What is Rigid Flex?
The evolution of Printed Circuit Boards (PCBs) has come a long way. From the first half of the 20th century, when the first true PCBs were created, to the complex circuits of now, there have been numerous changes in several different aspects of the circuit. This includes different substrate materials, different masking techniques, the switch from mechanical drilling to laser drilling, etc.
Almost all features of the PCBs have changed over the years to make them more efficient and fulfill the needs of the market. One of these demands is flexibility because a lot of circuits (like wearable electronics) are impossible to make without moving beyond the conventional rigid circuit. Flex circuits were invented to fill this need. Then, rigid-flex PCBs were conceived to offer the best of both worlds on a single circuit.
What is A Rigid-Flex PCB?
As the name suggests, a rigid-flex PCB is any PCB that contains both rigid and flexible elements. A rigid PCB is any conventional PCB board that offers almost no flexing capabilities, i.e., no bending. In contrast, flex PCBs are made up of a flexible substrate that can be bent (one time or several times) to accommodate complex shapes or constant movement needs (like wearable electronics).
A rigid-flex PCB contains both of these elements. It's a circuit that's partially on a rigid board and partially on a flexible substrate. This can be a single rigid PCB connected to a single flexible circuit or multiple rigid PCB layers connected to each other through flex circuits. In most rigid PCBs, the bulk of the circuit and circuit components are on the rigid PCB side, with relatively few on the flex circuits, but the concentration of components doesn't undermine a PCB's rigid-flex status.
It's important to note that rigid-flex PCBs where the primary role of the flexible part of the circuit is to connect rigid PCB elements (as long as all rigid and flex elements collectively form one circuit) can be classified as rigid-flex, even if there are no components directly placed on the flexible parts.
Rigid-Flex Board: Major Strengths and Differentiating Characteristics
The major strengths of rigid-flex boards include:
Best of Both Worlds: The most prominent strength of rigid-flex circuits is that they offer the best of both worlds. Rigid PCBs are easier to place components on and drill vias on and may allow for a much denser design on the same surface area compared to flex PCBs, where designers/fabricators have to take the bending of circuits (and the stress it places on conductors, components, and vias) into account. Whereas flex PCBs can take on different shapes and accommodate complex design and placement requirements. Rigid-flex boards combine both these strengths, making them ideal for a diverse range of use cases.
3-D Design: Like flex PCBs, rigid-flex boards can accommodate a three-dimensional design, which means they can occupy space in the device they are designed for much more efficiently. Or, they can help you form a much more complex and layered circuit without expanding the available space in a device. The simplest example could be a circuit that occupies four to five walls (including the "floor") of an enclosure instead of a conventional rigid PCB installed on just one side of the enclosure.
Lightweight: Rigid-flex boards allow for the creation of one comprehensive circuit rather than multiple rigid circuits connected to each other through wires and connectors. This makes them significantly lighter and easier to assemble. The reduced weight of wires (and their insulation) and connectors can make the device relatively lighter as well, which can be highly beneficial in devices and equipment where weight is a critical factor, like drones and satellites.
Small Form Factor (Compact Design): In most cases, circuits made on rigid-flex boards would be smaller compared to the same circuits made on rigid-only or flex-only PCBs. This small form factor can be critical in the development of smaller devices like implants, micro-drones, etc.
Reliable Design: The strengths mentioned above combine to make rigid-flex PCBs more robust, reliable, and resilient than other PCBs. By replacing wires and connectors, rigid-flex boards significantly reduce the number of potential points of failures in a circuit, making them more reliable and resilient, especially against harsh environmental conditions. The flex part of the rigid-flex board offers mechanical resilience by absorbing vibrations and jerks. With the right substrate, a rigid-flex board may also have a higher tolerance against higher temperatures than its purer counterparts.
Enhanced Signal Integrity: The integrated design and smooth signal transition from the rigid side of the PCB to flex traces can significantly reduce noise and minimize signal loss, preventing integrity. This “property” of the rigid-flex can be enhanced through both design considerations as well as the materials chosen for both rigid and flex parts of the PCB.
Better Heat Dissipation: A rigid-flex board doesn’t just spread out the circuit over a larger area, benefitting in heat dissipation, but it's also about material thickness. The thin polyimide of the flex part of the circuit is much better in heat dissipation than its thicker counterparts, leading to better thermal management.
Rigid-Flex PCB vs. Rigid PCB
Rigid-flex PCBs have several benefits over conventional/rigid PCBs, and most of them pertain to the flex elements of the PCB, such as 3-D design, resilience against harsh environments, complex design, etc. While rigid-flex PCBs tend to cost more than rigid PCBs and design is usually more complex, the long-term benefits of rigid-flex PCBs might outweigh the higher upfront cost. However, that should be evaluated on a case-by-case basis.
There are other, more nuanced differences as well, like component density. If a rigid-PCB is confined to certain dimensions based on the available space and you have to stretch component density to its limit, which may introduce new failure points, undermine signal integrity, and create thermal dissipation problems, a simple rigid-flex alternative can forestall all these problems. It may allow you to spread your PCB over a larger area, giving your components more breathing room, which may also translate to an easier design with looser trace and via tolerances. It may also have better heat dissipation.
