HOW-TO: Polygons and ground fills for PCBs in Eagle

In Eagle polygons are used to make big copper areas on a PCB that aren’t necessarily traces. We use them all the time to fill blank space on a PCB with copper connected to ground. Less frequently we use them to make large power traces such as with the ATX Breakout Board. Here’s some notes on how to use and customize polygons in Cadsoft Eagle.

How to use
Open Eagle and start a new schematic and PCB. Go to the board so we can lay down a polygon.

• Click ‘polygon’ icon  located in the toolbar
• In the top menu bar make sure the top (red) or bottom (blue) copper layer is selected

Route the polygon in the shape you want it. Make sure to close it at the starting point or you’ll have problems generating gerbers later.

A polygon with a dashed line will appear (picture 3).

Click the ‘Ratsnest’ icon , and the polygon will fill with a solid copper area.

This will now be a solid copper area on your final PCB. Next we’ll look at a few ways to customize it.

Polygon Name

Sometimes it’s useful to make the polygon part of an existing trace or electrical net. For example if you wanted to beef up a power trace.

To connect the polygon to the ‘net’ you want, right click on it’s edge, and select ‘Name’. Here make sure ‘This Polygon’ is selected, and type in the net name you want it to connect to. Hit ‘Ok’.

Polygon properties

Right click on the edge of a polygon and select properties. This opens the ”Polygon properties” menu. From here you can customize the polygon.

Width

This is the width of the polygon’s perimeter when you draw it on the board. It also affects some properties below.

Pour

Here you can select between two versions of fill for inside the polygon, solid or hatched.

Hatched line widths are adjusted in the ”width” property. Line spacing is adjusted in the ”Mesh Distances” property. We only use the solid fill option in our projects.

Isolate

This is the clearance between the polygon and neighboring objects, such as other traces, pins, and other polygons.

Don’t torture your board house here. Low isolation values lead to PCBs with electrical faults from under-etching. Leave as much room as you can.

Ranks

Rank determines if one polygons if above or below an overlapping polygon.

Assigning a lower rank to one polygon will make that polygon dominate the other as seen in the pic above. When using a ground fill over the entire board it is important for it to have the highest rank so other polygons can be drawn.

Thermals

With thermals on, a pin will connect to the polygon through small traces extending from the pin center in 4 directions. Thermals make soldering easier, the part heats faster because the heat is not dissipated as quickly.

Disable thermals will make a solid pour trough the pin/pad. We usually use thermals. We only turn it off on high power traces that require more conductivity between the pad and the polygon.

Uses

Ground planes

Ground planes fill-up the empty spaces of a PCB with copper connected to ground. The ground plane connects all the ground pins on a PCB automatically, which usually makes routing easier. It can also reduce electrical noise on the board.

Filling the board with a ground plane is the first thing we do when  designing a PCB.

1. Draw a polygon
2. Give it the same name as the ground connections on your schematic, usually ”GND”
3. Click the ratnest button to refresh the ratnest after adding parts or traces

Any parts placed in the ground plane with pins names GND will automatically connect.

Power planes

Power planes are no different than ground planes, they just carry a power supply instead of ground. We use these for fat power traces where the board will carry a lot of current.

Power distribution for multiple voltage powered chip

Multiple supply polygons, good for power distribution especially on CPLD and FPGA chips.

Although a normal traces can be used here, concentric C shaped polygons  provide both a power supply access, and give you more freedom with it’s shape.