Flex Circuit Board
Flex Circuit Materials and Construction
Flexible PCB can be used in a variety of applications. They can safely bend into small and complicated electronics, making them suitable for modern devices like smartphones. They absorb shock and vibration easily in high-stress applications.
Rigid-flex circuits combine standard circuit board construction with flexible design for situations that require more durability. This article will provide a quick overview of common materials and constructions in flex and rigid-flex PCBs.
Materials Used in Flex Circuits
While most standard printed circuit boards (PCBs) use fiberglass or metal as their base, flex circuit cores consist of a flexible polymer. Most flex PCBs feature polyimide (PI) film as the substrate. PI film remains flexible after thermosetting and does not soften under heat. While many thermosetting resins become rigid after heating, PI continues to be a preferred material for flex PCB construction. However, standard PI film has drawbacks such as limited resistance to humidity and tearing, though advanced PI variants help alleviate these issues.
To bond layers in a flex PCB, manufacturers use either adhesives or specialized base layers. Earlier production methods relied heavily on adhesives, but this approach often compromised PCB reliability. To address this, adhesiveless PI was developed, allowing copper to bond directly without additional adhesives. This innovation supports thinner designs and lowers the risk of via breakage. Instead of traditional solder masks, a PI-based coverlay film is used for protection. If certain sections of a flex PCB require extra rigidity, a stiffener can be laminated onto that area; however, electrical signals cannot pass between the flex section and the stiffener.
Rigid-Flex PCB Materials
Rigid-flex PCBs combine rigid PCB materials with flexible sections, enabling specific areas to bend while maintaining overall durability. If electrical signals must transition between rigid and flex regions, a rigid-flex PCB design is required. The flexible section functions similarly to a standard flex circuit, while the rigid segments are composed of materials akin to those found in conventional rigid PCBs. Rigid areas often incorporate fiberglass substrates, and multilayer rigid-flex PCBs frequently include prepreg fiberglass layers as internal reinforcements.
Typical Layer Stackups for Flex Circuits
Single-layer and double-layer flex circuits are commonly used in various electronic devices. A single-layer flex circuit generally includes stiffeners made from pressure-sensitive adhesive (PSA) and FR-4 fiberglass. The FR-4 elements add structural reinforcement at the PCB’s ends, while a thin PSA layer enhances central stability. Double-layer flex PCBs, widely used in computing applications, often incorporate zero insertion force (ZIF) connectors. Using PI stiffeners at the ends provides the necessary flexibility for secure attachment to ZIF connectors.
Rigid-flex PCBs are available in multiple stackup configurations, with one common approach featuring four rigid layers and two flexible ones. This structure includes a core composed of adhesiveless PI to mitigate breakage risks. Above this core and its copper foil, two prepreg layers attach to the flex section’s coverlay adhesive and coverlay layers. The rigid portions receive additional copper, fiberglass, and solder mask layers for enhanced durability.
Selecting a Trusted PCB Manufacturer for Your Needs
A good Flex PCB supplier can understand your needs and provide you with quality products. If you are planning to find a manufacture for you flex PCB project, Hitech Circuits is a great choice. Not only can we provide service of flex PCB fabrication, but also provide the services of components sourcing and assembly. Email Perlina Tang<sales3@hitechpcb.com\> for a quick quote.