# FEDEVELPlatform forum

## 20H Rules

FurkanDursun , 05-20-2020, 02:35 PM
Hello Everyone,

When I watched the second lesson of the Advanced PCB Layout Course, I saw 20H rules. But I did not understand actually. Therefore I would like to ask a question.

I know H is the distance between the ground plane and power plane. But What is the exact value of H? For example 0.1 mm or etc. And Is 20H rules useful?
robertferanec , 05-21-2020, 11:15 AM
You would find H from your stackup (you need to know the exact dimensions in your stackup). If it is useful - I am not 100% sure ... on internet you can read anything. But I always "hide" power planes a little bit under GND planes, I think it can help (my opinion). PS: I like to do it also because sometimes the edge of PCB can be damaged and I like to have PWR planes deeper, so there is no chance to have short circuit between gnd plane and power plane on the edge of PCB.
chitransh92 , 05-22-2020, 01:47 PM
Hello,
20H rule was discussed in book "Printed Circuit Board Design Techniques for EMC compliance" and this explains that for the Power and Ground Plane, if the Power plane is pulled back 20H times than the Ground Plane edge, this would help in reducing EM radiation. Where H is the distance between the plane.

For example:
if the distance between the Power and Ground Plane is 10 mils then 20H = 20*10 = 200 mils.
Hence Power Plane edges should be pulled back 200 mils on all sides compared to Ground Plane edges.

This being said, There is no experimental proof of this and experts say that this does affect the EMC performance of the design.

I would agree to @robertferanec , that both the power and ground planes should be pulled back from board edge as this could lead to other issues not particularly EMC related.

You can use 20H or not but it will not affect the EMC performance.

Thanks.