We are giving up mils for board design

Tables and a python script to make life a little easier in converting board designs to all metric (mm)

updated: Jan 02, 2024

Overview

We decided to give up on inches / mils for circuit board (PCB) design because it's a hindrance to keep going back and forth between mils and mm. Prior to the switch, we were calculating our design rules, trace widths & gaps, board outline and drill holes in mils and everything else in mm. Hopefully, we're leaving some headaches behind, and we'll use the tables and Python script below to help us with the conversion.

Base Conversions:

1 inch = 2.540 cm = 25.40 mm
1 mil = 0.001" (one thousandth of an inch)
1 cm = 10 mm = 0.3937 inches
1 mm = ~39.37 mils

Chip Scale Discretes

No more 0402's and 0603's, it's now 1005 and 1608. Keep in mind that an 0402 is approximately 40 mils X 20 mils ( 0.04" x 0.02")

inch	metric	L (mm)*	W (mm)*
0201	0603	0.6	0.3
0402	1005	1.0	0.5
0603	1608	1.55	0.855
0805	2012	2.0	1.25
1206	3216	3.2	1.6
1210	3225	3.2	2.5
* nominal

Typical Finished Board Thickness

mils	in	mm	mm rounded
62 mils	0.062"	1.576 mm	1.6 mm

+/- 10%

Other Common PCB Dimensions

	µm	mm	mils
Typical Silkscreen Thickness	25	0.025 mm	~1 mil
Typical Soldermask Thickness	25
T4 Solder Paste Particle Diameter	20 - 38
Nano Coating ¹	1 - 4

1: SAMP nano coating is ~ 2 nm

Some common mils to mm conversions

mils	mm	mm rounded
3	0.076	0.075
4^*	0.102	0.10
5	0.127	0.125
6	0.152	0.15
8	0.203	0.2
12	0.305	0.3
15	0.381	0.4
20	0.508	0.5
30	0.762	0.75
50	1.270	1.25

* 4 mils (100 µm) and less is considered HDI

Some common mm to mil conversions (handy for dealing with tape & reel component pitches)

mm	mils	mils rounded
2	78.74	80
4	157.48	160
8	314.96	320
12	472.44	470
16	629.92	630

Other fun facts:

1 oz copper (CU) is one ounce over one square foot, which is 1.37 mil thick, 0.0347 mm (35 µm)

conv.py

Provided below is a simple but handy python3 script for converting back & forth between mils and mm.

##############################################################################
#
# Copyright (c) 2017 Mind Chasers Inc.
# All Rights Reserved.
#
#    file: conv.py
#
#    convert between mm and mil
#
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE.
#
##############################################################################

import os, sys

INCH_TO_MM = 25.40

def conv_to_mm(mils):
    """ convert mils to mm """
    return 0.001*INCH_TO_MM*mils

def conv_to_mils(mm):
    """ convert mm to mils """
    return (1000/INCH_TO_MM)*mm

if __name__ == "__main__":
    if len(sys.argv) < 3:
        print ('usage: ' + os.path.basename(sys.argv[0]) + ' <value> <unit>')
        sys.exit()

    value = float(sys.argv[1])
    unit = sys.argv[2]
    
    if unit.startswith('mil'):
        print("%.3f" % conv_to_mm(value)  + ' mm')
    elif unit.startswith('mm'):
        print("%.3f" % conv_to_mils(value) + ' mils')
    else:
        print('unit not supported')

Download conv.py

References

NIST: Approximate Conversions from U.S. Customary Measures to Metric
U.S. Metric Association: Commonly used metric system units, symbols, and prefixes
Vishay: Standard Thick Film Chip Resistors

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