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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    Short answer, I use mil. – Kellenjb Jul 6 '10 at 13:21
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    Short answer, I use mm. – XTL Jul 8 '10 at 14:03
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    Mind linking to that PDF? Seems interesting – cksa361 Dec 16 '10 at 3:58
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    Short, correct answer: use whatever the majority of your device package's mechanical drawings are done in. – Connor Wolf Dec 16 '10 at 6:02
  • Of course, most modern software can export to either from either, so it really doesn't matter anyways. – Connor Wolf Dec 16 '10 at 6:03
up vote 8 down vote accepted

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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    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. – Kevin Vermeer Dec 16 '10 at 17:11

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.

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.

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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    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. – The Photon Jan 27 '13 at 17:33
  • @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. – Olin Lathrop Jan 28 '13 at 14:19

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.

  • 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. – supercat Jan 28 '13 at 18:11

Brazil uses metric system but my layouts are in mils because because it is the most widely used.

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

  • 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. – Robert Jul 13 '10 at 18:55
  • Yes, the PCB fab house uses mil for everything - traces, panel sizes, drill sizes, etc. They are not the assembly house, though. – ajs410 Jul 14 '10 at 4:41

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.

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?

protected by W5VO Jan 28 '13 at 1:31

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