I’d like to share this trick. If you’re new to Eagle, placing SMD parts on your bottom copper layer seems to be a mystery.But it’s real easy and I’ll show you.
In this example, we want to put C6 and C6 (both 0.1uf capacitor) to the bottom of the PCB. As you can see, the solder pads on C5 and C6 are colored “red”, which means they’re still on the top (default) layer.
To move these capacitors to the bottom copper layer, the trick is to use the MIRROR command. So from your Eagle toolbar, select the MIRROR command.
After selecting the MIRROR tool, click on both C5 and C6 (make sure your top origin tOrigins layer is turned on). Your screen should now look like this. Notice on our C5 and C6 capacitors, the solder pads are now colored “blue”, meaning they’re now on the bottom copper layer.
Continue routing your parts, but this time make sure to select the bottom layer when routing traces for your bottom mounted SMD parts. You may have to adjust the placement and position of component references and values after you used the MIRROR command.