Defining the Layer Stack in Altium NEXUS

The Stackup tab details the fabrication layers. Layers are added, removed and configured in this tab. For a rigid-flex design, layers are also enabled and disabled in this tab.

• The properties of the currently selected layer can be edited directly in the grid, or in the Properties panel.
• Right-click in the layer grid, click the  button, or use the Edit » Add Layer commands to add a layer. The new layer will be added next to the layer currently selected in the grid. Adding a Signal or Plane (copper) layer will also add a dielectric layer when an existing adjacent layer is also a copper layer.
• If the Stack Symmetry option is enabled in the Board section of the Properties panel, layers are added in matching pairs, centered around the mid-dielectric layer.
• The layer Material can either be typed into the selected Material cell or selected in the Select Material dialog, which is accessed by clicking the  button.
• Surface Finish can be added to an outer copper layer by using the appropriate right-click sub-menu and adding a Surface Finish layer.
• Internal copper layers include a Copper Orientation option which defines the direction that the copper is bonded to the core (and then etched from). Configure this to ensure the impedance calculations are accurate.
• Copper layers also include an Orientation option. Configure this when a rigid-flex design has an internal/flex layer with components mounted on it, or when the design uses embedded components, to indicate the direction the component is oriented relative to that copper layer.
• The selected layer can be moved up or down within the layers of the same type using either the right-click or Edit menus.
• The Board section of the Properties panel includes options to enforce Stack Symmetry and Library Compliance, which are discussed below.
• The Substack section of the Properties panel displays a summary of the currently selected stack (or substack for a multi-stack rigid/flex design).

► Configuring the Stackup in the Properties panel

This tab is used to define impedance profiles, which can then be used with routing design rules.

• Click on the Impedance Tab at the bottom of the Layer Stack Manager to configure the Impedance Profile requirements. Once the impedance profiles have been configured, the required profile can then be selected in the Routing Width or Differential Pairs Routing design rules.
• Click (or the Add Impedance Profile button if no profiles have been added yet) to add a new Impedance Profile, then define the required Type, Target Impedance and Target Tolerance in the Properties panel. The Description is optional.
• The next step is to define which layers the currently selected profile is to be available on. The grid is divided into two zones: the layers in the stackup are displayed on the left, on the right are the layers that the currently selected impedance profile will be available on. Use the layer checkbox in the Impedance Profile region to enable that layer to be available for the selected impedance profile.
• When you select an enabled layer in the Impedance Profile region, all layers in the layer stack are faded, except those being used to calculate the impedance for that selected signal layer (show image).
• Once the layer has an Impedance Profile assigned, edit that layer's reference layer(s) in the Top Ref and Bottom Ref columns. Note that reference layer(s) can be of Type Plane or Signal.
• The impedance calculators support forward and reverse impedance calculations. If you enter the Target Impedance, the Width will change automatically (forward calculation), or enter the Width and the Target Impedance will change automatically (reverse calculation).
• For a differential impedance calculation, lock either the Width or Trace Gap by clicking the appropriate button. The unlocked variable will then be calculated as the Target Impedance value is changed. Alternatively, edit the unlocked variable to change the Target Impedance.
• The Etch Factor = Thickness/[(W1-W2)/2] (hover the cursor over the ? in the panel to display the formula)
• The impedance calculator supports multiple adjacent dielectric layers. These layers can have different dielectric properties.
• The differential impedance calculator supports an asymmetric stripline structure.
• Single and differential coplanar structures are supported by the Simbeor impedance calculator.
• All calculations use a frequency of 2 GHz.
• To improve calculation speeds, impedance profiles are calculated in separate threads (when available).

► Configuring the Impedance profile in the Properties panel

The Via Types tab is used to define the allowed Z-plane layer-spanning requirements of the via(s) used in the design. The diameter and hole size (X&Y properties) of the vias placed in the design continue to be controlled by the default preferences if the via is placed manually, or the applicable Routing Style design rule if the via is placed during interactive routing.

• The Layer Stack for a new board includes a single, thruhole via span definition in the Via Types tab of the Layer Stack Manager.  For a two-layer board, the default via is named Thru 1:2, the naming reflecting the via type, and the First and Last layers that the via spans. The default thruhole span cannot be deleted.
• Click the  button to add an additional Via Type, then select the layers that this Via Type spans in the Properties panel. The new definition will have a name of <Type> <FirstLayer>:<LastLayer> (eg, Thru 1:2).
• The software will automatically detect the type (e.g., Thru, Blind, Buried) based on the layers chosen, and name the Via Type accordingly.
• If a µVia is required, enable the µVia checkbox. This option will be only available when the via spans adjacent layers, or adjacent +1 (referred to as a Skip via).
• If the Layer Stack has the Stack Symmetry option enabled, the Mirror option will become available. When Mirror is enabled a mirror of the current via, spanning the symmetrical layers in the layer stack, is automatically created. 
• Vias placed in the workspace include a Name property dropdown, which lists all of the Via Types defined in the Layer Stack Manager. All vias used in the board must be one of the Via Types defined in the Layer Stack Manager.
• When you change layers during interactive routing:
  • The Properties panel will display the applicable Via Type (show image).
  • If multiple Via Types are available to suit the layers being spanned, press the 6 shortcut to cycle through the available Via Types.
  • The proposed Via Type is detailed on the Status bar (show image).

