This sort of board damage also used to be common in old CRT TVs. Vibration from various sources including the internal speaker(s), mangetostriction in inductors or piezeoelectric deformation of capacitors would fatigue solder joints. Eventually one or more would go dry and a barely visible micro-crack would form in an annular ring round the component lead. If it was in a high current circuit, Joule heating due to the increased resistance at the crack would eventually melt the solder on the pin and the annular ring of fillet remaining on the pad, but due to the crack surfaces having been oxidised due to the slow heating, the joint would not reform, and surface tension would pull the solder on the pin away from the pad and the remaining solder on the pad away from the pin. The crack would blacken and become far more obvious. If enough voltage was available for the crack to arc over, the melting, burning and resulting track and component damage could become extensive, typically with severe laminate damage and carbonisation for typically up to 1/2" around the original faulty joint.
In your case, the vibration from the relay, and possibly the blower if both are in the same cabinet will have contributed to the joint going bad. The relay should be replaced as the pin heating has probably compromised its mechanical support in the relay body, and if its loose, contact movement will fatigue your repaired joint. Remove all carbonised PCB, make good with Epoxy (with a non-conductive filler) and repair the track. Make sure your repair is well secured as if the fault recurs, the wire or foil track repair could detach and cause a short circuit.
If there was a wave soldering process problem, its very common to see suspect joints across much of a board. If you are trying to actually locate an intermittent dry joint fault rather than 'shotgunning' it by bulk re-soldering, and not being certain you've actually found all the faulty joints, try heating the suspect pin, on the pin, not the joint fillet with a freshly tinned bit, without applying any side force. If the solder on the pin reflows without the annular ring of solder on the pad melting, the joint had a complete full-circle crack and was certainly bad. If the whole joint reflows it was probably O.K. - touch it up if it needs it and keep on looking for the fault. Once the fault is located, repaired and soak tested, any remaining dodgy looking joints can be resoldered with reasonable confidence the fault wont recur in the near future.
Home
Help
Search
Login
Register
