The Printed Circuit Board Primer

Test Of The Routed Circuit

To ensure that the circuit works properly after the wires have been routed, it has to pass a final check. This check will also verify that no connections have been incorrectly routed, i.e. that all components have been connected according to the schematic.

Creating Manufacturing Files

Since there are numerous different CAD tools for designing PCBs, the manufacturer needs a standardized set of files as input to the machines that produces the boards. There are a couple of different standards, but the most common is Gerber files. A set of Gerber files includes photo plots for all signal, power and ground layers, photo plots for the solder mask and the silk screen, drill files, and pick-and-place files.

Electromagnetic Compatibility Issues

An electronic device that is designed without considering Electromagnetic Compatibility (EMC) is likely to radiate electromagnetic energy that can cause undesirable interference in nearby electronics. EMC is a design requirement that has maximum limits for Electromagnetic Interference (EMI), Electromagnetic Fields (EMF) and Radio Frequency Interference (RFI). This requirement ensures both the proper operation of the electronic device itself and other nearby devices. It compels the design to limit the radiative or conductive emission from one device to another and reduce the device's susceptibility to external sources of EMF, EMI or RFI. In other words the goal is to prevent stray electromagnetic energy from entering or leaving a device. This is a rather difficult issue to deal with. Common techniques are the use power and ground planes, and place the PCB inside a metal box. Power and Ground planes tend to shield emission to and from the signal layers, while a metal box also shields the components. We will not go more into detail about that here.

The maximum speed of a circuit depends on how well the EMC requirement is met. Internal EMI like leakage currents between conductors increases in magnitude when the frequency of the circuit increases. The distance between connectors must therefore be increased if there is a big potential difference across them. This also tells us that it is important to avoid high voltages and keep the power consumption of the circuit at a minimum. The latency in the wires is also crucial, so their length must be as short as possible. Hence, a small and well-routed PCB tends to be capable of running at higher speeds than a large PCB.

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