The Role of CAD Tools
An important aspect of any engineering design course is learning to use CAD software, and PCB design is no different. PCB designers use specialized CAD tools to create the physical layout of a printed circuit board. When we say “physical layout”, we’re referring to the arrangement of components, pads, traces, vias, and any other component or physical element that will be placed on the PCB.
Technically, you could use any CAD program to create a physical layout of a PCB. However, the industry has developed specialized software, called ECAD software, that helps automate many of the tasks involved in creating drawings of a physical layout. Altium Designer is one such program, but there are many others that use similar workflows and have similar tool sets for creating PCB layouts.
The files you create in your ECAD software will store all the information needed to understand and manufacture a design. The PCB layout is most important because it is like a picture of the board that will be fabricated and assembled by a PCB manufacturer. However, a PCB layout is not the only document you’ll work on in your CAD tools. It is also important to understand the primary set of files you’ll need for a design. Different ECAD applications use different sets of files to store design information, but there are a few common file types that are used in every design.
Common PCB Files
When designing a PCB, there are four major types of documents you will need to create a design:
- Schematic sheets: Your schematic sheets are like the electrical blueprint for your design. These documents show components, nets, and other information needed to understand the design.
- Bill of materials: This document is a list of all components used in the design. It will be used to order components and to aid assembly.
- PCB layout: This document shows the physical placement of components and copper connections between them. The PCB layout document will also describe the layer stack used in the design.
- PCB libraries: Your libraries store all of the CAD data for your components (schematic symbols, PCB footprints, 3D models, and SPICE subcircuits). Libraries also store procurement information for each component, including MPN, distributor information, a description of each component, quantities of each component, and possible alternative components.
These four types of documents are the minimum needed to create the physical design for your board, but they aren’t the only documents used in PCB development. The design can’t be prepared for fabrication until the PCB layout is finished. Once the layout is complete, there is another set of fabrication and assembly files that are used in PCB manufacturing processes. We’ll cover manufacturing files in more detail in the next section.