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What is Ratsnest in Eagle PCB Layout?

Ratsnest is a tool or command in Eagle PCB layout that optimizes unrouted wires (or air wires) to the shortest path. It is also helpful in creating a ground plane through the Polygon tool.

Optimizing Unrouted Wires through RatsNest

Consider the following schematic:

Eagle sample schematic for Ratsnet Feature

Here we have multiple endpoints connected to ground. We convert this to an initial PCB layout:

Eagle PCB layout before ratsnest

As these components are manually placed, some of the air wires take longer paths. Clicking the ratsnest button (shown below) will optimize the air wires:

Ratsnest button

Eagle PCB after ratsnest

The air wires here are now easier to trace. To save time in converting the air wires to copper paths, we will be using auto-router:

Eagle PCB after auto-routing

This board will work as intended but in production, a lot of copper from the PCB needs to be removed.

Creating Ground Plane

We can further optimize this board by using ratsnest again. First, create a top-layer polygon around the board:

Eagle PCB creating polygon

Note that you can also use the bottom layer for a double-sided PCB. Once the path is closed, Eagle will ask for the name of the signal where the polygon is to be connected. We will connect it to GND:

Connect polygon to GND

After that, the polygon is converted into dashed lines:

Polygon now dashed lines

Click ratsnest and all GND copper wires will be connected in a single plane:

Eagle PCB with ground plane

We now have a ground plane where all endpoints to GND are connected. Here’s a closer look at the ends of the resistors:

Zoom in on plane connection

The same goes for other pins that are connected to GND:

Zoom in on plane connection 2

Aside from aesthetics and cost-effectiveness, creating ground planes also help reduce trace resistance. This is an unwanted resistance introduced to the circuit and is dependent on copper trace dimensions. In our example,  the ground plane reduces the distance between pins and GND and thus reduces trace resistance.

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