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Rigid-Flex Command Set-up and Layer Stack Design

Created: July 9, 2018
Updated: September 9, 2020

Microchip on a dark PCB

Once you take that nose dive into PCB design, you start to realize how many of your home electronic devices actually work. From DVD drives to laptop monitors, and just about anything that folds, it is all made possible with rigid-flex PCBs. Rigid-flex PCB design can be a tricky beast depending on the software you’re using to create it, but at the end of the design, your printed circuit board will bend and flex like a gymnast.

A rigid-flex PCB, at first, can be looked at like any other PCB: circuits, copper, vias; however, when you get into the thick of your circuit you’re going to want to have the software you can trust to work through both the flex PCB portion of your board and the rigid PCB portion.

Nowadays, with multilayer counts reaching anywhere from 4 to 30 and with form factors becoming more specialized and demanding, rigid-flex boards appear more often in electronic devices. The circuit that you’re needing to fulfil your design might be extremely difficult to draft. Don’t get left scratching your head or setting up your layers by hand; defining your layer stack is easy and accurate when you work in a unified design environment.

Layer Stackup, Ribbon Design, and Routing

Rigid-flex circuits are not just relegated to flip-phones. Many devices use rigid-flex ribbons to make connections between multiple boards in a single device. Odd-shaped electronic packaging and devices with folding components like laptops routinely use rigid-flex boards in order to route high-density connections through a single connector, rather than using a messy bundle of copper wire.

When designing rigid-flex circuits, you should always consult your manufacturer and assess their fabrication capabilities. You want to ensure that your design can actually make it off the fabrication line and operate according to your requirements. You’ll need to decide whether your ribbon flex will be static or dynamic, whether your ribbon will require multilayer routing and how you want your ribbon to connect between your printed circuit boards.

Some manufacturers will send you a pre-configured stackup file that is built according to their capabilities. Your PCB design software should be able to import and reuse these stackup files. This will help ensure that your rigid-flex designs will meet your manufacturer requirements and help prevent redesigns before moving to fabrication. This is also a major time saver and it takes the guesswork out of your circuit and PCBs.

Working with rigid-flex designs requires a PCB design software package with an intuitive stack manager and powerful routing tools. Your stack manager defines the material arrangement in each rigid and flex portion of your PCB. Once you have your layers defined and the flex PCB is arranged, your routing tools should allow routing across the flex ribbon as easily as it does on a rigid board.

Screenshot of layer stackup design in Altium

Layer stackup for rigid-flex design in Altium

Realizing Where the Pain Comes From

When you work in your stackup manager to define flex stackups, the stack manager should use an intuitive interface that contains all the required controls in a single window. Separating your stackup into multiple windows kills your productivity and makes it difficult to locate the required commands for defining your stackups. When you build your stackup it should actually resemble the device you are building and should not impose odd placement rules for top and bottom coverlays.

The same issue applies to ground and power layers. Most rigid-flex boards will have power and ground that spread across the flex ribbon just like it was another section of PCB. This is, again, where the stack manager shows its true value. When you build your stackup, you should be able to define power and ground layers in the same window as the remainder of your stackup.

When your flex zone connects to the rigid portion of the PCB without an external connector, you need to define a transition zone. What is odd is that some software programs have created a special process for defining this zone. In reality, a transition zone is just a slightly different stackup than your flex region, and defining this region shouldn’t require its own set of commands.

Rigid Flex Design Done Right

The rigid-flex design begins with defining your layer stacks for each board and your flex regions and requires an intuitive layer stack manager. You’ll need to define each layer on the board and ribbon, overlays and polyimide layers, as well as how these layers interface with each other. Defining rigid and flex regions shouldn’t take 5 different windows and dozens of clicks. All of this should happen in one easy-to-access window that includes all the required commands and options.

Once you define the layer stacks for your rigid and flex portions, you should be able to define the flex regions directly in your PCB layout. Once you define the overall outline of your board, you should be able to quickly assign layer stacks to each portion of the board. Routing your connections across the ribbon should be no different from routing within a single board or any circuit.

Once you have defined your stackup, laid out your components, and defined your connections between boards and circuits, you should be able to audit your design using thorough design rules checking features. You should be able to customize which rules are essential to your board, and your design rule checking features should display any errors or conflicts in an easy-to-read window.

Design validation takes more than checking against design rules, it requires diagnosing problems through simulation and verifying the form factor with a 3D viewer. Working in a unified environment means that you can immediately build and run simulations for your design without moving to an external program. Form factor and clearances in rigid-flex boards can be verified using a 3D view of your device, and all without exporting to a different program.

Screenshot of the 3D viewer in Altium

The potential of 3D viewers in a unified design environment

Rigid-Flex Design in Altium’s Unified Design Environment

Altium Designer’s unified design model integrates routing, layer stackup, and design rule checking in a single intuitive interface. Powerful CAD tools, an ultra-accurate 3D viewer, and customizable routing and via features give you the tools you need to design cutting-edge rigid-flex PCBs. All these design features are intuitive, and the required commands are easy to locate within the program.

If you are working in Altium’s unified environment, define your flex regions in the layer stackup manager is easy and intuitive. All the required options and commands are located in one window, making it quick and easy to define all of your layer stacks. Working with multilayer boards can be time-consuming, and software that includes the ability to save and reuse your layer stack will make you more productive.

If you’re looking for the best rigid-flex design software package, you need a PCB design software package like Altium Designer. The multilayer and multi-board design features naturally integrate with rigid-flex design, and the powerful design interface makes it easy to link and visualize your layer stack up. The 3D viewer feature also lets you view your rigid-flex board and helps mechanical engineers design packaging.

Altium wants to see you succeed as a designer, and they back it up by providing you with resources. From design examples to thorough documentation, Altium will help you take advantage of all the advanced tools Altium has to offer. You can also access the AltiumLive forum, user groups, video, and webinars provided by industry experts. This dedication to their customers sets Altium apart from other PCB design software companies.

Now you can download a free trial of the PCB layout software that you want to use for your designs. If you want to learn more about Altium and its rigid-flex design capabilities, talk to someone at Altium today.

Start the journey to switch over to Altium Designer today.

“We were using Orcad schematic with PADS layout. Keeping our designs in sync was always an issue and extra work for our layout guys. I like the integrated package of Altium. I like the ease of use of the user interface.”

-Design Engineer at Industrial Electronics Company

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