measures delivered power
Let's think.. so a delivered energy counter?
1) Take the iron out the stand, magnetic stand-by function triggers a counter reset.
2) iron going to temperature --> counter should stay zero since power is delivered but it has nothing to do with soldering
3) Iron touch metal or melt solder ---> counter going up
Not sure when they turn the green LED on, if it is a simple counter reaching a threshold... how can they know I am touching a ground plane metal or I am melting solder? In both case the iron is dropping energy somewhere...
The point 2) can be managed knowing the typical profile of each tip.
Maybe they sense the drop in energy delivered when the joint reach the temperature tip --> I vote for this one.EDIT: I was wrong, it's the IMC formation.
That said, who needs that? You can't desing an idiot proof iron.
Oh I know, those quality eng. department will push to have a reproducible and documented hand soldering process... maybe it's smarter to save the money and invest in checking the final board?