RE: Electronic/Circuit board experts, what causes this?
It’s called a “cold” joint, because it either didn’t really get hot enough when the board was assembled, or the surfaces weren’t clean enough. Time/environment can cause the solder to crack and/or fall out. The boards are soldered with a wave bath, so the larger connections are harder to get hot without overheating the smaller connections.
Fixed far more than I can count on electronics, but I’m leery of fixing them on something like a furnace where it could cause a house to freeze or something of that nature where I could be held liable for a repair. But it can be done easily with a good pencil and rosin core solder.