We can use Flex circuits in a variety of applications. They can be safely bent into small and complex electronic devices, making them suitable for modern devices such as smartphones. They are easy to absorb shock and vibration in high stress applications.
Flex circuits combine standard circuit board structure with flexible design to cope with situations requiring higher durability. Have you ever thought about the materials and layouts involved in creating these multifunctional PCBs? This blog post will give a brief overview of common materials and structures in flexible and rigid flexible PCBs. Rigid Flex PCB is widely used. They are important components for assembling small consumer goods into complex military / defense systems.
The range of materials used in PCB manufacturing varies according to its application field. Rigid flexible PCB assembly has a variety of materials. However, the choice must be made after careful analysis of the application of the circuit board. In addition to the field of application, several other factors need to be considered when selecting materials, such as:
- Reliability requirements
- The duration or shelf life of the application
- manufacturing method
- Cost and budget
Materials used in Flex circuits
Although most standard PCBs have a glass fiber or metal substrate, the Flex circuit core consists of flexible polymers. Most Flex PCBs are based on polyimide (PI) film. The PI film does not soften when heated, but remains flexible after thermosetting. Many thermosetting resins (such as PI) harden when heated, making PI a superior material in flexible PCB structures. The standard PI film does not have good moisture resistance and tear resistance, but the upgraded PI film can alleviate these problems.
Each layer of Flex PCB also needs adhesive or special substrate. Manufacturers used to only use adhesives, but this method reduces the reliability of PCB. To solve these problems, they developed PI without adhesives, which can adhere to copper without adhesives. This material allows for thinner designs and reduces the risk of through-hole rupture. Instead of using solder mask to cover and protect flexible circuits, the manufacturer uses the same covering film made of PI. If you want the area on the flexible PCB to be rigid, the manufacturer can laminate the harder part to the part, but the signal cannot be transmitted between the flexible and the rigid part.
Flex printed circuit board materials
A rigid flexible PCB connects a rigid pcb material to a flexible material. The result is that it bends only in some places, making the board stronger but still flexible. If the signal is to be transmitted between the rigid part and the flexible part, it is necessary to design a rigid flexible PCB. In rigid flexible design, the flexible part of circuit board is similar to typical flexible circuit. At the same time, the material of the rigid part is similar to that of the standard rigid pcb. Like standard PCB, these rigid areas are usually based on glass fiber. Multilayer rigid flexible PCB also includes prereg glass fiber as the intermediate substrate layer.
Popular stack for layer Flex circuit construction
Each of the single – and double-layer flexible circuits has a stack that is common in many electronic products. A single-layer flexible circuit usually includes a reinforcing plate made of pressure-sensitive adhesive (PSA) and FR-4 glass fiber. Several FR-4 can stabilize each end of the PCB, while a thin layer of PSA can increase the firmness in the middle of the board. Due to the wide range of computer related applications, double layer flexible PCBs tend to have zero insertion force (ZIF) connectors. The use of Pi as a stiffener at the end gives the board the flexibility it needs to connect to the ZIF connector.
Flex PCB has various structures, but a common method involves four rigid layers and two Flex layers. It includes a core made of a glueless pi to reduce the risk of fracture. Above the layer and its copper film, two layers of prepreg are connected to the covering layer adhesive and the covering layer of the bending part. The rigid part receives additional copper, glass fiber and solder mask.