Flex Circuit Boards
A flex circuit board is a printed circuit that can be bent or shaped to fit the required area. This allows the design to be smaller, lighter and more compact than rigid PCBs with similar functionality. It also offers significant design freedom in areas where space or weight limitations would otherwise prevent the use of other technologies.
The flex PCB can be built with a variety of conductor materials including copper. The copper is bonded to the dielectric material with an adhesive such as epoxy or PSA (pressure sensitive adhesive). Insulators, typically polyimide and polyester, are used to separate the conductors from one another and to provide support. The board can be made with various coatings such as solder mask, silkscreen and a choice of finishes including tin, gold or silver.
There are many different types of flex circuit board and they are categorized by their levels of inspection, testing and performance requirements. The highest level of flex circuit is known as military-grade which meets stringent standards to ensure continuous reliability and service in critical applications. These include life support items and flight control systems.
What Are Flex Circuit Boards?
Flexible circuit boards can be used to replace wire harnesses in electronic devices. They can be bent, rolled or even folded and are suitable for dynamic applications such as cell phones that are frequently flexed while in use. They can also be integrated into the device to reduce the weight and size of the final product, and they can make assembly easier and more efficient by eliminating the need for hand wiring.
A flex PCB can be made in either single or double-layer configurations. The stackup of the conductive layers and other layers is defined by the application for which it is being designed. Single-layer flex circuits are usually designed with stiffeners created from FR-4 fiberglass and pressure sensitive adhesive (PSA). They may include zero insertion force (ZIF) connectors on both ends of the board. The sturdiness of the stiffeners allows the ZIF connectors to be attached to the flex circuit without applying excessive force.
Multi-layer flex circuits are created by combining single-sided and double-sided flex circuits that are interconnected through plated-through holes or surface mount technology. They are ideal for complex connections, additional shielding and high component density.
The copper in a flex circuit is typically annealed rather than polished to increase its durability and tensile strength. The process elongates the crystal structure of the metal and makes it more resistant to fatigue. This is particularly important when the flex circuit is likely to be twisted, bent or flexed repeatedly over time. It is also beneficial if the copper is coated with tin or gold to protect it from corrosion and improve its electrical properties. The tin and gold also help to resist heat and shield electromagnetic interference (EMI).