Rigid flex printed circuit boards, or PCBs, serve a vital role across multiple industries where reliability is a must in less-than-ideal situations. They are an excellent go-between between the high-density circuitry of hardboards and the durability and flexibility of flexible circuits.

This guide highlights rigid flex PCB design guidelines, and for even more information, refer to our rigid flex design for manufacturing guide, and our Valu Builds brochure on low cost, high reliability rigid flex material layups.

Alternatively, if you’re a customer in need of custom rigid flex PCB solutions, contact us today to get a quote or set up a design consultation.

Pain Points Solved Through Rigid Flex Circuit Design

Environments that expose circuit boards to significant shock, vibration, or heat can all benefit from rigid flex design solutions. Rigid flex circuit design also allows for developing solutions for small spaces such as pacemakers and hearing implants. The most common equipment using PCBs are military, aerospace, medical devices and cameras of all types. Other industries are starting to use them as well.

Consumer goods requiring weight, shock absorption, and complexity, such as drones and wearable tech, have seen the use of rigid flex designs increase. The aerospace/aircraft and satellite industries have been using them for some time, and PCBs are great for enduring subpar environments such as high-shock and/or high-vibration situations.

When space and weight are at a premium, rigid flex PCBs are a great go-to because of their very thin materials, compact and lightweight nature.

Environments that undergo high mechanical stress benefit from rigid flex PCB designs. They are less susceptible to failure after bending, twisting, or vibration exposure. The same principle applies to other environmental stressors, such as exposure to certain chemicals or high heat.

Rigid flex PCBs have fewer points of failure. There are fewer connection points, eliminating connectors which are prone to failure in challenging environments, making them more reliable overall. This is why they are used in many medical, military, and aerospace applications.

Rigid flex circuit design allows for greater flexibility overall, allowing the designer to fit the electronics to the device, rather than make the device fit the electronics. Since they can be bent or folded without damage, the possibilities for use cases are expanded. By incorporating multiple PCBs onto a single rigid-flex board, assembly tasks are prone to fewer errors, saving time and money.

All Flex Solutions Valu Build Guide includes low cost, battle-tested rigid flex solutions. Our products’ dynamic flex properties ensure that our flexible circuits can handle hundreds of thousands of bend cycles without failing.

When to Use Rigid Flex Circuit Design

We briefly touched on the industries most frequently using rigid flex PCBs. Rigid flex PCBs are ideal when a designer needs to route circuitry between multiple hard boards, and wants to eliminate connectors with flexible cabling. They also are ideal in confined areas, where the PCB must match the existing design.

Despite being generally more costly than a rigid board, All Flex Solutions rigid-flex PCB boards are excellent as a cost-saving measure when combining multiple hard boards. The low weight of our rigid flex PCB boards makes them great in low-weight applications such as drones, aerospace applications, and wearables. The added flexibility of rigid flex boards gives them unmatched performance in high shock and high vibration environments.

Rigid Flex PCB Design Guidelines

Given the benefits of rigid flex design systems, being able to effectively design solutions using them is a skill in growing demand. Like any component, there are multiple factors to consider while designing them. Here is a quick overview of rigid flex PCB design guidelines.

Flex-to-Rigid Transitions

Rigid flex PCBs are a combination of both flexible and hard circuitry components. In addition to this, it’s important to note that the flexible portions run internally through the rigid sections. There is no connection point for the transitions between flexible and rigid components.

With this in mind, it’s crucial to have smooth transitions where the flexible and rigid sections meet. The flexible portion should not be exposed to excessive stress at the rigid board interface, or they run the risk of failure.

Depending on the fabricator and component, a keep-out area will provide a safety margin for all aspects of the design process. This means you must not include any pads, VIAs, or traces outside this margin. Drill holes should be slightly further from the margin area so the drill bit doesn’t cross the margin.

Rigid Flex Bend Radius

The core purpose of any rigid flex design’s flexible areas is to allow the entire circuitry component to bend and avoid damage or stress. Your specifications should have a large enough bend radius to prevent stress on conductors.

