A high-quality PCB can be extremely powerful. It can achieve a plethora of functions simultaneously, with so many independent connections being available at all times. However, to be able to do that, a PCB needs space. Regular PCBs can be up to 20” x 23” (500 mm x 600 mm) in size. Let’s face it — that’s a lot of space. What happens when you need to place such a PCB in a smartphone? What if you need it to be inside of a smartwatch?
Obviously, that’s not possible. It simply cannot fit. However, there’s a solution to that! There are flexible printed circuit boards (also called flexible circuits)! With flex PCBs, you can bend your PCB so that it fits the space you have available. In other words, you no longer have to build your device around your PCB. Instead, the PCB will fit the device the way you want it to!
But how do you make a circuit board flexible? Let’s take a look!
Let’s Talk Material
Before we get into the benefits of a flex PCB design and when you should use it, let’s talk about the material. The first time anyone hears about a flexible circuit board, their first question is almost always the same: how do they do it? For regular PCBs (rigid ones), we most commonly use FR4.
Obviously, with flexible PCBs, we want something that’s more agreeable to conforming. The two most frequent choices are polyester and polyimide. While polyester is cheaper, polyimide is a better choice overall. First of all, it’s non-flammable, so you don’t have to worry about your PCB overheating. It stays fire-resistant in harsh environments, and it’s more reliable in terms of performance. It also comes with a lower dielectric loss factor and better elongation features. All in all, unless you’re looking to shave the costs, polyimide should be your preferred choice. Here at MKTPCB, we use DuPont PI: Pyralux AP, LF, FR, high-speed Pyralux TK (Teflon-Kapton).
Generally, a single-layer flex PCB starts off with polyimide as its base material. Then, there’s a layer of adhesive and a conductive layer (usually copper). After that, there’s another adhesive layer, followed by a coverlay (polyimide and PET film). This is the basic structure as you can add various additional features.
When it comes to adhesives, flex PCB manufacturers usually use acrylic adhesives. You should bear this in mind during the PCB manufacturing process as acrylics can get soft when heated. To avoid any problems, make sure your copper pads are quite large (larger than normal).
Coverlays and Finishes
A coverlay (or flex circuit overlay) is a process of lamination during which we encapsulate and protect the external circuitry. Essentially, it’s a solder mask, with the exception that it’s flexible. You laminate the coverlay with a lot of heat and pressure, thus making the adhesive soft. Soft adhesive fills in all the gaps more freely, making sure that you don’t trap any air inside.
Regarding surface finishes, things don’t change much from rigid PCBs. A surface finish is there to give the final touch and make sure the copper you’ve used stands the test of time. Surface finishes you can use include (lead-free) HASL, ENIG, and Immersion Tin.
There are several other features you can add to your flexible circuit. For instance, you can use a graphic overlay. That’s basically a polyester or acrylic cover for your PCB. With it, you can simulate a UI to an extent. The overlay consists of some displays, LEDs, and switches, allowing you to interact directly with the PCB.
Electromagnetic interference (EMI) is an issue that has occurred with PCBs since virtually forever. If you expect your PCB to come across this interference (for example, you plan to put it in a mobile device), you need some sort of protection. You can add EMI shielding (usually out of copper or silver ink) to reduce the impedance.
Another handy feature comes in the shape of stiffeners. Now, the whole idea of a flex PCB is that it is flexible and can bend. However, you don’t want that to happen all over. Sometimes, bending can cause damage to your pins, surface mount areas, or hole patterns. In order to avoid that, you can use a stiffener to make that section more rigid. Consequently, you will increase the longevity and strength of your board.
Other features you may use that are worth mentioning are hot bar solder (more efficient connections) and panelization (assembling in panels of multiple units).
If you want to get deep inside the world of flex PCB thickness, we suggest you give this in-depth guide a look. Here, we’ll focus only on the most important parameters. The board thickness can vary from 3.93 to 31.5 mil (0.1–0.8 mm). The thickness is more important than you might think.
The thickness of a board will let you know how many times that board can bend, i.e., what its bendability is. For instance, a static board can bend around 100 times in its life cycle, while a dynamic one can do so tens of thousands of times. You can calculate your board’s bend radius (the minimum level of bendability) by how thick your board is. Generally, for a single-layer PCB, the radius equals thickness x 6. For double layers, you multiply by 12, and for multilayers by 24.
Flex PCB Types
As always, there are many different subtypes of this PCB. Let’s mention a few of them and see what they can offer.
- Single-sided Flex PCB. This board consists of a single layer of conductive copper. It can either go between two other layers or work on its own as a single-layer board. If the latter is the case, it has an uncovered side.
- Double-sided Flex PCB. As the name suggests, double-sided PCBs have two layers of copper, with a polyimide layer sitting in between them. The outer sides of the conductive layers can either be covered with a copper pad or be exposed. We usually connect the layers with plated-through holes.
- Multilayer Flex PCB. Similarly to the above, a multilayer PCB just goes another step further. To get a multilayer flex PCB, you combine two or more single- or double-layer boards. It’s a great choice if you’re looking to avoid shielding, impedance, and crossover requirements.
- Rigid-flex PCB. A rigid-flex board is a combination of two different types of boards — flex and rigid. Rigid-flex PCBs tend to use multiple conductive layers but use rigid material to hold the two together. Rigid-flex circuits are unique due to their use of conductors on rigid layers.
Main Advantages of Flex PCB
By far the biggest (and the most important) advantage of a flex PCB is its space and weight efficiency. A flex PCB is usually lighter by a whopping 95%, while at the same time, it uses around 10% of the space a rigid PCB requires. If you need to save space, you will fall in love with a flex PCB.
Additionally, given the fact that the board is lighter (in weight) and flexible, it is more resistant to shock, vibrations, and stresses. It also uses fewer interconnections than a rigid PCB, which makes the flex circuit much more durable. Signal reliability also profits from this.
Flex PCBs contain a lot of smaller parts and thinner layers than rigid boards do. This allows manufacturers to put a lot more components and traces on them. As a result, we have high-density boards with more capabilities. Due to the nature of flex PCB fabrication, there’s less variation when it comes to assembly methods. This means that flex PCBs usually have fewer problems with wiring errors.
Finally, and not less importantly, as flex PCBs go through fewer assembly processes, they are usually cheaper than rigid PCBs.
When to Use Flex PCB
Flex PCBs can be of use for a wide range of devices. They are ideal for non-stationary electronic devices, such as mobile phones, smartwatches, cameras, and other gadgets. Flex boards are also great because of their adaptability and the fact that they are resistant to tremors and vibrations. As such, they are perfect for transportation and automotive needs as well as some other industrial electronics that require their PCBs to absorb shock.
Many medical and military electronic systems also utilize flex PCBs as they are durable and small in size.
To summarize, flex PCBs are unique due to the fact that they can bend and conform to the space they are given. Unlike with rigid PCBs, you don’t have to plan ahead and leave empty space. Instead, you just take a flex circuit and put it where it suits you.
It’s a lightweight board that takes about 10% of the space a rigid PCB would take, making it perfect for smaller devices. Additionally, due to the material used, they can resist vibrations easily. This way, you can put your PCB in a non-stationary device and not worry about any damage occurring from shock.
At MKTPCB, we use our expertise and experience to create flex PCBs of the highest quality. All of our boards go through extensive quality checks to ensure that they will perform impeccably. If you want to get a free quote or to ask about anything else, feel free to contact us!