► Configuring the Via Types in the Properties panel

In a high-speed design, when the barrel of a via extends beyond the signal layers that the signal is routed on, signal reflections can occur. This can lead to signal degradation and signal integrity issues. One approach used to resolve this is to drill out the unused via barrels using controlled depth drilling, which is a technique also referred to as back drilling.

• Back drill properties are configured in the Back Drills tab, this tab appears when Back Drills are enabled in the Tools » Features sub-menu or by clicking the  button then choosing Back Drills.
• The Back Drills tab is used to define the layer-spans that are required to be back drilled when there is a pad or via stub present. These settings are used in conjunction with the Max Via Stub Length design rule, where the maximum stub length and the drill oversize amount are specified. The Where the Object Matches setting in the rule can be used to restrict stub-removal to specific nets.
• Click the  button to add a new back drill definition. The definition will be named according to the First layer and Last layer selected in the Back Drill section of the Properties panel, for example, BD 1:3. First layer defines the first layer to be drilled, Last layer defines the layer that drilling stops before (Last layer is the first layer in the layer stack that will not be back drilled).
• If the Substack Properties has the Stack Symmetry option enabled in the Properties panel, the Mirror option will become available in the Back Drill section of the panel. When this is enabled, a mirror of the current Back Drill is created, for example, BD 1:3 | 6:4. 

► Configuring the Back Drills in the Properties panel

Using modern printing technology, it is possible to print conductive and non-conductive layers directly onto a substrate material, building up an electronic circuit. This is referred to as printed electronics.

• The layer stack is configured for printed electronics by selecting the Tools » Features » Printed Electronics option. In this mode, all tabs are replaced by the single Printed Electronics Stackup tab.
• Traditional dielectric layers are not used in printed electronics. Instead, local dielectric patches are printed where routing must cross over. When the Printed Electronics option is enabled in the Features drop-down, all dielectric layers are removed from the layer stack and instead, the dielectric patches are defined by placing suitably shaped region objects on non-conductive layers.
• In printed electronics, copper signal layers are referred to as conductive layers, and insulating layers are referred to as non-conductive layers.

► Configuring the Printed Electronics stackup in the Properties panel

The Board tab is used to configure the different substacks required in a rigid-flex design. The Board tab is automatically displaying when Rigid-Flex (Advanced) mode is enabled. Note that it is not used/available when the standard Rigid-Flex mode is chosen.

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

• Add Substacks. Additional substacks can be quickly created from an existing substack using the Shift+Click shortcut to select the required layers, and then dragging the selection horizontally to position it in the set of 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.
• Switch to editing a specific substack - double-click on it in the Board tab to do this.
• Add additional Branches. Branches are used when the design has multiple flex sections radiating from a single rigid section. Learn more about Branches.

The selection of layer materials and their properties should always be done in consultation with the board fabricator.

• Mirror top half down - the settings of each of the layers above the central dielectric layer are copied down to the symmetrical partner-layer.
• Mirror bottom half up - the settings of each of the layers below the central dielectric layer are copied up to the symmetrical partner-layer.
• Mirror whole stack down - an additional dielectric layer is inserted after the last copper (Surface Finish) layer, then all of the signal and dielectric layers are replicated and mirrored below this new dielectric layer.
• Mirror whole stack up - an additional dielectric layer is inserted before the first copper (Surface Finish) layer, then all of the signal and dielectric layers are replicated and mirrored above this new dielectric layer.

• Enable the Rigid-Flex option by selecting the Tools » Features » Rigid/Flex command. You can also access the command in the Features () menu.
• If you are working in standard Rigid-Flex mode the display will remain on the Stackup tab, except the Substack selection and management buttons will appear at the top, as shown in the image below.
• There must be a unique Substack defined for each unique set of layers needed in the rigid and flex regions of the overall board, click the  button to add a new Substack, as shown in the image below.
• Using the Substack Selector, select each Substack in turn and use the checkboxes to enable/disable layers, to give the layer-set required for that Substack.
• For flex Substacks, enable the Is Flex option in the Properties panel. Flex-specific coverlay layers can only be added in a Substack that has the Is Flex option enabled and does not include a Soldermask layer.

• Enable the Rigid-Flex option by selecting the Tools » Features » Rigid/Flex (Advanced) command. You can also access the command in the Features () menu.
• In Advanced Rigid-Flex mode the display will switch to show the Board tab, as shown below.
• There must be a unique substack defined for each unique set of layers needed in the rigid and flex regions of the overall board. Board mode is used to configure the different substacks required in an Advanced rigid-flex design.
• Additional substacks can be quickly created from an existing substack using the Shift+Click shortcut to select the required layers, and then dragging the selection horizontally to position it in the set of substacks, as shown in the image below.
• 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.
• Switch to editing a specific substack - double-click on it in the Board tab to do this.
• Add additional Branches. Branches are used when the design has multiple flex sections radiating from a single rigid section.
• For flex Substacks, enable the Is Flex option in the Properties panel. Flex-specific coverlay layers can only be added in a Substack that has the Is Flex option enabled and does not include a Soldermask layer.