I use a PWB layout program where I can see the internals. Seems if you inp ut the data in inch units you can convert to metric perfectly. 1 inch =
25.4 mm. But if you input data in metric and try to store it in inches it doesn't convert so well and round off has to happen. 1 mm = 0.0393700787 4016... inches. As a consequence, this package stores everything as intege rs in nanometers.At one time trace widths were convenient in thousandths of an inch because they were typically whole units, 6/6 mil was a common trace and space. Now trace and space are commonly fractional thousandths, so metric is just as good. Some BGAs require 3.5/2.5 mil or 0.09 mm/0.0635 mm or 90um/63.5um.
I think on my last layout my basic grid was 0.325 mm which was a bit more t han 12 mil for 6/6 mil trace and space which turned into 0.15 mm if I recal l. This was close enough to 12.5 mil that it worked ok for routing to TSSO Ps and such. Routing a 0.5 mm QFP I had to switch to a 0.5 mm grid.
Does anyone still use inch measurements for anything other than mounting ho le locations on PWBs?
Rick C.