Why do so many PCB-designers use non-metric units? [closed]

Question

TLDR: Why do so many PCB-designers use non-metric units?

Sorry for asking this closed question again: what is the advantage of non-metric units

But I do not understand, and there is also no other answer for the linked question, why non-metric units used in so many places in electronic designs, for example for PCB and track sizes in mils, AWG for cables or copper thickness in oz/ft² (which is often written oz). And not mm, mm² or µm.

I see several disadvantages with imperial units or advantages of metric units:

• The SI-units use only Factors of 10^N, the imperial "system" have a lot of weird factors (like 12 for foot (for cable length) and inch (for track length))
• All complex Units exist only in the metric system like Volt: V = W/A = J/(s * A) = N * m/(s * A) = kg * m²/(s³ * A). And i think it would make sense to only use a single unit system. (so you don't end up with conversion errors or with a unit like H/ft = kg * m²/(A² * s² * ft) = 127/5000 kg * m/(A² * s²) ).
• Every one should know the metric system, but not everyone should know the imperial system. The Most imperial units are defined with metric units (example: 1 inch is defined with 25.4mm, so you must understand the meter before you can understand the unit inch).
• Every length written in inch or mils with a finite number of decimal digits can also be written exactly with a finite number of digits in a metric length, but not the other way around: (20 mils are exactly 508µm, 300µm are about 11.811023622 mils) (which can be a Problem when your PCB-tools use a internal inch-based grid).
• Copper thickness: in imperial units, the copper thickness is often written in oz, which is very confusing: oz are a units of mass (or one of many volume units). And you can not give a thickness in a unit of mass. There should write oz/ft². Which is still not very useful when you not know the density of copper. A thickness in µm is much more useful (for example, to calculate the resistance of a track).
• Some units in the Imperial System (like Ounce) still have significant different definitions and sometimes used for different physical quantity. For every metric units, the definitions did never change significant.

For the unit AWG, i see even more disadvantages to mm²:

• The unit only works with Natural number, when the wire is too big, the number do not go from 0 to -1, which would make sense, it go from 0 to 00. Which is unnecessary complicated, for example to handle it in a database (it is not a number, it is a string, and need a extra sorting algorithm (00>0>1>2)). If you use mm², You can just specify any real number and sort it correctly.
• The units have a wierd complicated formula of 127µm * 92^(36-n)/39 or 5mil * 92^(36-n)/39, with more or less random factors (92,36 and 39) (only works as long as n is positive), to get the thickness of a cable.
• With mm², the DC resistance is directly proportional to 1/A (A is the cross sectional area), and the maximum DC current can you estimate with about 6-9A/mm².

The people who start designing PCBs were engineers, so the did know all this things, why did they still choose to use imperial units? (It was even worse at this time, with more different definitions of imperial units).

Why i think this question is relevant for electrical engineering:

• The most common PCB-design tools, like Altium use a non-metric internal grid (which can lead to rounding errors).
• Shops labels there cables often in foot and AWG
• PCBs manufacturers often write there rules (for example minimum track size and minimum clearance) in mils or copper thickness in oz (when there really mean oz/ft²).

Answer 1

As with a lot of electrical engineering, the answer is tradition.

For one: it simply doesn't matter which system is used. The only advantage metric offers at all is that units of the same measurement factor by 10^N. Well, an inch is 10^3 mills, so that point is taken care of. Other than that, you never need to convert between units for anything. Copper weights stay in ounces, track sizes and board dimensions are all in mills or inches (which, again is scaling by a factor of 10^N).

Two: On your AWG point, that also doesn't matter because wires are manufactured in discrete sizes. You don't need a continuous unit like mm^2.

Three: Most of this was pioneered in the US, where imperial units were and still are dominant. It would take a ludicrous amount of effort for very little benefit to the people who work on them. All our standards etc. are in imperial. If we converted them, then what? It's all being produced and interpreted by American engineers who are used to imperial anyway, so what good is that?

Four: Any complex calculations are not done by hand, they're simulated in software so again, there's no need to ever convert between anything.