Electronics > PCB/EDA/CAD

Imperial or Metric for PCB design? Its time!

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I like Dave Jones's latest EEVBLOG on the KiCad review. Looking forward to the PBC layout follow up! Very interesting and a no cost altertive to the expensive packages.

But in the blog, Dave said he prefers mils than mm. Why work in mils when the world has has moved away from imperial long ago? The country which failed to adopt the smarter system - the USA - will become far less relevent in the world of electronics as China produces 1 million engineering graduates per year. The US was arrogant in keeping the archaic imperial system when everyone else on the planet changed to a much more simpler system. They are in self imposed exile from the world standard of metric.

The term mils is slang for "thou" which is milli-inches or thousanths of an inch. Many people use use the slang mils to mean millimeters. What a mess. 

I do a fair amount of PCB design and prefer to use metric, although I reluctantly use imperial form time to time. Altium makes it easy to use either. Metric is easy when doing component footprints. All companies use mm in their datasheets. But many don't use inches. So it makes more sense to use metric. It is simple arithmetic to use metric. The numbers you use often are 5.08, 2.54 and 1.27 for the traditional pitches - it is easy to convert and to set your grids to say, 0.254 or whatever. The only pain I have is when PCB manufacturers advertise their PCB design limits in mils rather than mm, but I have gotten used to it.

Sorry to sound hot under the collar over this, but at least it is in Celsius, not Fahrenheit.

Agree of disagree? I am interested to know what others use in PCB design - imperial or metric?


You use the units needed for the current pcb parts or schematic symbol you are using.  If you are laying out a PCB with parts with 0.1 inch pin spacing, you will use one of the mil (inch) grids. If you are using library schematic symbols with mil pin spacing, it would be hard work trying to use a metric grid. If you want to make your own schematic parts, then using a metric grid is no problem, but for the PCB, you will find it hard to use only parts with metric pin spacings.

Kicad allows you to change units and grids pretty easily so you can dimension in mm, and then work on tracks on a inch-based grid without any problem.

So is it time to switch completely to metric in PCB design? Not any time soon.



--- Quote from: VK3DRB on March 05, 2012, 01:19:53 pm ---Agree of disagree? I am interested to know what others use in PCB design - imperial or metric?
--- End quote ---

Generally disagree. I prefer to think in mils because 6/6 or 8/8 is easier to comprehend than 0.15/0.150 or 0.2/0.2. Easier the same way someone being 6 feet tall is compared to 1.8m.

I don't really have any problem mixing units and don't generally need to work to a grid.

The only thing that really pisses me off is unitless specification of footprints like 0805 being 2012 metric. In my experience imperial is more popular and assumed, metric versions need an 'm' or something to differentiate them and I wouldn't care if imperial version got an 'i' or something at the same time.

It depends very much on the job. If you are mainly using DIP and SO packages, imperial makes sense, with a 25 or 12.5mil grid, however nowadays I'm increasingly using find that I'm using mostly metric packages - 0.5mm QFPs and TSOPs, FFC connectors, 2mm connectors etc., and it makes much more sense to use a metric grid (usually 0.25mm, down to 0.125 for tight spots) with these.
Fortunately I can avoid having to remember all the common track sizes in metric as my PCB sw allows me type 'mil' or 'mm' after a text entry regardless of currently selected units.

I have used solely mm's for some years now, never looked back. Our PCB designers at work seem to do the same thing. And some guys from some domestic PCB factories used mm's in their slides. All modern SMD component packages are designed around metric dimensions. So world looks pretty metric for me. Imperial dimensions can be expressed accurately in mm's (given enough decimals), vice versa not so much. PCB software I use keeps all dimensions internally in 2 nanometer increments (IIRC), there must be a reason doing that instead of imperial units.

KiCAD version I have installed, seems not to have option to output gerbers in metric dimensions. I don't know if that has changed. I think that is due to that it internally uses mils (or thous). That makes it impossible to output accurately millimeter dimensions, for example, 100x100 mm comes out something like 99.9999x99.9999 etc. when translated back to metric world. Not a practical problem usually but I think that in year 2012 we should be able to express that accurately without round-off errors. But on the bright side, gEDA switched internally to metric recently, I guess there is still hope for KiCAD, too:

--- Quote ---- Internal coordinate space is now metric.  This should fix the "trace
  nubs on metric grids" issue.  Also, units are allowed on pretty much
  every "coordinate" that PCB accepts.  Example: 5.4in, 10cm, 0.55mm.
  New configure option --enable-coord64 to force (slower?) 64-bit type
  for coordinates on 32-bit systems (if you need a board bigger than 1
  meter across).  The internal precision is 1 nanometer, the internal
  type is "Coord".

--- End quote ---



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