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If it is important, how crucial is it that the polygon completely overlaps the ground plane beneath it? A power polygon can hang off the side a of a power plane. Complete overlap isn't crucia...
Answer
#2: Post edited
by
Nick Alexeev
·
2025-02-02T20:19:09Z (1 day ago)
- > If it is important, how crucial is it that the polygon completely overlaps the ground plane beneath it?
- A power polygon can hang off the side a of a power plane. Complete overlap isn't crucial.
- > [...] how significant is it that the ground plane remains continuous and is not split by traces?
- Keep the ground plane as intact as possible. Splitting ground plane by traces leads to increased parasitic inductance of all the signals which have to run across the split. Don't "Swiss cheese" the ground plane and run signals over voids. It's more importnat for signals than for power.
- > For power polygon that primarily carry DC, how critical is it that it is located above a ground plane?
- Why is this even a question? : )
- How many layers are you planning to have? These days there are too few reasons to do a 2-layer board instead of 4-layer. With a 4-layer board you'll have ground plane everywhere.
When a polygon overlaps the power plane on the adjacent layer, you get the benefit or power plane capacitance. The capacitance value is modest on the order of hundreds of pF per square inch. But the value of power plane capacitance is that it comes with very low equivalent series inductance (ESL).- As a general rule, the lower the speeds (rise times) of your signals, the less important power plane capacitance becomes for power integrity.
- As a general [philosophical] rule "Treat power as signal."
- > If it is important, how crucial is it that the polygon completely overlaps the ground plane beneath it?
- A power polygon can hang off the side a of a power plane. Complete overlap isn't crucial.
- > [...] how significant is it that the ground plane remains continuous and is not split by traces?
- Keep the ground plane as intact as possible. Splitting ground plane by traces leads to increased parasitic inductance of all the signals which have to run across the split. Don't "Swiss cheese" the ground plane and run signals over voids. It's more importnat for signals than for power.
- > For power polygon that primarily carry DC, how critical is it that it is located above a ground plane?
- Why is this even a question? : )
- How many layers are you planning to have? These days there are too few reasons to do a 2-layer board instead of 4-layer. With a 4-layer board you'll have ground plane everywhere.
- When a power polygon overlaps the ground plane on the adjacent layer, you get the benefit or power plane capacitance. The capacitance value is small on the order of hundreds of pF per square inch. But the benefit of power plane capacitance is that it comes with very low equivalent series inductance (ESL).
- As a general rule, the lower the speeds (rise times) of your signals, the less important power plane capacitance becomes for power integrity.
- As a general [philosophical] rule "Treat power as signal."
#1: Initial revision
by
Nick Alexeev
·
2025-02-02T18:38:58Z (1 day ago)
> If it is important, how crucial is it that the polygon completely overlaps the ground plane beneath it? A power polygon can hang off the side a of a power plane. Complete overlap isn't crucial. > [...] how significant is it that the ground plane remains continuous and is not split by traces? Keep the ground plane as intact as possible. Splitting ground plane by traces leads to increased parasitic inductance of all the signals which have to run across the split. Don't "Swiss cheese" the ground plane and run signals over voids. It's more importnat for signals than for power. > For power polygon that primarily carry DC, how critical is it that it is located above a ground plane? Why is this even a question? : ) How many layers are you planning to have? These days there are too few reasons to do a 2-layer board instead of 4-layer. With a 4-layer board you'll have ground plane everywhere. When a polygon overlaps the power plane on the adjacent layer, you get the benefit or power plane capacitance. The capacitance value is modest on the order of hundreds of pF per square inch. But the value of power plane capacitance is that it comes with very low equivalent series inductance (ESL). As a general rule, the lower the speeds (rise times) of your signals, the less important power plane capacitance becomes for power integrity. As a general [philosophical] rule "Treat power as signal."