The Future of Flexible Circuits
As predicted by RUSH PCB UK Ltd, the market for flexible circuits is going to continue to expand steadily in the future, just as it has been doing for the past three decades. The reasons for this are not hard to find, as, on one hand, flexible circuits continue to support the existing technology so important to different industries, while on the other, advanced flexible circuits are able to comfortably meet the futuristic demands being made by up-coming industries, including the military, avionics, aerospace, telecommunication, consumer electronics, medical, and automotive. To interpret the future of flexible circuits appropriately, it is necessary to consider the subject from three angles:
- Newer configurations of flexible circuits 
- Newer applications where flexible circuits are useful 
- Newer technology being used for manufacturing flexible circuits 
Newer Configurations of Flexible Circuits
Depending on the market demand, manufacturers are always willing to add higher flex layer counts along with prevalent blind and buried via structures, embedded components, integrated connectors, sculptured flex, and more.
Again, based on application, manufacturers can offer flex designs requiring shielding for EMI/EMC in specific areas, asymmetrical constructions, and varying thicknesses between different rigid areas.
Apart from the regular rigid-flex PCB methods of constructions, there have been recent advances in newer configurations of flexible circuits available from different manufacturers. The standard rigid-flex PCB is rather a symmetrical construction, acts as a baseline for building upon, and offers good control over impedance.
Fabrication of standard rigid-flex PCB usually has flex layers at the center of the construction and even layer counts in both, the flex and the rigid areas. Although four to sixteen layers are common in designs, there can be more. Placing the flex layers at the center offers the maximum level of flexibility. However, manufacturers such as RUSH PCB UK Ltd also offer variations in configurations such as:
- Odd Layer Counts
- Asymmetrical Construction
- Varying Layer Count
- Integrated ZIF Tail
- Via-in-Pad Design
- Air-Gap Construction
- Multiple Rigid Area Thickness
- Shielded Layers
Although most designs prefer an even layer construction, manufacturers do offer odd layer counts, and this has its own advantages. For instance, a rigid-flex PCB may have seven layers in its rigid portion, and three flexible circuit layers. Requirements of stripline impedance control mainly drive designs of this nature, where the flex area requires two-sided shielding. The construction in the flex area usually has ground layers on the two outer layers sandwiching a signal layer between them—offering large numbers of interconnects between the rigid sections.
The major factor in the odd layer count design is both rigid and flex areas may have odd layers, and the layers counts in the rigid and flex areas may be mutually independent. Manufacturers offer other variations too—even layer count on one side of the core, and odd layer count on the other. The advantage being higher flexibility and higher reliability of bending both in short-term and long-term. Leaving out unwanted layers also reduces the cost of the design.
Complex design requirements such as blind via construction and or widely different dielectric thicknesses within the same PCB calls for an asymmetrical construction. Manufacturers prefer to shift the flex layers towards the bottom of the stack rather than place them in the middle. Although this does raise some concerns of warp and twist during manufacturing, using hold-down fixtures handles them easily.
Manufacturers offer another variety of construction with varying flex layer counts between rigid sections. For instance, the first and second rigid sections may have four to six layers of flex between them, but have only one or two flex layers between rigid sections two and three. Leaving out unnecessary flex layers helps to improve the bend capabilities significantly for the portion with lower number of layers.
By integrating a ZIF tail into the rigid end of a rigid-flex design, the manufacturer eliminates the necessity of mounting a ZIF connector. This is a boon in high-density designs, as it saves both real estate and cost, while producing a thin design.
High-density designs often require blind and buried vias, whereas close-pitch BGA may require via-in-pad design along with via fill and capping. Although dimensional tolerances of materials and manufacturing methods limit the number of lamination cycles in multi-layer rigid-flex PCBs, manufacturing them with via-in-pad design is possible by placing them within the rigid parts of the board.
Manufacturers also offer flex layers in separately configured independent pairs with an air gap in between. This has the advantage of improving the flexibility of the flex part substantially. Of course, this design is only applicable where there are more than two layers of flex. The absence of adhesives within the rigid areas offers greater reliability of the vias therein, resulting in long-term operation of the board.
Although an expensive and complex stackup design, construction of flex circuits with different thicknesses in multiple rigid areas is possible, but presently limited to two rigid areas with different thicknesses.
Special flex circuits requiring shielded layers for reducing the effects of EMI and RF interference use specialized films rather than copper layers. Using copper layers as shield is an expensive proposition. Instead, the special films act as effective shielding material while keeping the thickness of the flex down, thereby improving the flexibility.
Newer Applications Where Flexible Circuits Are Useful
One of the latest applications that flexible circuits have independently triggered as an explosion is the wearable electronics market. Wearing electronics on the body essentially calls for comfort, and flexible circuits guarantee this. Some examples of wearable electronic applications prevalent on the market are wrist-worn activity and body function monitors, foot-worn sensors, wearable baby monitors, medical sensors, pet monitors, and electronics on worn clothing. By bending and forming flexible circuits to suit the curve of the human body, the applications provide comfort for long wear and use.
Newer Technology Being Used for Manufacturing Flexible Circuits
Manufacturing flexible PCBs still follows the traditional methods of photo-lithography and etching to get rid of the excess copper. To make even thinner flex circuits, researchers at the McCormick School of Engineering at the Northwestern University are dispensing with the copper layer altogether. Rather, they are using a graphene-based ink sprayed onto the substrate in the required pattern to provide the electrical connections.
The advantages of using graphene are twofold. One, graphene can exist as only one atom thick, and its two-dimensional characteristics makes it both flexible as well as transparent. However, the researchers are spraying it on as 14-nanometer thick layers for creating the tracks and patterns. The second advantage is graphene being 250 times more conductive than copper is, only very thin layers are necessary. This improves the flexibility significantly, reduces the weight, and allows for even thinner flex circuits. The next attempt is to allow doping of graphene so that apart from its use as a conductor, it can be used as a semiconductor to make embedded transistors.
The combination of configuration, technology, and application is making rigid-flex circuits a formidable force in the electronics field. It has already overtaken the applications of traditional rigid printed circuit boards, and is threatening the more sophisticated uses of special PCBs in military, aerospace, medical, and consumer electronics industry.
For further information, refer to RUSH PCB UK Ltd.