multiple planes (fills) in eagle

Question

I have some connections in my single-sided PCB which could benefit from a plane especially all connections to power and all connections to ground. I can easily create a polygon for only one plane (for example to intelligently flood the remaining bare spots (areas with no copper) of the PCB with ground), however I want to also be able to fill applicable bare spots with power (vcc). I tried creating a polygon named gnd then on top I tried creating a polygon named vcc and after running rats nest, it seems that the gnd and vcc are shorting each other. If I play around with the rank values, then either gnd or vcc will fill the bare spots, but not both.

Is there a way I can have both a gnd and vcc plane without resorting to non-overlapping of plane creating and without making each plane use only a small portion of the board?

Answer 1

I have some connections ... which could benefit from a plane ... especially all connections to power

Quite unlikely. Ground planes are certainly useful, but most instances of power planes are due religious convictions rather than physics. What makes you think you need a power plane? In some cases of high current, it may be a suitable way to deliver that current without much of a voltage drop, but then you still have to consider the high current connection points to the plane.

Most likely you don't need a power plane at all. Instead make sure that each use of power is well bypassed to the ground plane. That gives you low impedance at high frequencies. Fat enough traces give you low resistance at DC.

However, to answer your question, the RANK setting in Eagle is exactly for the purpose you seem to be describing. With the right relative rank settings, the power polygon will dominate in its small region within the larger ground plane.

From the Eagle help page for the POLYGON command:

Defines how polygons are subtracted from each other. Polygons with a lower 'rank' appear "first" and thus get subtracted from polygons with a higher 'rank'. Valid ranks are 1..6. Polygons with the same rank are checked against each other by the Design Rule Check. The rank parameter only has a meaning for polygons in signal layers (1..16) drawn in a board and will be ignored for any other polygons. The default is 1.

This really is quite clear.

Answer 2

You can't just draw them both as a box and have them fill as required because Eagle won't know which to put where. If you give them both the same rank it will simply fill both in the same area and hence you get shorting.

Once you give one a different rank, the one with higher rank (a.k.a. lower number) will take priority wherever they overlap. In other words if you draw two boxes over the top of each other, the lower ranked plane will simply cut out around it.

What you need to do is draw the higher ranked plane to roughly the required shape so that it doesn't overlap and hence cut away the lower ranked plane(s) in areas where you want the lower ranked plane(s) to fill.

Answer 3

You can use vias to add 'disappearing' planes.

Just remember to NAMe the planes and vias the same.