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I have been using 10 mil trace with 10 mil spacing. The PCB vendors quote that they like traces down to 7 mil. But then I ran across a PDF showing how to fan out a QFP to get all the signals accessible. They use millimeters because the QFPs are packaged with 0.4mm or 0.6mm pitches.

They also make an argument that using a 0.05mm grid approximates mils, but mm allows you to route buses in between the vias and pads.

Should I be using mil or mm when I am routing a PCB?

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    \$\begingroup\$ Short answer, I use mil. \$\endgroup\$
    – Kellenjb
    Jul 6, 2010 at 13:21
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    \$\begingroup\$ Short answer, I use mm. \$\endgroup\$
    – XTL
    Jul 8, 2010 at 14:03
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    \$\begingroup\$ Mind linking to that PDF? Seems interesting \$\endgroup\$
    – cksa361
    Dec 16, 2010 at 3:58
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    \$\begingroup\$ Short, correct answer: use whatever the majority of your device package's mechanical drawings are done in. \$\endgroup\$ Dec 16, 2010 at 6:02
  • \$\begingroup\$ Of course, most modern software can export to either from either, so it really doesn't matter anyways. \$\endgroup\$ Dec 16, 2010 at 6:03

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While the metric system is arguably saner, those that have been doing PCBs for much longer than myself have are pretty adamant about using mils. I think it has something to do with the machines used by manufacturers. That being said, if you have a board house in mind, and they spec their capabilities out mm, then yea you'll probably get a neater routing by picking a grid that is "natural" to your components. Typically though, you're going to have a mix of parts that are mm and mil based, so you'll likely have a "grid mismatch" regardless. I pretty much always use a 25mil grid and things usually turn out "good enough."

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    \$\begingroup\$ I'm not sure that historical arguments are valid here, because an increasing number of new parts are dimensioned in mm, while older parts were dimensioned in mils. \$\endgroup\$ Dec 16, 2010 at 17:11
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I think the system used by the majority of your active parts is the system to choose. Older parts were all 0.1 inch pitch, but at least some newer parts have ball or pin pitches specified in mm. I'd default to metric because it's the world standard, but if you're manufacturing in the US, you might default to inches.

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Tom at PCB Matrix advocates using metric exclusively, as the majority of high-tech parts are metric. I actually use both, in the same design sometimes, if I have a mixture of metric and Imperial parts. It doesn't really matter all that much with the software I use, as it can make connections off-grid, but I get neater results.

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Use whatever is more convenient for you. It doesn't matter what units packages are defined in because any decent software decouples package definitions from layout and routing dimensions anyway. Use whatever metrics the datasheet uses when defining the package footprint, since that will minimize errors. Even then, I switch back to inches when defining the silkscree and documentation layers because that's what I can relate to more directly.

For layout and routing, use whatever you are comfortable with. I learned this stuff thinking in inches and mils, so that's what I use. I can picture 8 mils in my head, but I'd have to convert the equivalent in mm to get a mental picture. That's what computers are for. They are very good at multiplying or dividing by 25.4, so let them serve you, not the other way around.

When exporting files to a board house, I'd use inches. That seems to be more of the univeral standard, probably due to history. The board house's computers can multiply by 25.4 too, so those board houses that use mm internally won't be bothered. A few years back, some board houses wanted all input in inches, but I suspect that's no longer true today.

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    \$\begingroup\$ Furthermore, your tool ought to be able to switch back and forth at the push of a button. In Altium, I only need to hit "Q" to switch units, and "G" to change to a different snap grid. \$\endgroup\$
    – The Photon
    Jan 27, 2013 at 17:33
  • \$\begingroup\$ @TheP: Yes, I use Eagle and I have hot keys set up for several different units and grid arrangement. In the package editor, for example, F19, F10, F11, and F12 all set up different grids, from .1 inch lines with 12.5 mil courseness to 1 mm lines with .1 mm courseness. Nowadays you have to be able to accept input in various units. \$\endgroup\$ Jan 28, 2013 at 14:19
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Typically I place parts on a 25mil grid and route on a 5mil grid. For low density boards I can usually route on a 10-25mil grid.

When I route I have the traces snap to the mid-point of the pad so that traces end in the center of the pad for all components.

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  • \$\begingroup\$ If one is running a trace between pads of a component, is there generally any good way of making the trace snap to the middle of the space between components? I would think that on software which can't easily do that, it would be better to have the grid set for whatever type of components are most likely to have space for traces between pads. \$\endgroup\$
    – supercat
    Jan 28, 2013 at 18:11
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This question is a decade old, but still highly trafficked, so I figured it's worth adding my (more recent) insight as a PCB designer by trade (U.S.-based).

