Introduction to Rigid-Flex PCB

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Due to technological changes, the type of assembly steps and materials used in any production tends to change. The change experienced is either a change that will bring solutions to a market or create a foundation for the next technology. Rigid circuit boards existed in the market place and they were doing quite well. Later rigid-flex circuit boards were introduced. They were designed to provide solutions and act as a replacement for rigid printed circuit boards. Rigid-flex circuit boards, as the name suggests, are a hybrid of flex and rigid circuits. This technology ensured that it serves both the rigid and the flexible parts. Many years ago before the invention of printed circuit boards, electronic circuits existed as flexible and free-wired, with possible additional use of solder strips. Mechanical anchorage points were components such as potentiometers, variable capacitors, switches having their solder lugs, and socket electron tubes.

In this article, we are going to talk about rigid-flex printed circuit boards. Everything, from design, applications, advantages, and many more.

How Rigid-Flex PCB came to Existence

  • Before the introduction of rigid-flex PCBs, the existed the rigid PCB. This was a solid printed circuit board that was not flexible. This type of circuit board could not be bent or forced out of shape it was a solid-state board.

  • The trend in aerospace products development includes miniaturization lightweight, multifunction, assembly densification, etc, a high requirement was seen being startup for PCB technology and the process of manufacturing.
  • A flexible printed circuit board is a type of electronic circuits board that is made of a flexible substrate substance featuring more advantages compared to rigid printed circuit boards. They include;
    • Smaller thickness
    • Lightweight
    • More space was saved
    • High freedom in electronic and mechanical design
    • Bending feature
  • It was made of a solid substrate having copper tracks and components layouts where through-hole components, surface mount devices, and mixed PCB assembly could be soldered either automatically or by use of through-hole technology.
  • These flexible PCBs are used everywhere since they can fit in any housing. They just need some folding to fit in the space available.

  • Later designers realized that they needed a printed circuit board that was more advantageous to them and that’s how rigid-flex printed circuit boards came into existence.
  • The combination of both the rigid and flexible PCBs brought so many advantages for the user in terms of signal transmission, size assembly, stability, and many more.
  • Electromagnetic signals are capable of running faster and softly for flex PCBs because of the perfect electrical and thermal performances of the flexible substrate materials. The flexibility is widely used in many manufacturing areas such as; instruments, automobiles, medical care, aerospace, and military
  • Due to further development of flexible printed circuit boards, their next step should be the Rigid-Flex printed circuit boards, the HDI Flex PCBs, and embedded flexibles amongst which Rigid-Flex private circuit boards attract the most applications.

Materials used in Rigid-Flex Printed Circuit Boards Technology

  • When it comes to the rigid-flex PCBs performance, it will always depend on the substrate material that flexible dielectric, and flexible adhesive films.
  • Flexible dielectric films contain;
    • Polyester (Mylar) is common for products of low-end.
    • Polyimide (Kapton) is the most common type.
  • Fluoropolymer (PTFE) for aerospace and military products

When the three flexible substrate materials are put into comparison, polyimide will feature the topmost dielectric constants with excellent mechanical and electrical properties and resistance to high temperatures. Almost similar to polyimide when it comes to performance, polyester has bad high temperatures resistance.

  • Fluoropolymer or Polytetrafluoroethylene (PTFE) is used in products with high frequencies accompanied by the low constant of the dielectric. Their performances can be demonstrated in the table below:

Types of Rigid-Flex Printed Circuit Boards

Only two main types of rigid-flex PCBs are existent in the market or are rather popularly known.

  • Flex to install

This board only folds once when the device is removed or assembled for the first time. The flex part remains stable throughout usage, besides some movements that might occur in case of high oscillation applications are experienced.

Flex to install

  • Dynamic flex

This printed circuit board is a special category of the rigid-flex PCB family. It can be folded and bent many times as per the design or the product needed during the end product phase. This type requires special consideration to ensure the flexible portion of the board can withstand repeated wear and tear. If proper design procedure is followed using the correct bend radius, dynamic flex can be bent over ten thousand times.

Manufacturing Technology of Rigid-Flex PCBs

Technologies used to manufacture every PCB tend to differ from each other. It depends on the type of PCB one needs and the performances they require it to deliver. In the case of rigid-flex PCBs, there is a difference in the fine circuit manufacturing technology and the micro via manufacturing technology. Since every electronic equipment and product is developing towards lightweight and miniaturization, multifunction and assembly densification, advanced printed circuit boards are attracting the most attention including rigid-flex PCBs, HDI flex-rigid PCBs, and embedded flex-rigid printed circuit boards.

