The advent of flexible and rigid-flex circuitry technology has revolutionized many aspects of modern electronic design. Components rely on flexible circuitry solutions for a range of applications that must process critical electronic signals, perform flawlessly in extreme environments, manage heat and power distribution, enable national defense, reduce automobile crashes, and do so via a miniaturized design.
The wide range of technologies enabled by customized flexible circuit designs and advanced flexible circuit material options has fueled advancements in automotive, medical, consumer, industrial and military/aerospace products and programs. Consider this cross section of examples:
 State-of-the-art technology allows doctors to quickly diagnose early stage stroke and other critical health events via MRI. Flexible circuitry advances provide critical rapid-scan analysis, increased machine movement dynamics, and step-change image resolutions.
·  Down-hole oil and gas measurement rely on flexible circuitry to measure and process information extracted thousands of feet below the earth’s surface.
· Miniature cameras driven by ultra-thin flex circuits are swallowed by patients and enable physicians to reduce diagnostic costs by providing real-time gastrointestinal images.
·  Engine maintenance units and anti-lock brake systems in vehicles use polyimide circuitry to provide weight and cost savings over traditional wiring harnesses while withstanding under-hood environments.
· Rigid-flex printed circuit boards (PCBs) are produced with multiple layers and in circuit patterns down to 0.001” spacing, driving many aspects of next-generation military aircraft, ships and satellites.
· Smartphones use 3-D antennas and advanced optical drivers to provide clear videos while also dissipating heat.
· To be sure, flexible circuit technology is not new. Patents issued in 1903 show early researchers envisioning the use of flat copper ribbons sandwiched between insulating materials for telephone switching applications. But the technology momentum of using flexible printed circuits in lieu of wiring bundles is broadly a product of aerospace and defense industry developments in the 1950s. Printed circuit board applications then grew quickly across the industrial and consumer electronic sectors. Technology advancements and the demand for rigid board replacement in flex applications, as well as shrinking wiring design footprints and other space constraints resulted in the growth of flex solutions after 1990.
· Traditional dielectric material cores for copper-trace insulation in flexible circuits included polyester, polyethylene naphthalate, fluropolymers, and polyimides. These materials are formed by bonding copper to the dielectric materials under high heat and pressure with adhesive “glues” such as acrylics.
A technology leap occurred around 1998 when a new adhesive-less all-polyimide material option was introduced. It enabled thinner circuit profiles without the need for acrylic adhesive layers and improved the coefficient of thermal expansion match between the dielectric core material and the copper-circuit traces during high temperature PCB fabrication processing. The improved uniformity of the coefficient of thermal expansion in the all-polyimide materials provided a springboard for greater complexity and density, and widely expanded the use of flexible-circuit designs in demanding applications.
The move to all-polyimide circuitry for environments where temperatures exceed 200 degrees centigrade (such as under the hood of an automobile) opened still more markets. In recent years, all-polyimide flexible circuits have played a role in enabling the exploration of Mars. Some of the first panoramic views and the search for water were driven by flexible circuitry components on NASA vehicles roaming the Martian surface.

Flexible Circuit Constructions

There are four main types of flexible circuit constructions, each of which works best in certain applications:
Single-Sided or Single-Layer Flex Circuit:
· Single layer conductor (usually copper) bonded to a dielectric film
· Basic construction allows component connections to one side only
· Chemical etching forms designed circuit patterns, and a polyimide overlay is used to environmentally seal component traces
· Most economical flex circuit construction
· Many applications in automotive lighting, consumer devices and other cost-sensitive, high-volume applications where dynamic flex and thickness profile are required, but circuitry design isn’t complex
Double-Sided Flex Circuit:
· Two conductor layers of copper, double side bonded to a dielectric film
· Versatile construction can be customized to match physical and electrical application needs by varying copper type and thickness, and dielectric insulator thickness
· Plated thru-holes provide layer conductivity with double-sided solder connection compatibility
· Primarily used in bend-to-install applications due to dynamic flex capabilities provided by thin core construction and rolled-annealed copper conductors
· Cost to manufacture is higher than rigid PCB due to pattern loss and manual assembly of customized designs. Upsides include improved power/ground integration, 3D flex and vibration resistance.
Multi-Layer Flex:
· Three or more conductive layers combining multiple double sided or single sided circuits to create complex interconnection designs with required electrical shielding, often using surface mount technologies
· Higher cost due to multiple lamination steps required to bond layers together
· Specialized fabrication of discontinuous bonding in key areas allows for maximum flexibility while maintaining advanced properties such as impedance and crosstalk control.
· Common applications include connector cabling for medical/industrial applications and antenna flex.
Rigid Flex:
· Traditionally used in military/aerospace applications for highly specialized and demanding needs.
· Recent uses for industrial and commercial designs.
· Hybrid of both rigid and flexible circuitry for applications where extra support is needed in the core board area, but also requiring flexible capability in some areas
· Conductors in both rigid and flex areas are connected via plated thru holes across multiple layers
· Frequently exceeds 15 layers
· Often applied in low volume, mission critical applications due to its high cost and reliability

Next-Generation Flex: No Limits

New fabrication techniques and flex circuit material such as ultra-thin copper conductors (less than 5 microns), and high speed/low-moisture absorption dielectrics (such as liquid crystal polymer) will likely shape a new generation of design options.
Heaters and thermal management components present unique opportunities for flex circuits that are capable of delivering precise heating via circuits using non-copper metal traces such as inconel or cupro nickel. New technologies for aircraft wing deicers are being tested that use flexible materials to quickly heat and shed ice and prevent in-flight build-up. The technology may save on costly liquid deicing procedures and improve airline safety.
Other possible applications include hospital newborn incubation chambers that require uniform and consistent temperature control, surgical room and equipment support, deep-space satellite heating in low-power environments, military electronics for vision systems, and automotive seat heating and cooling.
Wearable flex continues to capture the imagination of designers and consumers alike. Fitness devices and watch phones are already a reality on many wrists, but on the horizon are technologies that may take the next step in health and human performance analysis. Flexible circuits are being developed for clothing-based biomedical sensors, displays and communication devices.
Such advances could relay real-time vital signs to doctors and hospitals for high-risk patients, provide first responders with rapid feedback in emergency situations, and equip military pilots and soldiers with next-generation vision and information tools. Smart and flexible fabrics are being targeted for wearable consumer electronics and even computers.
Flexible circuitry continues to evolve and be used in more electronic devices. Begun as an alternative to bulky wiring harnesses, flex solutions are expanding in lightweight, rugged and efficient ways. In doing so, they are helping to miniaturize complex electronics and enable next-generation communications.