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The Flex PCB Assembly Process: A Step-by-Step Guide


Flex PCB assembly involves mounting the required electrical components onto a flexible printed circuit board (FPCB). With their flexing nature, special considerations apply when assembling these circuit boards. Find out more about the flex PCB assembly steps (and techniques) by reading below.

What is PCB Assembly?

PCB (printed circuit board) assembly is the process of combining components such as capacitors, resistors, and integrated circuits onto a PCB in order to create an electronic device.

This is done by mounting and soldering parts to the board so electrical connections are made and the end-product functions properly.

Usually, the type of circuit board, whether flex or rigid, determines the best way to assemble the board. Here, we will concern ourselves with flex PCB assembly. As you’ll learn, due to the nature of flex circuits, special considerations must be taken when assembling them.

Flex PCB Assembly

Flex PCB assembly means the adding of components to a flexible printed circuit board. FPCBs are quite different than rigid printed circuit boards, and the flex PCB assembly process differs slightly from that of rigid PCBs.

The flex PCB assembly process follows distinct steps that range from component preparation to soldering and surface finish application. Here, we’re referring to the complete PCB that only requires assembly and no design work.

Assembled flex PCB, SMT process
Assembled flex PCB, SMT process

Flex PCB Assembly Process

The flex PCB assembly process, from start to finish, is made up of the following main steps: preparing a bill of materials, PCB baking, solder paste printing, component mounting/placement, re-flow soldering, inspection and inspection, and packing.

Step 1 – Flex PCB Bill of Materials

The first of the flex PCB assembly steps is to identify the components that will be mounted, in a document called the bill of materials. The bill of materials (BOM) includes details like part numbers, reference designators, and quantities needed for each item.

Step 2 – Flex PCB Baking

The next step is called baking. Flexible circuits contain humidity-sensitive components and materials, such as adhesives and insulation layers, which require a certain level of dryness before assembly is possible.

The flex PCB baking process aims to remove moisture from the board — by subjecting it to around 120°C. This is done for a duration that depends on the amount of moisture on the board’s size or thickness.

PCBs up to 1 mm or with thickness up to 39 mils are heated for about 2 hours, while PCBs up to 1.8 mm, 70 mils, require around 4 hours of baking. Thicker boards (up to 4 mm or 157 mils) take longer, around 6 hours.

Step 3 – Flex PCB Solder Paste Printing

After baking, the next step is to apply solder paste so components can be mounted onto the printed circuit board. This involves placing a stencil over the PCB and transferring a pattern of solder paste onto it.

This is done by manually or automatically printing solders paste onto pads of the FPCBs with a stencil printer. Solder paste is a combination of solder and flux, and used to connect the electrical components on a circuit board without creating any dry joints.

Step 4 – Flex PCB Assembly

Next, the components are mounted on the printed circuit board. This is the actual flex PCB assembly. It’s either done by manually or by using an automated process to place all parts onto the flex PCB, ensuring that each component is in its correct location.

Flex PCB assembly methods in common use are surface-mount technology (SMT) and through-hole technology (THT).

The flex PCB SMT process is used when components are too small or there isn’t enough space to mount them through holes. The components are placed directly on the surface of the board and soldered in place with a wave solder machine.

THT is the method used when components are too large for surface mount technology. Through-hole components are inserted through holes and soldered in place with a hot air gun or an automated machine.

Step 5 – Flex PCB Soldering

Once all the components are placed, the next step of the flex PCB assembly process is to solder them in place and make sure that each joint is thoroughly soldered. This process is called re-flow soldering.

Flex PCB reflow soldering involves heating the board for a specific amount of time. The temperature level and duration must not be too high as to damage the components.

The heat causes the solder paste that was applied earlier to melt, resulting in electrical connections between the components and the printed circuit board. The temperature, time, and ramp rate of the flexible PCB soldering process must be carefully controlled to avoid damaging components.

Step 6 – Inspection and Testing

Inspection and testing are key flex PCB assembly steps as they ensure that the board meets all performance standards. The board is inspected for any physical defects such as shorts or opens, incorrect component placement, poor solder joints, and other issues.

The inspection is usually done through automated optical systems (AOI) or manual visual inspection. Testing can be conducted to ensure that the board meets all electrical performance parameters and that components work as expected.

Step 7 – Flex PCB Packaging

The final step in the flex PCB assembly process is packaging. This involves wrapping the board in protective material, labeling it, and boxing it up for shipping.

The proper packing of flexible circuit boards ensures their protection during transit and that they arrive at their destination in perfect condition. It also helps keep track of which boards have been shipped and their destination.

Flex PCB assembly is a complex process. However, with the right tools and expertise, it can be completed quickly and efficiently.

Here at ABC, we do more than manufacture flex PCBs; we also offer complete flex PCB assembly services to ensure your boards meet all of your requirements. Contact us today and get your flex PCBs manufactured and assembled in one place.


Flex PCB assembly is a comprehensive process that requires a careful attention to detail and precision. Following best practices when assembling these types of PCBs ensures they will perform as expected and provide long-lasting reliability. Not only that, but it can also help streamline the assembly process and reduce costs in the long run.


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