What Is The Difference Between A Flex And Rigid PCB?

PCBs can be classified and divided in a number of ways, including based on the flexibility or rigidity of the PCB board’s substrate. Conventional PCBs are mostly built upon a rigid board, earning them the title Rigid PCBs. However, many applications require PCBs to be built upon a flexible substrate, which may allow the circuit to bend one time or frequently. These PCBs, called flex PCBs, offer certain benefits over rigid PCBs. Then, there are rigid-flex circuit boards, i.e., PCBs, that incorporate the strengths of both rigid and flex PCBs.

Let’s go over the differences between rigid PCBs and flex PCBs and how they are minimized in rigid-flex circuits.

Material

One of the main differences between a flex and a rigid PCB is the material of the PCB itself and a few other materials (like the ones used for coverlay). The most significant of these materials is the substrate. It provides structural support to the PCB components as well as conductive layers. Its electrical characteristics are also important as it serves as the insulator/dielectric in the PCB as well.

For rigid PCBs, the most commonly used substrate material is FR-4. It's a fiberglass-reinforced epoxy laminate that is preferred for its amazing blend of electrical and thermal characteristics, cost-effectiveness, and physical properties.

For flex PCBs, the most commonly used substrate is polyimide. The primary reason this material is preferred is its physical characteristics/flexibility. It easily bends, but this bending, even when it's at relatively sharp angles, doesn't impact its mechanical integrity. That, coupled with its electrical characteristics and thermal resilience, are among the major reasons why polyimide is among the choice materials for flex PCBs.

Now rigid-flex circuits, since they include both rigid and flexible elements in their structure, often include both materials. The FR-4 is used for the rigid part of the PCB, while the polyimide makes up the flexible parts. The different materials come together at certain levels, where the flexible polyimide usually serves as one of the layers of the rigid parts of the PCB, blending in seamlessly into its structure.

But that's not the only material difference in rigid and flex parts of a rigid-flex circuit. There is also copper. For the rigid parts of the rigid-flex circuit boards, the copper used is mostly electro-deposited. This type of copper is evenly distributed, making it an even layer over or in between the substrate (for multi-layer rigid-flex circuit boards). However, this kind of copper is relatively inflexible, making it unsuitable for the flex part of the rigid-flex circuits. They use annealed copper, which is heat-treated and more pliable in nature than electro-deposited copper. This allows it to bend without cracking, which can significantly undermine the electrical makeup of a rigid-flex circuit board.

Then, there are different coverlay materials for the rigid and flex parts of the rigid-flex circuit. For the rigid part of the rigid-flex, the coverlay material of choice is usually the solder mask. In contrast, flex circuits are mostly covered with a thin polyimide coverlay that is just as flexible as the substrate underneath.

Design

Both rigid and flexible PCBs have different design considerations, and both have to be taken into account when designing rigid-flex PCB circuits. The design considerations vary based on complexity and whether you are focusing on the output and functionality of the circuit alone or if you are also adhering to good design for manufacturing (DFM) practices. There are several design considerations that are the same in both, like the component placement. However, there are several differences as well, and they are also apparent when designing rigid-flex circuit boards. This includes:

• The overall design area you have to work with. Compared to plan rigid PCBs, you may have more flexibility and area to work with when you are designing rigid-flex circuit boards. You can connect different rigid PCB segments with flex PCB elements/layers, allowing you to design in three dimensions.
• The stackup design is different in rigid, flex, and rigid-flex circuit boards. In rigid PCBs, the stackup is about the total number of conductive, substrate, and adhesive layers you can get within the allowed thickness of the circuit. In flex circuits, you also have to consider the areas where the circuit will bend, how it may impact the layers, and how it will stress the conductive surfaces. In a rigid circuit board, the stackup takes both elements into account, and you can decide where you want to concentrate your layers and components and which part would simply serve as an electrical and thermal connection.
• Trace width and spacing also differ between rigid and flex PCBs. You can have tighter tolerances in rigid PCBs when it comes to trace widths and mostly have to focus on the electrical requirement of the circuit as a whole and the components you are placing on the circuit. However, in flex PCBs or rigid-flex circuit boards, you also have to take the mechanical impact of bending into account. Trace widths have to accommodate the mechanical stress, especially in the bending areas. In rigid-flex circuit boards, the transition of traces between rigid and flex parts of the circuit is an important factor to consider.
• Via types and placement is another point of difference between flex and rigid PCBs. In rigid PCBs, the design is mostly about its aspect ratio, plating, and the layers it has to penetrate through and connect. In flex PCBs, via placement, it has to take bending areas and radii into account. Through-hole, vias might not be placed in the bending area because the mechanical stress would undermine its plating. There is also the stiffening requirement for certain vias in flex PCBs or flex parts of rigid-flex circuit boards. Via type is also different in flex and rigid parts of the rigid-flex circuit, with through-holes concentrated in the rigid areas and flex mostly populated with blind and buried vias.
• Ground plains are critical circuit components that are treated differently in the rigid and flex parts of the rigid-flex PCBs. They are crucial for heat dissipation and signal integrity. In rigid parts of the PCB and rigid PCBs, grounding plains can be all copper, but that won't work on flex PCBs because a solid copper layer will prevent the circuit from bending. A meshed copper grounding layer is used instead to achieve the desired electrical characteristics without compromising on mechanical flexibility.
• Bending radius and bending cycles, i.e., how many times a circuit has to bend, is another important factor to consider, but it's only limited to the flex part of the rigid-flex circuit board. Both designers and rigid-flex pcb manufacturers have to ensure that they understand the angle at which the flex part has to bend and how many times it has to bend since it can have a significant impact on the mechanical integrity of the substrate, copper layer, and even component placement.
• Component choice is also different between rigid and flex parts of the circuit. As most rigid-flex PCB suppliers would recommend, designers should choose relatively lighter components for the flex part of the PCB. The rigid parts would be able to handle heavier components because they have more mechanical/physical leeway. So when designing, it’s important that your component placement and component choice both factor in the weight of the component and where it’s going on the circuit (rigid or flex part of the circuit).

