PWR & GND planes

It is good practice to use "power planes" to distribute power across your board. Using power planes can drastically reduce the power wiring inductance and impedance to your components. This can be vital for high speed digital design for instance. It is good design practice to use power planes whenever possible. They can even be used on double sided boards, if most of your signal tracks are on the top layer.

A power plane is basically one solid copper layer of board dedicated to either your Ground or Power rails, or both. Power planes go in the middle layers of your board, usually on the layers closest to the outer surfaces. On a 4 layer board with complex power requirements it is common to dedicate one layer to your ground plane, and another layer to your various positive and negative power tracks. Your ground rail is usually your signal reference line, so a ground plane is first preference before a power plane is considered.

Many PCB packages have special Power Plane layers that are designed and laid out in reverse to your other normal tracking layers. On a normal tracking layer, your board is assumed to be blank, and you then lay down tracks which will become your actual copper tracks. On a power plane however, your board is assumed to be covered with copper. Laying down tracks on a power plane actually removes the copper. This concept can take some getting used to.

A simple power plane will not have any "tracks" (or removed copper bits) at all on it, but will just be one solid layer of copper. In which case you don’t need to lay down any tracks to remove any copper. However, it is common practice on more complex boards to "split" the power plane by laying down tracks. This may be done to separate an analog and a digital ground, which will reduce the amount of digital ground noise which is coupled into the more sensitive analog circuitry. A typical split power plane would involve a "track" being placed from near your input power connector or main filter capacitors and the opposite edge of the board. Be careful not to accidentally cause a power "loop" on your board by inadvertently connecting the two halves of your plane on the other side of the board.

As a matter of course, you should place "tracks" completely around the outer edge of your board. This will ensure that the power planes do not extend right to the edge of the board. Power planes on the edges of your board can short to not only one another, but also to any guide rails or mounting hardware. You don’t have to use the actual Power Plane layer on your PCB package if you don’t want to. You can use a regular signal layer and lay down copper fills and tracks yourself. Power Planes layers though often have some advantages that will vary from one PCB package another.