For one and two layer flexible components, the minimum bend radius should be 6 times the thickness of the flexible portion of the board. For all flex areas three layers or greater, the minimum flex radius should be 12 times the thickness of the flexible composite.. All Flex Solutions offers some of the most durable and ultralight flexible board components. Request a design consultation or quote to see where we can make your PCB designs stand out.

Material Selection in Rigid Flex PCB Design

The materials used in rigid flex PCB applications affect multiple aspects of your design. These include overall flexibility, durability, and thermal resistance. One of the most common materials used in rigid flex PCB design is polyimide on the flexible components and FR4 on the rigid ones.

No flow prepregs are all but required in rigid flex PCB design. No flow prepregs prevent uncured resin from flowing into flexible areas during lamination, which would prevent the flexible material from doing its job.

Unfortunately, the most widely used laminates do not have a no-flow prepreg in their product offering, so laminate selection is limited. Be sure to reference our design guide for more information on material and laminate selection in rigid flex circuit design. Fabricators may also be able to point you in the right direction regarding laminate and prepreg selection.

We have the largest library of rigid flex constructions, all approved to UL 94 V-0 and RoHS compliant. Get your All Flex Solutions rigid flex design & manufacturing guide today.

Layer Count and Stackup

To ensure the effective longevity of the final build, designing the proper arrangement and number of layers is crucial for maintaining flexibility over use. Remember, the flex layers extend throughout the rigid boards, which enhances their performance during exposure to sudden shock and excessive vibration. Optimal designs feature a balance between rigid and flex areas. Regarding fabrication package creation, there are a few differences between hardboard design and rigid flex design.

For situations with very high routing density, we offer layer counts of over 20 in some scenarios with flexible layers typically ranging from 1-4. However, All Flex Solutions can accommodate more as needed. Refer to our Valu Builds Guide for more information.

Trace Routing Layout and VIA Placement in Rigid Flex PCB Design

Trace routing is another key consideration in the rigid flex design process. Because traces will encounter different amounts of stress depending on their location (on the flexible components vs. the rigid components), trace routing should accommodate bending without needing stress. In most cases, the flexible portions will have wider traces, and you should account for the extra spacing required.

One aspect of this design process that is more complicated is maintaining impedance through transitions in trace width. Also, note that neckdowns should occur after traces have passed beyond the margins on rigid board components. Again, individual fabricators may set other requirements, so involving them throughout the rigid flex circuit design is critical.

Via placement is also an aspect of the planning and design process that must be carefully considered. Once again, we see the transition points between flexible and rigid components being the primary focal point during the design. Vias should have some distance between the flex to rigid transition area and fall within the safety margin. In some situations, Vias placed within this minimum distance – typically .050” or 1.27mm – will create reliability issues that will affect the overall performance of your board.

All Flex Solutions rigid flex boards offer flexible solutions for component routing, small space, and low-weight applications. Contact us today to start designing your own rigid flex solutions.

Heat Management

Consider that different parts of a PCB design will respond differently to heat, given their different materials and densities. Generally speaking, if possible, components with higher heat generation should be placed in areas with better heat dissipation properties. Rigid panels have more design options in terms of heat dissipation. Flexible components don’t have this luxury. However, their lower overall density helps them retain less heat. Moreover, their placement may also aid in heat dissipation to some degree.

All Flex Solutions are Experts in Rigid Flex PCB Design

All Flex Solutions has been the go-to for high reliability, never fail, rigid flex electronic packaging, that excel in high shock, high vibration applications. Our packaging solutions are also ideal for weight reduction and cost reduction when connecting four or more rigid boards electronically. As an engineer, you need the best rigid-flex solutions for these mission-critical components, and All Flex Solutions is your partner.

We carry the largest library of rigid flex constructions, and our ultra-thin dielectrics help you save weight in your designs. Read more about the latest in All Flex Solutions Rigid Flex products and design specifications on our blog, or order your rigid flex design guidelines today!

Contact All Flex Solutions

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