Congratulations! You'll find out really quick that the metric system is superior to imperial units.

I have a personal guarantee that if you use these rules and do 5 PCB designs in metric, it will be impossible for you to switch back to imperial, because you will discover the beauty of the metric system and the ugliness of the imperial system.

Here are the things you need to be concerned about:

1. Make sure your PCB Library is completely created in metric units. Mixing imperial unit footprints in a metric PCB layout is very messy.

2. It’s best that your Board Outline is in metric units, but that’s up to the mechanical engineer.

3. Make all Mounting Hole pad and drill sizes in 0.05 increments. Placement grid is up to the mechanical engineer.

4. Create all your PCB design elements in 0.05 mm increments.

1. Library parts

                                                               i.      Line Width and Gap for Silkscreen 0.15

                                                             ii.      Line Width for Assembly 0.10

                                                           iii.      Line Width for Courtyard 0.05

                                                           iv.      SMD Pad Length & Width

                                                             v.      Pad placement

                                                           vi.      PTH pad and hole diameters

                                                          vii.      Note: the worst metric SOP/QFP/QFN/SON is a 0.65 mm pitch. This screws up a great PCB layout. Be on the lookout. 

2. Part Placement – use numbers in 0.10 mm increments

                                                               i.      Big parts snap to 1.00 mm grid

                                                             ii.      Medium parts snap to 0.50 mm grid

                                                           iii.      Small parts snap to 0.10 mm grid

3. Routing traces – use 0.05 mm increments

                                                               i.      Best Signal Trace Width = 0.10 mm – this allows differential pairs for a 1.00 mm pitch BGA

                                                             ii.      Power Trace Widths = 0.20, 0.30, 0.40, 0.50, etc.

                                                           iii.      Routing Grid = 0.05 mm

                                                           iv.      Via Pad Size = 0.50 & Via Hole Size = 0.25

                                                             v.      SMD Via Fanout should snap to a 0.10 mm grid if possible