Flexible circuit boards

Flexible printed circuit boards offer the following advantages over rigid printed circuit boards:

• Thin and flexible – can be folded or rolled to fit into tight spaces
• Flame-retardant – material class UL 94V0
• Easy to solder – even in lead-free reflow processes
• Space-saving – in many cases, they replace cables and connectors
• Customisable – structure, thickness and reinforcements can be tailored to the specific application

This combination makes flexible printed circuit boards particularly attractive where weight, installation space or movable connections are a factor – traditional cable harnesses can often be replaced by a single, more robust solution.

Most flexible printed circuit boards are based on polyimide, a material with higher temperature resistance than FR4, which is commonly used for rigid printed circuit boards. In the adhesive-based construction method, the copper layer is bonded to the polyimide via a thin layer of epoxy – a tried-and-tested, cost-effective process that is entirely adequate for most applications.

With adhesive-free material, this epoxy layer is omitted: the copper is rolled on directly. This results in a thinner construction, and the temperature resistance remains closer to that of pure polyimide. Adhesive-free material is therefore often the more suitable choice, particularly for applications involving a very high number of bending cycles or elevated temperature requirements. The coverlay itself also usually contains an epoxy layer, via which it is pressed onto the polyimide – regardless of whether the base material is adhesive-free or adhesive-based.

When rigid and flexible sections are combined on a single printed circuit board, this is referred to as a rigid-flex printed circuit board (RFPCB). Technologically speaking, this is a discipline in its own right, with additional requirements regarding layer structure and design – find out more on our page about rigid-flex printed circuit boards.

The manufacturing process for flexible printed circuit boards is similar to that for rigid printed circuit boards, but differs in a few steps. Instead of solder resist, a cover film is usually heat-pressed onto the board, into which the openings for the solder pads have been cut beforehand. Due to the sensitivity of the material, we also tend to use chemical rather than mechanical processes for the final surface treatment. When separating the boards, we cut the flexible printed circuit boards from the panel using a laser or by punching, depending on the requirements, as mechanical milling is usually unsuitable for such thin material.

For connector areas or for mechanical reinforcement, we offer stiffeners made from polyimide or FR4. For easy attachment during final assembly, double-sided 3M high-temperature adhesive tape is also available, which withstands the lead-free soldering process without damage. Both options can already be selected in the online quotation tool for flexible PCBs. Detailed technical information on bend radii, layer structure, stiffener thicknesses and adhesive tape can be found in our design guide for flexible and rigid-flex printed circuit boards.