Designing and building your own board

Ground Pours and Trace Width

The sizing of trace lines is beyond the scope of this article, but a few general comments can be made. Regular signals can be routed anywhere between 8 and 10mils (10-thousandths of an inch). If you have lines that carry more than about 300mA, then you need to have wider traces. This board is essentially a low-current board, so I could use the default 8mil traces. Traces that carry significant current should be wider than signal traces. Table 1 gives some rough guidelines of how wide to make a trace for a designated current.

The ground pour step involves filling all available space with copper connected to ground, and it does two major things: It reduces noise on the board caused by high-frequency harmonics of the digital signals. Interrupts are often very sensitive to this kind of noise, so it is good to reduce it. Additionally, doing a copper pour reduces the number of lines that the autorouter has to route.

Generally, you do the pour first and then the autorouter knows to connect all the ground lines to the pour. Notice in Figure 3 that none of the ground lines have any traces connected to them. That is because the autorouter is actually connecting the ground pins to the ground pour. Figure 4 shows the layout with the ground pours visible.

Figure 3: Final trace routing (with ground pours removed for clarity).

Figure 4: Board with top and bottom ground pours.

A good tutorial on how to create ground pours in Eagle can be found on the BuildElectronicCircuits website [4].

DRC Checking

The next step is to run the design rule check (DRC), usually supplied by the manufacturer. A good ruleset for general use can be found at DFRobot [5]. You will generally find some issues that need to be corrected as well as some issues that don't matter – specifically, stop mask errors. These can be safely ignored, but you need to look at all the others.