Peeling Back the Layers: Electronic PCB Stackup

March 29, 2018 Altium Designer

Distinguished rock layers on a cliff face

One of the first things you notice about the Grand Canyon when you visit is the incredible scenery. The canyon walls with their histories inscribed for all to see through the different colored layers of rocks and minerals are truly inspiring. I’m sure you could spend years and not even come close to discovering all the connections, materials, and creatures that live or have lived within those layered rock-faces.

Because it’s hard for me to stop connecting things to what I’m passionate about, the Grand Canyon reminded me of printed circuit boards with their fiberglass, solder masks, and other finishing coatings on outer layers. Electronic PCBs can be complex and the must use enough layers to meet the required complexity. Despite their size, PCBs seem to have as much depth and interconnectivity as even the layers of rock in the Grand Canyon.


Determining Layers of a Printed Circuit Board

With a PCB, you’ll have internal layers between routing, traces, and vias but also layers of protection between copper planes such as fiberglass or similar material, forming layers called the core and the prepreg. The core is manufactured with the tightest controls so it may be used to specify dielectric constants for your electric signals to decouple and keep noise out of the design. The prepreg is less controlled and instead is used to vary PCB thickness as necessary.

The layers of a printed circuit board evolve based on the amount of copper a needs to realize the design. The engineer or the study the density and amount of components and traces to determine the number of layers needed.

The next step identifies critical signals and where they will need to be placed to optimize their performance. This establishes core thickness, placement and thickness of traces, and ideal location of . The materials for manufacture need to be kept in mind during this phase of the design process, to ensure that the design is manufacturable to minimize cost.


Use Layers to Build the Stack and Define Overall Structure

You can then use layers to build the stack before laying down traces. Each layer may be visualized and defined as either copper, prepreg, or solder mask. The prepreg material may be selected for both its electrical and manufacturing characteristics, in addition to fulfilling the requirements of your reliability and safety departments, for the design.

The location and placement of vias are accomplished at this time, including plating for the via dependent on its current-carrying requirements. If blind vias are needed for the design, these are included in the appropriate locations within the PCB.

Surface of a circuit board with a blue light

Knowing how to plan for and stack your layers will yield a better PCB.


Continue Construction of the PCB Within the Stack

By taking the components from schematic to layout, you can place the components onto the virtual printed circuit board in the locations that will optimize the design. This is followed by adding traces from component to component and/or to connectors and test points within the design.

The more components, the more traces. More traces means more copper and more layers so traces that are traveling in the same directions can be placed on differing layers to avoid conflicts. I like to imagine different colonies of ants within close but separate layers of rock, if they know each other and communicate or remain isolated.  

The colonies may merge by punching tunnels between planes to visit one another. The same process occurs in printed circuit boards with vias. Should a copper trace need to go across the printed circuit board without interference from other traces, a via would be designed into the board so the trace could continue across the board on another plane.

Altium’s Layer Stack Manager

Using smart tools in your design software can ease your design process.

If you’re looking for board layout features that can guide you through all steps of the design, try a Visual Layer Stack Manager with a design rule system to define the requirements within the tool. Each layer may be visualized and defined as either copper, prepreg, or solder mask. The prepreg may be selected for both its electrical and manufacturing characteristics, in addition to fulfilling the requirements of your reliability and safety departments, for the design.

So the next time you start your layout, think about the layers of a canyon to visualize how your cross-sectional view of the board may be realized. The nets to connect your components have the ability to be both complex and elegant when using sophisticated design tools found in Altium Designer®. The tool allows you to organize your components on outer layers, and it allows you to define planes onto which you can route your nets using vias.

Altium’s tools can enable you to realize the picture in your mind of an organized printed circuit board that contains the elegance found in nature. If you want to learn how, try talking to an expert at Altium today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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