There are several other design differences that you have to take into account when you are designing a rigid PCB, a flex PCB, or a rigid-flex PCB. A good rule of thumb is to follow the guidelines and recommendations of the rigid-flex PCB suppliers you have decided to work with. Rigid-flex PCB manufacturers usually define their tolerances and good design practices to encourage the design for manufacturing (DFM) approach. This approach can reduce or even eliminate multiple design iterations and ensure that the first complete design you have developed is ready for manufacturing and there are no challenges in the manufacturing or assembling stage that might require you to revise the design.

Cost

The cost of rigid and flexible PCBs and rigid-flex PCBs differ as well. If both of them contain the same number of layers and almost the same circuit, a flex PCB may cost more than a corresponding rigid PCB because of the materials. This is especially true if you require a more complex design to achieve the same results, like a layer of buried and blind vias replicating the top-to-bottom layer connectivity of a through-hole via. Or if you have to choose more expensive components because they are lighter for your flex PCB. However, for most practical considerations, cost has to be analyzed in a much larger context.

If Rigid-flex PCB fabrication allows you to keep the design limited to a specific size (or to limited dimensions) and allows you to add more sophisticated functionalities in the circuit without increasing the size of the device it’s going in, that’s more than enough to justify a costlier circuit. Also, a rigid-flex PCB supplier may offer a more attractive quote than a pure-flex PCB supplier since rigid-flex design allows you to incorporate the strengths and design flexibility of rigid PCBs, which may warrant a lower price tag. 

Manufacturing/Fabrication

Rigid, flex and rigid-flex PCB fabrication have several differences. From a fabrication perspective, rigid PCBs are the easiest ones. The process usually involves drilling (as per design), then plating, then etching, and finally solder mask application. The process is slightly different for flex PCBs. The base material (polyimide) is first prepared to support proper adhesion; then, copper patterns are defined on the substrate using lasers. Unlike in the fabrication of rigid PCBs, copper can be both added or removed (based on the approach you are taking) in the fabrication of flex PCBs.

The rigid-flex PCB fabrication process is different from both of them. It’s a hybrid process that combines the elements of both, which also makes it more complex and often more expensive, especially compared to conventional rigid PCB fabrication. Rigid-flex PCB fabrication has to contend with challenges like zoning and seams, i.e., areas where both rigid and flex parts of the PCB come together. It's made up of different materials with their own mechanical, electrical, and heat dissipation properties, and to keep these characteristics uniform, the design and fabrication have to make some allowances, like expanding traces from rigid to flex areas.

Assembly

The assembling process also differs in rigid, flex, and rigid-flex PCB fabrication. Things are relatively simple in the rigid PCB assembly. The assembling process is usually "pick and place," where components are automatically picked up and placed on the rigid PCB board, and a reflow oven is used for soldering all the components in their place. That's not an option in flexible PCBs.

Applying soldering paste is challenging in flex PCBs, and since using a reflow oven might damage the flex circuit, selective soldering, and a few other techniques are used. Rigid-flex PCB suppliers face even more challenges in assembly. The two different assembly processes and techniques already make assembly more challenging and costly. Managing the transition points/zones adds to the complexity of the process.

Applications

Finally, the most important differences between rigid, flex, and rigid-flex PCBs are reflected in their applications. Rigid PCBs are the most common type of PCBs there are and cater to a comprehensive range of applications where there are no bending requirements, adequate space available for the circuit, and no undue mechanical stress on the circuit.

Flex PCBs are used when the circuit has to be kept as light as possible, has to endure mechanical stress, or has to bend, one time or multiple times, because of its dimensions or use case. Rigid-flex PCB manufacturers develop circuits that incorporate the strengths of both, i.e., sophistication, layers, thermal dissipation, and other strengths of rigid PCBs and the flexibility and  3D design elements of flex PCBs.

Final Words

The differences between flex and rigid PCBs can be explored in a more exhaustive manner, as well as the differences between the two pure types and the hybrid rigid-flex PCBs. Rigid-flex PCBs take advantage of the differences and different strengths of the two PCB types, making them ideal for a comprehensive variety of use cases despite their higher cost and complexity.