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Parent page: More about PCB Design
As the name suggests, a flexible printed circuit is a pattern of conductors printed onto a flexible insulating film. Rigid-flex is the name given to a printed circuit that is a combination of both flexible circuit(s) and rigid circuit(s), as shown in the image.
Flexible circuit technology was initially developed for the space program to save space and weight. It is popular today as it not only saves space and weight - making it ideal for portable devices such as mobile phones and tablets - it can also reduce packaging complexity, improve product reliability, and reduce cost.
Flexible circuits are normally divided into two usage classes: static flexible circuits, and dynamic flexible circuits. Static flexible circuits (also referred to as use A), are those that undergo minimal flexing during assembly and service. Dynamic flexible circuits (also referred to as use B), are those that are designed for frequent flexing, such as a disk drive head, a printer head, or as part of the hinge in a laptop screen. This distinction is important as it affects both the material selection and the construction methodology. There are a number of layer stackup configurations that can be fabricated as rigid-flex, each with their own electrical, physical and cost advantages.
Designing a flex or rigid-flex circuit is very much an electromechanical process. Designing any PCB is a three-dimensional design process, but for a flex or rigid-flex design, the three-dimensional requirements are much more important. Why? Because the rigid-flex board may attach to multiple surfaces within the product enclosure, with the attaching and folding process happening as the product is assembled.
The traditional approach to confirm that the folded board fits within its enclosure has been to create a mechanical mockup - known as a paper doll cut out. By its very nature, it's difficult to achieve the accuracy and realism required with this approach. Altium is helping to solve this challenge with CoDesigner, a sophisticated mechanical - to - electronic design interface technology. CoDesigner allows the engineers to pass the board shape and component changes back and forth between the ECAD and MCAD design domains.
A printed circuit board is designed as a series of layers stacked on top of one another. For a traditional rigid printed circuit board, the board shape defines the board in the X-Y plane, and the stack of layers defines the board in the Z plane. The X-Y board shape is defined in the main PCB editing window, and the layers are configured in the Layer Stack Manager. In a rigid-flex PCB, there is more than one zone or Region in the finished printed circuit board, and each of those Regions can use a different set of layers.
To design a rigid-flex board, you need to:
To support the complex structures present in a modern rigid-flex printed circuit board, the Z plane editor - the Layer Stack Manager, provides different display modes for editing the structure of your board. Select the Design » Layer Stack Manager command to open the Layer Stack Manager, where you can create and align the Substacks needed in your rigid-flex design.
When the Layer Stack Manager opens, it will show the current board layer Stackup. For a new PCB, this will be a simple two-layer board. To enable the features needed to design a rigid-flex board, select Rigid-Flex from the Tools » Features sub-menu or the Features button.
Enable the Rigid-Flex mode to configure a Rigid -Flex board; either via the Tools menu or by clicking the Features button (hover the cursor over the image to show this).
Once rigid-flex has been enabled, the display will change from Stackup mode (shown above) to Board mode (shown below). Use the Navigation bar at the upper right of the Layer Stack Manager to move back and forth between the Stackup and Board modes, as highlighted in the image below.
The Board mode of the Layer Stack Manager is used to define the Substacks in a rigid-flex design.
The Board mode of the Layer Stack Manager is used to:
A Board can include any number of Substacks. One approach that helps with visualizing the overall board structure is to define a Substack for each Region of the board. This is not a requirement though, the minimum requirement is to create a Substack for each unique set of layers needed in the overall design. Multiple Regions can then be assigned the same Substack, if required.
The video below shows a rigid-flex board with nine Board Regions, which use three unique Substacks.
Each Substack can be assigned to a Board Region as many times as required.
Each Substack is created within a section. Why do you need sections? Because you can also create multiple Substacks within one section, a feature you use when you are creating a bookbinder-style rigid-flex board (two rigid regions connected by multiple flex regions). The image below shows two flex substacks, named FlexUpper and FlexLower, in the center section of the layer stack.
A bookbinder style rigid-flex PCB, note that the center section has two Substacks.
Working in Board mode in the Layer Stack Manager:
As well as being used to add and remove Substacks, Board mode is also used to configure if a Stackup uses common or individual layers.
A new substack is created from the currently selected layers.
Creating a new Substack:
Enable the Individual Material Usage option to allow different adjacent layers.
A rigid-flex design often has copper and dielectric layers that are common through the rigid and flex regions, but different outer dielectric layers, such as the coverlays. To help the designer manage this, the Properties for the selected Substack includes a Material Usage option.
If the design has a structure of stackups that cannot be modeled in the Board view, then it requires the Branch feature. In the example shown in the image below, there are four flexible regions radiating from different layers on the main board, with each flex region having a small rigid region at the end. Although it is possible to connect the four flex regions to the MainBoard without using Branches, it is not possible to create the small rigid region at the end of each flex region.
The selected Region, ConnectorRegion4, is getting the Substack ConnectorRigid4 assigned.
This board requires the use of the Branch feature. A Branch grows from a Substack, one Substack can have multiple branches radiating from it. In this example the MainBoard Substack has four branches; FirstFlexBranch
, SecondFlexBranch
, ThirdFlexBranch
and ForthFlexBranch
.
Use the controls in the Navigation Bar to switch from one Branch to another.
