Imperial or Metric for PCB design? Its time!

[VK3DRB]

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?

cheers,
Dave

[amspire]

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.

Richard.

[Rufus]

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

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.

[mikeselectricstuff]

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.

[jahonen]

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:

- 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".

Regards,
Janne

[sacherjj]

Personally, I lay out a through hole board in in/mil and an SMD board in mm.  If there is a combination, I use mm and deal with the 2.54mm pin spacing.

The tough thing with saying "Its Time" is the inertia.  The US would have trouble going to metric board sizes, as 8'x4' plywood is a standard and would cause new board to not work in a repair situation if we had the slightly smaller 1.2 m × 2.4 m.  We have gone to metric thicknesses in many instances.  Same with A4 vs Letter (A4 is 210 by 297 millimetres (8.3 × 11.7 in) vs 8.5 x 11in letter). 

This same problem exists with any 0.1" based common component.  The inertia maintains some in based legacy.  Although, I agree that the math isn't too hard to just get 2.54 multiples in your mind.

[Rufus]

PCB software I use keeps all dimensions internally in 2 nanometer increments (IIRC), there must be a reason doing that instead of imperial units.

No it keeps them in 0.00001/127 inch units or 0.00001/5 mm units allowing exact representation of 1/100,000th parts of an inch or mm.

[FreeThinker]

The main problem is historical.The first components were designed in inches and so the machines to handle them were tailored to this fact. As time went on metric units were introduced but pin spacing maintained at 0.1 inches to be compatible with existing machines. As new machines were built they had to maintain compatibility with the existing components and so on. So unless BOTH the component manufactures and the machine manufactures introduce a DUAL standard (at a great extra cost) and slowly migrated every one over to metric only, then the imperial pitch for through hole components will always remain. Just think of the Arduino and its erm 'novel' connector. Multiply this by thousands (millions?) and you can see that chaos reigns.

[jahonen]

PCB software I use keeps all dimensions internally in 2 nanometer increments (IIRC), there must be a reason doing that instead of imperial units.

No it keeps them in 0.00001/127 inch units or 0.00001/5 mm units allowing exact representation of 1/100,000th parts of an inch or mm.

Actually, just checked it, PADS seems to use 2/3 nanometers as a basic unit.



Regards,
Janne

[Jon Chandler]

When creating part footprints, it's important to do so in its "native" units.  It's true that data sheets generally show metric and imperial units, but the conversion between units is not exact and rounding errors will create problems.

The illustration below is from the Eagle Microchip library.  The two rows of pads are for components with the same pitch but one is built in native units (metric) and the other built in rounded (imperial) units.  The rounding error across 10 pads makes them almost one-half the pitch off from one end to the other.

[Pat Pending]

6/6, 8/8 and the like, I agree, are easy to remember, but when geometries shrink further, the step size in the imperial system is huge in comparison to the greater resolution offered by converting to a finer metric measurement system, at which point rational ratios of microns might be more prevalent and as easily recanted.

That aside, using two measurements systems becomes a problem when a track endpoint does not terminate exactly on a pad center - thank heavens for 'snap to nearest'.

[Rufus]

That aside, using two measurements systems becomes a problem when a track endpoint does not terminate exactly on a pad center - thank heavens for 'snap to nearest'.

If a track connects it connects, electrons are not so pedantic.

[EEVblog]

Old habbits die hard.
When you've been doing PCB design for decades you just get used to working in imperial for many things, even on "metric boards".
Many PCB manufacturers will still work in mils for capability, like 4/4. So if you use a nice round 0.1mm (3.94mil) for the trace and you've got no recourse when you get breakouts. And when blame is to be had, that stuff counts. So my traces are still almost always in mils partly for that reason, regardless of what board I'm working on, but also because it feels comfortable to me and I have a gut feel for it.
I've converted entirely to mm for dimensions and hole sizes because that's what the manufacturers have used for a long time.
But the grids and pad sizes etc depend on the parts used and the need, so swapping back and forth on the same design is common here.
And my SMD parts are 0805, 0603 etc.

Dave.

[gxti]

I've started switching my PCB layouts to metric because it's almost exclusively SMD work. I'm also the kind of weirdo who uses Celsius while living in the U.S. But that said, I resent the tone of the orignal poster considering the USA "arrogant" for continuing to use an "archaic" system. There's little evidence that having multiple systems is significantly restraining innovation or commerce, and aside from a few high-profile incidents that everyone loves to mention, it doesn't seem to be doing much harm. Especially for PCB layout where mostly you're dealing with thousands of an inch vs tenths of a millimeter, both systems are effectively "metric" and the weirdness of the imperial system never comes into play. Any software worth using is dual-system (or more!) capable anyway and can fluidly transition from one to the other.

Even if there were greater motivation to switch, remember that the UK has been changing over for nearly 50 years and they still speak of distance in miles and weight in stone. The US has 5 times the population and is much further from Europe and its "harmonizing" influences, plus it has a much larger manufacturing base (if not as big as it used to be) that is going to have significant momentum. Don't hold your breath, it's going to be a while.

[Rufus]

I suggest to check this very informative blog (19 part !) about PCB design, by one of the expert at Mentor (also member of some IPC committees):
http://blogs.mentor.com/tom-hausherr/blog/2010/07/08/metric-vs-imperial-measurement-systems/

Beside this Imperial/metric open debate, this series of articles is a must-read to learn the schematic/layout best practices.

