Layout for this board was really hard since I don't have a professional version of Eagle. So I was confined to 2 layers.

With some scrupulous care, I managed to layout the back of the board in a way that needed minimum vias. This meant that the top of the board could focus on the 78 APA102 LED chips. And the bottom of the board could focus on the SAMD21 (Arduino compatible circuitry.)

Getting this whole thing to route was a real pain in the ass. However, I started to learn how the Eagle route optimizer works. I had never realized it before, but Eagle basically sets up several different routing jobs and runs them in parallel trying to find the one that gives the best result. Each of these jobs typically just varies the direction of the vias and the routing grid. 

I knew from hours of empirical tests trying to get this board to route that a routing grid between 0.1 and 0.15 was necessary to make the APA102s happy. Therefore, I set up the router manually to vary just that setting:

This acheived some reasonable success, however, there were still some glitches where the router kept painting itself in the wall, unable to "untangle" itself from vias from the back to the front. Eagle has a strange quirk in the way it handles routing. It first tries to just get any 100% net route that it can. And then it optimizes trying to cut out routes or wigging lines to avoid vias and long traces. The problem is that once Eagle "lays down" a via during the initial pass, its is reluctant to get rid of them. However, I found that increasing the router cost for a via was quite successful at prevent this from happening, forcing Eagle to get the routing "right" up front:

Doubling the cost from 8 to 15 did the trick.

Now I got a fully routed board!

Now it was off to OSH park to get this to the fab!

A few days later, I held in my hand something very cool:

Call me a softie, but there is something really cool about hold this little object seeing all of the places for the leds!