Unlimited-length FPCs are set to unlock benefits for a wide spectrum of applications
Flexible printed circuits (FPCs) are a popular choice for designers of a wide range of industrial and consumer electronic products, owing to their many benefits – including weight, space and cost savings, ruggedness and electrical performance. Traditional FPC manufacturing techniques have, however, been unable to produce FPCs more than one metre in length, restricting their use to smaller applications such as cameras, mobile phones, printers, instrument panels, control systems and medical instruments.
FPCs are available in six main types, depending on the needs of the application, but their construction is best understood by examining the single-sided FPC. This commonly used variant is a flexible laminate, made up of four individual layers, each of which contributes to the overall performance of the FPC, as shown in figure 1.
· The FPC’s mechanical properties are derived from the base substrate, a dielectric film which also acts as the insulation between the conductive tracks.
· The conductor layer consists of these conductive tracks which, as well as providing the electrical connectivity, are responsible for the electrical characteristics of the FPC.
· The adhesive layer creates a laminate by binding the conductive layer to the base substrate, and it can also be used to bind layers of laminate together in multilayer FPCs.
· The cover coat, also known as the cover lay, is applied to the surface of the FPC, giving a protective finish which protects against moisture, contamination and abrasion.
Figure 1: Basic FPC construction
The adhesive layer also helps to reduce stress during flexing, a key property of an FPC, which can be further defined according to the flex requirements of its application. A static flex application requires that the FPC is folded or bent during assembly in order to get it to fit inside a tight or difficult-to-access location. An FPC in a dynamic flex application may have to withstand constant flexing during its lifecycle, for example in a car-door hinge or printer cable.
A wide range of materials is available for each of the above four layers, and choosing the optimum material for each layer is key to ensuring that the electrical and mechanical performance requirements of the application are met.
Polyimides and polyesters are the most popular choices for the base layer, which gives the FPC its mechanical stability and also influences its thermal resistance, tear resistance, electrical characteristics and cost.
As well as ensuring the electrical performance of the FPC, the choice of material for the conductor layer determines its overall fatigue life, stability and mechanical performance. While copper is the most commonly used material, a range of options exist, including aluminium foils and metal mixtures such as stainless steel, beryllium-copper, phosphor-bronze, copper-nickel and nickel-chromium resistance alloys. Additionally, various printing techniques have emerged in recent years, including polymer thick-film (FTP) ink-based processes and lithography ink deposition.
Similar options exist for both the adhesive and cover lay materials, and selection of these is based on factors including cost, temperature resistance, chemical resistance and mechanical strength, as well as compatibility with the other FPC layers.
With IHT now enabling the production of FPCs of unlimited length, a whole spectrum of applications are set to benefit from their properties. In the automotive industry, for example, car makers are rising to the challenge of reducing emissions and are investing heavily in electric vehicle (EV) technology. Efficiency is key in EV design and, with weight savings critical, replacement of complex and heavy wiring harnesses represents a significant opportunity for FPCs. It has been shown that FPCs can reduce weight by up to 75% over traditional wire harnesses, making them ideal candidates not only in modern car design but also in the aerospace industry, where unlimited-length FPCs can span aircraft wings, again significantly reducing weight, enabling substantial fuel savings.
Figure 2: A 26-metre-long, multilayer flexible printed circuit for an unmanned aerial vehicle (UAV)
To find out more about the opportunities and benefits of using FPCs, contact Trackwise or consult its recently published white paper – ‘The engineer’s guide to multilayer flexible printed circuits (FPCs): technologies and applications’. This comprehensive and invaluable paper examines all aspects of FPC design.