What is a flexible printing circuit board?
Flexible printed circuit boards, also called Flexible PCBs or Flex PCBs, derive their name for their ability to enable the circuitry to be designed to fit the electronic device or product. Flex boards are characterized by a distinctly patterned printed circuitry and component arrangement highlighted by a malleable base material.
Flexible printed circuit board technology offers a multitude of possibilities for different types of products and designs. It can help to reduce the size and weight of products, improve electronic packaging, withstand high temperature and etc. However, the flexible PCBs are easily damaged by improper personnel operation.
Damage to flexible printed circuit boards can be related to two factors: the environment and human factor.
Storage in unsuitable environments will affect flexible printed circuit boards. The common environment factors are as follows:
A humid environment will let moisture in the flexible circuit board. It is well known that water conducts electricity. If there is water on the circuit board, it may connect the 2 circuits which should not be connected and cause a fault. Besides, the humid environment is suitable for the growth of microorganisms, which will also affect the quality of the flexible circuit board.
As same as rigid printed circuit boards, flexible circuit boards suffer from static electricity. The static charge will affect the etching on the PCB.
Dust is the insulator. If there is dust on the flexible PCB, it will reduce the heat dissipation capacity, and resulting in the failure of the PCB caused by high temperature.
Extreme cold or heat
The temperature is too cold or too hot will damage flexible PCB. The substrate expands by heat and contracts by cold. The extreme temperature deforms the substrate of flexible PCB, thus disconnecting the circuit and impeding the flow of current.
Chemicals can corrode both the surface and circuit of flexible PCB. It is important to note that chemical smog also carries risks. If the chemical reagents are stored in the warehouses and other storage areas with flexible PCB, the chemical fumes from the reagents will corrode the flexible PCB after long contact.
If the factory does not do a good job of pest control, the emergence of insects and rats is at great risk. They bite all sorts of things, including flexible circuit boards, and if they do, the flexible PCB will become completely unusable.
The design and manufacture of flexible circuit boards is a very complex task. There are many risks of accidental damage.
Design and manufacture errors
Flexible PCB needs to work properly when bent. So it’s much more stringent in terms of design and production than rigid PCB. Unreasonable circuit layout design, improper material selection and mistakes in the manufacturing process will affect the bending degree of the flexible PCB.
Impact of external forces
Although the flexible PCB can be bent, if it is folded at 180°, there will still be undesirable wire breakage. Falling from a height, extruding and other external forces may damage the components on the flexible PCB, thus causing the whole PCB to fail. Scratch from hard object can damage the circuit, and frequently plug and unplug the flexible PCB can even cause tear in the flexible PCB in severe cases.
Faulty PCB components may damage the circuit board by not working correctly. In some cases, these components cannot protect the PCB from overheating.
How to avoid damage?
The design process of flexible PCB must consider the number of layers, the product’s features, the circuit structure and the material. The designer must also consider how often the circuit bends and how the bending is formed, including how tight the bending is and how much it is bent. Designers can take full advantage of the technology’s potential to work with flexible PCBs. These include identifying unique requirements for the flex PCB, while carefully defining the application and design priorities.
It is generally recommended to keep the flexible circuit’s copper circuit bent in a vertical direction. If this is not possible, try to reduce the bending amplitude and frequency, or use conical bending according to the mechanical design.
Do not suddenly change the width of the wire, when the wire is connected to the pad, especially the flexible circuit terminal, will form a weak spot, with the time pass by, the copper sheet is very easy to burn-in. The tapering method is recommended.
It is very important to choose the right material to accommodate the bending and to distribute the bending force evenly. The risk of damage can be reduced by using more resilient materials.
Notes for bending Flexible PCB
Do not bend directly on the through-holes of flexible circuit board; the golden finger plucking area, although there are reinforcement design, but still not suitable for bending this area; silkscreen, marking and other soldermask areas are strictly prohibited from bending more than 90° during assembly.
Flexible PCB assembly
The substrate material used by FPC has the characteristics of moisture absorption. If the customer has high temperature or the SMT process in the manufacturing process, the PCB should be baked and dehumidified before assmbly, so that to avoid the undesirable appearance of bubble, stratification, blasting hole and etc. After baking, it shall be assembled on the SMT production line within 2 hours. If it is more than 2 hours, they shall be baked again, so as to prevent from moisture absorption again and affect SMT.
If manually soldering pins in dense areas, adjacent pins should not be continuously soldered. It is recommend that to move and take turn to solder the adjacent pins, to avoid local overheating.
The storage places and the packing of the flexible PCBs should be well protected from moisture. The factory should be kept clean and regular pest control. Anti-static work should be done in the factory strictly. Operators must wear anti-static clothing and wristband.
If the user needs to unplug the flexible PCB, he/she should hold the area of the connector, do not pull the FPC directly.
If the flexible PCB does not work, how to repair it?
FPC not working may be due to soldering, components and circuit problems. Through the observation of FPC, to check whether there is a burned place, apply copper whether there is a fracture of the place, the smell of the circuit board whether there is a peculiar smell, whether there is any poor soldering, the golden finger interface is become mildew or black.
All the components shall be tested until the defective components are found. If the components cannot be detected by the instrument, the elimination method can be adopted, using the new components to replace the components on board, to ensure that all the components on the board are working well.
Checking the soldering condition carefully and repair if there is bad soldering.
If the components and soldering failure is excluded, the circuit problem is the most complex.
If the circuit is burned out, or the manufacture itself is not done well, the user and the mounting factory cannot repair. They must return to the PCB factory to be repaired by professional technicians. If the circuit is damaged due to the unreasonable design, then there is no need to repair it. The designer should modify the design and reproduce the PCB.
There are many factors that cause damage to the FPC, but as long as the design, manufacture, storage and use of attention, the damage can be forbidden.
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