A flex circuit board is a printed circuit board designed to be flexible. These boards are used in many applications where the PCB must withstand harsh environments such as heat, moisture and chemicals. They are also often used in place of wire harnesses in products such as mobile phones and automobiles. In addition to requiring flexibility, these types of applications often require small form factor circuits or very high current density.
Most flex circuits are constructed with a base raw material known as a polyimide film. This material is bonded with various adhesives in order to create a laminate that provides the electrical and physical properties required for the circuit board. The choice of adhesive is important because it can significantly affect the performance characteristics of the circuit board. For example, certain adhesives are suitable for higher temperature resistance than others.
The conductors in a flex circuit board are typically copper foil, but other metals can be used as well. Copper is preferred due to its balance of cost and physical and electrical performance attributes. Copper foil is typically etched using electrodeposited or wrought copper processes. This process allows for greater control of the copper thickness and uniformity, which is beneficial in reducing stress on the circuit during bending and flexing.
Materials Used in Flex Circuit Boards
In a typical multilayer flex circuit, the layers of the substrate are separated by through holes and plated with copper. The thickness of the copper layer can vary depending on the application and design. A bare copper layer is sometimes used, while other times a copper-filled epoxy or acrylic flex adhesive is utilized. These materials allow for easier assembly of surface mount components and provide good conductivity even when the board is bent or flexed.
Some flex circuits use solid copper pours to serve as the ground or power plane, which can be fine if the designer doesn’t mind the resulting loss of flexibility. However, for most flex designs, it is more practical to use hatched copper. A hatched copper layout reduces the copper’s bias in the 0deg, 45deg and 90deg angle directions, which will help to alleviate the stress on the copper when the flex circuit is in these angles.
The versatility of flexible PCBs extends beyond their physical properties to their role in enabling advanced electronic functionalities. From high-speed data transmission and impedance control to signal integrity and electromagnetic interference (EMI) shielding, flexible PCBs offer a host of features that enhance the performance and reliability of electronic systems. Whether in consumer electronics, automotive applications, industrial automation, or beyond, the adaptability and functionality of flexible PCBs continue to drive innovation and shape the future of electronics.
The top layer of a flex circuit is the coverlay, which protects the copper from moisture and chemical exposure. The coverlay also provides some strength and visual appeal to the final product. Typical choices include polyimide, epoxy and a variety of acrylic-based flexible adhesives. The coverlay can be printed with a wide range of colors, patterns and textures, which allow for a variety of aesthetic and functional design options.