Some notes on building the board:
I used 0805 passive parts. They aren't the smallest but they are much easier to place by hand than 0603.
The MCU and SI1132 present a challenge to me as I don't want to do a solder stencil at the moment and the pads are way to small for placing solder paste by hand. You could just lay a line of paste down along the pads for the ATMEGA168 but I don't like cleaning up the resulting solder bridges.
What I do instead is:
- flux the pads (I use a ChemTool flux pen)
- wipe the pads with solder (small chisel soldering tip)
- flux the pads again
- place the parts (3x reading glasses + 10x loupe)
Then reflow solder as usual with the rest of the parts on the board. This leaves a little darkened solder flux residue on the board but has been 100% reliable so far.
Good job getting it to work. I was trying in the same thing with a Si1146, but somehow i cannot get it to work. I can read some registers properly, while some not. Readout is alwas zero.