Can we apply more voltage to a PCB heated bed to get faster heating? From my understanding, the power of heater must higher than heat dissipate to ambient air so the bed can heat up. The reason why a bed heats up too slowly is due to its heat capacity compared to heater power.
As the heater is a resistive load, I think we can put higher voltage to get more heating power.
The PCB heater has two parts: copper and laminate. The reason of failure is that the copper can come off the board due to high temperatures. In this case we can control temperature with firmware. The questions are:

Will this method work?
What can go wrong or what is the risk of this method?
Yes, this method will work. Some (LED) power supplies have a small potentiometer that you can use to slightly adjust the voltage. On a 12 V printer, adjusting the supply from 12 V to 14 V will give a 36% increase in power.
Obviously there is a limit to how much you can increase the voltage. The voltage itself is unlikely to be the limiting factor, but there is a limit to how much heat the bed can dissipate.

The PCB heater have 2-part copper and laminate. the reason of failire is the copper go off the board due to high temperature. In this case we can control temperature with firmware.

This is true, but you have to consider that firmware is not infallible. If you increase the voltage (and thus wattage) of the bed a lot, you risk creating a dangerous situation if something fails. Ideally, you should choose the wattage of the bed such that it does not create a dangerous situation even if it is left on permanently by accident. In extreme cases (e.g. 220V to a nominally 12V heated bed) damage will be almost instantaneous before the firmware can intervene.
You also have to consider that increasing the voltage also increases the current. The wires need to be thick enough to handle the additional current. You also have to pay special attention to the MOSFET that is used to switch the bed; it also needs to be able to handle the current (and voltage, though this is usually less of a problem). Usually the integrated MOSFETs on 3D printer control boards can only handle about ~10 A which is what the bed might normally draw. Also pay attention to fuses and to screw terminal connections. They might not be able to handle the increased current either.