PCB Design Rules

PCB

Some basic design rules are provided by Analog Devices, in their book “Linear Circuit Design Handbook“, which is free to download.

Regarding boards that have to handle large voltages and currents, but relatively low frequencies (< 500 kHz), the basic rules are the following:

TRACES

  • Width of a trace depends on the estimated current that will pass through. When current is passing through a resistor – and copper traces have resistance, it produces thermal heat. By defining the maximum current and desired temperature increase, there are plenty of calculators – either online [1] [2] [3] or in PCB Design Software [1] – that can determine the minimum trace width. Other factors, that play a role in this calculation, are the estimated length of the trace and the thickness of the copper – typically 0.035 and 0.07 mm.
  • Clearance between two traces depends on their maximum voltage differential. There are many protocols to follow and many online calculators [1] [2] [3] [4] that can give a rough and quick estimation.

    Figure 1: Clearance and Creepage

    • Creepage Distance, is the shortest path between two traces, measured along the surface of the insulation
    • Clearance Distance, is the shortest path between two traces, measured along through the air

Both of them can be seen in Figure 1.

  • It is necessary to separate analog and digital traces and that includes both VCC (Bias) and GND (Ground). The reason for this can be easily explained in Figure 2. Pulses from digital signals have higher order harmonics, which can cause unwanted voltages across the parasitic induction the traces. This can result to additional noise at analog circuits inputs and to fluctuations of the power supply.

Figure 2: How to Properly Design Digital and Analog Traces, Source

  • Loop minimization is another important issue. Every object (or living thing) is under constant electromagnetic radiation, the source of which can be the local power grid –

    Figure 3: Trace Loops, Source

    seeing 50 Hz noise is a common issue when the ground is not properly connected, SMPS (Switched-Mode Power Supplies), or any other device which radiates in e/m spectrum. As displayed in Figure 3, the large loop that is created between the positive and the negative terminals can lead to noise generation that will be expressed in the load. This may be important, especially if the signal is driven to a μC and it’s part of the control. Oscillations in the measured signal can lead to non-optimum control behavior, or even to instability.

  • The length of the trace must be as short as possible. This reduces the resistance and the inductance, hence providing greater efficiency and better behavior to rapid changes of the current.
  • Control and power traces must not be designed in parallel since it is possible for the latter to interfere with the first. So, control and power traces must have a respected distance or designed on opposite sides of the PCB and vertical to each other.

FOOTPRINTS

coming soon…