If you want to use a 220 VAC % Watt heater pad I would suggest, as mentioned, using a SSR (Solid State Relay) and use your uC for control. One potential problem is the SSR leakage current. Your 5 Watt 220 VAC heating pad only draws 5 Watts / 220 Volts = 22.7 mA. Not very much current at all. Should you choose a SSR solution I suggest you look at the data sheets in detail and understand leakage current.
Next you will need some sort of sensor placed at the point where you want temperature control to happen. Have you given any thought to a sensor? You may wish to consider a DS18B20 for simplicity and easy compatibility with a uC like an Arduino.
Finally the actual control which can be as simple as ON/Off or more complex like using PID (Proportional Integral Derivative). The latter will afford much tighter temperature control but requires working with it a little. A Google of Arduino Temperature Control should get you going and a Google of Arduino PID Control should get you some examples of PID control loops.
Also as mentioned there are low DC voltage heater mats available and easily controlled with a simple MOSFET circuit. I have seen 12 VDC mats for keeping coffee cups warm in automotive applications in addition to those linked to. A simple Google of heater mats should find you plenty.
A heater mat really doesn't care if you apply AC or DC as long as it is just a heater element.
So now you have some decisions to make.