A Complete Guide to Flexible Printed Circuit Boards

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Oct 2, 2020|PCB Maintenance, Repair and Troubleshooting, Product Innovations and Design

Originally designed to replace traditional wire harnesses, flexible printed circuits have evolved from early World War II to the exponential growth possible today. Ideal for simple to complex applications, flexible circuits possess an outstanding versatility to tackle nearly any circuit board job.

What Are Flexible Printed Circuits?

A flexible printed circuit is a light-weight electrical circuit that easily conforms to small spaces and contoured shapes. However, they are far more than merely a PCB that bends. Flex circuits present distinctive differences and benefits, requiring their unique design by an innovative PCB manufacturer.

Flexible printed circuit boards consist of conductive metal strips, usually copper, insulated by a dielectric material or solder mask. The circuit board’s critical function is to carry electronic signals while providing mechanical flexing, enabling the use of smaller connectors and shielding radiated noise for EMI.

The PCB is a substrate, or non-conducting board, used to connect electronic components to make an electrical circuit. 

Basic Types of Flexible Printed Circuits

The variety of available configurations, sizes, and functionalities highlight the versatility of flexible circuit boards. The basic types of flexible circuits include single-sided circuits, double-sided circuits, and multi-layer circuits.

Single-Sided

Single-sided PCB circuits only have a single layer of metal traced on one side of the dielectric layer. 

These circuit boards contain only one layer of conducting material on one side of the board so that the other side can be used to house different electronic components. As single-sided PCBs are made of a substrate layer, conductive metal layer, protective solder mask, and silk screen, they remain one of the most common flex circuits available. Their simple design makes them appealing to a wide variety of applications.

Because of this simple design, single-sided flex circuits require fewer resources and are of a lower density. They are cost-effective, allowing for a more affordable manufacturing process that can be produced at higher speeds and within larger quantities. There are fewer potential problems with single-sided flex circuit manufacturing, which also means shorter lead times for clients. Compared to multi-layer flex circuits, single-sided are an attractive option if they are for a simple design, such as radios, printers, and vending machines.

Double-Sided

In contrast, double-sided courses feature metal layers on both sides of the single dielectric layer. 

These flex circuits feature circuits with two conductive layers: one on each side of the polyimide base layer. Specific trace patterns can be created on both sides of the substrate, where they can be connected through copper-plated through-holes where desired. Because of their copper plated through holes, the double-sided flex circuit allows electrical connections through the layers.

Double-sided flex circuits hold many advantages such as reduced wiring errors, higher circuit density, and stronger signal quality. Because of the improved reliability and flexible design, these flex circuits are attractive for a wide variety of applications. They have decreased assembly time and also eliminated the high cost of routing, wrapping, and soldering wires. This means there are fewer wiring errors overall, further reducing the costs of assembly. The design freedom of double-sided flex circuits means they are used in many electronic devices, such as dashboard components and lighting.

Multi-Layer

In double-sided circuits, metal layers frequently connect through metalized through-holes. The same through-holes can be found on multi-layer boards, containing several separate copper layers encapsulated by dielectric layers.

Multi-layer flex circuits actually combine single-sided and double-sided circuits together by using complex connections. This multi-layer design uses shielding or surface-mounted technologies to enable maximum flexibility during the process with the option of continuous lamination. Multi-layer circuits are especially effective when considering specific requirements, additional shielding, and high component density. 

Multi-layer flex circuits greatly reduce assembly errors, with more accurate designs via automated production. Circuits are routed by accurate design plans, and because they require less manual labor during the assembly process there are fewer production errors overall. Flex circus eliminates the soldering and routing costs, and features a complete interconnection of systems that makes them different from individual PC boards. Because multi-layer circuits are not restricted to two dimensions, they offer endless design freedom and complex design challenges. Multi-layer flex circuits are used in many applications such as cameras, cell phones, hearing aids, and satellites.

Rigid Flex

A rigid flex PCB incorporates elements from both hard and flexible circuits to form a hybrid circuit board.

Rigid-flex circuits are made when layers of both flexible and rigid materials are used to create flexible and rigid areas within a single package. They are able to combine the best of rigid PCBs and flexible circuits, integrating them into one circuit that has a wide variety of complex applications.

Copper-plated vias connect the rigid outer layers to the inner flexible layers, allowing rigid-flex circuits to have a higher component density and overall better quality control. Their combined features allow them to be rigid for extra SMT component support, and flexible when they need to fit into tight spaces. Rigid-flex circuits have been use in the aerospace industry for over 40 years, providing the necessary components for military and commercial applications. They have many high-level benefits including more reliable connections, a lower part count, and flexible design options in high-density applications.

Benefits of Flex Printed Circuits

Compared to traditional rigid boards and cabling, flexible electronics offer several advantages. These include more accurate wiring, mechanical connector elimination, complete design flexibility, higher circuit density, a broader range of operating temperatures, more robust signal quality, improved impedance control and reliability, and size reduction.

