Electronics > PCB/EDA/CAD

Inches, mils, or millimeters?

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Sal Ammoniac:
Almost all PCB and schematic capture programs allow you to set units to inches, mils, or millimeters. Which one is the best choice nowadays?

I’m in the US, where we routinely use Imperial units, but I’m fully conversant in metric as well. A lot of components are laid out on 0.1” (100 mil) centers, so common grid sizes are 100mils, 50mils, etc., which results in grids like 2.54mm and 1.27mm in metric.

I think I prefer mils. Are there any drawbacks in 2022 to using mils instead of mm?

I use mm for layout, mils for schematics. Latter primarily because of Altium idiosyncrasies.

In my experience layout is most sensible in metric unit related scales such as mm because even though some components have
alignments based on imperial grids, even more components including most of the ones with the even finer pitches, are laid out in metric grids.

Also even the components with imperial grid aligned layouts as well as almost any other component should have data sheets and / or foot prints
and CAD models available which specify or convert to coordinates denominated in mm, so one can use almost any component "natively" in a metric

As for schematic the only sane thing would be for EDA systems to keep track of absolute / relative coordinates in some quite high resolution system, but the actual drawings should overwhelmingly be based on scalable vector graphics fonts / strokes so really the unit system should be irrelevant since some op-amp symbol or whatever should scale perfectly whether one wants the symbol's pins to aligned on a 5 mm grid or a 0.2 inch grid or a 3.14 pica grid, it is only a matter of scale and translation both of which should be able to be done perfectly assuming the symbol pins were defined to be periodically regular on *any* dimensionless relative spacing grid.

And in either case of schematics or layout one should have some relatively sane "align to grid" and "snap to grid" and "make a local or hierarchical grid / grid line" setup and be able to snap / align etc. to whatever one is operating relative to for a given region / component / sheet / board.

That said I don't usually go out of my way to redefine all existing schematic libraries to metric dimensions since there may be things imported that are on imperial but when importing stuff from library generators or something or creating things I'll specify metric, and when doing layout / footprint libraries I use metric.

layout st

Metric all the way for layout. Imperial in schematics for historic reasons. But since this is convention only and thus has no fabrication implications the units in schematics really don't matter too much.

One more point for metric:
One can express 100 mil as 2.54 mm exact, but 1 mm = 39.3700787 mil. So chances for rounding errors are higher.


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