alkali resistant performance, which can cause a decline in bonding strength. Though not visible, in the FPC electroplating process, plating solution is likely to seep from the edge of the cover. In serious cases, overburden stripping occurs.

During the final soldering, the solder can appear under the overburden.

It can be said that the pretreatment cleaning process will have a significant impact on the basic characteristics of the flexible printed circuit board.

(2) FPC Electroplating Thickness

The thickness of the FPC electroplating and plating metal deposition rate is directly related to the intensity of the electric field and with the shape of the line graph, the position relationship between electrodes. The general wire line width of the thinner, terminal parts of the terminal is pointed, the greater the closer the distance of the electric field intensity and electrode, and the parts of the coating is thicker.

In use related to flexible PCB, in the same line width difference, many conductors in certain conditions exist where it is easier to produce coating thickness not uniform. To prevent this kind of issue from happening, a diversion cathode graphics can be attached around the line to absorb the uneven current distribution in electroplating, ensuring the coating thickness is as even as possible across all parts.

So attention must be paid to the structure of the electrode.

A compromise is proposed here: for the part with high standard strict coating thickness uniformity requirements, and for the other parts where the standard is relatively relaxed. For example, the fusion welding lead, tin plating, and metal wire by (welding) of gold plated layer standard is higher, while for general corrosion of lead, tin plating, the plating thickness required is relatively relaxed.

(3) FPC Electroplating Stain and Dirt

Just been electroplated, the coating state shows no problem with the appearance, especially at the factory inspection. However, some surface stains, dirt, discoloration and other phenomena appear soon after. In particular, no abnormal appearance is found in the factory inspection.

This is due to insufficient rinsing — the coating surface has residual liquid that causes a slow chemical reaction after a period of time.

In particular, flexible printed circuit board, due to being soft and not very smooth, has concave areas where various solutions can accumulate.

To prevent the occurrence of this situation, not only sufficient rinsing, but also sufficient drying treatment should be carried out.

The high temperature thermal aging test can confirm whether the rinsing is sufficient.

2. FPC Electroless Plating

When the conductor of a line to be electroplated is isolated and cannot be used as an electrode, only chemical plating can be carried out.

Generally, the plating solution used for electroless plating has strong chemical effect. The chemical plating solution is an alkaline solution with a very high pH. This kind of electroplating process can easily occur under the coating layer, especially if the lamination process quality management is not strict and the bonding strength is low.

The chemical plating of replacement reaction is more likely to occur under the covering layer due to the characteristics of the plating solution.

3. FPC Hot Air Leveling for Flexible Circuit Board

Hot air leveling is a technology developed to coat lead and tin on rigid PCB.

Hot air leveling is the direct vertical immersion of the plate into the molten lead tin tank, with the excess solder removed by hot air.

This condition is too harsh for FPC. If the FPC does not take any measures, it cannot be immersed in the solder. The FPC must be sandwiched between titanium and steel wire mesh, and then inserted into the molten solder. In advance, of course, the surface of the flexible PCB FPC also needs to be cleaned and coated with flux.

Due to the harsh hot air leveling process, the phenomenon of solder drilling from the end of the overburden to below the overburden is also easy to occur, especially when the overburden and copper foil surface adhesion strength is low, which is more likely to occur frequently.

As polyimide film is easy to absorb moisture, the moisture absorbed during the hot air leveling process will cause the overburden to bubble or even peel due to rapid evaporation. Therefore, dry treatment and moisture-proof management must be carried out before FPC hot air leveling.