Automated production eliminates the human errors common in hand-built harnesses. Flexible circuits only route to the points directed by the design plan. With less labor involved during the custom PCB printing, a flexible PCB is cost and time-efficient to produce.

Flexible circuit boards can come in various customizations thanks to the freedom of a design process unrestricted by two dimensions. This flexibility extends to installation, allowing a third dimension of access to connect between two or more planes.

Flexible circuit boards’ functionality improves with increased airflow, heat dissipation, and application of a high-density device population in a point-to-point connection using simplified circuit geometry.

Fundamental Structural Applications

Before designing a circuit, two fundamental structural applications will dictate the selection of a conductor. The static application is an application that only requires flexible circuits to flex for installation. This application generally uses Electro Deposited (ED) copper, a less expensive option.

Dynamic flexing applications involve dynamic flexing of the flexible circuit during the daily use of the product. Flip-style cell phones, laptops, and computer arms all need an active application that uses Rolled Annealed (RA) copper. 

Applications of Flex Printed Circuits

Commercial Electronics

At least one-third of all flexible printed circuit sales come from computer technology: smart devices, printers, scanners, and more all utilize flex printed circuits. As the market changes and technology becomes smaller, handheld, and more advanced, flex circuits are necessary for providing heavy computing power without sacrificing space or weight.

Automotive

Vehicles are the second-largest sector for flex PCBs, and as the average car changes with the times, the hardware also grows more advanced. Cars include backup cameras, sensors, and dashboard electronics, as well as GPS software and interior climate control. Flex PCBs allow for better overall performance without sacrificing space.

Aerospace

Flex PCBs can handle the tests of extreme temperatures and vibrations, allowing them to be uniquely suited for many aerospace applications such as planes and satellites. Because flex PCBs can take on unusual shapes, they are suited for spacecraft that require connectors that might be put in situations more prone to failure.

Medical Devices

Wearable devices are a key feature in the medical sector, changing and updating as the technology evolves. Flexible PCBs have greater durability, and lower weight, and can mirror the key shapes unique in the human body. Printed circuits allow for great technological strides in hearing aids and heart monitors, offering higher performance with less wear and tear.

Flex Printed Circuits Capabilities

Single and double-sided flex circuits can feature reverse bar, through-hole, floating finger, and ZIF termination. Multilayer flexible PCBs can include up to 20 layers and handle a high circuit density with multiple conductive layers, through-hole assembly, embedded resistors, controlled impedance, and EMI shielding.

Flexible printed circuit boards use the smallest pitch and component size to improve signal integrity, electrical performance, and thermal performance when via diameters need minimization.

Flexible designs can also use the same high-density components in rigid PCBs by using blind or buried vias to route signal lines out from the device’s high-density areas. Controlled impedance improves significantly with fast signal switching, quick transitions, and high clock rates. Signal transmissions in all PCBs require uniform laminate materials in thickness and electrical signals.

Protective shielding is available from films that reduce noise and control impedance of signal lines, while stiffeners reinforce flexible circuitry where components need support for durability and mounting.

Materials for Flexible Printed Circuit Board

Flexible printed circuit boards are composed of similar materials to other PCBs. All circuits require a metal conductor to carry electricity, and copper is the most common material used in PCBs. Copper comes in a variety of thicknesses for different preferences. Other conductor options include aluminum, silver ink, carbon, and constantan.

Adhesives bind the layers of the printed circuit board together. The specific material depends on the consumer and conductor thickness needs, but all bonds must remain strong in the operating temperature range. Standard adhesives used in flexible PCBs include epoxy, acrylic, or pressure-sensitive adhesives.

Insulators encapsulate and separate the circuits with flexible substrates and coverlay materials. Polyamide, polyester, solder mask, and dielectric material all commonly act as insulators in flexible printed circuit boards.

The complete PCB receives a finishing touch at the end of the assembly process with a final finish that is customizable to the consumer’s preferences and the circuit’s function. Standard finishes applied to the final product include solder, tin, nickel, gold, silver, or carbon.

But are printed circuit boards toxic? Burning the plastic and metal found in PCBs releases toxins such as furans and dioxins, meaning that circuit boards in landfills can eventually contaminate groundwater if not appropriately recycled.

However, you can quickly repair a damaged PCB to return to its original function or recycle it in a completely different device.

Advanced Features

The flexible circuit board has advanced dramatically since its invention. As the market for flexible PCBs expands,  advances in technology enable the inclusion of advanced features to the circuit design. Advanced features include heat sinks that dissipate heat away from sensitive areas and extra components that expand mount capabilities and protection. 

As signal switching speeds advance, better impedance control is needed. Advanced impedance traces minimize electrical reflections to prevent error during the transition between interconnections and control the cable when optimized.

Other features are also available, including crimp pins, graphic overlays, laser skived slots, sculptured flex circuits, shielding, flexible heaters, and stiffeners. With so many possibilities, there is a flexible circuit board suitable for every need.