  • Rigid-flex PCBs are fabricated through selective and orderly layer stack-up of flexible PCBs and rigid PCBs with plated through holes technology responsible for the connection between its layers.
  • The advent of rigid-flex PCBs can very effectively lower electronic products’ volume and their quality. They can enhance this by replacing harnesses and connectors that were applied in traditional electronic products. In addition, rigid-flex PCBs are capable of solving contact and intense heat problems caused by harness and connectors, rapidly improving the reliability of these devices.
  • From the 1970s, rigid-flex PCBs were built by stacking up the rigid boards onto the flexible boards. Several signs of progress and optimization have experienced new and numerous flex-rigid printed circuit board manufacturing technologies.
  • For flexible substrate material, the polyimide (PI) double-layer plate covered with a copper layer is used as a flexible core and polyimide film is applicable to give protection to the flexible circuits. Adhesion will depend on the low flow and all the components are laminated for the printed circuit board to be manufactured.
  • equipment volume and quality are reduced so that they are greatly applied in electronic devices that need to be bent over and over again.
  • The flexible substrate material features excellent dielectric stability that they are fit for higher frequency signal transmission and independence control. It can also withstand radiation, temperatures, shock, and extreme environments. The dielectric ensures the smooth running of electronic devices.
  • Additional conductive layers use either flexible or rigid insulation. This depends on the product’s requirement specification. The outer layer on the boards despite their price contains exposed pads to ensure safety. Here conductors are used in the primary hard or rigid layer, while in the flexible plates through-hole technology is used for any additional layer of the flexible and rigid layer.
  • Other projects need traditional rigid technology and design, while others contain restrictions that prevent manufacturers from using the larger, less flexible boards. An example; if standard board design is used, portable devices and mobile will be affected. Very many moving parts and components don’t perform well when subjected to certain conditions. On the other hand, mobile devices need to be portable, have lighter weights, and can withstand harsh conditions such as cold, heat, and humidity at times.

Rigid-flex Design

  • The manufacturing process of the rigid-flex printed circuit boards nearly resembles that of the old hardboard circuit, but some layers are flexible circuitry running through the hardboard. The fabricator adds plated through-holes in order to make connectivity to the layers of the rigid circuit regions of the flexible circuit.
  • Some of the configurations will create the rigid-flex printed circuit board that is assembled just like the hardboards, but it is also able to fit in the product that has space constraints.  This makes them most used for continuously flexible applications which are also called dynamic flex applications. If rigid-flex PCB is well designed, it is capable of withstanding millions of flex cycles without a single failure. The flexible and the rigid substrates will form a single integrated unit which can be manipulated further to a three-dimensional subassembly.
  • It is very important at times to distinguish between rigid-flex and flex PCBs with stiffeners, which at times it can be used for the same applications. Stiffeners used on flexible circuits will always provide the required stability for the assembly.
  • Rigid-flex PCBs make the manufacturing process easier while giving a much high density for electronic components and schematic/ circuit routing. When this is not required, flex boards that have stiffeners might be enough and less expensive too.

  • Rigid-flex printed circuit boards can be quite more costly than their comparable hardboards. Their prices are usually twice or even thrice the cost of a flexible circuit that has a stiffener. Moreover, its high cost is justified by where it finds its application specifications and environments which include:
    • Highly reliable applications; in case an assembly might be exposed to too much or looping shocks, or high environmental vibrations, connectors that have flexible cables are most likely to be destroyed. Rigid-flex printed circuit boards offer high reliability even when it’s subjected to excessive environmental vibrations.
    • Highly dense applications; on very small enclosures, it’s very impossible at times to attach all the cables, components, and connectors that an electronic printed circuit board can be folded to very small shapes, offering wonderful space saves for these instances.

Production Flow

Since the soft and hard bonding board is a combination of both the flex PCB and the rigid PCB, the production of the plates should have both the FPC equipment of production and PCB equipment of production.

  1. First of all, electronic designers and engineers draw a trace and layout of the soft bonding plate with respect to the requirement specification.
  2. After the drawing part, the drawing is then forwarded to the flex-rigid PCB factory or production units. The fabricator deals with and plans the necessary documents and arranges the FPC manufacturing line to generate the required FPC, PCB line of production to produce PCB. The two soft and hard boards are then produced by the plan’s requirements of electronic and electrical engineers.
  3. Here there are several detailed inks. This is the final part of or rather the final process for the production of the soft and hard plates. It is one of the most important parts. The flex-rigid printed circuit board is tested. So many details are needed to be confirmed before shipping is initiated. Generally, a full smart inspection is carried out. This is done to allow the supply and the demand to cause the loss of related benefits, so the value is relatively high. Below is the manufacturing flow of the flex-rigid PCBs.

