Flexible PCB technology, also known as FPC (Flexible Printed Circuit), is continuously growing, with applications in the main electronics sectors, such as consumer, automotive, electro-medical devices, wearables, telecommunications and aerospace. The introduction of flexible PCBs has revolutionized the legacy electrical interconnection techniques, traditionally used to connect multiple parts of the same circuit or of multiple electronic devices. Thanks to the flexible nature of the connection, its compactness, and the high density of electrical connections that can be achieved, the solution based on flexible PCBs allows to obtain a considerable reduction of space, weight and costs compared to an equivalent solution based on rigid PCBs. Flexible printed circuits have replaced many types of wiring, often done by hand, in several applications, reducing the total cost of electrical wiring by up to 70%. The heart of an FPC is represented by flexible films and thin layers of conductive material that make the electrical connection by replacing the traditional wires (think, for example, of the connection between a microcontroller board and an LCD or OLED display), and on which electronic components can be directly attached via soldering or conductive adhesive. Figure 1 shows an example of flexible printed circuit.
Structure of a flexible PCB
As happens for rigid PCBs, FPCs can be divided into single layer, double layer or multi- layer circuits. The main elements of a single layer flexible printed circuit are the following:
·dielectric substrate film: the base material of the PCB. The most commonly used material is polyimide (PI), characterized by high resistance to traction and temperature;
·electrical conductors: made of copper, they represent the traces of the circuit;
·protective finish, made with cover lay or cover coat;
·adhesive material (polyethylene or epoxy resin), used for joining the various parts of the circuit together.
The first phase of the FPC manufacturing consists in etching the copper to obtain the traces, while the protective coating (cover lay) must be drilled so as to allow access to the soldering pads. After a cleansing treatment, the components are joined together by rolling. The external terminals/pins, necessary for the electrical connection of the circuit, are protected from oxidation by immersion in tin for welding or in gold. If the circuit has a high complexity, or requires the presence of copper ground shields, it is necessary to switch to a double layer or multi-layer FPC. The manufacturing technique is very similar to that used for single layer flexible circuits, with the difference that in multi-layer FPCs it is necessary to insert PTH (Plated Through Hole), so as to create, where required, the electrical connection between different conductive layers. The union of these materials forms a flexible circuit, in which the adhesive material serves to join the conductive tracks with the dielectric substrate or, in multi-layer flexible circuits, to join the individual layers together. In addition, the adhesive film can also be used for protective purposes, preventing moisture, dust or other external agents from corroding or oxidizing the flexible circuit.
Advantages of flexible PCBs
The fact that a flexible circuit board can be bent, folded, and configured in just about any shape or thickness imaginable gives the designer tremendous options when creating an electronics package. Size and space limitations are far less of an issue than traditional design using hardboard circuits. Assembly and handling costs can be significantly decreased because the entire interconnect system can be built as one integrated part. Add in All Flex’s ability for component assembly and testing, and supply chain management becomes greatly simplified.
This tremendous flexibility in design choices leads to electronic packages being smaller, lighter, and more functional.
In addition, the use of FPC can reduce the incidence of human errors during wiring, with consequent improvement in quality and reduction of costs. FPC technology contributes to significantly reduce both the size and weight of the application, a key factor for the creation of reliable, compact and highly integrated electronic devices.
Disadvantages of flexible PCBs
Although there are several and meaningful advantages, the FPC technology has also some drawbacks, or disadvantages. First of all, compared to traditional rigid PCBs, FPCs have a high one-time initial cost. Since flexible circuits are designed for very specific applications, the initial costs related to circuit and prototype design are higher than for rigid PCBs. If the cost is a determining factor in the choice of the type of PCB, it is better to use the FPC technology only for production volumes that are not too low.
Another disadvantage is the difficulty of repairing or modifying the PCB when it needs to be reworked. In this case, in fact, it is first necessary to remove the protective film that wraps the circuit, perform the intervention and then restore the protection. Flexible PCBs are a relatively new technology and not all manufacturers are equipped to provide this type of product to their customers. Furthermore, during the assembly phase, great care must be taken, as the circuit can be easily damaged if handled incorrectly, or by unauthorized personnel.
Conclusion
Flexible printed circuits are integral to many everyday devices, including smartphones, tablets, cameras, printers, and laptops. These circuits often replace connectors and cables, enhancing reliability, reducing assembly time and costs, and minimizing device size. Consequently, flexible PCBs have enabled innovative applications that rigid PCBs cannot achieve. Nonetheless, a thorough evaluation of the benefits and drawbacks of flexible versus rigid PCBs is essential during the preliminary design phase. While flexible PCBs are indispensable for some applications, for others, the choice hinges on a cost-benefit analysis.