What’s In a Circuit Board Layer Stackup: PCB Layers Explained
There is just something amazing looking about a trifle dessert. All of those layers of delicious food look like pure magic. Wouldn’t you agree? There are a lot of layered things in our lives that we are familiar with. Some extra layers of blankets during a cold night, the layers of wrapping paper concealing a wonderful gift, and (sadly) those annoying layers of paperwork involved to file our taxes.
There are also a lot of different layers that are used in the design and manufacturing of a printed circuit board. These layers might not be as familiar though and can sometimes cause confusion, even for people who work with them on a regular basis. There are physical layers in the circuit board for circuit connectivity, and then there are layers in the PCB CAD tools to design those layers. Let’s take a look at what all of this means with a description of PCB layers explained.
PCB Layers Explained in a Printed Circuit Board
Like the trifle dessert pictured above, a printed circuit board is made up of multiple layers. Even a simple single sided (one layer) board is made up of a conductive metal layer and a substrate layer composited together. As the complexity of the PCB increases, so will the amount of layers within it.
A multilayer PCB will have one or more core layers made up of a dielectric material. This material is often made up of a woven fiberglass cloth with an epoxy resin binder, and serves as insulation between the two metal layers immediately adjacent to it. There will be additional layers of metal and core materials depending on how many physical layers of the board are required. Each metal layer will have a layer of fiberglass pre-impregnated with resin known as “prepreg” between it. Prepreg is essentially uncured core material that will melt and join the layers together when put under the heated pressure of the lamination process. Prepreg will also serve as an insulated between the metal layers.
The metal layers on the multilayer PCB will conduct the electrical signals of the circuitry from point to point. For regular signals thin metal traces are used, while for power and ground nets wider traces will be used. Multilayer boards often use entire layers of metal to form a power or ground plane. This allows easy access for all parts into the plane through small holes filled with solder negating the need to route power and ground strips throughout the design. It also helps with the electrical performance of the design by providing electromagnetic shielding as well as a good solid return path for the signal traces.
A cross sectional image of PCB layers to explain the structure of a circuit board
The Printed Circuit Board Layers in PCB Design Tools
In order for the layers in a physical circuit board to be created, there needs to be image files of the metal trace patterns that the manufacturer can use to build the board with. In order to create these images, PCB design CAD tools have their own set of circuit board layers in them for engineers to work with as they design the board. Once the design is complete, these different CAD layers will be exported to the manufacturer in a set of fabrication and assembly output files.
Each metal layer in a circuit board is represented by one or more layers in the PCB design tools. Usually the dielectric (core and prepreg) layers are not represented by CAD layers, although this will change depending on what circuit board technologies are being designed as we will mention later. For most PCB designs however the dielectric layers will only be represented by attributes in the design tools so that the materials and widths are accounted for. These attributes are important for the different calculators and simulators that the design tools will use when determining the correct values for metal traces and spaces.
In addition to each metal layer of the board getting a separate layer in the PCB design tools, there will also be CAD layers devoted to solder mask, solder paste, and silkscreen markings. Mask, paste, and silkscreen are applied to the circuit board after it is laminated together, so they aren’t physical layers of the actual board. But to give the PCB manufacturers the information they need to apply these materials, they too need their own image files created from the PCB CAD layers. Lastly the PCB design tools will also have many other layers built into them for additional information needed for either design or documentation purposes. This could include other metal objects on or in the board, part numbers, and component outlines.
PCB cross-sectional editors like this one in OrCAD Designer make working with layers easy
Beyond the Standard PCB Layers
In addition to designing single or multilayer printed circuit boards, there are other PCB design technologies that CAD tools are being used for today. Flex and rigid flex designs will have flexible layers built into them requiring those layers to be represented in the PCB design CAD tools. Not only will these layers need to be displayed for working on in the tools, but advanced 3D work environments are needed in the tools as well. This will allow the designers to see how the flex designs will fold up and out while they are working on them as well as to what extent, what angle they will bend.
Another technology that requires additional CAD layers is printable, or hybrid electronics. These designs are fabricated by adding or “printing” metal and dielectric materials onto a substrate instead of using a subtractive etching process as in a standard PCB. To accommodate this, PCB design tools need to be able to display and design these dielectric layers in addition to the standard metal, mask, paste, and silkscreen layers.
To create the layers you need in your next PCB design, you need a PCB design system that gives you the control you need over the layers in your design. With its cross-section editor and other powerful utilities, OrCAD PCB Designer has the capabilities you are looking for.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.