A flexible circuit carries the same conductor as a rigid board on a base that bends — polyimide, Kapton or PET, thin enough to fold into the space the product already has. That replaces a bundle of wires, the connectors at each end of it and the assembly time that goes with both, which is why handset, camera, wearable and medical designs are built on flex.
What a flexible circuit replaces
- Cable and connector assemblies — one flat circuit replaces the wiring harness and its two mating connectors, removing the joints that fail first in a moving product.
- Volume and weight — a 12.5–50 µm base with 12–25 µm coverlay weighs a fraction of an equivalent round cable and takes up no more room than its own thickness.
- Assembly time — the flex is placed and soldered like any other component, so a populated section that used to be hand-wired arrives as a single part.
- Dimensional freedom — the circuit can be routed in three dimensions, folded, and fixed to the housing with a pressure-sensitive adhesive.
Materials and stiffeners
PI, PET and Kapton base material covers most work, bonded to FR-4 where a section has to be rigid. Where a connector or a component needs a flat surface, PI, PET, FR-4 or stainless-steel stiffeners are laminated on, and the stiffener thickness is chosen together with the adhesive system so the finished part does not curl. Copper runs from 0.5 to 2 oz, and the finish is ENIG, immersion tin, OSP or carbon ink.
Design rules that decide the yield
Minimum line width and spacing are 0.002″ (3 mil) and the smallest hole is 0.006″ — finer than most cable-based designs need, but the bend area is what decides whether a flex survives. Conductors cross a fold at right angles rather than running along it, plating is kept out of the flexing region, coverlay windows are placed away from the bend, and the minimum bend radius is agreed at DFM so the copper is not compressed past its limit. Impedance can be held to ±10 % where the design is controlled-impedance, and HDI or rigid-flex constructions are used when the same circuit also has to carry fine-pitch components.
Where flexible circuits are used
Mobile and wearable devices, camera and battery modules, medical equipment probes and monitoring patches, printers and moving heads in industrial control, and antenna and RF sections of telecommunications equipment. Where a rigid section is needed for the connector or the processor, a rigid-flex board puts both halves on one part number.
Getting a quotation
Send the Gerber set with the layer count, base material, copper weight, stiffener size and the bend radius the part has to survive. Assembly runs in the same plant, so a fully populated flex can be quoted as one order; the process limits are on the capability page and the issued certificates on the certifications page.
Flex circuits that continue into assembly are quoted with fabrication, sourcing and box build on the China PCB assembly page.
