Electronic Flex Circuit
As the names imply, a rigid PCB is a circuit board built on a rigid base layer or substrate that cannot twist or bend, while a flexible PCB, also known as flex circuit, is built on a flexible base that can be bent, twisted and folded.
Both rigid and flexible printed circuit boards (PCBs) serve to guide the movement of electrical current, connecting electronic components in various consumer and non-consumer products.
Rigid PCBs are relatively stronger than flexible PCBs but flexible circuit boards offer durability in flexibility.
The rigid-flex PCB is a hybrid board that combines the features of rigid and flexible PCBs to make a highly effective board that maximizes functional efficiency for various applications. It offers benefits from the use of both rigid and flexible material, providing an extra boost of reliability and a wider range of PCB capabilities. 
Though rigid and flexible PCBs both fulfill the same basic purpose of connecting electronic components for various devices, it is pertinent to note their many differences. Rigid and flexible PCBs are manufactured differently, presenting varying performance benefits and drawbacks. Their distinguishing properties and functions are presented below.
1. Base Layer: Rigid boards connect electrical components using conductive tracks and other elements, arranged on a non-conductive substrate. And this non-conductive substrate usually contains glass which gives it strength and thickness. Flex PCB also features conductive tracks on a non-conductive substrate, but it uses a flexible base material like polyimide.
2. Flexibility: the base material of the rigid board reinforces the board with strength and rigidity. On the other hand, the dynamic flex PCB uses a base that allows for flexibility, as it can be bent to fit the desired application and folded up into various shapes.
3. Conductors: Rigid circuits usually use electro-deposited copper as a conductive material, but because flexible circuits are meant to fold and bend, manufacturers may use more flexible rolled annealed copper.
4. Manufacturing Process: the solder mask application in rigid circuits is replaced with a process called overlay or coverlay in flexible PCB manufacturing, to protect the board's exposed circuitry.
5. Cost: Flex circuits typically cost more than rigid circuit boards. However, because of their ability to fit in compact spaces, flex circuits allow manufacturers to make portable-sized products in consumer electronics, medical devices, space, and automotive applications, which are in high demand, leading to more revenue and indirect savings for electronics manufacturers.
6. Durability: Although both styles of PCB offer decent amounts of durability, this durability manifests differently for each. Rigid PCBs offer higher levels of strength, while flex materials make the PCBs better absorb vibrations, dissipate heat, and withstand other environmental elements.
Flexible circuits can also be flexed repeatedly for hundreds of thousands of cycles without failure.
7. Weight: Their strength and thickness mean that rigid PCBs weigh slightly more than flexible PCBs, which are typically more lightweight. This is a beneficial feature for the electronics industry, which frequently creates small devices that require lighter components.
8. Resistance: Flexible PCBs possess high levels of resistance to high temperatures and extreme environments so they are less likely to succumb to such pressure. In contrast, Rigid PCBs do not have as much resistance, making them more prone to getting damaged or warped from heat, radiation, or chemicals.
9. Design Sophistication: For more basic consumer devices, such as toys or musical keyboards, the rigid PCB is a very good fit. However, flexible PCBs are more sophisticated in design, making them ideal for highly complex products.
Because of their structural versatility, flex circuits are popularly used in compact and innovative electronics. 
The rigid-flex PCB design features a combination of both rigid and flexible board technologies. In most rigid-flex boards, layers of flexible circuit substrates are attached to one or more rigid boards internally, externally or both, according to the application's design specifications. Typically, rigid-flex printed circuit boards are produced with flexible polyimide material on copper cladding substrate which is connected to the rigid board. The layers of the flexible part of rigid PCBs are usually built with padded through holes that secure interconnectivity between them.
Rigid-flex PCBs offer copious advantages, including being lightweight, easy to assemble and repair, as well as enhanced reliability, performance, and flexibility of electronic devices. These and more are discussed subsequently:
Electronic devices are increasingly designed to be smaller in size and more flexible. The versatility of rigid-flex PCBs makes them amenable to bending and folding to fit into smaller devices, with the proper connection of miniature components. This miniaturization also makes it lightweight.
