PCB Design


The PCB design process employs ECAD systems and PCB software packages that enable the design and testing of the PCB schematics on both the physical and logical level. and also incorporate additional facilities in to the design, such as, simulations, complex routing and many more. The schematics for both aspects are generated and stored in digital format with frequent backups.

Capturing of circuit schematic

After the circuit schematic is developed, it has to be captured by a schematic capture tool. The capturing can be done by the PCB design software suit or by a separate software tool whose output can be exported in a suitable format. Capturing of schematic implies that the design of the circuit is contained in a file which can be converted into a netlist file. A netlist file ( an ASCII file) is essentially the interconnectivity information of a circuit made of its components. The components of a PCB are generally gates, so the netlist is a connection of gates.

Simulation of the circuit is also performed at this stage to learn about the design viability before going into advanced stages.

Component placement

The next important step is to gain the rough idea of whether there is sufficient space on the board required for all the components and how they will be placed on the board. This will help to decide about the number of layers required to contain the circuit within the board.

Once the rough estimate is on hand it will be easy to make the detailed component layout on the basis of proximity between the devices which need to communicate with each other, and other information, pertaining to RF considerations , for instance.

Other relevant information at this stage, required for the PCB design include footprints for the PCB pads, drilling information, keep out areas, etc. Typically several devices may share the same footprint.

Linking of the components

After the components are placed, the next step would be to establish interlayer tracks between the components. This is done by the PCB software which makes physical paths between the components separated across the layers using the interlayer 'via holes,' in accord with the netlist. Often one layer will be allocated as a ground plane and the other as a power plane. This helps to reduce noise level and enables low source resistance connections to be made for the power.

This is a time consuming process to establish numerous conducting paths between the components and therefore takes a lot of computing power. If it's a complex design but the board size is small then it might take considerable amount of time.

Gerber files and PCB design considerations

A Gerber file is an image of how one layer of the circuit design looks. The circuit board consists of copper traces to connect the components together , silk-screen to show component values, etc. and solder mask to prevent solder bridges between the traces. Once the final Gerber data file is available, it is fed into a laser photo plotter to convert it into pixel image. A laser plotter writes this onto a film. The exposed film is automatically developed and unloaded for the operator. The film is now ready for the PCB fabrication process.