Hallo Simon,

single layer PCB will definitely NOT work reliably! What you need with todays fast microcontrollers is at least a double sided PCB, where one side makes a solid ground plane.

Connect decoupling capacitors with one terminal directly to Vcc, while other terminal runs to micro's ground pin via solid massplane. Only by this, unavoidable inductance of power supply decoupling is kept to a minimum. If, on the other hand, terminals of decoupling capacitors are connected to pins of micro via narrow copper traces or even thin wires (absolutely destreous!!), inductance of this arrangement is so high, that decoupling measure will show only a very little benefit. Keep in mind, that inductivity of solid ground plane can be up to about 100 times smaller than that of a single thin wire!

Also, copper traces, which connect chips with each other MUST run over a ground plane. This heavily minimizes inductiviy of traces, keeps loop area of currents and their return at minimum, introduces a shielding against electrostatic, magnetostatic and electrodynamic fields and minimizes emission of such fields. Solid ground plane also minimizes ground bounce, ground noise and improves noise margin, by guaranteeing extremely low impedance over a wide spectrum of frequencies.

If you don't use solid ground plane you will suffer from susceptibilities like you observed. Your designs will never pass any CE test, and connecting any cable to your PCB will result in a desaster.

So, please use at least double sided PCB with solid groundplane. There are many people even stating, that with modern microcontrollers use of 4 layer PCB is inevitable.

Kai