It's not 1.6mm, it's actually 1/16", and it's been the standard since the days of yore. It's a super common inch measurement so it's not really a surprise to see it here, if you know your inches. There's tons of 1/16" plywood, for example.
A "thick" PCB was commonly 3/32" (0.093") and a "thin" one 1/32" (0.031"). Now of course it's all made in Asia so the metric dimensions predominate (0.8mm, 1.6mm, 2.4mm), but the legacy remains.
And of course PCBs aren't really any of these thicknesses, they're whatever the glass weaves press down to in the lamination press....
That's really interesting, thanks.
Your answer prompted me to do some further research. It's true that IPC-2221A does reference 1/16th as a historical precedent.
That said...
Lawrence Berkeley is an American institution, and it's fair to say that your perspective is American-centric. Without rehashing the ages old argument, it is still true that the US stands with Liberia and Myanmar in your steadfast refusal to standardize on the metric system.
It's your prerogative to conclude that this is an Asian thing, but it's very much an almost everyone else thing.
I will concede that it drives me nuts that not only does the lumber industry still use inches here, it's also officially and somehow legally not even accurate in inches. Why we can't have nice things...
Where does LBNL come into this? The reference in IPC-2221 is to NBS Report 4283 from 1956. Neither of those have anything much to do with LBNL.
I reject your assertion that my comments are America-centric. This a technology that was primarily developed in America and is now predominantly no longer manufactured there (Asia is not in America). It is rather natural for each place to use the units of measure they favor, so it is natural for what was a very common customary unit dimension of 1/16" to become a reasonably round metric dimension of 1.6mm. This is how things develop in the world.
Thanks for calling that out; the IPC-2221A doc I was viewing was on the LBNL domain and my brain incorrectly inferred that the folks who had worked on the IPC spec were employed by LBNL.
Still, IPC is also based in America, which brings us to the same place. While every org has to be based somewhere, the notion that the entire world should use imperial measurements simply because America does is pretty much the definition of America-centric in my books.
The ground truth is that we're talking about a de-facto standard, not a hard one. Given the actual difference between 1.6mm and 1/16", the ease of working in base 10, PCBs are mostly made in Asia and the flogged horse that the entire rest of the planet thinks in metric... it's no surprise to me that 1/16" has become a historical footnote.