Top PCB Design Myths
1. A Complete Connection Is Just from Pin to Pin. Many PCB designers simply route their signal traces from pin to pin, and don’t worry about any timing issues except matching the traces to the delay. However, designing this way neglects one important fact: Current doesn’t just flow one way. It flows in a loop. And that doesn’t mean simply flowing back the way it came, either. The return current takes the path of least resistance, or, at high speeds, the path of least inductance, charging the local parasitic capacitance as it makes its way back through. Because of this, the ground plane can’t be used to dump unwanted signals.
2. Trace Widths Can’t Be Smaller Than 3 Mils. In the past, it was true that using trace widths smaller than three thousandths of an inch caused problems for anything more than a very short span. However, new developments in semi-additive manufacturing now allow trace widths to be much smaller without causing problems.
3. “Ground” Is a Region of Zero Potential. At regular DC voltages, ground has zero resistance and zero impedance at all frequencies. However, when it comes to high speed PCB designs, the term “ground” is no longer a viable concept. Any metal has some amount of resistance. It may be as near to zero as (seemingly) makes no difference, but as we covered in the first point, that current forms a loop, which then causes inductance. Once you get up to those higher speeds, these seemingly insignificant figures are no longer insignificant, and finding a place with completely zero potential becomes virtually impossible.
Credits: PCB Designers - Facebook Page
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