There are many different layers in the design and manufacture of printed circuit board. These layers may be less familiar and sometimes even cause confusion, even for people who often work with them. There are physical layers for circuit connections on the circuit board, and then there are layers for designing these layers in the PCB CAD tool. Let’s take a look at the meaning of all this and explain the PCB layers.
PCB layer description in printed circuit board
Like the snack above, the printed circuit board is composed of multiple layers. Even a simple single-sided (one-layer) board is composed of a conductive metal layer and a base layer that are compounded together. As the complexity of the PCB increases, the number of layers inside it will also increase.
A multilayer PCB will have one or more core layers made of dielectric materials. This material is usually made of fiberglass cloth and epoxy resin adhesive, and is used as an insulating layer between two metal layers immediately adjacent to it. Depending on how many physical layers the board requires, there will be more layers of metal and core material. Between each metal layer there will be a layer of glass fiber glass fiber, pre-impregnated with a resin called “prepreg”. Prepregs are basically uncured core materials, and when placed under the heating pressure of the lamination process, they melt and connect the layers together. The prepreg will also be used as an insulator between the metal layers.
The metal layer on the multi-layer PCB will conduct the electrical signal of the circuit point by point. For conventional signals, use thinner metal traces, while for power and ground nets, use wider traces. Multilayer boards usually use a whole layer of metal to form a power or ground plane. This allows all parts to easily enter the plane of the aircraft through small holes filled with solder, without the need to wire power and ground planes throughout the design. It also contributes to the electrical performance of the design by providing electromagnetic shielding and a good solid return path for signal traces
Printed circuit board layers in PCB design tools
In order to create the layers on the physical circuit board, an image file of the metal trace pattern that the manufacturer can use to construct the circuit board is required. In order to create these images, PCB design CAD tools have their own set of circuit board layers for engineers to use when designing circuit boards. After the design is completed, these different CAD layers will be exported to the manufacturer through a set of manufacturing and assembly output files.
Each metal layer on the circuit board is represented by one or more layers in the PCB design tool. Normally, the dielectric (core and prepreg) layers are not represented by CAD layers, although this will vary depending on the circuit board technology to be designed, which we will mention later. However, for most PCB designs, the dielectric layer is only represented by the attributes in the design tool, in order to consider the material and width. These attributes are important for the different calculators and simulators that the design tool will use to determine the correct values of metal traces and spaces.
In addition to getting a separate layer for each metal layer of the circuit board in the PCB design tool, there will also be CAD layers dedicated to solder mask, solder paste, and screen printing marks. After the circuit boards are laminated together, masks, pastes and screen printing agents are applied to the circuit boards, so they are not the physical layers of the actual circuit boards. However, to provide PCB manufacturers with the information needed to apply these materials, they also need to create their own image files from the PCB CAD layer. Finally, the PCB design tool will also have many other layers built in to obtain other information needed for design or documentation purposes. This may include other metal objects on or on the board, part numbers and component outlines.
Beyond the standard PCB layer
In addition to designing single-layer or multi-layer printed circuit boards, CAD tools are also used in other PCB design techniques today. Flexible and rigid flexible designs will have flexible layers built into them, and these layers are required to be represented in PCB design CAD tools. Not only need to display these layers in the tool for operation, but also need an advanced 3D working environment in the tool. This will allow designers to see how the flexible design folds and unfolds and the degree and angle of bending when in use.
Another technology that requires additional CAD layers is printable or hybrid electronic technology. These designs are manufactured by adding or “printing” metal and dielectric materials onto the substrate instead of using a subtractive etching process as in standard PCBs. In order to adapt to this situation, PCB design tools need to be able to display and design these dielectric layers in addition to the standard metal, mask, paste and screen printing layers.