Welcome to the CADSTARguys Blog - Information, hints, tips and my waffle on the CADSTAR Printed Circuit Board design suite.

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Tuesday, 2 December 2014

Imperial or Metric - Does it really matter?

There often seems to be discussions about which unit of measurement to work in, they pop up on the web several times throughout the year. Questions are asked on whether someone should use Millimetres or Thou as a measurement unit.

Which side of the fence are you on?

Being an old fart (or feeling like one!) and coming from a history of through hole components, crepe pads and tape etc (Not quite film and red/Blue lines). I have a history of using thou, to me it is an easier number system to use especially when I am trying to remember all the design rules, specifications, hundreds to thousands of connections in a design and so on. I find it easier to remember 100thou than 25.4mm maybe its something to do with the decimal place or just my bad memory.

The thing is though, its only a unit of measurement, what really counts is that whichever is used the dimension etc is accurate and pertinent to what it is being used for.

The Metric measurement system has been around for a few years now (about 200+ according to Wikipedia ) and is taking over imperial in the electronic world more and every year.
So despite a heck of a lot of the components that we use still being on an imperial pitch (even if the datasheet gives metric measurements) at least we should be using it in some of our PCB design work now (a common component pin pitch of 1.27mm is in fact 50th.).

I used a symbol library that was created many years ago, where all the symbols were created on an imperial pitch with 0.1" between pins. So changing them to a metric pitch was just not going to happen, and besides - for a schematic I do not really see the need for my dimensions to be metric - I can work with imperial quite easily in schematics.

For component footprints though, new footprints are being produced on a true metric pitch so when I create a component I do this using Millimetres rather than Thou, even if both values are present on the datasheet. Pad dimensions and drill sizes - these I will only use Millimetres for, especially drill sizes as I understand that board manufacturers no longer use imperial drills, also they are only available in 0.5mm increments. (the problem comes when the datasheet has both values but they do not add upto the same distance!)

Board outline, mounting holes, MCAD specific placement etc are all done using Millimetres because they all interact with other systems that are already metric. However there are still an awful lot of components that use imperial spacings even if the dimensions given are metric.

Things like track widths, outline widths, spacing rules etc I still use Thou for these - simply because either I'm lazy and its something I am so, so used to or I have too many other things to remember so do not need .somethings too. I should really change to MM for all of them, I guess that I will one day.

It seems that electronic circuit and PCB design is something that we can switch between measurement units quite easily (and often do) and still get the same results.


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