Rigid-Flex PCB vs. Flex PCB
A rigid-flex PCB may also offer several benefits over a simple flex PCB, starting with a relatively simpler design. By moving most of the components to the rigid side of the circuit, a rigid-flex PCB eliminates the problems associated with tighter tolerances, no-go areas in flex PCBs (bending areas), mechanical stress on copper plating of vias, etc. It's also possible for rigid-flex PCBs to eliminate or reduce the impact of many of the major cost drivers of flex PCBs, like stiffeners, layer count, tracing, etc.
However, unlike rigid PCBs, there are several use cases where pure flex PCBs are either irreplaceable by rigid-flex PCBs, or it's more feasible to stick to flex PCBs than convert the circuit to rigid-flex boards.
Rigid-Flex PCB Manufacturer’s Perspective
Rigid-flex PCB manufacturers have to take into account several important considerations to ensure that they produce electrically and mechanically (and thermally) sound PCBs as per design. But before we dive into the rigid-flex PCB manufacturer’s perspective, we have to take into account the designer’s perspective.
Rigid-Flex PCB Design
When designing a rigid-flex PCB, several design considerations and good practices should be kept in mind. This includes:
- Evaluating whether rigid-flex is the right fit for your project requirements. In most cases, rigid-flex is likely to be costlier compared to a similar circuit being designed on a rigid PCB, and if the requirements and operating conditions are not too demanding and cost is an important factor, opting for rigid-flex just because of its benefits may not be the wisest choice.
- Leveraging the full potential of rigid-flex. If you have determined that going forward with a rigid-flex is your best option, then it’s important to leverage its full benefits, like making the circuit as light and as durable as possible. This may require taking full advantage of the available space, for which the designer should have access to the dimensions of the enclosure/space the circuit will be installed in (or a rough idea, if circuit design preempts the device design.)
- Stackup is a critical part of a rigid-flex design. You have to determine the number of layers you want in total, how many of them would be rigid, how many flexes, and whether there will be variation between rigid layer count and flex layer count, and, if there is, how it may impact the electrical and signal characteristics of the final circuit.
- Where the circuit will bend is another important consideration in a rigid-flex design. You have to be careful with component placement near the band, as well as trace thickness/routing at the bend and angles, to ensure that you don't stress out the copper.
- The circuit design should also take into account the component requirement and placement. Components like ICs that may require BGAs and several layers might be better off on the rigid sections.
- Choosing the right material for both the rigid and flex sections of the circuit is also critical because you have to look at the circuit and its electrical and mechanical characteristics both as a whole and per their individual segments. For example, the electrodeposited or rolled copper of rigid PCBs may have different electrical and thermal conductivities compared to the annealed copper of flex PCBs, posing problems for both signal integrity and heat dissipation. This should be addressed at the design stage through the right material choice and tracing.
- The component and via placements are other major considerations. You have to keep the strengths and limitations of both rigid and flexible elements in mind when placing both components and vias. In the rigid section, the mechanical stress on filled or plated vias may be minimal, but it may be a factor to consider in flex PCBs. You should also provide adequate clearances, especially for both flex and rigid elements of the PCB meat.
Rigid-Flex PCB Manufacturer
The rigid-flex PCB manufacturers also have to take into account the material and design differences of the two segments (rigid and flex). The process can become significantly easier if the designer adheres to the design for manufacturing practices when designing a rigid-flex board.
Rigid-flex PCB manufacturers typically convey their capabilities when it comes to these PCBs, including elements like the total layers/typical layer counts they accommodate, whether they can manufacture High-Density Interconnect (HDI) rigid-flex PCBs, interconnection layers instructions, and what kind of vias (and proportions) they can accommodate on the rigid and flex layers. Adhering to these guidelines and recommendations can ensure faster turnarounds and no do-overs for the design.
Manufacturers have to take special care for certain elements of the rigid-flex circuits, like the areas where two different parts (rigid and flex) connect and the lamination of rigid and flex areas, because they have to choose the material that may work well with both.
Drilling is another challenging aspect of rigid-flex PCB manufacturing. While the rigid parts of the circuit are easy enough to drill into (even with mechanical drills), the flex parts may take a more careful approach. The most challenging, however, is the transition parts, where rigid meets flex in these PCBs, and special care must be taken to maintain a smooth transition.
Some of these challenges are carried into the assembly of rigid-flex PCB manufacturing as well. This includes soldering components on rigid and flex parts of the PCB, each of which requires a different technique.
Conclusion
Rigid-flex PCBs have a comprehensive range of use cases. A critical part of complex wearables is that they may require the component density strength of rigid circuits as well as the flexibility of the flex PCBs. They are also widely used in mobile phones, cameras, and several medical equipment, including implants. While the choice for a rigid-flex instead of a rigid PCB or a flex PCB with rigidified segments (using a stiffener) is clear in some use cases, it may be less transparent in others. Factors like cost, manufacturing and design complexity, compact form factor, and several other factors can help you determine whether rigid-flex really is the best PCB format for your use case or not.
