We can use flexible circuits 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. Have you ever wondered about the materials and layouts involved in the creation of these versatile PCBs? This blog post will provide a quick overview of common materials and constructions in flex and rigid-flex PCBs.
Materials Used in Flex Circuits
While most standard PCBs have a fiberglass or metal base, flex circuit cores consist of a flexible polymer. The majority of flex PCBs have a polyimide (PI) film as a substrate. PI film does not soften when heated, but it stays flexible after thermosetting. Many thermosetting resins like PI become rigid after heating, making PI a superior material in flex PCB construction. Standard PI film does not have good resistance to humidity and tears, but choosing upgraded PI film mitigates these issues.
A flex PCB also requires an adhesive or special base material for its layers to attach. Manufacturers previously used adhesives only, but this method reduced the PCB’s reliability. To resolve these issues, they developed adhesiveless PI that attaches to copper without an adhesive. This material allows for thinner designs with a lower risk of via breakage. Instead of using a solder mask to cover and protect a flex circuit, manufacturers use a coverlay film also created with PI. If you want the area on the flex pcb to be rigid, the manufacture can laminate a stiffer to that portion, but the signal cannot travel between the flex and the stiffer.
Rigid-Flex PCB Materials
A rigid-flex PCB connects rigid PCB materials to flex materials. The result bends only in certain places, making the board stronger yet still flexible. If you want the signal to transfer between the rigid and flex part you will need to design a rigid flex pcb. In a rigid-flex design, the flexible part of the board resembles a typical flex circuit. Meanwhile, the rigid sections have similar materials to standard rigid PCBs. Just like standard PCBs, these rigid areas often have fiberglass as a substrate material. Multilayer rigid-flex PCBs also include prereg fiberglass as middle substrate layers.
Popular Stackups for Layer Flex Circuit Construction
Single- and double-layer flex circuits each have common stackups found in many electronics. A single-layer flex circuit often includes stiffeners created from pressure-sensitive adhesive (PSA) and FR-4 fiberglass. Pieces of FR-4 stabilize each end of the PCB, while a thin layer of PSA adds sturdiness to the middle of the board. Since double-layer flex PCBs have multiple computer-related applications, they tend to have zero insertion force (ZIF) connectors. Using PI as stiffeners at the ends gives the board the flexibility needed to attach to the ZIF connector.
Rigid-flex PCBs have various constructions, but one common approach involves four rigid layers and two flexible ones. It includes a core made from adhesiveless PI that reduces breakage risks. Above this layer and its copper film, two layers of prepreg connect to the flex section’s coverlay adhesive and coverlay layers. The rigid sections receive additional layers of copper, fiberglass and solder mask.