What is Rigid PCB vs. Flexible PCB Board?
PCBs have been a staple of the electronics world for ages. They come in a wide variety of shapes and sizes - from large to small, crude to highly sophisticated, and hobby circuits to cutting-edge circuits incorporating powerful ICs and components. These PCBs can be divided into different categories or classes in a number of ways, including the number of layers, used surfaces, component density, and whether the circuit is rigid or flexible. There are rigid-flex circuits as well, but for this article, we will focus on rigid and flexible PCB boards and how the two differ.
What is a Rigid PCB Board
When we use the word PCB, a rigid PCB board is what comes to the mind of almost everyone. A rigid PCB board is made up of hard/inflexible conductive layers (usually copper) separated from each other by an insulating material that can be etched away to expose the copper (conductor) to create conductive paths (called traces) for the circuits. The characteristic part of this "type" of a PCB board is that it's rigid in nature. It doesn't flex or bend, and any attempt to do so may harm the circuit, which is often irreversible.
What is A Flexible PCB Board
A Flexible PCB board, in contrast, is designed to flex or bend, either continuously or a few times. Like a rigid PCB board, a flexible PCB board is also made up of layers of conductive materials divided by insulation layers. The traces can be formed on the flexible PCB in nearly the same way (through etching). However, there are several mechanical, electrical, design, manufacturing, and use-case differences between the two.
Differences Between Flexible Circuit Boards vs. Rigid PCB Boards
To give you a full idea of the scope of the differences between the two PCB types, the differences have been divided into several categories.
Benefits of a Flexible Printed Circuit Board vs. Rigid PCB
It's important to understand that while both a flexible printed circuit board and a rigid PCB have use cases where it's either one or the other, there are several instances where you can actually choose to design a flexible printed circuit board instead of a rigid one and vice versa. That's where understanding the inherent benefits of either one can be useful and help you make a smart choice.
Let's start with rigid PCBs, and the first benefit is simplicity. It's relatively easier to design for, and most of the complexity related to design comes from the circuit itself, not the PCB material and its mechanical/electrical characteristics. However, this simplicity is only a benefit of rigid PCBs (compared to flex circuit PCBs) for relatively unsophisticated circuits. When you start designing complex, high-density circuits with several mechanical/electrical environmental considerations, this benefit doesn't stand out. Instead, some of the benefits of a flexible printed circuit board may outshine a rigid board in those contexts.
One such benefit is that in fixed mechanical dimensions, a flexible printed circuit board may offer you more surface area than a rigid PCB. Let's say you have to make a circuit that has to fit in a box that's two inches in width, length, and depth. You will have less than 2x2 inches of area for your rigid PCB, whereas a flexible printed circuit board may circle the board, covering all four sides, giving you far more surface area to work with. This can lead to easier component placement without increasing density, which leads to easier design. A flexible PCB board can significantly reduce the dimensions and weight of a rigid PCB-based device by over 50% in some cases.
For a relatively simple circuit, rigid PCBs are likely to cost less than flexible printed circuit boards. For more complex circuits, the cost-benefit is influenced by several other factors and can lean either way.
Both PCBs can be durable in their own ways. Flexible printed circuit boards may be more durable in certain harsh environments, whereas rigid PCBs may be more durable in stationary circuits and use cases.
Limitations of a Flex Circuit Board vs. Rigid PCB
Both circuit types/PCB types have their limitations as well. Most of the rigid PCB limitations are associated with their inability to bend and flex, which makes it infeasible for specific use cases. But rigid PCBs can also be heavier and larger, making them less than ideal for use cases where weight and dimensions are of paramount importance.
One clear benefit that Rigid PCBs have over flexible printed circuit boards is the total number of layers they can achieve. This is a limitation of the flexible PCB board. While it may be possible to design and fabricate a flexible printed circuit board that may have several layers, for many fabricators, the limit is usually under ten layers because of mechanical and electrical constraints ingrained in a flex circuit board. Rigid PCBs, however, can go up to well over 100 layers, and even for relatively simpler use cases, rigid PCBs can have dozens of layers.
Use Cases of A Flex Circuit PCB/Bendable Circuit Board vs. Rigid PCB
Flexible printed circuit boards are most commonly used in wearable electronics like smartwatches, fitness trackers, and VR headsets. They are also used in a wide variety of medical devices that need to connect to or get into our bodies, like light pacemakers and hearing implants (Cochlear implants).
There are also use cases where flexible PCB boards are preferred or are the only viable choice because of their inherent shock resistance and vibration resistance. Since rigid PCBs are typically physically conjoined with the body of the device, they absorb all the shock and vibrations. Flexible PCB boards may be able to handle these environments far better.
Similarly, there are use cases where a device has to bend repeatedly, like smart glasses or bendable smartphones, that would not be possible without bendable circuit boards. How many times a flexible PCB board can bend is determined by its flex cycles/bending cycles.