Background

At this point, mils/inches are starting to feel somewhat obsolete for a handful of reasons:

  • All new SMT footprints are defined in metric. Routing in mm allows for slightly more optimized fanouts between pads (not super relevant at low density)
  • Most off-the-shelf enclosures are specced in mm (especially true if you are 3D printing one)
  • All IPC guidelines are in mm, as well as any new IEEE/IEC/JEDEC/etc specifications for package, connectors, etc
  • Ucamco (creator/maintainer of the Gerber format) has made it clear that mils/inches are deprecated and will be eventually be removed from the Gerber standard (worth noting though that this doesn't actively affect routing in mils, they round to the nearest nm just fine)
  • All new equipment related to assembly, and almost all new equipment related to fabrication, is configured and operates in metric, often without the ability to change
  • All FCC/CISPR/NRTL testing (or any sort of post production verification) uses metric exclusively
  • The standard board thickness has moved from 62mils (1.57 mm) to 1.6 mm, even at board houses that still primarily use mils for traces. And even those that do still spec in mils have a perfectly competent understanding of mm.
  • Ounces are now just a placeholder, copper thickness is controlled to the micron (oz/ft2 has always been a silly unit for people who don't make the foil)

Grid

If you are looking for your board to be compatible with all output formats, everywhere, then a 0.05 mm grid (or its multiples) is technically the objectively correct answer, as it's been agreed upon by dozens of organizations and will work without rounding for all outputs.

However, we're at a point where you can pretty much throw grids out the window unless you're going for visual appeal. Gerber X2 recommends 10nm or 1nm grids for all outputs, small enough that any routing grid is huge by comparison. When I'm doing HDL I usually place on a 0.05 mm grid (for assembly) then route with grid off since it serves little purpose.

Summary

In short, if you're seasoned at designing and used to mils, there's really not much motivation to change (though I was, and I did), especially on the hobby level. And everyone should certainly use what they are comfortable with. However, if you're just starting out, my recommendation at this point would be to learn to route in metric. 10 years ago I would have said it really doesn't matter, but the industry (currently a messy mix of both units) is slowly but surely moving toward metric only, so it's worth at least getting a feel for that.

Exceptions

For completeness, here are few exceptions I can think of:

  • U.S. based dielectric manufactures use an annoying mix of units. Not critical for hobby level but mildly irritating for high speed boards
  • Many U.S. boardhouses will still give you overall board dimensions or stackups in inches on drawings, even if they communicate in mm. That's just old software, shouldn't affect how you design your board
  • 0.1" / 0.05" connectors, are still out there en masse (in addition to DIPs and 1.27mm SOIC), but this is hardly relevant when today's routing pitch is much smaller
  • Some datasheets still give trace length / matching specs in mils, though most list both and some list mm only. (Not relevant for hobbiests)
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    \$\begingroup\$ I would add that if you use metric and have to include the odd inch dimension, it works out better than the reverse. For example, 2.1" is 53.34mm exactly. But 50.0mm is about 1.968503937007874015748031496063". So you'll round it off, of course, but it won't be exact. That might have consequences if you are right up against some limit in the design rules. \$\endgroup\$ Jul 9, 2021 at 4:00
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    \$\begingroup\$ That's very true and worth accounting for, even if it is just a coincidence of USC units being defined off of metric. Some software does a terrible job handling this (looking at you, Altium) so converting from mils to mm is easier than the reverse. \$\endgroup\$ Jul 9, 2021 at 4:19
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Brazil uses metric system but my layouts are in mils because because it is the most widely used.

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I use mil, because that's what they use at the company I outsource the PCBs to.

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  • \$\begingroup\$ Is the outsource company the PCB fabrication house? Is it the drill sizes? Is it the panel sizes? Is the outsource company the assembly house? Sorry for the list of questions. \$\endgroup\$ Jul 13, 2010 at 18:55
  • \$\begingroup\$ Yes, the PCB fab house uses mil for everything - traces, panel sizes, drill sizes, etc. They are not the assembly house, though. \$\endgroup\$
    – ajs410
    Jul 14, 2010 at 4:41
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I have the same opinion as Olin, since I'm used to think in metric I choose mm; if I were born in the US I'd probably go for mils. But I keep a conversion table for the most common sizes/distances at hand, so I don't have to make calculations every time. Oddily enough, my PCB fab requires some dimensions in mm, some in mils, so I'd have to convert either way. Even if not for that, you'll from time to time stumble upon values that need to be converted no matter what you choose. Why not pick the choice you're most comfortable with, then?

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It depends on the parts you're using and the capabilities of the fabricator. If the fabricator is working with mils, then making the design on an mm grid may cause minor alignment issues. These issues though, are only a problem if you're really testing their clearance specifications. Even then, its unlikely to actually result in a non functioning PCB.

Personally, I use mils. The vast majority of my parts use mils for pin spacing specification. Mils also lets me think in terms of integers for things like wire width, clearance, and so on. And my manufacturer deals with mils anyway.

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