The blog and comments look more like rants and drivel to me, like this from one of the comments

Millimeters allow finer (and greater) granularity in the grid system to optimize the board real-estate, placement, via and routing grids.

[FreeThinker]

As Dave said old habits die hard and in truth there is little or no compelling reason to use imperial and by the same token no good reason not to. I think imperial will slowly fall out of favor and die a natural death, but as others have said 'no time soon'.

[jahonen]

What I like most about metric (SI-) system, is that everything is related by factor of 10 (or power of 10) to next bigger or smaller unit of same kind, unlike in imperial system. For example, it is pretty trivial to calculate how many micrometers there are in a kilometer (or even more exotic ratio) but it is much harder exercise to calculate how many thous there are in a mile (at least for me) .

I agree that it is that people are reluctant to change their habits, since learning new "reference scale" and developing that "gut feeling" takes quite a while. Maybe it is not worthwhile. Or should I say that old dog can't learn new tricks

Regards,
Janne

[Rufus]

However, my intent when I posted the link above was not to start a flame war on the Imperial/metric debate, but thought that this blog series may be also very informative for EEs on the schematic/PCB design techniques.

There isn't enough of a problem to have a war about so IMO anyone claiming things would be so much better with one system or the other is ranting.

After successfully transitioned all the engineers then the PCB design process got really easy, faster and simpler. Then when I give the PCB manufacturer all my nice clean metric drawings and metric Gerber and Drill data the first thing they do is convert all units to Imperial to panelize and CAM the job with their mil based DRC rules. Wow, it took longer to CAM my job because of the translation.

Designing a PCB gets really easy, faster and simpler because you call something a 2012 instead of 0805? - rant.

My preferred board house doesn't give a rat's arse if I supply them metric or imperial data.

I do think it is bloody stupid calling the same thing 0805 and 2012, but, it would be easily fixed by adding 'i' and 'm' suffixes, it is probably resistance from the 'one true unit' zealots on either side preventing this happening.

When the PCB fabrication shop starts recommending metric units preferred to their customers that’s when true electronic product development automation will really kick in and maybe we’ll start creating faster, better, more accurate, cheaper products or rather products that today cost $100,000 will only cost $1000.

So 99% of the cost of products is converting design units from imperial to metric? I didn't notice previously but the comment I quoted in my last post was actually from him. I don't care if he is a supposed IPC expert, I judge people by what they say not who the 'are' and I judge him a ranting moron.

What I like most about metric (SI-) system, is that everything is related by factor of 10 (or power of 10)

No one uses feet or yards in PCB dimensions, the inconvenience of other imperial units isn't relevant in this discussion.

Staying irrelevant what people like about units related by a factor of 12 is 12 is divisible into halves, thirds, quarters and sixths while 10 only halves and fifths. Decimal systems exist because we have 10 fingers, shame we didn't evolve with 12 and we would have the best of both worlds.

[G7PSK]

Units of measurement change with time as something better comes along it is gradually used and a good job too otherwise we would be working in cubits.

[Ajahn Lambda]

Darn, and here I've been using smoots for God knows how long....   


I'll use either system.  I have tools for both, i.e. I'm a total gear-head.

[FreeThinker]

My Daughter had an Austin/Rover Metro car and strangely the brakes were held on with imperial bolts.Why? because the cost of tooling up to change them to metric was prohibitive. (I think they used the same brakes as the old Mini). What may seem like a small issue to you and me can be a big issue to a mass market producer, so imperial will be with us for many years to come.

[FreeThinker]

My Daughter had an Austin/Rover Metro car and strangely the brakes were held on with imperial bolts.Why? because the cost of tooling up to change them to metric was prohibitive. (I think they used the same brakes as the old Mini). What may seem like a small issue to you and me can be a big issue to a mass market producer, so imperial will be with us for many years to come.

Perfect demonstration: Austin/Rover is dead now, and the new Mini has been designed by BMW, which uses metric bolts

However check out the number of failures in the 'new' minis due to engine/gearbox problems. I've been told by someone in the trade that it's mainly due to wear in the tooling producing the engines which is well past it's sell by date... so once again it all boils down to money!
But my point still stands as the Metro was built from old mini designed parts where as the new Mini does not (I don't think the brakes or engine are from the old mini) if they were you can bet that they would be imperial.
<Umm! Engine appears to be French >

[sonicj]

1 ft lb = 1.36 N m

I memorized that one earlier this week while wrench'n on my truck (mitsubishi) with torque tools built for the US market.   

converting from thou to fractions is painful. metric is easy.
-sj

[Monkeh]

The US would have trouble going to metric board sizes, as 8'x4' plywood is a standard and would cause new board to not work in a repair situation if we had the slightly smaller 1.2 m × 2.4 m.

Actually, you'll find that's rounded down. They're 2440x1220mm. 8'x4' down well into manufacturing tolerance, let alone natural shrinking and swelling.

[BBQdChips]

The blog and comments look more like rants and drivel to me, like this from one of the comments

Millimeters allow finer (and greater) granularity in the grid system to optimize the board real-estate, placement, via and routing grids.

It isn't just drivel, there's some truth to it.  Depending on how the software is written, the rounding error involved with conversion can become significant.  Just as shown with the example above (page 1) showing footprints not matching up because of the units used for their layout.

It's a lot like using a jewelers scale with a resolution of .002 grams.  Change units to grains (imperial) and now the resolution is .05 grains.  Do the math on those two, and you find that set to imperial, that scale has roughly 1/3 less resolution.  Same A-D, same sensor, same scale.  Change units, and it's bad karma.