What is Flexible PCB

Our dependability on electronic devices is increasing day by day. Once our phones used to just a mode of communication, now they contain a small world of their own that is available to us on our fingertips. Our mechanical watches have transformed into smart devices that can monitor our vitals, count our steps, and read our texts.

A lot of these changes are made possible because of the continuous evolution of PCB, the underlying backbone of most of these electronic devices. And one of the significant steps in that evolution is the Flexible PCB.

Flexible PCB

The name flexible refers to the mechanical flexibility of such PCBs, which allows them to bend. This one trait offers a world of difference; because that means that you can design your circuit in three dimensions. So instead of developing the housing of the device with PCB's accommodation in mind, the designer can focus on the device's mechanics, and the circuit can adapt to that.

It’s not always as simple as that, because with this mechanical power, comes a lot of electrical responsibility. The mechanical prowess of a flexible circuit sometimes clashes with the electrical characteristics of the circuit, and the designer has to weigh one against the other.

Further improvement upon the flex is the Rigid-Flex circuit, but there are still devices that require a simple flex circuit.

Distinct Characteristics of a Flex PCB

There are some characteristic differences between a Flex circuit and other types of PCBs.

1. Material selection for a flex PCB is different from a Rigid PCB, mainly the substrate that forms the core of the circuit. The most commonly preferred material is polyimide because of its flexibility, strength, and thermal resistivity. The conduction layer is usually copper, but how it’s deposited on the substrate is different from what it is on a Rigid-Flex. For increased flexibility, thinner and hatched copper layers are usually preferred over thick planer layers.
2. The thickness of the board is an essential characteristic. For a rigid board, the thickness usually impacts the through-hole and microvia ratios. But for a flex circuit, the thickness has a severe impact on the overall flexibility of the circuit. So designers try and make the flex as thin as they possibly can.
3. Component placement limitation. Conventionally, plated through holes are used for Flexes, but they have certain restrictions. You can't place plated through holes wherever you want like you can in a rigid PCB. You have to consider the bend radius because a plated through hole in its vicinity has a very high chance of developing a fracture line in the copper. This is somewhat mitigated by focusing on the use of SMTs in the flex circuits. Still, in a flex PCB, you can’t use real estate as efficiently as you can in a rigid PCB. In most cases, if you take a double-sided flex and a rigid of the exact same dimensions, you might not be able to place the same number of components on both of them.
4. Trace width is a bit different in flex is a bit different. It is advisable to keep the traces as wide as possible, which further reduces the available real estate for components and pads. Also, in a multilayer flex circuit, you can’t have conductors vertically over one another (the I-Beam construction), since it limits the flexibility of the circuit. Instead, a good practice is to stagger the conductors to reduce the stress from both the compression and tensile side of the bend.
5. SMT placement limitation. In a flex, the circuit is covered in a polyimide (or PET) based cover-lay, compared to a rigid circuit that is covered with a photo-imageable mask. If the fabricators use a couple of different drills to get around the problem, it adds to the tooling cost of the flex fabrication. This is why it’s a good idea to work with fabricators that employ cutting edge technologies and modern techniques, to reduce cost and increase efficiency as much as possible.

Advantages of Flexible PCB

Even though it has some fabrication and design limitation, and a relatively high cost, the advantages of flex PCBs are impossible to ignore.

1. Adaptability. A flex PCB adapts to the shape, dimensions, and requirements of the device. Its best example is probably wearable smartwatches. Packing so many sensors and features in a compact, lightweight device like this would not have been possible without an advanced flex circuit.
2. Highly reliable. In applications where the device has to go through many jerks and vibrations, flex circuits provide a much more reliable alternative to rigid PCBs. Since flex PCBs are kept as thin as possible, and they use lightweight substrate, adhesive, and cover-lays, they are usually very light compared to their rigid peers. Their lighter weight allows them to handle vibrations in a much better way, without damaging any component on the circuit.
3. Dynamic flex, where a circuit has to bend repeatedly (like the laptop screen and bendable cellphones), is only possible because of flexible PCBs.
4. Flex circuits are highly bio-compatible, which is why they are preferred in many biotech devices, surgery tools, and implants.
5. Flex substrate (though relatively costly) are better at handling heat than the conventional rigid PCB construction materials. This helps in high-frequency and high power circuits.

Conclusion

Flex PCBs are no longer just another option; they are a necessity for most circuit designers. If you haven't come across them yet, you eventually will. The way the electronics are shrinking, many devices can not be conceived without designing in flex.

Flex capabilities genuinely shine when they are combined with rigid, in Rigid-Flex, and HDI technology. Microvias fix many problems with designing in flex since a lot of them are there because of through holes.

If you are designing your first flex PCB, make sure you go through all the common design practices that are prevalent in the industry. Also, familiarize yourself with different material choices, and which ones would be best for your application, as well as budget. Understanding the fabricator’s design guidelines is very important as well. You don't want to design something that the fabricator can't fabricate or something that will cost you too much.