How do I determine the traces width when designing a PCB?

Trace width and trace (or copper) space go hand in hand for a minimum value.

4/4	Impact yield and increase cost
6mil	With very high yield for most PCB houses
8/8	Better
12/12	Can be made by PCB houses which is even not good at QA

What is the finest pitch of parts? If you are using 0.4mm pitch BGA, you will need very narrow traces. Only for THT you will probably use larger traces.
Designing with a grid.In the days of thru-hole with 0.1″ typical component placement pitch, a grid of 0.025″ make routing easier. These days good PCB software will ‘push and shove’ so grid, and trace width/space becomes not important. I have typically routed on a 1 mil grid. Guess how does this effect trace width? The trace + space will equal your grid setting.
The most important factors are current carrying requirements and signal integrity. You need wider traces for higher current. Use a PCB calculator to determine your requirements or look to an IPC standard. For signal integrity, if you need controlled impedances the trace width will be a critical factor.
Last is copper thickness.If you use 1/2 oz, 1 oz, 1 1/2 oz, 2 oz, 3 oz, or even higher, the spacing and minimum trace with will increase. Etching is not limited to vertical removal of copper so the PCB manufacturer will have to edit your Gerber (or other) file to compensate.

#PCB Design

Oliver Smith

Oliver is an experienced electronics engineer skilled in PCB design, analog circuits, embedded systems, and prototyping. His deep knowledge spans schematic capture, firmware coding, simulation, layout, testing, and troubleshooting. Oliver excels at taking projects from concept to mass production using his electrical design talents and mechanical aptitude.