As the root cause seems to be the board layout with large form factor SMT parts soldered over a position with high mechanical load (or more accurate strain), the problem will not be limited to a few units. I am afraid all units with this board layout problem have a weak spot there.
Agreed. When looking at the photo Kean took of a board outside the case, I couldn't help but think "what were they thinking". And they did it knowing full well that it might not be that strong, they tried to counter it a bit by not having the slots under L2 and L3 sit in line with eachother, but they are offset.
I would consider this a point that should be addressed at the next board revision. It might be even a reason to start a new board version.
Indeed, especially if dropping the meter, or inserting a fuse leads to problems in that area.
Some may fail early if the soldering is not the very best quality - failing solder may be the better case as resoldering can make it work again. With good solder joints the inductor itself can break - just like in the unit Dave showed.
I think now that maybe Dave's conclusions on the inductor were wrong. He speaks of black goo under the inductor, and indeed when you look at the remains of the inductor at first it looks like black goo. But if you look closer you can see the light shine through a piece of it sticking out from the underside of the inductor (first attachment, it's clear not black). There's also quite clearly residue on the good end cap of the inductor, which I previously called contamination
I think I should stand corrected on my remarks that these are good solder joints. They are still good in the sense that there has formed a meniscus, and the slightly gritty nature of the joint is inherent to some alloys of leadfree solder. But there's also some yellowish residue covering the joint that you can see at 15:35 when Dave scrapes it off the joint with his pointer (second attachment).
I have seen this residue form on the boards we produce as well, but they should not form in such a thick layer under that inductor, but rather puddle around the pads like you can see at 12:28 around the unpopulated transistor pads.
I don't think the solder profile was wrong, but perhaps the solder paste was past its use by date, or something like that. And in the weakest area of the board this will be the first where it will lead to problems. So perhaps the inductor was fine when it came from the tape after all, but it was damaged by strain later on.