Rigid PCBs have a much wider range of use cases. Almost all electronic devices where the circuit doesn't have to move and operate in normal conditions (in some cases, even extreme conditions) use rigid PCBs. As the level of sophistication of the PCB grows and it requires more layers and tighter electrical tolerances, rigid PCBs may be a more viable choice than flex circuit PCBs.
Conductive Material
While both rigid and flexible PCB boards have the same basic structure, i.e., conductive layers separated by insulations, both conductive and insulation layers are different in rigid PCBs and flexible circuit boards.
Rigid PCBs usually rely upon copper that's electrodeposited. It's neatly stacked but inflexible and may crack if you try to bend even relatively thin layers. In contrast, the copper (conductive layer) for flexible circuit boards is made from annealed copper, i.e., copper that's heated to a high temperature and then allowed to cool off, making it much more pliable and flexible.
The insulation material, called prepeg in the context of PCBs (short for pre-impregnated), is made from fiberglass fabric that is impregnated with resin (hence the name). There are a wide variety of prepregs available, with different resin content, and they can differ significantly in thickness, composition, etc., between rigid and flexible circuit boards.
Design
Rigid PCBs may be more straightforward to design, at least for less sophisticated circuits. However, the same circuit may be designed on a flexible circuit board to be lighter and more compact compared to its rigid counterpart. That's because, due to its bending capabilities, the flexible circuit board might occupy the same space while offering the designer significantly more surface area to work with.
This leads to more design flexibility.
However, for complex circuits, you may have to take into account the mechanical and electrical limitations of flexible circuit boards, like the number of layers. These can be offset (to an extent) by the larger surface area they offer, but there comes a layer count where flexible printed circuit boards are simply not feasible. There are also some design considerations characteristic of flexible PCB boards. This includes:
- Material considerations. For most flexible PCB boards, the standard choice is a polyamide film, but choosing the right one can be tricky as it's influenced by multiple factors, including its dielectric constant. It's usually better in flexible PCB board materials compared to rigid PCB material (FR-4).
- Bending radius. Even though a flexible PCB board can bend, certain modes of bending can damage its insulation layer, weakening conductive paths. The bending radius is influenced by the layer count in the flexible PCB board, the thickness of the material that coats its exteriors (called the coverlay), and it should ideally be calculated from inside, not outside of the bend.
- Conductive elements (traces, vias, PTHs, etc.). While flexible PCB boards can accommodate vias, the process can be quite different compared to rigid PCBs. The reason is its flexible nature. To ensure that the via and its electroplating remain intact and its structural integrity (and conductivity) isn't compromised, vias have to be relatively bigger, placed well away from bend areas, tear shaped, and take into account the mechanical integrity of the flexible PCB board as a whole. Similarly, the traces have to be perpendicular to the bend; no sharp bends should be present in the flexible PCB board, and pads should be tapered. The gist of it is that while you may get more surface area with a flexible PCB board, you may have to give much of it away to a flexible circuit board's design considerations.
Simply put, designing a flexible circuit board may be more complex than a rigid PCB or, at least, has a more "mechanical dimension" that's not present in rigid PCBs.
Fabrication and Assembly
The fabrication process of a flexible circuit board is also different from that of a rigid PCB, and while many of the steps might be the same in the two, there are some steps that are characteristic of a flexible circuit board.
One main difference (from a fabrication standpoint) is that flexible circuit boards require a stiffener. Since flexible circuit boards are "flexible," it becomes difficult to mount electronic components on them during the assembly process. The easiest way to solve this problem is to apply a stiffener to parts of the flexible circuit board, making them rigid and easier to work with.
Another difference is the coverlay
When rigid PCBs are finished, they are coated with resin, which acts as a solder mask, protecting the surface(s) of the PCB. The exposed pads and traces on the PCB may be vulnerable to shorts and other electrical damages, often from environmental elements, and the solder mask prevents that. The problem is that it's too rigid for flexible circuit boards. So for a flexible circuit board, this job is taken by a coverlay. It's essentially a polyamide (flexible) that's pasted on the flexible circuit boards via an adhesive. The amount (thickness) of adhesive and the type/thickness of the polyamide used can differ based on use cases.
Cost
With a more complex design, additional fabrication steps, and different material considerations, it’s easy to see why flexible circuit boards might cost more than rigid PCBs. But that’s just one dimension. If you evaluate the cost from the perspective of the entire device and the benefits a flexible circuit board might offer compared to a rigid PCB (a more comprehensive cost-benefit analysis), the additional cost might seem justified.
Conclusion
Understanding the differences between rigid PCBs and flexible circuit boards can help you make the right choices for your design needs. Your primary consideration should always be your use case. Flexible printed circuit boards might be inherently suitable or the only viable option in some cases, but for others, where both PCB types may serve the same purpose, you have to analyze the two from different perspectives, including design complexity, environment the circuit will be used in, and cost.
