EMC Question of the Week: June 3, 2024

flat sheets of metal

The walls of a particular shielding enclosure have a nominal thickness of 60 mils. This is equal to  

  1. 0.06 inches
  2. 0.17 inches
  3. 6 mm
  4. 6 cm


The correct answer is “a.” One mil is equal to 0.001 inches. This may be obvious to anyone who works with shielding materials or circuit board layouts on a regular basis. However, many people seem to think mil is a verbal shorthand for the word millimeter.

Yes, it would be nice if the whole world could agree to use metric units. And while the use of units like inches and feet has declined significantly, mils are often the unit-of-choice (even outside the U.S.) when it comes to specifying material thicknesses. Perhaps this is because the mil is conveniently sized for specifying thin materials. For example, it's easier to say that a circuit board dielectric thickness is 10 +/-1 mil than to say it's 0.25 +/- 0.025 mm. 

It doesn't help that people in the electronics industry often mix Imperial and metric system units. For example, it would not be uncommon to see a circuit board described as being 10 cm x 6 cm with a thickness of 63 mils.

Perhaps the use of mils as a unit of thickness will eventually die out. But for now, it's a unit of length that EMC and board-layout engineers need to recognize.

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