When I know the current is 3A, which calculations will I perform to find pad clearance, trace width and via diameter to setup design rules for a group of footprints?
Current has not much to do with clearance. KiCad has a calculator to which you can give different known values. But you have to know the thickness of the copper, ambient temperature and allowed temperature rise etc. That’s because it’s (probably) more about temperature and cooling than voltage drop.
You probably know where the high current flows. Put a filled zone there instead of a track if you can. It’s as wide as it can be and better for cooling. Add some vias between zones when you have to switch side. Remember that the copper “width” of a via is actually quite large, about 3*diameter.
But thickness less then at PCB surface.
Base copper at standard PCB is 18um. Then during metalization it becomes 35um. I suppose that via near the PCB surface has thickness of (35-18 = 17um) but in its middle probably less.
But vias can be designed to be filled with solder, increasing their ability to conduct both current and heat. And sometimes you really ARE concerned with voltage drop rather than trace heating - especially if you have cleverly designed a length of trace to function as a current sensor.
Yes, @ANDREAS_THEODOROU, it is complicated. In addition to the calculator that comes with KiCAD there are several on-line calculation tools. Before using the results from any calculator in a critical application, look into the assumptions it is based on.
Many years ago, this Rule of Thumb was drilled into me:
For day-to-day work in most applications using FR-4 board substrate, a 10 mil (0.25mm) wide trace of “1 ounce” copper (a bit less than 1.5 mils (35um) thick) can safely carry 750 mA - 1000 mA. And, it scales linearly through all common copper thicknesses, as well as for a decade or two of current ranges in each direction.
114A/mm² - sounds impossible, but I don’t have real knowledge in this subject.
Nor do I. I must rely on the work of others for guidance. The KiCAD calculator says that a 10 mil (surface) trace of 1 ounce copper will have a temperature rise of 10 deg C when carrying 875 mA.