PCB Design Units – Imerial vs Metric?

The short and sweet answer is both! In the recent years there has been a big move towards the metric system, but truth be told the circuit board manufacturing houses haven’t made the switch. Many of todays components are dimensioned in millimeters yet most of the board design happens in inches.

Before we go any further you should become very familiar with a few terms. ‘Mil’ and ‘thou’ are two terms which can be used interchangeably. Both mil and thou stand for 1/1000th of an inch, so 100mil is 0.1″. Mil should never be confused for millimeter also sometimes referred to as mil. Many try to stick to thou to alleviate any confusion.

As you begin your design you’ll need to create pcb footprints of your components. Many times these pcb footprints will be dimensioned in metric. You can make the conversion to inches to be consistent, but if you want to design your footprints in metric be sure your pcb design software makes the conversion when you go to board layout. Any respectable software should have no problems making this switch.

Once you begin designing your board its best to stick with inches for pads, grids, traces, and spacings. If you need to use mm for mechanical and manufacturing, which includes board dimensions and mounting holes, it is permissible to do so. Grids and traces are much easier to work with in inches. It may be easier for you to stick with inches throughout your design and just make the conversion. In time you’ll begin to see several commonly used conversions like 100mil = 1.27mm. In general 100mil is the basic reference point for all aspects of pcb design. You’ll find that most every component pin spaces are a fraction of 100mil which is why it’s so easy to work in mils.