can it be that brass eyelet are used instead of copper?
I have seen them both fail in the same way.
I have seen eyelets fail near a PCB mount transformer. I wonder if it was maybe the heat like your saying not the mass of the transformer.
I thought their reliability was supposed to be superb if they are staked properly and soldered well. I wonder if maybe they don't get soldered good because its a mechanical component stored like screws that might be more tarnished then usual (the board is usually fresher) and it might not get soldered well enough. People might not treat it like a connector
I thought it would be better then a via because its normal copper.... I noticed that plated copper is not that great.
But I did notice they don't flare out as much as I like, that is, the rolled side of the eyelet is always ALOT better then the staked side in terms of being solderable. Big lap joint vs weird cone thing. If you don't press them down enough they will still solder but its a far cry from a 'nice' lap joint like you get on top on the rolled side.
But it does make me feel alot better given how much time and money i spent in trying to learn how to do electroplate vias, rather then just forgetting about it and regresing to eyelets haha
Did you ever see a turret terminal go bad? They are staked like eyelets. Not sure I ever ran into that, oddly enough. Could it be that people don't mess around with them, and stake them with the proper tools, wheras with eyelets they get creative with a center punch to save on a die? Or am I just lucky ?

Because when I started with eyelets, I used my own BS tools to press them. When I got turret terminals, I thought that its pretty specific so I got all the right tools. If you try to get cheap with the, its easy to damage them.
could it just be half assed staking? The tools are pretty expensive. I can totally see factory managers trying to hack that cost. Like zero doubt about it.
