The table 8.1 in UL840 doesn't consider clearances for voltages less than 50V (for example). I have to use a clearance of 0.2 mm on the PCB for the case of overvoltage category 1 and pollution degree 2. This is a problem indeed, since the distance between the microcontroller pins is only 0.175 mm. The standard doesn't say I am allowed to extrapolate. Does anyone know how to deal with this problem of UL 840 not considering voltages less then 50V? P.S.: Coating could be a solution but I don't want to use it since it's expensive compared to the price of the electronic.
I believe you are misapplying this table.
Instances where clearance (and creepage) is a concern are where the voltage/power source can be a shock or fire danger to the user. If your 5V is created by an internal regulator, then you don't need to worry about clearances to other traces powered by the same 5V regulator. Trace spacing at these voltages is more a function of PCB technology. That's why your microcontroller spacing is less than 0.2mm; it's more than enough.
Tables like you're dealing with are commonly (but not exclusively) applied to instances where you're dealing with mains voltage, where some insulation is required to protect the user from transients and the resulting stress on insulation. Certainly you can have a hazardous 5V rail if it's capable of overvoltage transients, but those are less common.
A final disclaimer: I don't know the full details of your design, and can not be responsible for the safety of your product. The responsibility falls first to you as the designer to determine whether your 5V is hazardous to the user.