Ground and Power Planes

Design Rules:
1: Stack power planes next to ground planes so they capacitively couple. Coupling the power plane to the ground plane reduces the number of by-pass capacitors required in the circuit. However it would seem that most engineers never reduce the amount of by-pass capacitors used in a design; but instead, still gain the benefit of an overall reduction of a uniform drop in impedance across the board introduced by the layer stackup.
2: When possible use wide traces for power and ground. Thicker traces reduce the trace impedance and allow the line to carry higher current loads.
3: Avoid placing slots in ground planes. Slots in ground planes force the current to flow around the slot, separating the return current from it signal trace. Slots may be placed at the edges of the board where signal trace aren't run.
4:It is highly recommended that high speed signals do not cross reference planes. Whenever a signal has to cross reference plane, it is recommended that a low value bypass capacitor between the planes be used as close as possible for decoupling.
5: Avoid Ground Loops when possible, and avoid ground loops in low-level, low-frequency circuits. A ground loop is an unwanted current loop. The current flowing in the loop generates additional noise and provides no benefit to the circuits operation. Usually a secondary current return path is unintended and in many cases hard to find. So the effort in proper design needs to occur before the circuit is fabricated.
6: Board Stack-up; Adjacent Power and Ground planes form a capacitor. It is highly recommended to use the power planes as a low value, high speed, bypass capacitor. This can be accomplished by reducing the thickness of the core between adjacent power planes. So when determining the Printed Wiring Board [PWB] stack-up try to place a power plane next to a ground plane to form a 'board-wide' capacitor. Forget about the older text that talk about a power and ground plane grid. The grid approach was used when two layer boards were common. Except for high power low-speed industrial control applications, it's hard to come up with a valid reason, other than cost, to produce a PWB with only two layers.

There are three fundamental grounding techniques;
1. Floating Ground System: The ground plane is not returned to earth.
2. Single-Point Grounding System: A single physical point in the circuitry is designated as a ground reference point.
3. Multipoint Grounding System: The ground plane is taken to earth, effectively taking each point connected to the ground plane to earth.


Capacitor Info
{Cap Layout Info-Material-Temperature-Chip Sizes}

Design Considerations:
Chart of PWB External Etched Copper Current Capacity: current rating based on trace cross section in square mils, temperature rise and current in amperes.
Chart of PWB Internal Etched Copper Current Capacity: current rating based on trace cross section in square mils, temperature rise and current in amperes.
Notes on Chassis Grounding

Circuit Design Section / Design Pitfalls, or back to the Logic Design page.

Chassis Design Steps / Chassis Grounding Recommendations / Rack Grounding Recommendations

Of course board grounding is a different topic from chassis grounding and rack grounding.
And Ground planes on a printed circuit board is a different topic than board grounding.

Related pages on this site:
PWB Info Component Chip Sizes Capacitor IC By-Pass Design Information IC Package Type Definitions PWB Production Vendor Services

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Modified 1/23/12
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