The evolution of flex printed circuit boards is very interesting. The concept of PCBs started in the 1900s with construction of electrically conducting paths on insulated surfaces or boards. Manufacturers used these for developing equipment such as radios and gramophones. The through-hole technique came into existence around this time. Manufacturers in the US later introduced the fabrication of PCBs and automated soldering techniques. Major developments took place once the US army adopted PCBs for use in their equipment.
From the 1960s, when NASA introduced the use of rigid-flex PCBs in their applications, to the 1990s, when Military Avionics used REGAL flex for 24-layer back panels, the demand for customized PCBs increased significantly in various industries. This included high-speed analog and digital designs and multi-layer boards with high pin count such as for BGA packages. Users discovered several methods of assembly such as the backplane assembly process, cable harness, box build, and electro-mechanical assembly methods. This led PCB manufacturers to look for new ways to differentiate themselves from through innovation.
As a result, several new technologies came into being including Integrated Circuits (IC), Surface Mount Technology (SMT), and Hybrid Circuits. In parallel, the PCB industry also kept pace with a range of new designs, all fabricated to meet the demands of their customers. Older methods of design of rigid-flex PCBs underwent a change in favor of increased number of layers, multiple assembly cycles, and the introduction of RoHS.
All the changes introduced a lot of flexibility in the methods of fabrication of flexible and rigid-flex PCBs. These included special capabilities in solder masking, surface finishing, PCB profiling, material thickness, and micro-drilled holes. Fabrication of multilayered boards was mechanized, with high-end customization and improvements in their functional layouts.
Around the same time the concept of rigid PCBs was gaining ground, researchers in the telephone industry at the beginning of the 20th century found that alternating layers of conductors and insulators produced standardized, flexible electric circuits that suited their purpose very well. The first patent in this area was from England, and it described paraffin-coated paper with flat metal conductors on it to provide the circuits. Even Thomas Edison experimented with linen paper coated with cellulose gum, with circuits traced on it with graphite powder.
It was only in the late 1940s that mass production techniques resulted in a deluge of patents involving photo-etching circuits on flexible substrates for replacing wiring harnesses. Recently, flexible silicon technology has added active and passive components to flexible circuits. This is the ability to include thin-film transistors and other semiconductors onto the flexible substrate.
The technology combines the traditional advantages of flexible circuit construction with onboard sensing and computing capabilities, leading to exciting developments in various areas—especially in consumer-electronics, medical, and aerospace fields. Beyond 1950s, commercial high volume circuit fabrication involved stamping metal foil, printing conductive paste, and copper etching.
Although the entire process was automated and high volumes of circuitry could be manufactured at high speeds, hot stamped copper circuit had limitations such as trace density and configuration. With more demands being placed on flexible circuits, such as from the automobile manufacturers, hot stamped copper circuits lost their appeal. They were replaced by printed conductive paste, which is used even now.
However, fabricators of flexible circuitry rely more on etched copper, as they offer finer and higher resolution. This technology first appeared in the 1960s, as etched copper circuits on flexible composite substrates for telecommunication. Until now, the most universally used techniques of mass-producing flex circuits on polymer thick film are conductive printing and copper etching.