Yeah so I was able to reflow it back in place. It looks like the root cause was the combination of two issues.
The lack of solder mask caused by my overlooking the solder mask options in board setup.
A heavy amount of HASL on the back side of the board.
Out of curiosity, are the bridged footprints all in the same orientation? The hot knife of the HASL only blows in one direction. I wonder if it wasn’t blowing as strongly as the other side, given one side had more solder than the other…
Yes. All the affected footprints were on the back of the board, same orientation. Always with the square pad connected to the middle pad.
More microscope pr0n follows, since people seem to like that.
Here’s a 2.5mm mounting hole, front side.
And here’s the same hole on the reverse.
Regarding my open via (sorry actually I think it was a thru hole pad for a transformer). This was a 2 layer board with interconnect trace on the top side. The transformer pin was soldered on the bottom but TH plating was broken. Solder (wave soldering process) did not wick up so the pin was not connected to the topside trace. I guess it is possible that mechanical stress might have broken the thru hole plating during or after the transformer was installed. But finding this open connection drove me nuts, and it was probably difficult to solder the top side of the pcb under the transformer to fix the problem.
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