PCB Manufacturing

Flex PCB Manufacturing Process: An Overview for Beginners

Flex PCB Manufacturing Process

Flexible PCBs have many outstanding advantages, such as excellent flexibility, reduced interconnections, lower weight, cost-effectiveness, and improved Reliability. So they are more and more widely used in many high-tech electronics products, such as consumers, telecommunications, medicals, automation, automotive, aerospace., etc.  The flex PCB manufacturing process determines the quality of flex PCB, so here we are going to introduce the detailed process for beginners.

What is the Flex PCB Manufacturing Process?

The flex PCB manufacturing process involves designing the circuit layout, selecting the appropriate flexible material, and applying the copper foil layer. The flex PCB substrate is first coated with a photoresist during the flex PCB manufacturing process, exposed to ultraviolet light through a photomask, and the circuit pattern is developed. The copper is then etched, leaving traces that we designed to keep. The next step is to drill and plate the holes, then apply and cure the protective covering. Finally, solder resistance and surface treatment are performed to improve durability and weldability. A reliable Flex PCB manufacturing process guarantees a high-quality Flex PCB output and ensures a high-quality presentation of its subsequent performance.

The Detailed Process of Flex PCB Manufacturing

Flex PCB Manufacturing Process

The flex PCB manufacturing process is different from RIGID PCBs in that flex PCB production involves additional steps and specialised equipment that is necessary for flexible material flow. Here we would like to talk about the general steps in the flex PCB manufacturing process from from the view of beginners.

1. Design and Layout

The first step of the flex PCB manufacturing process is to create a flex PCB circuit design. By using such specialised software as Altium Designer, Eagle or KiCad this is done through the creation of detailed layouts for the circuit itself. It involves locating components, determining where electric wires should run and specifying how many levels should exist within the motherboard while also referring to its structure.

2. Material Selection

Material selection is the most important step for the flex PCB manufacturing process. Flexible printed circuit boards differ in terms of the materials used compared to rigid ones. Typically, these circuit boards are the substrates consisting of flexible polyethene (FE) (such as polymers such as polyimide or polyester). Polyimide is the choice material mainly due to its very impressive electrical and thermal performance. Thereafter, a thin layer of copper is laid on the flexible base in the flex PCB manufacturing process to make the conducting path.

3. Copper Circuit Production

Copper Circuit Production

After design completion and selection of material, the next step of the flex PCB manufacturing process is to put the copper layer into the printer to enable printing the circuit pattern on the copper layer.

The photolithographic method is the one normally utilised in the flex PCB manufacturing process. A photoresist is deposited on the copper layer, which is followed by the procession of a specific design using a photomask. It is further proceeded by the introduction of ultraviolet light in the etching process.

  • Photoresist Application: Photoresist material which is sensitive to UV light is applied onto the copper-clad flexible substrate.
  • Exposure and Development:  In the flex PCB manufacturing process, a substrate that is covered with photoresistive material is first exposed to UV light through a mask to determine the size of the circuit design. The photoresist sensitive only to light is transformed into areas where the exposure it gets from light creates harder parts, the other areas unaffected by the lightness of the room remain soft.
  • Etching: The substrate is next taken and immersed in the etching solution so that the exposed copper is removed, leaving the well-required circuit pattern.

4. Lamination and Curing

Lamination and curing is another important step of the flex PCB manufacturing process. For the layers of the multilayer flex PCB, several flex cores are stacked and then mashed together under heat and pressure. This multiple-layer lamination process is the making of the PCB’s own construction process; lamination connects the different elements and ensures a solid and coherent assembly. Besides that, at this step of the flex PCB manufacturing process, the flex PCB is put through a curing process once the lamination process has been finished, being exposed to heat that will cure and bind the layers altogether. This is a piece of the puzzle in terms of the final product gaining structural integrity and reliability.

