How Flexible Printed Circuit Boards Enable Complex Routing

Flexible Printed Circuit Boards

In flexible printed circuit boards (FPB), the conductor patterns are formed on a base material that is much thinner than rigid PCBs. This allows traces to be routed in complex three-dimensional configurations and at angles that cannot be achieved with ribbon cables or hand wiring. Combined with the ability to bend and twist, these attributes make flex circuits ideal for dynamic applications that require a significant degree of flexibility.

The flex circuit design process starts with a virtual model of the layout using simulation software that accounts for all the components, traces, and vias in the final product. This significantly reduces the need for costly and time-consuming physical prototype iterations. Once the flex circuit design has been verified, gerber files are sent to the manufacturer and the board is fabricated and components installed.

To produce a flex circuit, a base layer of polyimide or polyester film is coated with copper foil. This layer is a thin film that can be as little as 12 microns thick, which is substantially thinner than rigid PCBs. To isolate this thin copper film from the base material, a layer of prepreg is applied, similar to that used in rigid PCBs. This layer insulates the conductor layers and provides mechanical strength. Conductors are then etched on the flex circuit, either single or multilayer as required by the design. A coverlay is then applied to protect the traces and copper from moisture, dirt, and damage. This is typically a polyimide or PTFE film.

How Flexible Printed Circuit Boards Enable Complex Routing

While a flex circuit can have many advantages over traditional rigid printed circuit boards, the choice of substrate material is critical to its performance. Different materials have varying dielectric constants and loss tangents that can impact signal propagation and attenuation. In high-speed applications, signal degradation and data corruption can result in system failures, so it’s important to carefully consider the material selection for a flex circuit.

Another key factor in the performance of a flex circuit is via size and spacing. Smaller vias can help control impedance, but they also create manufacturing complexity and the risk of reliability issues due to via barrel cracking when flexed. To mitigate these risks, it’s important to carefully consider the placement of vias during flex circuit design and ensure that they are properly filled and capped with conductive material after fabrication.

The flexibility of a flex circuit enables it to conform to the shape of a product, which can save space and weight. This is particularly advantageous for dynamic applications, such as mobile phones that fold during use, or for connecting subassemblies to replace bulkier wire harnesses. In addition to their structural benefits, flex circuits offer significant cost savings by eliminating the need for connectors and wires that would be necessary with ribbon cable construction.

The insulating properties of a flex circuit also make them attractive for use in harsh environments. While conventional ribbon cables are insulated with Teflon and other plastics, which outgas fluorine or other reactive gases at high vacuum conditions, the Kapton-based insulation of many flex circuits is virtually unaffected by this phenomenon.