While growing up I watched my dad create all kinds of different projects around the house. Backyard play structures, birdhouses, bookshelves, desks, and even a complex layout of railroad tracks for a model train set. Each of these projects started first as a sketch on gridded paper in order to account for all the materials and dimensions that would be needed. It was my first exposure to a schematic, and the example of my dad’s meticulous attention to detail served me well when I started my own career in electronics.
Each circuit board that you see being produced by a PCB contract manufacturer first started its life as a schematic. Someone placed the symbols on the schematic and connected nets to the symbol pins in order to create the design data that would drive the layout of the PCB. Let’s take a closer look at what PCB schematic capture is, and some of the do’s and don’ts that you need to be aware of as you design your printed circuit boards.
Symbols and Components, and Why They are Important
Each physical component on a circuit board is represented by a symbol or combination of symbols on the schematic. Symbols can have specific shapes, such as the jagged lines of a resistor so that they are easily identifiable to anyone reading the schematic. These shapes are specified by industry standards such as IEEE 315-1975, and IEC 60617 so that there is a commonality between schematics. Larger symbols, however, such as 600 pin ball-grid array devices, are usually depicted with a simple square or a rectangle shape.
The symbol represents the logic of the circuitry for design engineers, but in order for the CAD system to correctly interpret that logic, the symbol will also include the following information:
- Reference designator: This is an alpha-numeric designation to identify each unique part on the schematic. Resistors are usually designated with an “R,” capacitors with a “C,” connectors with a “J” or a “P,” and ICs with a “U.” The trailing number distinguishes one part from another, as in the case of R200 and R201.
- Values: Depending on the component, it will have different values. For example, there may be a 150K ohm resistor or a 22pF capacitor in the schematic.
- Tolerances: Some parts, such as a 22pF capacitor, may be rated at 25 volts DC with a 5% tolerance.
- PCB footprint: As each symbol represents a physical component on the circuit board, the symbol must also contain the name of the CAD footprint model associated with it for the PCB layout. These footprints will either be thru-hole or surface mount parts.
- SPICE model: Although not a requirement for circuit board design, an associated SPICE model is needed if the schematic is going through automated circuit simulation.
Another critical part of the schematic symbol is the pin. Each physical pin on the component needs to be represented by a logical pin in the symbol. This symbol pin is either named or numbered, and it is the connection point for the nets, or electrical connectivity, that will make up the circuitry of the PCB. In the resistor picture below you can see where pins 1 and 2 are displayed. There may also be additional properties attached to the pin that will control whether or not they can be swapped with other pins that are functionally equivalent, or if they are purposed for specific nets such as power or ground.
Once the schematic symbols are created, checked, and stored in the appropriate CAD libraries, the design is ready for schematic capture.
A resistor symbol showing the reference designator and other properties
Defining Electronic Circuitry in PCB Schematic Capture
Schematic capture is the process of designing electronic circuitry in a PCB design CAD system. As the CAD system will allow you to copy, cut, paste, or delete circuitry, design engineers often do many iterations of their work before the design is complete. There is however a basic work-flow for schematic capture which looks like this:
- Component placement: The designer will start by placing the symbols on the schematic. The design tools will give the user the ability to rotate and maneuver the symbols where desired to achieve the most optimum placement. The goal here is to keep the symbols neat and organized with enough space between them to be easily readable without wasting space that could be used for other parts.
- Drawing nets: Here is where the designer will connect the different component pins together in the schematic so that the nets can be routed as traces in the PCB layout. Usually, this is a manual drafting process of drawing each line, or net, from one pin to another. The designer is aided in this task with definable grids in the CAD system plus many other automated features. In some cases, the CAD system will allow the user to combine multiple nets into a single net group so that they are drawing one line instead of 8, 16, or more. Again, readability is the key, and the CAD system will usually add visible net names such as VCC or GND to clarify which net is which.
- Documentation information: In addition to being an electronic database, the schematic is also a document that will be used for manufacturing, test, and fieldwork. As such it needs to have all of the necessary corporate, legal, and tracking information on it to fulfill your company’s requirements. This can include board and project names, part and revision numbers, sheet titles and numbers, and company contact information and logos.
- Design rule checks: Most PCB design CAD systems have automated checking capabilities that can be set up and run for the schematic. The importance of using these DRCs can not be understated as they can save you from a lot of unexpected difficulties. For example, they will look for dangling nets, unconnected pins, and other potential problems that could create serious design errors.
Once the schematic process has been completed, the next step is PCB layout to design the physical board that will be manufactured.
Fully Prepared for Layout
In order for the circuit board layout to go smoothly, it is imperative to start with a clean schematic. To accomplish that however, requires that each step in the schematic capture process be completed correctly. This level of detail can add a lot of work to the design schedule though, and engineers are often looking for help to complete their schematic and have it ready to go for PCB layout.
Fortunately, there is a resource already available that can help designers who are in this predicament. Your local PCB contract manufacturer can not only lay out your circuit board design, but they can help with the schematic capture part of the job as well. At VSE our engineering team has years of experience with both circuit board design, and they are ready to help you with both your schematic capture and PCB layout.