In the common flex PCB manufacturing process, Flex PCBs are joined with the rigid section of Rigid-Flex PCBs in most applications. The laminating of the flex circuit is achieved through the rigid PCB with additional adhesive layers and alignment.  There are normally a lot of flex sections concerning the flex PCB manufacturing process that we install. However, these are almost always kept apart by prepreg layers and a core that remains flexible. The flexible sections represented with the holes and vias in the flex sections are done simultaneously most often with the rigid sections to make sure of the proper alignment. Other than the flex PCB manufacturing process, the flex-rigid process is a multi-level operation that needs careful planning and accurate execution to fulfil the final product as per the performance and reliability expectations set by the customer.

5. Drilling

Drilling

Once the circuit pattern is printed, the next step of the flex PCB manufacturing process is to proceed on the printed circuit board by drilling through the holes as well. Technically, those holes are vias that connect the layers of PCB. They are also used as a base for some components (in the future). In flex PCB,  drilling is performed by using laser drilling in the flex PCB manufacturing process due to the precision required and the delicate nature of the materials.

6. Metallization

In the subsequent flex PCB manufacturing process, to obtain the proper connections between the walls and the surface of the metal that is alloyed with, it should be metallised. This is a process that involves the two following steps:

  • Shadow/Black Hole: As an aid for the electroplating process, a layer of conductive carbon powder is spread all over the hole walls.
  • Electroless Plating: During the procedure, copper is inertly chemically absorbed onto the board surface along with the hole walls.

These actions then constitute the groundwork for those future processes which will be the physiosorbed thermal-based plating and the electrodeposition process respectively.

7. Solder Mask Application

Solder mask application is an important step of the flex PCB manufacturing process for the protection of the PCB. A solder mask is used for the protection of the circuit from oxidation and it also prevents solder bridges during soldering of components. Solder mask is basically a polymer coating that is applied over the entire surface of the PCB except for the place where components will be soldered. The solder mask also enhances the mechanical strength of the PCB.

8. Surface Finish

Once the circuit pattern is defined, the exposed copper pads then undergo a surface finish treatment to prevent oxidation. There are many surface finish methods used in the regular flex PCB manufacturing process. The most common surface finished for flex PCB include:

  • Immersion Gold: A thin layer of gold is placed on copper pads to provide excellent soldering ability and corrosion resistance.
  • Immersion Tin: In this process a thin layer of tin is deposited on the copper pads. It offers good soldering ability and it is cheaper in cost compared to gold.
  • Anti-Oxidation Film: A protective film is applied to the copper pads to prevent oxidation and maintain solderability.

The choice of surface finish in the flex PCB manufacturing process is the determination according to the specific application and the performance that can be expected.

9. Silkscreen Printing

Silkscreen Printing

Silkscreen printing is another step of the flex PCB manufacturing process that can not be ignored. Silkscreen printing like visual markings, logos, and ID numbers can be done on a whiteboard. The flex PCB is inserted into the silkscreen printer, and the wanted designs are printed with spray hard ink. After baking the printed silkscreen, the ink is fully cured and bonded to the flex PCB’s surface.

10. Profiling and Cutting

The flex PCB manufacturing process is completed when a board is cut to the final shape and size. Two methods normally used are :

  • Cutting Dies: This is the process used to make straight edgewise profiles by the usage of hydraulic punch and die set that is used, as the flex PCB is cut out in a cost-effective and high-volume method.
  • Blanking Knife: A blanking knife (a long razor blade bent to the desired profile) is a tool that is used when a nonstandard or custom shape is required to be formed by hand without using a machine. In the flex PCB manufacturing process, these machines are often used for low-volume or prototype runs.

11. Inspection and Testing

Before completing the flex PCB manufacturing process, the cut-out flex PCB is checked, tested, and then packed to be shipped to the customer. Generally speaking, open and short tests, AOI inspections, microsection checks and visual inspections will be done after the whole flex PCB manufacturing process before packaging.

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