Since solder joints and board-to-board connectors are reduced, there is less chance of connection impedance. Also, the connection of the attached rigid and flex layers is secure and reliable, which ensures less circuit failures.
Because of the interconnecting circuit built into the flexible substrate, there is more space on the rigid-flex PCB to route the traces around. This makes the board suitable for applications with compact designs as they don't provide much room for wire harnesses and high-profile connectors.
Even though rigid-flex PCBs are more expensive than normal rigid PCBs, it is more cost-effective in terms of assembly charges. Rigid-flex PCB assembly requires less material due to the smaller size, and fewer connections. The fewer parts and connector assemblies reduce the purchasing and assembly costs of the final product and increase revenue. This means that overall manufacturing cost from assembly and logistics is significantly reduced with the rigid-flex PCB.
The already interconnected subcircuits of the rigid-flex PCBs make it easy to perform automated testing. This testing allows manufacturers to eliminate connectivity issues before the components are assembled, thereby preventing needless wastage and expenses.
3D design and multiple layers of flexible circuits, make the rigid-flex PCB highly flexible to be able to fit into small devices. This way, devices are not restricted to specific PCB designs as it is with rigid boards because rigid-flex boards have an impressive bend radius and so can be flexed into various applications.
This type of PCB can withstand high temperatures since polyimide which has high thermal stability is used in its design. Also by combining the best features of both rigid and flex boards, it possesses considerable resistance to radiation, harmful oils and chemicals, as well as extreme shocks and vibrations, and other severe industrial conditions. This makes it perfect for devices that are likely to be subjected to excessive movement and vibrations during their use.
The rigid-flex PCB might have lots of merits, but it also has quite a few deterring disadvantages that manufacturers weigh with serious consideration before diving in. For one, it has a long and difficult production process, so it is necessary to invest in more materials and manpower to gain some assembling speed, incurring costs.
If there is a need to make adjustments to the actual design during the testing phase, it is notable difficult with a rigid-flex board, as one might need to go back to the Gerber programming to correct the error. This translates to added expenditure and time since it is practically reworking the entire design. It is also known to have a low yield rate. 
Several characteristic and functional benefits of Rigid-flex boards, such as reduced interface connections, minimal packaging weight and size, and resistance to extreme conditions make them widely sought after for application in the commercial, industrial and domestic divisions. Apart from small consumer products like phones and multimedia players, they are used in the following industries:
Medical Industry: for devices like pacemakers, cochlear implants, handheld monitors, imaging equipment, drug delivery systems, wireless controllers, and others.
Aerospace: this application produces devices such as radar equipment, GPS, radio communication systems, control tower systems, sensors, noise, and vibration testing systems, motion sensors, and environmental and climatic test chambers.
Telecommunications: for this sector, there are base stations, handheld units, communication satellites, wireless communication systems, signal processing systems, transmission media, routers and servers, online signal expansion systems, and so on.
Consumer Appliances: this is a common application for rigid-flex boards, seen in ovens, TVs remote controllers, washing systems, lighting systems, solar power systems, UV water purifiers, electronic irons, etc.
Automotives: this industry is not left out, applying rigid flex for products like electronic control modules, transmission controls, LCDs, comfort control units, as well as air conditioning, music, traction control, entertainment, and navigation systems.
Military Sector: rigid-flex printed circuit boards bear significant application here for the production of weapons guidance systems, communication systems, GPS, aircraft missile-launch detectors, surveillance or tracking systems, and many others.
Manufacturing Industry: In this sector, rigid-flex boards are applied for test equipment, electrical switches, industrial automation systems, control panels, industrial air conditioners, and CCTV surveillance systems, among others. 
The original rigid PCB paved the way for technological advancements in board design, and electronics manufacturing by extension, manifested by the advent of the flex circuit and the rigid-flex circuit. Each of them is significant for specific device applications due to their individual combination of properties and functions which ensure their continued relevance.
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