Since I deal with US colleagues a lot on the job (but rarely discussing electronics), I am curious: In the common US pronounciation, is the 'L' suppressed in all forms of the word? E.g. would you say "soddering iron" too?
Yes. That is the normal American pronunciation.
The only reason I asked my question was to inquire about whether the L pronunciation is starting to pop up in America. Apparently so, and the obvious reason is spelling pronunciation (that is, people want to pronounce the word the way it is spelled) together with YouTube suddenly enabling us to hear the word pronounced by people from elsewhere (many of them outside the USA). I did find one person on another forum for whom the L pronunciation had been normal, in Savannah, Georgia, since at least the 1970s.
The quote from Fowler (just above) is interesting. We know that the L originally was *not* pronounced when this word came into English. The history is:
(1) English borrows the French word 'souder' and pronounces it 'sodder'
(2) Spelling is changed to 'solder' on the basis of the Latin etymology (solidare)
(3) Pronunciation adds the L because of influence from the spelling.
Fowler must have observed British English before step (3) had run to completion over there. Step (3) never took place in America -- or so I thought -- but it seems to be doing it now.