content: Honestly just make sure you solder it cleanly. A good contact patch without any excess solder on it is more important than just about any other connection on your device. I haven't even looked to see if your board design has an oscillator, some don't as the ATmega can run on a less precise internal clock.  Every bit of copper you lay out has resistance, inductance, and capacitance. Basic physics rules apply. While your 20mhz oscillator has a 10 metre wavelength and this is of only minor concern in your design; a cpu operating at 4 ghz has a wavelength of 7.5 cm, much smaller than the width of a typical motherboard.   Remember that external electronic interference, depending upon operating environment, can also be a concern. A nice groundplane helps greatly: enclosing your board in a grounded conductive enclosure is a good approach.      23c