What is it
What to take note of
Where to get it
The printed circuit board or the PCB for short, is the foundation of the keyboard. It communicates the signals from the switches to the computer.
Image via Keyboard treehouse
Image via Keyboard treehouse
The size and shape of the PCB will determine the shape and kind of case of your keyboard. When shopping for PCB's its best to make sure it fits in the case you are looking to get. This also extends to the plate that you are getting for your keyboard.
Hot swap PCB
Standard PCB
A hot swap PCB would allow you to install your switches without having to solder them. These are really awesome for people who do not want to solder and it makes changing switches out really easy. The only downside of a hot swap PCB is that you are confined to the standard ANSI layout.
If you are getting a keyboard from a group buy they usually have an option to purchase a PCB with it. If you are purchasing the PCB on its own, make sure to check if it fits into the case and plate you are getting. They usually state the compatibility in its description, or you can always check r/mechanical keyboards.
Links:
Thocstock - For In-stock parts
Mechgroupbuys - They track keyboards group buys