English term (edited): 30 u"

u is a common replacement for μ, " is inch

References:

Wiktionary:
microinch ‎(plural microinches)
A unit of measurement, the length of one one-millionth of an inch, equivalent to 25.4 nanometers. Symbol: µin
https://en.wiktionary.org/wiki/microinch

"How Many? A Dictionary of Units of Measurement":
u- a common replacement for µ- as a symbol for the SI prefix micro. The symbol is frequently seen in combinations such as ug for the microgram (µg). The use of u- was approved by the International Organization for Standardization in its standard ISO 2955, issued in 1974. The problem at that time was that many of the character sets in common use did not include the Greek letter µ. This is much less of a problem today, so ISO 2955 has been withdrawn and is no longer in effect. The message from all standards agencies now is: use µ-, not u-, for micro-.
© Russ Rowlett and the University of North Carolina at Chapel Hill
https://www.unc.edu/~rowlett/units/dictU.html

See also this article about printed circuit boards and their different finishes (for example gold):

"Hard Electrolytic Gold consists of a layer of gold plated over a barrier coat of nickel. Hard gold is extremely durable, and is most commonly applied to high-wear areas such as edge connector fingers and keypads.
Unlike ENIG, its thickness can vary by controlling the duration of the plating cycle, although the typical minimum values for fingers are 30 μin gold over 100 μin nickel for Class 1 and Class 2, 50 μin gold over 100 μin nickel for Class 3."

http://www.epectec.com/articles/pcb-surface-finish-advantage...