Design For Manufacture; Metric vs. Imperial?

Community PCB Design
1

The question here is not about which system of measure is better; the question is about which produces a shippable quality, cheaper to manufacture board.

Assuming you have parts in both Imperial (Inches & Mills; obvious case, headers & Through-Hole parts) and Metric units (Millimeters and Micrometers; obvious case, SMT parts), how do you draw the line between where you transition from Metric to Imperial, if anywhere, to produce a board that is as cheap as possible to manufacture?

An underlying assumption here is that, ideally, you want your board capable of being made on both Imperial and Metric equipment, and not limited to just one or the other, which presumably increases the competition available to produce your board. If it’s fundamentally cheaper to have a board made in one unit of measure than the other, that is worth noting, but please also remember to answer the original question in the assumed context; you never know when market conditions are going to change due to improved automation or increased labor costs.

2

It doesn’t matter what unit of measure you use. It’s still the same size board… Most datasheets have both. Most CAD programs will allow you to use both.

3

As above, it does not matter.

What matters more is you making the measurements easier to manage.

For instance, if your mounting holes are exactly 3" apart its easier for someone making a chassis to measure with a steel ruler, yet if its 78.59mm - I cant see that on a ruler.

As much as we may think that everything is done with cnc machines etc, a lot of work is still done manually, or checked manually etc.

The board manufacturers will only have a teensy weensy problem when you send imperial drill sizes as they all only use metric now so will have to convert them.

4

Now, see, that’s a case where it matters, doesn’t it? There are probably other little “gotcha”s like that.

5

There are lots of little gotchas, things that make not just the PCB cheaper but the entire product.

Make it so that there is less PCB manfacturing problems so less wasted boards and the PCB is cheaper, make it so that component assembly is easier without any problems, no tombstoning, minimum PTH, no solder balls, no bad joints due to wrong size pad choice etc and the assembly becomes cheaper.

make it so that the board can be easily and quickly tested, then easily repaired, easily packaged and the whole production of the product is cheaper.

You can choose to use cheaper laminate I.E. paper board instead of FR4, cheaper finishes I.E. OSP than gold plated and your PCB will be cheaper, however it will have knock on effects for the life of the PCB, the higher defect rate in later life etc. So by having a cheaper PCB the actual lifetime product cost can be more.

Whether you choose 1 measurement method or the other is just a fraction of the equation.

As a PCB designer, we often have to look at the bigger picture

That said, I refer imperial because its simpler, easier to remember numbers for an old fart like me, 8 thou track and gap I can easily remember, 0.216mm - forget it :slight_smile:

6

Well, again, this question is not about which unit system is more convenient, but where using one unit system over another potentially results in a cheaper board (or, at least, more manufacturers being able to deliver a competitive quote).

7

It doesn’t make any difference.

8

After doing some research, here’s what I’ve found.

Photoplotters come in many kinds of resolution; it’s not even as simple as mils vs. um. Many photoplotters for PCB use use repurposed paper printing components, and thus, use printing resolutions. Still others use binary (power of 2) resolutions. And some of the blasted things even use a mix, with one for x and another for y, just like printers. For a specific boardhouse, ask them whether they use a mill, and whether it’s imperial or metric, or use a photoplotter, and what the photoplotter’s resolution is in both axes, to ensure your quad parts get done right.

Assuming resolution is in dpi:

If the photoplotter resolution is divisible by 1000, it’s imperial; use mils.

Otherwise, if the photoplotter resolution is divisible by 300 (eg. 1200, 2400, exc.), it’s printing dpi; use inches divided by that number.

If the photoplotter resolution is divisible by 25.4, it’s metric; use micrometers (microns).

If the photoplotter resolution is divisible by a power of two, it’s binary; use inches divided by that number.

For where x and y are different resolutions, my best recommendation is to tell them to round the dimension that’s out of measure in the right direction.

Of course, this still begs the question of what to do if you want to cast the net wide, and have your board manufacturable as cheaply as possible and by as many places as possible. It would be nice to have a nice neat report that lists installed base numbers of milling machines and photoplotters by their resolution, but I haven’t found anything.

9

Essentially, the whole PCB world is imperial.

Its just that in the last 30 years or so metric values for imperial measurements have become the norm :slight_smile:

The old equipment that many of the longer established fab houses uses imperial, but they like to tell you that they use metric lol

The resolution of your data is important when you are working with fine pitch, 2.3 is just too coarse.

ISTR Gerber format is recommended by UCAMCO to be to 6 decimal places. A new Gerber format is currently in draft, changing RS274-X to X2.

I suspect this is to combat the new IPC format.

And its all going to be ignored by the old guys doing boards that will continue to use RS-274 and imperial. Yet they can still get their boards as cheap as those done using metric measurements.