The traditional approach to circuit board design has been to place the components on the top and bottom surfaces of the board. This process is well supported by circuit board assembly houses, which typically use automated pick and place machines to position each component, ready to be soldered onto the surface. The ever increasing demand for smaller and more integrated electronic products, combined with the higher frequencies of the signals within these devices, drives the ongoing research into better ways to fabricate and assemble a circuit.
One technique that delivers both higher density and improved support for higher signal frequencies, is to embed components within the layers of the circuit structure. For example, embedding discrete components directly under an integrated circuit can result in shorter signal lengths, reduced resistance and parasitic inductance, leading to lower noise and EMI, and improved integrity of the circuit signals. These improvements deliver smaller and more reliable products, supporting faster signal speeds and higher bandwidths. Combined with the ongoing improvements being achieved in fabrication processes and technologies, they can also lead to a reduction in the product size, and lower fabrication and board-level assembly costs.
Embedding components places a number of unusual demands on each stage of the process from design to fabrication, to assembly, to testing and maintenance of the finished product.
Embedding components introduces a significant difference in manufacturing a circuit board - no longer is there a simple delineation between fabricating the bare-board; then loading components onto that bare-board during assembly. These two jobs were often handled by different companies, due to their distinctly different process and technology requirements. If embedded components are used, they must be fitted to the board during the fabrication process. While this was once the domain of highly specialized fab houses, the processes are well understood now and there are many fabricators that can manufacture circuits with embedded components.
There are two ways a component can be embedded: an open cavity is created such that the embedded component sits within that cavity and remains visible on the completed board, or they can be placed on an internal layer during fabrication then covered as the upper layers are added to the board during the fabrication process so that they are not visible on the completed board.
There are numerous approaches to fabricating a board with embedded components, the description and image below show one approach.
In the PCB editor, components can be placed on any signal layer, not just the traditional top or bottom surface signal layers. If they are placed on an inner copper layer that is covered, the components are called embedded components. There are two approaches to embedding components:
When a component requires a cavity, this cavity can be completely enclosed within the board, or it can extend to one side of the board to create an opening. The image below shows 3 embedded components, the outer 2 have a cavity defined that results in them being open on the top side of the board. The middle component is on a lower layer, resulting in it being completely enclosed. From the designer's perspective, the component placement process is the same for both open cavity and closed cavity components.
If a component is to be embedded and it requires a cavity, the cavity is defined as part of the component footprint in the PCB Library editor. Note that adding a cavity does not stop that component from being used on a surface layer, in this situation the software will ignore the cavity.
To define a cavity:
The image below shows the PCB library editor, with:
To embed a component, edit the component properties and set the Layer to the required internal copper layer. The direction that the embedded component orients (up or down) is defined by the Orientation specified for that copper layer in the Layer Stack Manager.
Related article: PCB Layer Stack Management
The Cavity that you define in the PCB Library editor has a Height attribute. This height defines the distance that the software will remove all layers above the surface of the copper layer that the component is placed on.
To simplify the interaction between the cavity and the layers that it cuts through, the software ensures that a layer is not partially cut. If a cavity enters into a layer, such as a dielectric layer, but does not extend all the way through that dielectric layer, then the software automatically extends the cavity all the way through that layer.
The image below shows this, the darker drawing lines have been added to show the corners of the cavity and how the component sits below the surface of the last-cut layer, but the layer cutout continues all the way through that layer. This behavior applies to both internal and open cavities.
The hover image below shows the Layer Stack Manager for a rigid-flex design that includes embedded components. Each separate zone or region of a rigid-flex design can be made up of a different number of layers. To achieve that you need to be able to define multiple stacks, referred to as substacks.
When you embedded a component, the PCB editor 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. The PCB editor checks for embedded components, tests if any of the currently 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 the PCB editor to check if new managed stacks are needed, switch between 2D and 3D Layout Modes.
Like User Stacks, Managed Stacks are listed in the PCB panel when it is set to Layer Stack Regions. The image below shows the managed stacks for two embedded components, R1 and C15. Use this feature to examine the extent of each Managed Stack in the X, Y plane.