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All About Flex: FAQs for Extended Length Flexible Circuits

Extended length flexible circuits are larger than typically offered sizes in the interconnect industry. The length of these oversized circuits can be anywhere from two feet to 10 feet or longer. A long, continuous flexible circuit can offer design advantages over using normal sized circuits where multiple connections and connection assembly steps are required. By eliminating connectors and the assembly labor required for the connections, the long length circuit can be lighter, more flexible, lower overall cost and more reliable. At All Flex, we have branded our extended length circuits as Maxi-Flex.

The following are some common questions we have received regarding extended length circuits.

Q. Why are there so few flexible circuit fabricators that offer extended length circuits?
A. Most of the standard equipment used in the industry is geared toward 12” x 18” or 18” x 24” panel sizes, based on standard rigid board process technology. Oversized circuits are a relatively small niche within the industry and require custom fabrication equipment and techniques that many fabricators have avoided due to capital expense.

Q. What are the biggest challenges in producing an extended length circuit?
A.  There are a number of process steps where the standard equipment and tooling need significant modification. Many of the challenges are related to material handling. Since opens and shorts render a part useless, any defect in the length of a circuit reduces panel yield.

Q. In what applications are extended length circuits commonly used?
A. We see a variety of applications. Any application where having a long, thin, continuous interconnect system would be ideal. Unmanned aerial vehicles (UAV) and medical probing systems are a couple of applications driving existing demand, but like other flex circuit applications, demand is often generated by the imagination of a creative engineer.

Q. Can an extended length circuit have a multilayer construction?
A.  It is quite common for extended length circuits to have several layers, but limitations occur because of unique processing requirements. As a design layout consequence, the higher density region of the circuit may need to be confined to some subset area of the overall circuit size.

Q. Will feature-to-feature tolerances be different for extended length circuits?
A. Feature-to-feature specifications are often restricted to within small zones of an extended length circuit. Tolerances within smaller zones are similar to tolerances held on standard-sized circuits. Dimensional tolerances spanning the length of the circuit will need to be reviewed on a case-by-case basis.

Q. Is impedance control offered on extended length circuits?
A. Yes, it is very common for extended length circuits to be built to meet impedance specifications. Impedance control often requires thinner copper and thicker dielectric. As circuits become lengthy, material construction options are important considerations. Since oversized circuits are usually built on laminates that have been fabricated in a continuous roll process, limitations in thicknesses and constructions are possible.

Q. What are the line width and spacing limitations for extended length circuits?
A.  Like all good questions about capability, the answer is usually “it depends.” The probability of a circuit trace having a flaw is a function of its length and width. As trace runs get lengthy, and especially if combined with multiple traces, the chance of a circuit flaw increases and yield can be an issue. We have produced extended length circuits with <3 mil lines and spaces.

An extended length circuit can be fully customized to meet most requirements. These circuits provide customers with a single interconnect solution for physically large electronic systems and can be built in a variety of customized constructions including heater circuits.

Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.