Altium Layers

[Reuquen Boyett]

Fastforward to today, where almost all PCB designs have multiple copper layers. Two to 10 copper layers are common, but it is possible to fabricate a board with over 30 copper layers. These copper layers are defined in the design environment as part of the layer stack, along with the insulation material that separates them.

To design a single printed circuit board, it is only necessary to define a single layer stack, which defines the entire board area in the vertical direction, or Z plane. However, technological innovation and refinements in the processing technology have led to a number of revolutionary concepts in PCB fabrication including the ability to design and manufacture flexible PCBs. By joining rigid sections of PCB together via flexible sections, complex, hybrid PCBs can be designed that can be folded to fit into unusually shaped enclosures.

Since it is fabricated as a single entity, a rigid-flex PCB must be designed as a single entity. To do this, the designer must be able to define multiple PCB layer stacks and assign different layer stacks to different parts of the rigid-flex design.

A limiting factor in how small a component can be made is the challenge of providing access to the component interconnects, or pins, that were traditionally arranged around the edges of the component. The greatest jump in the density of the interconnects came with the introduction of grid array interconnects - where the component's connection points are arranged in rows along the bottom surface of the component. Component packages, such as PGA (Pin Grid Array) and BGA (Ball Grid Array), use this arrangement of interconnects.

Within the component, the silicon die may still present its connection points around the edge of the die, so these die connection points must be internally routed to the grid array of interconnections on the bottom of the PGA or BGA package. This is often done using an internal PCB. It is this internal PCB that has driven much of the High Density Interconnect (HDI) technology development that is now available for use in the broader printed circuit board design and fabrication market.

An excellent reference for building a good understanding of the materials and techniques of both the design and the fabrication of printed circuits is Right First Time - a Practical Handbook on High Speed PCB and System Design, by Lee Ritchley. A free PDF version can be downloaded from

All layer stacks are defined in the Layer Stack Manager. To open the Layer Stack Manager select Design Layer Stack Manager from the main menus. For a new board, its single default stack comprises a dielectric core, two copper layers, and the top and bottom solder/coverlay and overlay layers as shown in the image below.

The Layer Stack Manager is used to define the board structure in the vertical, or Z direction. Defining the board in the horizontal, or X-Y direction, is done by first defining the overall board shape. If it is a rigid-flex design, the shape is then divided into the required zones or board regions. Once the board regions have been defined, each region is assigned a layer stack.

For rigid-flex PCBs, you need to be able to create and manage multiple stacks. This is performed by entering the dialog's Advanced mode by clicking the Advanced button at the bottom-left of the dialog.

The stack currently selected in the lower region of the dialog has its name highlighted and the stack is displayed in the upper layer region of the dialog. To enable or disable layers in a stack, first select the required stack.

The set of available layers and the defined layer stacks can be transferred from one board design to another by clicking the Save button to write them to a *.stackup file. Click the Load button to replace the current set of available layers and defined stacks, with those defined in an external *.stackup file.

Stacks are added, removed, and their order is configured in the lower half of the Layer Stack Manager. Note that this section is only displayed if the Advanced button has been clicked.

When you click on the cell associated with a particular layer in the layer region of the Layer Stack Manager, that layer will be highlighted in the graphical display of the currently selected stack (to the left) and also across all defined stacks that use that layer in the stack region of the dialog.

Layers are added, removed, and re-ordered in the upper half of the Layer Stack Manager using either the Add Layer button (for adding) or the right-click menu (for all types of changes).

Layer stacks can be saved and loaded using the relevant buttons at the top of the Layer Stack Manager. You have the ability to control how the layers will be added (controlling how the two stacks are merged) to arrive at your desired stack setup.

In addition, if you are connected to a server, you have the ability to save your layer stack as a managed item in that server, and load a stack from a server. By extension, if your company is using environment configuration management, layer stack usage can be managed through that system, allowing the use of all or specific layer stack items stored in the server and shared with the defined role(s).

When you load layer stack data, from a *.stackup file or a Layer stack Item in a server - the Merge Layer Stacks dialog will open. This dialog is used to control how the current layers are to be merged with the layers in the stack being loaded.

The New Layer column corresponds to the layer stack being loaded/imported, while the Old Layer column corresponds to the existing, current layer stack for the board. The Used column shows which layers currently have primitives placed on them in the board design. Use the Create column to specify that an imported layer be used (checked) or not (unchecked).

If you have a layer that is in the current stack but not in the new stack, it will not be removed. In addition, primitives are not deleted, but rather moved to the newly created layer. If you disable the creation of a layer and that layer is used and has primitives on it, you will be alerted to this and asked to confirm.

In Altium Designer, there is a single set of layers defined and any layer can then be used in any layer stack. This set of layers includes all of the layers that are used in the overall PCB design regardless of whether the design is a single PCB or a rigid-flex design incorporating numerous rigid and flex sections. A variety of types of layers can be included in the layer stack: copper, dielectric, surface finish and mask layers. Each layer must be completely specified in terms of its material and mechanical requirements including the material used, the thickness, the dielectric constant, etc. The selection of materials and their properties should always be done in consultation with the board fabricator.

1) Protective layer that restricts where solder can be applied to the circuit. A cost effective and proven technology, suitable for rigid and flex use class A (flex-to-install) applications. Suitable for finer features than flexible film coverlay.

When you double-click on a split plane area the Split Plane dialog will open. Select the required net. This process is the same if you are assigning a single net to the entire plane, or are assigning a net to a split region of the plane.

When you embedded a component, Altium Designer has to manage how that embedded component affects the layer stack not only in terms of how it is displayed, but also in terms of calculated data such as solder mask openings and design rule checking. It does this by creating a stack for each unique combination of placed + cut layers needed by the various embedded components included in the design. These stacks are referred to as Managed Stacks.

The Managed Stack is created automatically when a component is embedded within the layers of the board. As managed stacks are created automatically, there is no user-input needed in their creation and management. Altium Designer checks for embedded components, tests if any of the current managed stacks are suitable and if not, creates a new one. The same applies when embedded components are removed; if a managed stack is no longer needed, it is automatically removed. To force Altium to check if new managed stacks are needed, switch between 2D and 3D Layout Modes.

Note that the stack selector on the bottom of the dialog is set to Show Managed Stacks. When this is selected, the dialog displays the managed stack. Note that the stack selector setting is not persistent and when the dialog is reopened, it defaults to Show User Stacks.

Documentation is a key part of the design process and is particularly important for designs with a complex layer stack structure, such as a rigid-flex design. To support this, Altium Designer includes a Layer Stack Table, which is placed (Place Layer Stack Table) and positioned alongside the board design in the workspace. The information in the layer stack table comes from the Layer Stack Manager dialog.

Altium Designer also provides a dedicated documentation editor - Draftsman. Draftsman has been built from the ground up as an environment for creating high-quality documentation that can include dimensions, notes, layers stack tables, and drill tables. You can either continue to create documentation using the approaches described here, or explore the documentation capabilities of Draftsman.

The Layer Stack Table can also include an optional map of the PCB. This is an outline of the board showing how the various layer stacks are assigned to regions of the board. Use the Show Board Map option and slider bar to configure the settings desired.

Altium Designer includes an intelligent Drill Table that is placed like any other design object. The table displays the drills required for a specified layer pair, you will need to place a drill table for each layer pair used in the design.

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