Power Distribution – To Route, or to Plane

Power distribution on a PCB can come in a number of forms. The three most common methods are:

  • Route power and ground.
  • Use surface layer floods.
  • Use internal planes.

After component positioning, you’ll need to look at power and ground distribution. With a two-layer board, your options are limited to individually routing power and ground, or using a polygon fill, also called a flood or pour.

 

 

 

 

For simple low-speed layouts, it’s common to route power just like any other signal. You’ll typically use a wider trace, which you can set manually, or with design rules. Drawing a polygon in the board shape, and giving it the same name as your power or ground signals may make the job easier. Keep in mind though, that you can end up with parts of a ground plane disconnected from the rest of the board. This is called an orphan. Some CAD error checks will spot such a problem and some won’t.

I made that mistake not long ago, as describe in this blog post.

If you have a four (or more) layer board, common practice is to designate one of the internal layers for ground, and one for power.

 

 

 

 

 

 

Doing so can leave more room for signal routing, can reduce EMI, and can leave a cleaner-looking, easier-to-debug board. It also reduces the chances of having orphan ground or power areas, as I warned against in the prior post.

Duane Benson
Chocolate layer cake with coconut frosting will not help with EMI

http://blog.screamingcircuits.com/

Pads on Ground Plane

Generally, small pads for passive parts are connected  with a single PCB trace of equal size to each pad. That’s the right way to do it.

Top pads solidly connected to copper pour

However, sometimes, circumstances dictate a little different approach. The illustration on the right shows something of a worst-case. This is for a snubber (resistor, capacitor pair) between two power planes.

A couple of things will likely happen. The power plane will act as a heat sink, preventing the solder paste on one side from melting, resulting in a poor connection. Or, the unequal melting could lead to surface tension pulling the part up, causing tombstoning.

Most designers are aware of that, but sometimes, thermals will be deliberately turned off to allow for better current capacity to and from the large power Mosfets (not shown).

Thermal pads on side connected to pour.

If that’s the case, make sure that you can turn the thermals (see figure at bottom right) on or off by the part, rather than just by the plane.

Duane Benson
The rain falls mostly on the ground plane due to static attraction

 

http://blog.screamingcircuits.com/

 

Pour Or Not — Just What Is My Opinion?

I posed a question about using copper pours (AKA flood) a not long ago. The premise was a simple microcontroller board with a 20MHz clock and no special requirements.

I had a couple of different comments on the post with some very good insight. Myself, I generally don’t use copper pours. My only reason is that I think it usually looks better without (although I do like the look of the cross-hatch pour on the Arduino). A well done flood can be pretty cool, but still my inclination is to only use it if it’s needed. If it’s a shop doing the PCB, the metal will be recovered and recycled, so the conservationist in me is pleased.

If it’s a home-etched deal, then a pour is probably a better idea because it will reduce the amount of etchant needed. Although you do need to be careful to keep plenty of space between things to prevent solder bridges. Solder bridging isn’t such a big deal on a PCB with a good solder mask, but it certainly is on a board with no mask or thin mask.

If there is a good reason, I will. Like a high-current motor driver — I use the pour to keep the current capacity up and the kelvons mellow. Heat-sinking is a good reason for a pour. High-speed stuff usually benefits from a flooded plane of some sort too and in four-layer boards, using the innerplanes for power and or ground is nice and convenient. But you all know that. I’m just rambling now.

Duane Benson
Does high speed stuff on a flooded plane require a speed boat?
Will too much heat sink it?

http://blog.screamingcircuits.com/