A Complete Guide to Flexible Printed Circuit Boards

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Oct 2, 2020|General

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. 

Double-Sided

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

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.

Rigid Flex

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

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.

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.