I was at the appliance repair shop the other day and saw heaps of repair orders piled on the front counter--electronic motherboards of various kinds.
I asked the tech what usually failed. I expected him to say "surge type stuff," but he answered "relays."
I grabbed the first board at hand to look at and sure enough, one of the ice-cube relays was plainly bad, but, not for the reason you'd think.
The solder connection had failed mechanically, from temperature cycling, then the open gap arced and carbonized some FR-4, and sputtered some carbon plume on parts nearby.
I also saw this same connection failure mode in my Acura's "main relay," where several connections failed. PbSn solder, multiple pins disconnected as if by a circular cutter.
When coil dissipation is high enough, temp cycling eventually cracks the solder around a connection pin. Once started, a crack propagates on a circle path until it encircles the pin. That being completed, the connection becomes intermittent (and exceedingly temperamental).
FYI, for long-term designs using relays. Caveat emptor.
Cheers, James Arthur