Flex Primer for IoT & Wearables

Mainstream technology is headed toward small form factor electronics, mostly dominated by IoT devices and wearables. IoT is made possible with the help of flex circuits in combination with rigid flex PCBs.

It is important for designers to learn all there is to know about the new terminologies and various design considerations as they develop IoT applications.

Bendability of Flex Circuits for IoT

All flex circuits need to be bendable and pliable regardless of their applications. However, most experts are divided on the bendability of flex PCBs. At the time of writing, the Institute of Printed Circuits is rather ambiguous on this topic to take into account various factors that may impact circuit durability.

In other words, industry experts have not figured out the preciseness of bendability for flex circuits. This stance of the industry probably won’t change because there is no definitive answer.

The best solution for designers is to rely on their experience or consulting experts who have worked with IoT circuit designs. You can get in touch with Hemeixin PCB consultants for help in determining the extent of flexible circuit bendability for your project.

With that said, it is important to be aware of terminology such as bend ratio, strains, and bend radius, all of which are closely related. Let’s explore them below.

Bend Ratio

Flex circuits are only as reliable as the ratio of the bend radius to circuit thickness. This is known as the bend ratio. A multi layered circuit for IoTs is required to have a bend ratio of 20:1 at a minimum. Flex PCBs can be constrained to minimize the damage done due to flexing if they are close to the minimum bend ratio.

Bend Radius

Designers can determine the bend radius by taking the distance of the center of the radius from the inner surface of the bend. Where circuit bending is concerned, it is useful to know whether the PCB is subject to static bending or dynamic flex involving repeated bending operations.

For static bending, the bend radius should be, at a minimum, 10x the thickness of the circuit with a maximum strain of 2.2% on the critical layers.

Dynamic flexible PCBs should have a bend radius of 25x or less. For example, dynamic flex circuits such as BGA packages should have a bend radius under 0.4% for one million cycles.

Strain

In the case of flex PCBs, strain is already present in the circuits due to the manufacturing process. This means that strain is an inherent feature of various circuit layers but you can relieve it with the help of stiffeners and other relief devices. You can also use a strain measurement kit for quality assurance purposes.

These kits should ideally come with software packages that allow you to measure your flex board’s strain and stress. 

Dielectric Thickness

Flex PCBs may be subject to increased strain because of dielectric materials. As such, it is important for designers to choose a dielectric material based on the application. Designers can adjust dielectric thicknesses and conductor widths to meet the impedance requirements of their application.

This brings up the concept of board modulus. Softer structures have a low modulus while harder circuits with stiffeners have a high modulus. Stiffeners can be used to create rigid areas on flex circuits for various purposes such as soldering.

Bend radius is particularly important in high modulus boards with stiffeners. This is because bend ratio calculations should also account for the parameters of the stiffeners because they contribute to the overall thickness of the flex circuit. Maintaining a small bend ratio is crucial to the reliability of flex circuits.

This requires designers to become fully aware of the strain at various levels of their flex circuit boards. In other words, they must know how much copper is used on different layers. The amount of copper can have a major impact on the strain.

For example, copper with a thickness of 17.5µm on a flex circuit with a specific bend ratio will be subject to different levels of strain. However, doubling the thickness of copper to 35 µm will reduce the flexibility and limit the bend ratio because of the thickness of the copper. This highlights the importance of carefully measuring the bend ratio.

Moreover, it is good practice to measure the thickness of the copper at various layers of the flex board. This is because thickness can affect the strain factor and bend ratio. Note that there is no one-size-fits-all rule for the type of flex material you can use on your application. Consult your manufacturer for advice on the type of flex material to use.

Via Placement

Designers should follow a few rules when it comes to via placement because they are vulnerable to curvature and blinds. Maintaining via integrity becomes more difficult because you have to maintain adherence within the multiple layers in your circuit.

Take an 8 layer flexible circuit, for example. You must make sure that each layer adheres to all other layers, irrespective of whether the circuit is flexing, bending, or stationary.

Vias must be placed strategically by accounting for the flex part of the circuitry. As a rule of thumb, vias should have a clearance of 20 mil between each other if they are installed on the same circuit.

The Role of HDI PCB in IoT Devices

HDI allows for more versatile routing and component placement in flex circuits. The main advantage of HDI is that it minimizes drill to copper. Using vias (both buried and blind) allow de singers to avoid the denser areas of the board.

As a result, HDI flex circuits have an improved wiring density with smaller trace widths. This is done with the help of stacked microvias, blind vias, and buried vias.

Rolled Annealed or ED?

Most IoT designers may not encounter this aspect of circuit design. However, this is a topic that you may consider if you want to minimize design problems. For example, it is not recommended to use ED copper in flex circuits because they are better used for rigid PCBs. Flex circuits should use rolled annealed copper instead.

RA copper is considerably superior for flex circuits because they are smooth and tolerate repeated cases of flexing and bending.

Wrapping Up

Reliability is absolutely essential to IoT applications, especially if the sensors are difficult to access. Considering the challenges in repairing and replacing IoT devices, designers should go above and beyond to ensure a high quality standard.

Are you working on IoT projects for your company and need help with rigid-flex circuits? Get in touch with the consultants at Hemeixin PCB here.