What is PCB Design?
PCB design is an integral part of any electronic product development project. PCB design uses a CAD software tool to create a 3D rendering of a PCB from a 2D schematic which is then used to manufacture the board. An optimized PCB design ensures the circuit board performs as desired and within the expected specification. PCB designers must follow a specific process that follows the steps of mechanical definition, critical placement, placement, critical routing, routing, final design rule checks, and creating outjobs.
PCB Design Software Tools
Many computer-aided design (CAD) software packages are available, some free and others requiring payment. Altium Designer, PADs, Allegro, and OrCAD, are the most commonly used PCB design software tools. For more simple designs and engineering students and hobbyists, free CAD tools such as Kicad and ExpressPCB. These tools are limited in handling more complex designs and have limited features in comparison.
Features and capabilities of PCB design software can vary quite a bit from tool to tool. An engineer or designer may often choose a particular tool because it has the necessary features other tools do not. The more basic PCB design tools provide much more basic functions and features. Software that companies must pay for offers more features and abilities, such as complex routing with design rules specified and routing signal simulation.
As PCBs become increasingly more complex and often on a much smaller footprint, paid-for software packages are often preferred, if not required.
Schematic Capture
One of the first steps in electronic product development is the creation of a design specification. This document describes the board’s requirements and identifies critical components, signal speeds, differential pairs, etc. Once the specification is complete, schematic capture can begin. PCB schematics used to be drawn by hand and sometimes still are, but most schematics are drawn or “captured” using CAD software.
A PCB schematic is a two-dimensional drawing that shows which components connect to other components and provides instructions on how to layout the traces on the board. Using CAD software, schematic symbols are created by the engineer for each component, which is then linked to a PCB footprint or land pattern. The footprints are created by the PCB Designer and define each component’s physical dimensions, pin locations, and if it is SMT or thru-hole.
Component Placement
PCB design is taking the two-dimensional schematics and creating a three-dimensional rendering. Once the schematic is complete and mechanical specifications such as board size and shape, constraints, and drill information have been set, component placement is the first step in the PCB design process. PCB footprints must have all of the required information input before they can be placed in the design; they must have all of the relevant information defined.
Before delving too deep into component placement, it is essential to roughly place components, often in related groups, to get an idea of where they will be located and whether or not they will all fit on the board. It is essential to consider how different components will communicate and the speed with which signals need to maintain. A rough placement exercise can also help determine how many layers will be required to obtain a stackup. Next comes critical routing, meaning traces with very tight constraints that the PCB designer will not move later. Once the critical routes are locked down, a more general placement is done. The final placement must be thoroughly reviewed because placement changes after routing has begun can result in having to rip up and redo work.
Routing
Routing is connecting the components with traces as directed by the schematic. For complex designs with many constraints and requirements, some software tools allow the designer to input a set of design rules. If a rule is broken, it will be flagged during a design rule check. Then a netlist, a text-based file, is generated from the schematic. The netlist contains information such as reference designators and pin numbers. It also shows which components need to connect to other components.
The first step of routing is to lay down the critical routes. These are routes where signals must travel at a particular speed, make the connection within a required time constraint, or differential pairs. These will be locked down when complete. Critical routes are completed first, where the size and length of the trace are crucial. The rest of the routes are then laid down, usually in the order of difficulty or complexity. These traces often move up and down through layers through different types of vias. The PCB designer must do a final detailed review, and all DRC violations are either fixed or cleared.
Manufacturing Outputs
Once the PCB design process is complete and approved, the data for manufacturing are generated. The Gerber files are images used to show the different layers and will be used with a photo plotter for fabrication. Other files needed for fabrication are silkscreen, soldermask, and NC drill and routing.
For assembly, a whole other set of files is used to program the various machines involved in the process. These include a bill of materials (BOM), so the components can be sourced and purchased, a pick-and-place file used to program the pick-and-place machine, and the netlist for functional testing and inspection.
911EDA offers PCB design services using Altium, PADs, Allegro, and OrCAD.
