Flexible printed circuit boards: flex and rigid-flex

Printed circuit boards are an extremely versatile technology of building electronic circuits. Their ubiquity is the reason why almost all electronic components are designed to be mounted on them. They are not without their disadvantages though, which become apparent especially in the case of space limitations or high design requirements. However, these limitations can be overcome by using flexible and rigid-flex circuits. Both of these technologies are valued in specialized applications. We are observing their steady growth in consumer electronics as well. In today's article, we'll look at flex and rigid-flex circuit technologies, the advantages they offer and their applications.

Construction and types of elastic circuits

Printed flexible circuits are built almost identically to typical PCBs. Likewise, they consist of one or more conductive copper layers, of which the outer layers may be covered with a solder mask. The main difference is that instead of a rigid fiberglass substrate, they use a much thinner, naturally flexible substrate. In most cases it is made from plastics with high temperature resistance, including, for example, polyimides (Kapton). Because of this, the board can be bent or shaped freely without the risk of breaking or damaging. On the other hand, rigid-flex printed circuit boards are a hybrid of the previously mentioned technologies. They consist of areas of both rigid and flexible substrate, laminated together into a single structure. The rigid areas most often carry electronic components, while the flexible parts serve as connections between them. Advanced manufacturing processes allow the creation of multiple regions with rigid substrates with varying numbers of layers, as well as the derivation of flexible parts from specific layers of a multi-layer board. Read also: Electronic component assembly technologies Rigid-flex boards should not be confused with stiffened flexible circuits. The latter are just fitted with an additional stiffening element – it is often a piece of metal sheet glued to the underside of the flexible circuit. The purpose of the stiffener is usually to strengthen and stabilize the area where components are soldered.

Where and why are flexible circuits used?

Sometimes, we do not have enough space to fit all of the components on a single PCB. Instead of dividing the circuit into many sub-assemblies, we can use flexible boards. Not only are they much thinner, their shape can be also easily adapted to the space available inside the enclosure. A single flex or rigid-flex module allows us to replace a large number of conventional PCBs, along with connections between them. Flexible circuits also allow the size and weight of the circuit to be reduced. This is especially useful in small devices with a high degree of complexity, requiring a large number of components and connections in a limited space. Digital cameras and cell phones are the best examples of such solutions.   [caption id="attachment_61823" align="aligncenter" width="930"] An example of flexible circuits in a CD-ROM drive head. The ability to adapt the board to the shape of the part allows all components to be connected without the need for wires or connectors.[/caption]   Flexible circuits, due to their properties can also replace cables and connections. We come across this application in most LCD displays and their touch panels. A piece of flexible tape coming out of them is usually terminated with a ZIF (zero insertion force) connector. The tape can be easily bent and deformed, which allows for some freedom in the installation of the display. Like a regular PCB, the flexible tape can also accommodate additional components such as a touch controller or voltage regulation circuit. Read also: All about TFT LCD displays Flexible circuits are also used in assemblies that are required to move or flex during use. A good example of such an application is a hard drive head. Recently, OLED displays based on flexible substrates have made their appearance. Their flexibility allows them to be freely shaped, bent, or even rolled while in use.  

Rigid-flex – the solution for the most demanding applications

Rigid-flex circuits allow us to achieve seamless and reliable connections between rigid and flexible board areas. They provide better signal integrity than even the highest quality connectors, especially in case of high-frequency data lanes. All this helps us to enhance protection against electromagnetic interference and achieve controlled path impedance, while simultaneously meeting high spatial requirements. The ability to derive flexible circuits from selected layers of the rigid board broadens the ability to adapt to specific design requirements even further. However, the use of rigid-flex technology is quite expensive and requires a lot of knowledge and experience. For this reason, it is the preferred solution in areas where reliability and spatial efficiency are key factors. Their applications include medical and military equipment, as well as satellites and space probes.  

Difficulties in the design and fabrication of flexible circuits

Designing flexible printed circuit boards can be quite a challenge. Bending causes stresses, which in case of incorrectly designed boards can cause damage to the circuits. Therefore, some specific design rules must be followed and the phenomena occurring during the deformation of the board must be taken into account. An example of a design feature of flexible circuits is the use of "hatched" areas (in the form of a grid) instead of uniform fill regions. It is also a good practice to cover the edges of pads and solder points with solder mask to minimize the risk of their detachment. In applications where the shape of the flexible circuits must be properly adapted to the enclosure, it is essential for the board outline to be drawn correctly and accurately. The designer must consider the three-dimensional shape that the board will take when bent in the desired manner. In addition to extensive knowledge, designing this type of circuits also requires special computer software. Usually, flexible boards must be designed along with mechanical components, and the two teams must work closely together. Difficulties also arise when assembling the boards. Flexible circuit parts make precise positioning and soldering of parts difficult. Often it is necessary to use specially prepared carrier plates, and in case of rigid-flex circuits –  properly attaching the rigid parts to the panel using breakaway tabs. Diagnosing and repairing flexible circuits manually is also considerably difficult and sometimes even impossible. The issue of the high cost of manufacturing flexible boards is also unavoidable. Their production is a much more complicated process, requiring special tooling. Due to the often non-standard shape of the boards, the use of the panel surface is worse, which increases the waste of materials. All of these things translate into significantly higher production costs. For this reason, making a custom flex or rigid-flex circuit often exceeds the capabilities of smaller companies and hobbyists.  

From concept to finished product – with InterElcom

InterElcom offers a wide range of services related to the design of electronic devices, PCB production and assembly, as well as programming of embedded systems. Many years of experience and competence of our engineers are the reasons why you should pass your project into our hands. We will make sure to choose the most appropriate technologies and components, as well as to provide professional advice and support. We invite you to contact us for a free pricing of your project!