Creating a Substack for Your PCB in Altium Designer

Created: April 12, 2022 | Updated: June 2, 2022
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Parent page: Designing a Rigid-Flex PCB

There are two rigid-flex design modes available in Altium's PCB design software. The original, or standard mode, referred to as Rigid-Flex, supports simple rigid-flex designs. If your design has more complex rigid-flex requirements, such as overlapping flex regions, then you need the Advanced Rigid-Flex mode (also known as rigid-flex 2.0). The mode is chosen in Tools menu in the Layer Stack Manager.

The fundamental difference between the modes is that in the original mode, the board shape is split into separate board regions by placing a Split Line, and that Split Line remains as an object defining where one board region ends and another begins. In the Advanced mode each board region is placed separately, or if a larger region is Sliced it becomes two separate region objects. In the Advanced mode, the edges of adjacent board regions define where those two regions meet, not the presence of a Split Line. Because of this difference, there is no Split Line object when the board is in the Advanced Rigid-Flex mode.

Learn more about Designing a Rigid-Flex PCB

Defining the Substacks (Standard Rigid-Flex Mode)

When the standard rigid-flex mode is selected in the Layer Stack Manager the Substack Selector will appear in the display (as shown below), as well as buttons to add a new substack and delete the current substack.

The first step is to configure the layers required in the rigid-flex design.The first step is to configure the layers required in the rigid-flex design.

Creating a new Flex substack:

  • Configure the layers required across the entire rigid-flex board, and Name the stack in the Properties panel. Learn more about working in the Layer Stack Manager.
  • Click the  button to add a new substack, as shown above.
  • Check or clear the Checkbox for each layer so that only layers required in this flex substack are enabled.
  • Enable the Is Flex checkbox and Name the substack in the Properties panel.
  • To add additional layers, such as a Coverlay, right-click on the layer in the main editing window and select the required command from the context menu.

Enable/disable the layer checkboxes to configure the layers require in the flex substack, then add any additional layers needed in this substack.Enable/disable the layer checkboxes to configure the layers require in the flex substack, then add any additional layers needed in this substack.

Defining the Substacks (Advanced Rigid-Flex Mode)

If the Rigid-Flex (Advanced) mode is enabled, the display will change from Stackup mode (shown above) to Board mode (shown below). Board mode is used to visually edit and organize the various substacks in the rigid-flex design. An individual substack can be edited by double-clicking on it, when you do the Stackup mode is displayed for that substack.

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 define the Substacks in a rigid-flex design.

The Board mode of the Layer Stack Manager is used to:

  • Add Substacks.
  • Configure the relationships between layers in adjacent Substacks - do they share layers (Common), or are the layers unique in that Substack (Individual).
  • Configure if adjacent layers intrude into the neighboring Substack.
  • Add additional Branches (Branches are used when the design has multiple flex sections radiating from a single rigid section - more on this in the Creating a Branch section).

Configuring the SubStacks

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 board 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.A bookbinder style rigid-flex PCB, note that the center section has two Substacks.

Working in Board mode in the Layer Stack Manager:

  • Once Rigid-Flex mode has been enabled, the Layer Stack Manager opens in Board mode, where new Substacks are added and aligned with the adjacent Substacks.
  • To edit the layers and layer properties of the selected Substack (selection is indicated by a pale blue outline, as shown above), either double-click on the Substack or use the Layer Stack Manager navigation bar to select the required Substack and open Stackup mode, where the individual layers in that Substack can be edited. Add and configure the layers as required.
  • To return to the Board mode after editing a Substack, click the Home button  in the Layer Stack Manager navigation bar.

Adding and Editing a New Substack

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:

  • A new Substack is created from the currently selected layers (highlighted in pale blue) - the first step is to select the layers required in the new Substack:
    • Left Click to select an entire Substack.
    • Shift+Click to select a single layer in a Substack. Continue to Shift+Click on other layers to build up the set of selected layers. Alternatively, Shift+Click on the first layer you require, then move straight to the last layer and Shift+Click to select it; all layers between will also be selected.
  • To add a new Substack using the selected layers, click the  button and select the appropriate command:
    • Insert Selected before - create the new Substack to the left of the currently selected Substack
    • Insert Selected after - create the new Substack to the right of the currently selected Substack
    • Insert Selected into - create the new Substack in the same section as the currently selected Substack.
    • Branch - create a new branch, starting from the currently selected Substack. More about branches below.
  • Alternatively, you can drag and drop the selected set of layers horizontally in the Board view to create a new substack of those layers (as shown in the video above). 
  • The new Substack will be created, double-click on the Substack to open it for editing in the Stackup mode of the Layer Stack Manager.
  • If you are planning on adding Coverlays to a flex region, enable the Is Flex option in the Properties panel when the flex Substack is open in Stackup mode.
  • Configure the Substack layers as required, then click the Home button  in the Layer Stack Manager navigation bar to return to the Board mode.
  • Edit the Substack Name in the Properties panel to reflect its function in the overall design.
  • Continue to add a Substack for each Region in the rigid-flex design.

Material Usage

Enable the Individual Material Usage option to allow different adjacent layers.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.

  • Common - material usage means: all layers in the Substack with the least layer count must have identical layers (and properties) as the layers in adjacent Substacks. This includes both the common copper/dielectric layers as well as any special purpose outer dielectric layers, such as coverlays.
  • Individual - enable the Individual mode to allows different combinations of materials in this Substack.

Creating a Branch

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.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.Use the controls in the Navigation Bar to switch from one Branch to another.

Working with Branches:

  • A Branch is created with the currently selected Substack as its base, select the required Substack before adding a Branch.
  • Click the  button and select Branch from the menu. The new Branch will appear, containing just the common Substack being branched from. Enter a suitable Branch Name in the Properties panel.
  • Add new Substacks to the Branch, as required.
  • Use the Layer Stack Manager's Navigation Bar to switch from one Branch to another.
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