Designing Flexible Printed Circuit Boards

General purpose flexible circuit boards are those with electronic components that can be bent to fit in tight spaces. Most are just one or two layers thick and are meant for “flex to install” applications, as they will tolerate limited flex cycles. Circuit boards like this are often found in a variety of medical and consumer products.

The more difficult flexible circuits have three or more layers and are based on specifications that require, for instance, high flex cycles, or boards that must be bent or flexed to fit into unusual packaging. What constitutes high flex cycles? It’s is hard to say? There have been tests done in the commercial industry where they are well into 100K bend cycles, but it all depends on the length, type of materials, copper weight on the boards, type of bends etc. The government, on the other hand, can and does use many flexible circuits that are in one-time bends and high flex cycles. Electrical engineers are getting so creative that many new flex circuits are specified with unusual features that can take a little research and experimenting before they are manufactured in quantity. This article tells of one such flex circuit, and although it was not for a medical product, it could be used for medical applications.

A Few Unusual Circuits

Most people think of a flex circuit as a board consisting of conductors sandwiched between layers of insulating material. Although true, that description fits many types of flex circuits. A few examples would include:

• Single-sided flexible circuits that have one conductive layer of copper on a dielectric (insulating) material. These circuits are simple circuits that could be used in a hinge circuit on a laptop computer or maybe on the hinge of a flip open phone. These can be built by a number of different fabricators.
• Double-sided circuits with copper on both sides of the dielectric. These have similar applications as above, but are used where more interconnections are required in a small space.
• Flexible circuits with several layers. Each is registered to the other layers and bonded together with a suitable adhesive cured layer. The layers are all interconnected by plated throughholes. The number of flex layers can be up to the engineer’s imagination, but too many layers and too much copper will turn it into a rigid board.
• Sculptured flexible circuits are single or double-sided designs with thicker copper to allow connections such as fingers or pads as rigid extensions of the flexible conductors. These circuits have thicker copper plated on exposed areas where the conductors could be inserted into a mating connector.
• Molded flex are flex circuits that are thermally molded into simple shapes. A good example of this would be a formed dome on a flexible circuit.
• Multi-layer rigid flex circuit boards combine flex circuits and rigid boards where the flex circuit extends from the rigid board area. These boards are the most complex and are used to interconnect two or more rigid or multilayer boards together.

Fermi National Accelerator Lab - oratory is home to the Tevatron, a fourmile circumference high-speed particle accelerator. The lab needed a flex circuit for an unusual application – a particle detector inside the accelerator. The circuit had to work as a controlled impedance board with a low loss material because the circuit would be detecting low amplitude signals. Also, the circuit could tolerate no out-gassing because the researchers at Fermi didn’t want the detector finding particles that had been out-gassed off the circuit. In addition, the circuit had to be flexible enough to be inserted into an enclosure.

The lab needed a controlled impedance board that was flexible, even though the design required four copper layers — two layers of signals and two layers of shielding. What’s more, the circuit would have to withstand soldering to make all the electrical connections.