Working with Branches:
The layer stack defines the board in the vertical direction, or Z plane. In the PCB editor, the area that the board occupies in the X and Y planes is defined by the Board Shape. The board shape can be a polygonal region of any shape, with straight or curved edges that lie at any angle, that can also include cutouts (internal holes) of any shape.
There are two techniques that can be used to defining the overall board shape and the various rigid and flex regions:
The final shape can be created using a mixture of the two techniques.
The overall Board Shape is defined by the set of Board Regions, the video shows two ways these regions can be created.
Defining the Board Shape:
► Learn more about defining the Board Shape
A bend in a flexible section of a rigid-flex board is defined by placing a Bending Line. A bending Line is a linear object, whose properties are edited in the Bend mode of the Properties panel.
To edit the properties of a Bend, select it and edit the settings in the Bend mode of the Properties panel:
The Bend zone is displayed in a green-orange color, click anywhere within the zone to select that Bend.
Each Bend can be named so it can be easily identified.
Confirm that the Bend is being applied to the correct Substack Region, available regions are listed in the Stack Regions section.
Set the Bend Zone Radius and Bend Angle as required.
Bends are folded in the order of their Fold Index, use this feature when the folding order is important to check.
To move a Bending Line, click and drag on each end handle.
Bending Lines can be applied to board cutouts edges.
A selected Bend can be removed by clicking and holding on one of the vertices, then pressing Delete on the keyboard.
► Learn more about placing Bending Lines
A common feature on rigid-flex boards is the selective use of coverlay material. This insulation layer is cut and laminated onto specific areas of the board, and because of this selective use, coverlay is also referred to as bikini coverlay. Coverlay layers are added in the Layer Stack Manager and the shape of coverlay objects can be manipulated in Board Planning Mode, once coverlays have been enabled for that region of the board.
Coverlay layers are added in the Layer Stack Manager. To add coverlay layers:
Add the coverlay layer(s) into the substack and configure the layer properties in the Layer Stack Manager.
If coverlay layers are added to the substack before that substack is assigned to a Board Region, the coverlay objects will be present when the substack is assigned to a Board Region.
If coverlay layers are added to the substack after that substack has already been assigned to a Board Region, then coverlays must be added to that region. Coverlays can be added in Board Planning mode by selecting the Board Region and then either:
If the Board Region had the substack assigned before the coverlays were added in the LSM, use the right-click command or the panel button to add them to that region.
Coverlay is automatically added to cover the entire area of the Board Region it was added to, as shown in the image below (in accordance with the Coverlay expansion value defined in the Layer Stack Manager). Behaving like an additional solder mask layer, openings are automatically created for component pads in accordance with the applicable Solder Mask Expansion design rule, or else the settings defined for the Pad if the Solder Mask Expansions setting has been configured to override the design rule.
Openings in the coverlay for the component pads is controlled by the applicable Solder Mask Expansion design rule (which can be overridden by local pad settings).
To edit the coverlay:
A section of custom coverlay has been placed and a cutout is about to be defined in it.
When the output is generated, each layer is output as separate data. For example, when Gerber is generated, the top solder mask is written to one Gerber file, the top coverlay is written to another Gerber file.
The output for a coverlay can be divided into 2 categories:
Examining the Gerber output in a CAM viewer - the blue is the coverlay layer, the purple is the top solder mask layer.
Zoomed in, a custom coverlay cutout is selected - as with the coverlay outline the cutout is also defined as a closed polyline.
Coverlay output is supported by all of the appropriate output formats available in Altium Designer.
The PCB editor includes a powerful 3D rendering engine, which allows the presentation of a highly realistic three-dimensional representation of the loaded circuit board. This engine also supports rigid-flex circuits, and when used in combination with the Fold State slider in the PCB panel, it allows the designer to examine their rigid-flex design in the flat state, the fully folded state, and anywhere in between.
To switch to the 3D display mode, press the 3 shortcut key (press 2 to return to 2D or 1 to return to Board Planning Mode). The board will be displayed in 3D. If the component footprints include 3D body objects that define the mounted component, these will also be displayed. In the image below, you can see that the board includes a battery and a battery clip.
To apply all of the Bending Lines, slide the Fold State slider in the PCB panel when set to Layer Stack Regions mode as highlighted in the image below. Note that the bends are applied in the order defined by their sequence number. Bending Lines can share the same sequence number; it simply means that those bends will be folded at the same time when the Fold State slider is used. The board can also be folded/unfolded by running the View » Fold/Unfold command (or by pressing the 5 shortcut).
Use the Fold State slider (or the 5 shortcut key) to apply all Bending Lines in the order defined by their sequence value (Fold Index).
The ability to fold a rigid-flex design can also be captured as a 3D movie. It is very simple to do and does not require the use of movie key frames during the folding sequence.
Refer to the PCB 3D Video page for a detailed description of how to make a 3D movie. As a basic guide:
The video below was created using this process. It has the two key frames described above, plus one additional key frame that was added at the end to hold the final position for a second.
A simple 3D movie created from three key frames; the folding behavior is defined by the Bending Line Sequence values.
► Learn more about 3D PCB Video
Below is a summary of key design areas that must be considered when designing a rigid-flex PCB:
Typical suggested documentation requirements include:
Flex Circuit Design Guide - Epec Engineering Technologies
Flexible Circuit Technology - Joe Fjelstad
Flex Circuits Design Guide - Minco Products Inc
Machine Design website:
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