How are rigid flex PCBs made?


When drilling a single-sided soft board, pay attention, and ensure the glue side is up to prevent nail heads. If the nail head faces the glue surface, the bonding force will be reduced.

2 Desmear

Generally, there are four methods for removing drilling pollution: sulfuric acid method, plasma method, chromic acid method, and potassium permanganate method.

PI produced less drilling stains in the rigid-flex board, while modified FR4 and acrylic acid produced more drilling stains.

Modified epoxy drilling dirt can be removed with concent rated sulfuric acid, while acrylic acid can only be removed with chromic acid. Polyimide is inert to concentrated sulfuric acid and is not resistant to strong alkalis (potassium permanganate). PI will swell in strong alkalis. The same chemical treatment method cannot remove the drilling dirt of the rigid-flex PCB.

Plasma uses a radiofrequency energy generator to make ions, electrons, free radicals, free radicals, etc., lose their electrical properties under vacuum, and show neutrality. At this time, the drilling of various resin types can quickly and evenly pass from the hole. The wall is removed, and a certain bite is formed to improve the reliability of the metalized hole.

When using Plasma to remove the hard and soft board hole drilling dirt, the biting speed of various materials is different, from large to small:

Acrylic, epoxy, polyimide, glass fiber, and copper,

From a high-power microscope, it is evident that there are prominent glass fiber heads and copper rings. In order to remove the fiber heads and copper rings, it is usually adjusted with a very low concentration of alkali after degreasing of PTH (usually KOH), of course. Rinse with high-pressure water. (PI is not resistant to strong alkalis)

3.Chemical copper deposition:

PTH of flex board commonly used black hole process or shadow process (Shadow)

The electroless copper of the rigid flex PCB is the same as the principle of the chemical copper of the rigid board.

The flexible material polyimide is not resistant to strong alkalis. As a result, the pretreatment of copper sinking should use acidic solutions. Acid colloidal palladium should be used for activation rather than alkaline ion palladium.

At present, most chemical copper deposits are alkaline, so the reaction time and the concentration of the solution must be strictly controlled. If the reaction time is long, the polyimide will swell. The insufficient reaction time will cause cavities in the holes and poor mechanical properties of the copper layer. Although the board can pass the electrical test, it often fails to pass the thermal shock or the user’s assembly process.

4.Copper plating:

To maintain the flexibility of the flex board, sometimes choose the plated copper, called Button Plate. (The pattern transfer of the plated hole is done before the selective plating)

The electroplating principle is the same as the hard board.

5.Graphics transfer:

Same process as the rigid board.

6.Etching and film removal:

Etching: The etching solution mainly includes acidic copper chloride and alkaline copper chloride etching solution. Since there is polyimide on the flexible board, acid etching is mostly used.

Film removal: same process as rigid PCB

Pay special attention to the infiltration of liquid at the rigid-flex joint, causing the rigid-flex pcb joint board to be scrapped.


Laminating is to press copper foil, P sheet, inner flexible circuit, and outer rigid circuit into a multilayer board.

The lamination of the rigid-flex board is different from the lamination of the soft board or rigid board. It is necessary to consider the problem of easy deformation of the flexible board during the lamination process. Also, the smooth surface after the rigid board is laminated is of concern as well. For the issue of sex, we must also consider the protection of the flexible window at the junction of the two rigid areas.

Lamination control points:

1)The amount of glue flowing in No-Flow PP prevents excessive glue from

2) Because No-Flow PP has an opening, there will be a loss of pressure during lamination, so use a conformal sheet and release film when laminating.

No Flow’s PP needs to open a window at the rigid-flex joint (using a gong or punching method). After the outer green oil is completed, the rigid part of the rigid-flex joint is uncovered and laminated during the appearance processing.

3) Before lamination, the rigid outer layer and flexible inner layer must be baked to eliminate latent thermal stress and ensure the quality and dimensional stability of the hole metallization.

4) Appropriate cushioning materials should be selected. The ideal cushioning material should have good conformability, low fluidity, and non-shrinking characteristics during cold and hot processes to ensure that the lamination is free of bubbles and flexible materials during the lamination process. This is to make sure no deformation can occur.

Quality inspection after lamination: Check the appearance of the board to see if there are quality problems such as delamination, oxidation, and glue overflow. Also, the peel strength test should be carried out to ensure quality.

8.Surface Finish

After the protective film (or solder mask) of the flexible board is laminated, the bare copper surface to be soldered must be made of organic soldering preservatives (Organic Solderability Preservatives; OSP), hot air leveling (HASL), nickel gold or Electric nickel gold)

Quality control points on the surface of the rigid-flex board

  1. Thickness,
  2.  Hardness,
  3. Porosity,
  4. Adhesion

Appearance: exposed copper, copper surface pinhole/dent/scratch/yin and yang color.

9.Profile processing:

Most of the profile of high volume flexible board is mold-cutting.small volume is laser. The process is as follows:

Mold design → mold making → test beer → (first board) measurement of size → production.

When CNC the rigid-flex board, special attention should be paid to the uneven shape and rough edges caused by the easy distortion of the flexible part.

In order to ensure the accuracy of the outline processing size, the process method of adding the gasket and the thickness of the rigid plate is adopted and should be fixed or compressed when CNC.

In addition, low feed and high speed will cause the edges of the board to be burnt. Meanwhile, the high feed and low speed will break the knife and the edge of the board.

Rigid-flex board and flexible board are glued to open windows, cover film open windows, base materials open windows, and reinforced processing with CNC and punching methods can also be used.

Leave a comment

Please note, comments must be approved before they are published