What Is the FPC Board Manufacturing Process?

FPC or Flexible Circuit Board, is a reliable and highly flexible PCB designed with polyester film or polyamide. The thin, lightweight, soft board has a high wiring density. Available in single-sided and double-sided versions, the FPC board has a wide range of applications. However, the Flexible Printed Circuit manufacturing process involves some steps.

What Materials Do You Need to Manufacture Your Flexible PCB Board?

FPC PCB is made from 2 types of materials- protective film and substrate. The most common base materials are PET and glue. You also need a copper foil with weight of around 1oz.

Glue or adhesive materials are essential for bonding the FPC material (such as PET or PI) to the copper. Most commonly, engineers use epoxy or acrylic adhesives and thermally activated films for bonding. 

The coverlay comprises a polyimide sheet with an adhesive layer. Its role is the same as that of the rigid PCB. Polyimide laminates with adhesive coating and a solder mask are useful for the FPC manufacturing project.

Besides, you need stiffeners like FR4, polyimide, and aluminum inserts.

FPC Manufacturing Process – A Comprehensive Guide

You should go through some steps for designing the flexible PCB board.

Use Copper Clad Laminates

Copper clad laminates are the base material for your flexible PCB board. They are affordable and customizable to any space. One advantage of using copper clad board is that it helps you form and retain the PCB structure. It ensures components are held in place and prevents them from getting damaged. 

Prepare the Inner Layers for Designing the FPC Board

With the etching process, you need to remove the unwanted copper and design a proper circuit pattern. While etching precise patterns, you should pay attention to your etching device and other conditions. The leftover traces of copper will help you manage connections between different PCB elements. 

If you want to create a multilayer FPC, you should deal with every imaging inner layer. Use lithography for copper patterning and strip the photoresist masks. Evaluate layer quality visually before proceeding with subsequent steps for the FPC manufacturing process.

Stack Up Layers and Consider Lamination 

You need to stack FPC material layers and bond them together. Maintain proper sizes while cutting materials. You can then clean the layers to eliminate debris. Foils and films need to have proper alignment. After applying pressure and heat, you should let them become cool. It will lead to the formation of a laminate that fuses different layers into a board.

Drilling the Holes

As you have created FPC copper laminates, you should create PTH holes with the drilling process. There are various drilling technologies used for the flexible printed circuit manufacturing process. The minimum hole aperture needed for your project is 0.2mm.

You can apply the mechanical drilling technique to create holes on the FPC PCB coverlay.

Laser drilling, body etching, chemical etching, and punching with apertures are common drilling methods. You should also create the hole walls to go through the plating process.

Hole Metallization of FPC Boards

The hole metallization of flexible PCB boards is the same as the metallization process of the rigid printed board. Professionals implement the direct electroplating method and use technology to create a carbon conductive film. However, as the board is soft, you need to use specially designed fixing fixtures, which keep the plating solution stable. Or else the copper plating’s thickness will not be uniform. You may face disconnection issues while etching. You have to tighten the board in the fixture. 

Creating Conductive Patterns

You can use a UV exposure machine to apply the photosensitive method and create a circuit pattern on the pre-coated layer on the copper foil’s surface. In the case of the single-chip FPC board, you should focus on overlapping positioning. Choose a fixture for positioning pattern masks. Some FPC manufacturers design them on their own for convenience.

You can now check the steps for the patterning process. Using fully prepared substrates, photolithographic imaging lets you use circuit conductors. So, go through these steps for successful patterning.

• Photoresist layer application into copper
• Exposure to UV using the patterning artwork
• Removal of the resist selectively
• Etching of the exposed copper parts
• Strip the resist remaining after etching

If you want to create a double-sided circuit, you should repeat these steps.

Using Solder Mask for Fabricating the FPC Board–

Solder mask inks, also known as solder mask bridges, are isolation tapes designed to prevent issues like short circuits. Instead of using the original cover layer, you can choose the solder mask ink. The board ink needs to be printed on the foil circuit.

After producing the circuit board, it has to undergo different processes. To prevent the circuit from getting damaged during production and transportation, you have to apply a protective layer. Let the solder mask ink be cured properly and prevent the contact of foreign objects and the circuit.

Consider the Silkscreening Process

There are different methods for printing the silkscreen legend.

For instance, liquid photo imageable is the most commonly chosen method where you can use light-sensitive ink.

The inkjet printing method involves no masking. Inkjet-deposited graphics need direct application. The minimum layer thickness needs to be 0.1 mils.

The silkscreen is highly useful because it minimizes the risk of errors and reduces the time needed to detect the error. The PCB’s passive components need to be labeled easily.

While dealing with the silkscreening process, you should focus on some important things. For instance, you must not print silkscreen epoxy over PCBs that you want to solder. It may cause the solder joint to melt. So, you should apply it over your solder mask.

Epoxy ink is highly durable and resistant to wear. If you have a black mask, you can choose a white silkscreen legend.

Attach Stiffener for Your FPC Board

Polyimide is the most commonly chosen stiffener for flexible PCB boards. The thickness tolerance of the stiffener is around +/- 0.002. You can use the stiffener to control the bending potential of the circuit board. Solder resistance and high bond strength are the major reasons for choosing polyamide stiffeners. But, thin polyimide substrates may also pose some challenges. They may get damaged due to repeated twisting stresses and flexing. Excessive vibration can also lead to failures in the FPC components. 

The best attachment process is to create a thermal bond between the circuit and the stiffener. This method involves using pressure and heat. You will find a long-lasting bond with the manufacturing technique. 

Pressure Sensitive Adhesive is another method for stiffener attachment. If your design is not compatible with the flex adhesive, you can choose this process. 

Test Your Board –

Every board should undergo electrical validation. During the tests, you need to identify open and short circuits, fabrication issues, and other problems. You should also focus on

• The covering layer and substrate film surface
• Deviation of the covering layer and the connecting plate
• Electroplating combination standards
• Any missing coating

You can ship your flexible PCB board only if it functions properly.

The comprehensive guide on the FPC manufacturing method will be helpful for engineers. Buy high-quality raw materials to design the board and optimise its design for better usability. A good fabrication process will result in the development of high-end PCB boards.