Advantages of Rigid-flex Printed Circuit Boards

The combination of flexible and rigid boards makes it an extremely beneficial product for both applications as well as to the customers. Some of the advantages include:

  • Dynamic stability: Rigid-flex circuit boards fives the customer the liberty of packaging geometry while retaining the precision density and looping of printed circuits.

  • Money saving and space: Using rigid-flex circuit boards can bring an immense reduction in total computer circuit boards costs and expenditure. However, rigid-flex PCBs are costlier compared to flexible PCBs and rigid PCBs, but their assembly charges are less. this is often thanks to their small sizes and also the few connections in them. Rigid-flex computer circuit boards hence require less material for assembling. The few parts and connectors during assembling don’t only increase the yield but also reduce the acquisition and assembly costs of the ultimate product. Having a reduced number of connections also eliminates the risks within the logistics of a device. In simple terms, logistics and assembly costs are reduced when it involves rigid-flex computer circuit boards.
  • Security: Rigid-flex computer circuit boards provide secure connections for other components, providing stability and polarity. These circuit boards also help to scale back the number of connector components in an application.
  • Mechanical stability: The circuits tend to own an interchanging factor for the layer which forms a powerful foundation. Flexible boards will give the flexibility required for installation in small spaces while rigid boards will give stability.

  • Flexible design option: Rigid-flex computer circuit boards are often significantly deformed to any shape with no breakage. Its standard materials can withstand as many bending cycles as possible. thanks to the 3D design and therefore the multiple layers of flexible circuits, the rigid-flex computer circuit board offers increased flexibility to suit small devices. In summary, you don’t need to specifically design a tool in keeping with the desired specification of the PCBs as within the case of the rigid boards. The corners of the rigid-flex PCBs are generally to supply a versatile connection. it’s also possible to feature rigidness in its design when extra mechanical support is required.
  • Reduced packaging size: Product miniaturization has been applied in rigid-flex computer circuit boards thanks to the compacted size of the circuited boards. it’s easily manufactured to suit small electronic devices, that’s why it’s used is in designing several sophisticated circuits. Here the electronic devices are designed to be smaller in size and more versatile, so they fully serve the necessity of proper connection of small components. Thus it is often folded or bent to smaller devices and it’s also lightweight thanks to its small size.
  • Repair and maintenance are convenient: 3D designing has made the repair and maintenance easier and it’s not a tiresome task, unlike for rigid or flexible boards. The rigid-flex board is bent, folded, and twisted allowing technicians to mend and touch the issues anywhere they’re diagnosed during troubleshooting easily. Besides, the connection between rigid and flex sections is thru the board to board connectors, hence the circuit is evident and simple to debug. This makes it easy to detect the fault within the rigid-flex board and repair it during routine maintenance.
  • High chilling and aggressive conditions: Polyimide being the substrate material that’s used, has high thermal stability. This material can withstand high temperatures. Hence the rationale it’s suitable for stylish applications for military and defense purposes. It can retain the most effective features for both rigid and flex boards, and so it demonstrates extreme resistance to ultraviolet and radiation exposure and even harmful chemicals. it’s also ready to withstand environmental shocks and vibrations with other harsh conditions from the industries.

Applications of Rigid-flex Printed Circuit Boards

These types of circuits are found and applicable anywhere due to the technology used in every field. Some of the areas where its applications include;

  • Commercial applications
  • Military applications
  • Industrial applications
  • Digital cameras
  • Cell phones
  • Smart devices
  • In the medical industry, they are used to develop pacemakers for weight reduction, space, and flexibility.

CONCLUSION

Traditional rigid-flex computer circuit boards are hard to manufacture since there’s a comparatively low yield rate and high density and are difficult to repair if there’s a breakdown. within the manufacturing process, the rigid plate requires to be embedded into a fashionable flexible substrate material so that the staple waste rate remains high hence manufacturing technology is difficult. The substrate material contains a comparatively high coefficient of thermal expansion and high moisture absorption rate of large-area flexible substrate material, resulting in size tolerance accumulation. this may further affect circuit pattern, drilling, layer up, plating, and cleaning which results in a lower yield. Circuits that have embedded flexibility are capable of effectively reducing and avoiding the above-mentioned issues.

Rigid-flex PCB brings about the main explanation for rising costs, meaning raw materials and products should be utilized. If technicians, designers, and engineers can cooperate within the first stages of the project design, they’ll make sure that design work is completed at an occasional price hence avoiding excessive and unwanted costs.

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