Why won't Eagle route this airwire?

I’m using the autorouter and it is leaving just this one trace unrouted. The airwire connects two pins that are named GND and there is also a ground plane. As you can see, those pins are already connected to the GND plane so the airwire seems redundant.

Note: those pins are not connected to each other on the schematic but to GND.

It looks to me as though the groundplane on the bottom of the picture may be completely cut off from the ground plane at the top, in which case this airwire is the only thing connecting some portion of the board to GND. (Although that doesn't explain why the autorouter didn't just connect them with trace on the TOP layer...)

You probably got a "polygons may have fallen apart" message when autorouting. Try adding a groundplane on top of board too...

Yes, Ground plane on both layers is best. Add some vias and name them GND also to connect the layers in places where there are no pins to connect the layers.

There is a via between pad 8 and pad 3 - if you move that to the left of pad 1, then the bottom GND layer will connect pad 8 and pad 1. Cross under the vertical trace, go on the top layer and then back to the bottom layer.

I don't recall the autorouter ever using traces to connect GND pads up; maybe it does that if there are no GND planes?

CrossRoads: Yes, Ground plane on both layers is best. Add some vias and name them GND also to connect the layers in places where there are no pins to connect the layers.

There is a via between pad 8 and pad 3 - if you move that to the left of pad 1, then the bottom GND layer will connect pad 8 and pad 1. Cross under the vertical trace, go on the top layer and then back to the bottom layer.

I don't recall the autorouter ever using traces to connect GND pads up; maybe it does that if there are no GND planes?

Yeah, I had a similar problem 2 boards ago where I had a ground plane on one side, but had to run large traces between some sections of it to connect it all together. The last board I just hand routed since I don't like the Eagle auto-router (Or I just don't understand how to properly set it up). Diptrace has a surprisingly good one though. I do usually end up going through and rearranging wires afterwards, making them all one plane to reduce vias, and such.

I would suggest running a rather wide trace between the two pins, at the very least, to connect the ground plane sections. Or run it to two vias at the edges between them.

Thanks for the prompt replies. As soon as I added a second ground plane it all came together nicely.

Regarding two ground planes... I've read forum posts were people mentioned that putting two ground planes could turn the board into a big capacitor and provoke undesired effects if you were not "careful" (not sure what their definition of careful was). On the other hand, the argument against that was that since these were two GND planes, there is no voltage difference therefore they couldn't form a capacitor. And yet again, the argument against that was that in reality there might be a tiny difference in voltage so we're back to square one.

My only interest in PCBs are at the hobby level so should I be worrying about these kinds of nuances? Or do you think I'm better off slapping two ground planes in and forgetting about it?

I put two on every board, with vias connecting them. Vcc traces over the plane adds some capacitance for the power signal. Hard to get any capacitance when Gnd is connected to Gnd tho. Boards are made taking double sided copper and removing the bits around traces & pads to isolate them. Any that's left can be ground plane.

CrossRoads: I put two on every board, with vias connecting them. Vcc traces over the plane adds some capacitance for the power signal. Hard to get any capacitance when Gnd is connected to Gnd tho. Boards are made taking double sided copper and removing the bits around traces & pads to isolate them. Any that's left can be ground plane.

If you can fit it, it is almost always better. My last project didn't have them, but it also isn't switching anything at high frequencies or powers, and I ran out of space for traces to try and keep the board as small as possible.

CrossRoads:
I put two on every board, with vias connecting them. Vcc traces over the plane adds some capacitance for the power signal. Hard to get any capacitance when Gnd is connected to Gnd tho.

Isn’t the capacitance a good thing though? I thought it was better to have one plane VCC and the other GND for the benefit of that extra capacitance.

The planes get all cut up by traces and vias and pads and thru holes tho, so a VCC plane turns out not to be that useful unless you have a 4 layer board with dedicated power plane and dedicated ground plane, and leave the outer layers for signals. But then 4 layer boards are pricier. So ground on both outer layers and wider Vcc traces are all that is needed for Arduino and 8 MHz clk for SPI as the generally the fastest signal to worry about when the crystal is properly place next to the uC.