Units of measure

The first important information a designer has to know regarding a PCB design is what measurement units are/has to be used. There are two systems of units: IMPERIAL (inch, module, mil) and METRIC (cm, mm, micron). The most used is IMPERIAL but an important translations is happening from this to METRIC, even in USA. IMPERIAL is used because the majority of electronic components were (and still are) manufactured with imperial pin spacing (the unit is the module, 1M= 0.1inch= 100mil).

Regading IMPERIAL system, an important unit is "mil", called also "thou", which is 1/1000th of an inch, and is universally used and recognised by PCB designers and manufacturers. Terms as "10mil spacing" and "25mil routing grid" are used all over the world. The term "mil" comes from 1mil= 1 mili inch. In practical projects the designer has to use both imperial inches (thous) and the metric millimeter (mm). As a general rule, use "mil" for tracks, pads, spacings and grids, which are most of your basic "design and layout" requirements. Only use mm for "mechanical and manufacturing" type requirements like hole sizes and board dimensions. Some manufacturers use metric size drills, so specifying imperial size holes really is a souce of errors.The best idea is to spek firstly with the manufacturer in order to create a perfect matching

It is recommended to know imperial-metric and metric-imperial conversions. For conversion, it is known that 1mm= 39.37mil. If the user accepts 1mm = approx. 40mil (but only till values and distances las than 700mil), you have to know that: 0.1mm= 4mil, 0.5mm= 20mil, 0.9mm= 36mil, 1,5mm= 60mil, 2,2mm= 88mil, etc. Exactly speaking, 100 mil (0.1 inch) = 2.54mm and 200 mil (0.2 inch) = 5.08mm. Values of 100 thou and above are very often expressed in inches instead of mils (0.2 inch is sometimes more commonly used than 200thou).1 inch is also commonly known as 1 "pitch". So it is common to hear the phrase "0.1 inch pitch", or more simply "0.1 pitch" with the inches units being assumed. This is often used for pin spacing on components.

Along with the rest of the world, IPC standards have all been translated to METRIC system, and only occasionally refer to imperial units.