This sounds typical of what I have seen on single sided circuit boards. The holes are not plated through so the solder joint is very weak. Thermal cycling in particular will cause the circular crack you describe. I have seen this often in TV's, on boards with hot devices on them. I suppose vibration might cause a similar problem. I wouldn't think that relay would run that hot with intermittant use. Not sure why they put that circuit through a relay anyway. Maybe that other circuitry in the relay runs hot for some reason.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra