Introduction
Flexible PCBs have substrates that make them easy to bend. During the assembly process, they often get damaged. So you must handle them with care. Errors during the assembly can break or make the board stop working. This article will guide you on how to assemble flexible PCB without damage.
Why Do Flexible PCBs Require Special Handling?
Flexible PCBs are not like rigid PCBs. They are made using bendable, flexible materials. Because of this, the board can fit into small and complex shapes. Yet, flexible circuit boards offer flexibility, but they also have a delicate structure. And that is why you have to handle them with special care.
For example, you must apply optimal force during assembly. Extra force can crack the board and materials. Heat is another factor that you should be more careful with when soldering the flex circuit. Because excessive heat can damage board materials.
Additionally, bending areas must be left empty. In cases where the board bends, the solder joints may be broken if there are components in empty spaces. Or they might damage the circuit.
Steps to Assemble Flexible PCB Without Damage
To assemble flexible printed circuit boards without getting damaged, follow these steps.
1. Design Verification & Preparation
Before the flexible PCB assembly process starts, it is necessary for you to check whether the PCB design meets the manufacturing requirements. And the desired flexibility requirements as well. Here, compliance checks should be done on bend radius, component placement, and routing of traces.
Bend Radius
For a 0.1mm single-layer flex board, the static bend radius should follow the 10x thickness rule (minimum 1mm). Dynamic applications typically require a 20x ratio to prevent fatigue.
Component Placement
The bend area should not have components on it. Components should be placed on static regions. Also, make sure to reduce strain. FR4 or polyimide stiffeners are recommended to reduce mechanical stress.
Trace Routing
Do not use sharp angles for traces, as they can increase stress. Instead, use curved traces. They will help you reduce stress level.
To give an example, in a wearable fitness watch, you can mount the microcontroller on a stiff section. The flexible part can then be used to route signals to the sensors. The radius of the bend must be kept at 1.5mm. This will make the PCB remain firm as the wrist moves.
To have help with PCB design checkup, CAD tools may be implemented. Two popular tools are KiCAD and Altium Designer. These will assist you in conducting a Design for Manufacturing (DFM) check.
2. Incoming Inspection & Material Storage
This step is critical before flex PCB assembly. Here, you have to perform Incoming Quality Control (IQC). Inspect boards for damage. Check if there are any cracks or any other visible defects.
To avoid contamination, store boards in a clean room. Wear ESD-safe gloves during handling and keep boards in antistatic bags at all times.
Flexible printed circuit boards should be stored in antistatic bags. Moreover, you can store them in trays, but at:
- 15–25°C temperature
- 60% humidity level
Note: Do not bend flexible PCBs too much before assembly. Otherwise, the substrate will become weak before soldering.
3. Jig Fixing & Preparation
Flexible PCBs are lightweight. During handling, they can move and slip easily. So, you must hold them firmly by fixing them on jigs. But do not make the jig over-tight. Make it hold the board gently. When you squeeze it too hard, it may be damaged.
4. Solder Paste Printing & SPI
At this step, you have to print solder paste onto the PCB pads. It connects electronic components to the board. The paste should be printed in a careful and even manner. A clean and accurate print helps create strong solder joints later.
After printing, check the paste using solder paste inspection (SPI). This check is to ensure that the paste is in the right places and in the right quantities.
5. Component Placement with Precision
The improper positioning of the components during flexible PCB assembly may cause stress on the substrate. So, you must place components in their accurate place with high precision. For this, you can use automated pick and place machines. Especially those with vision systems can help you place components precisely.
During placement, avoid applying too much pressure. Extra force can put a dent in the board substrate. To avoid this, set the pick and place machine to a low force, 1-2N is suggested.
Also, for areas where fine pitch components need to be placed, you can apply stiffeners, either temporary or permanent. Before positioning the component, applying stiffeners will assist you in ensuring that the board does not flex when the process of soldering is being undertaken.
Additionally, you need to be careful about alignment. To stay aligned, make use of fiducial markers. Fiducial marks will assist in positioning components in the right direction and eradicate location errors.
6. Soldering with Controlled Thermal Profiles
Next comes the soldering, which is a very crucial step of flexible PCB assembly. During soldering, heat is applied. And if that heat exceeds the limit, the substrate can get damaged.
You can use multiple techniques to perform soldering with controlled thermal profiles.
One technique is reflow soldering for SMT. This is done by putting the PCB in a reflow oven. It is in a controlled manner in which the board is heated by the oven.
Another technique is lead-free soldering. In lead-free soldering, the maximum temperature that you ought to maintain is 245 °C to 260 °C. Do not retain the board too long in the heat. This is to prevent the bending and warping of the board. The board should also be heated slowly. A slow increase in heat is useful in curbing thermal shock.
There are occasions when low-temperature solder can also be used. This is a solder that melts at approximately 138 °C.
7. Final Inspection & Testing
After assembly, you must check the flex PCB. This is done to identify problems in advance.
- Automated Optical Inspection (AOI): This is a system where the board is checked with the use of cameras. It helps you find wrong part placement, solder bridges, or lifted pads.
- Bend Testing: You test the board by bending it. This test is to test whether the PCB is able to bend without cracks.
- Electrical Continuity Testing: You check the circuits. This test will assist you in locating a short or open circuit.
- Pull Strength Test of Solder Joints: In this test, you pull the solder joints. This is a test that demonstrates whether components remain in their place on the board.
- Metallographic Microscope Inspection: In this inspection, you examine PCB cross section. You have to verify solder joint quality, plating thickness, and inner layer structure.
Mistakes to Avoid During Flexible PCB Assembly
A lot of errors occur when flex PCB assembly takes place. These errors can make your flex PCBs malfunction. The errors that are most likely to occur during the assembly process are as follows.
Mistake # 1. Overflexing
During assembly, many people bend PCBs too much for no reason. Flexing these bendable PCBs past their design radius will result in cracks in the board. So do not do that, remember to keep them flat when handling. Fixtures can be used to provide good support. They will also be used in preserving shape.
Mistake #2. Improper Stiffener Placement
Sometimes it is forgotten to apply stiffeners before placing the components. Other times, they are applied in an inappropriate manner. Both these can lead to component detachments. This can be avoided by using stiffeners in an appropriate amount uniformly.
MV Flex Circuit: Leading Turnkey Flex PCB Assembly Supplier
We are a professional flexible PCB assembly manufacturer with over 20 years of assembly experience. We have extensive project expertise across wearables, foldable phones, medical devices, aerospace, and more. Our factory is equipped with 8 SMT + THT lines, and we strictly follow IPC‑A‑610 Class 2/3 standards. We hold ISO, UL, REACH, and RoHS certifications to ensure our products meet global market access requirements. And our quality is strictly controlled through AOI, X‑ray, FCT, and functional test fixtures.
If you have a project, send us your Gerber files and BOM. Our engineering team will provide free DFM/DFA checks to evaluate manufacturability and deliver a detailed quotation.
Conclusion
Flexible PCB assembly requires attention at each stage. You must check the design, handling of boards, and heat in soldering. Avoiding damage requires that components be stored and placed properly. With the above steps, the assembly of flexible PCBs will be problem-free, and their maintenance will be preserved.
