Flexible and stretchable devices are a fast-growing class of technology that has the potential to revolutionise life by pioneering;

• Healthcare applications

• The interaction between humans and machines

• Internet of Things

• Other applications.[1]

In most cases, flexible printed circuit  (FPC) issues arise because the designers are more accustomed  to designing rigid circuit boards, so they design flex circuit boards according to the same principles. Of course, flex circuit boards are not rigid circuit boards, and they require their own flex-friendly design.

FLEXIBLE MATERIAL SELECTION

Flexible Materials can be divided according to the different functions of materials in flexible electronic devices, they can be divided into basic substrate materials, conductive materials, semiconductor materials, dielectric materials, etc. Flexible material in addition to considering the optimization of device performance, the selection of materials must also fully consider the requirements of the assembly process on material performance to achieve low manufacturing costs.

BASE MATERIALS

To meet the requirement of flexible electronic devices, the base material must have a number of attributes. It must be bendable but not fragile, impact resistant, high insulation and cost effective.

Roll-to-roll printing technology (R2R) is a fabrication method used in manufacturing that embeds, coats, prints or laminates varying applications onto a flexible rolled substrate material as that material is fed continuously from one roller on to another. In addition, you can also use R2R to greatly increase productivity and reduce production costs. Accounting for the above considerations, high molecular polymers are most used for flexible substrates. The common flexible substrate materials include polyimide, polyethylene naphthalate (PEN), polymer organic polymers such as polyester (PET) [2], these organic materials can meet the requirements of insulation, flexibility, and strength well.

Fig. 1 artistic impression of  a flexible electronic device prepared on a polyimide plastic substrate.

In this article, we will be looking into the following aspects of the FPC design:

1. Flexibility and number of layers of FPC

2. Flexible PCB traces

FLEXIBILITY AND NUMBER OF LAYERS OF FPC

Keeping the following in mind are very helpful during the design phase;

–Knowing the number of times your FPC will bend is crucial to your design. If a PCB is bent more times than the design allows for, the copper will begin to stretch and crack. Bend radius is the minimum amount the flex area can bend. 

-Keep the number of layers as low as possible

-The standard stiffener thickness varies from 0.002′′ to 0.059′′. The FR-4 stiffeners can be 0.008″ to 0.059″ thick while the Kapton stiffener thickness ranges from 0.002″ to 0.010″. The more support the PCB stiffener provides, the thicker it is. Each design requires a distinct thickness.

-Avoid to solder joints not too close to bend point

-Add flexibility to bend the radius of the circuit at the intersection of rigid and flex parts. This will avoid the stress on the flex part during multiple bending operations.

FLEXIBLE PCB TRACES

 The following points  are also important;

-The performance and longevity of a FPC can be directly attributed to the layout of the circuitry. Signals should never be routed at sharp angles.Ruiputuo  recommends the largest radius possible for your design when designing multi-layer FPC, stagger traces on the front and back.

-Avoid sharp corner traces, curved traces are more efficient.

-In a FPC, a trace entering a pad forms a weak spot in which the copper might get fatigued over a point of time. It is always recommended to taper down the pads towards (as shown below) the end in which they are connected to the traces.

-Support for pads on FPC. Copper is prone to be detached from the substrate due to the bending of the flex circuit. It is vital to provide mechanical support for the exposed copper. The through-hole plating in the vias inherently provides mechanical support to the vias in the flex region. Additional through-hole plating of up to 1.5mils is recommended for rigid-flex and FPC. 

-Keep certain areas of flexible circuits flat and stable.

-Ensure compliance with ZIF connector specifications.