Contact failures-Relay failures-Failure of
Soldered joints can fail in many ways like electromigration and formation of brittle intermetallic layers. Some failures show only at extreme joint temperatures, hindering troubleshooting. Thermal expansion mismatch between the printed circuit board material and its packaging strains the part-to-board bonds; while leaded parts can absorb the strain by bending, leadless parts rely on the solder to absorb stresses. Thermal cycling may lead to fatigue cracking of the solder joints, especially with elastic solders; various approaches are used to mitigate such incidents. Loose particles, like bonding wire and weld flash, can form in the device cavity and migrate inside the packaging, causing often intermittent and shock-sensitive shorts. Corrosion may cause buildup of oxides and other nonconductive products on the contact surfaces. When closed, these then show unacceptably high resistance; they may also migrate and cause shorts. Tin whiskers can form on tin-coated metals like the internal side of the packagings; loose whiskers then can cause intermittent short circuits inside the packaging. Cables, in addition to the methods described above, may fail by fraying and fire damage.
Every time the contacts of an electromechanical relay or contactor are opened or closed, there is a certain amount of wear. An electric arc occurs between the contact points (electrodes) both during the transition from closed to open (break) or from open to closed (make). The arc caused during the contact break (break arc) is akin to arc welding, as the break arc is typically more energetic and more destructive.
The heat and current of the electrical arc across the contacts creates specific cone & crater formations from metal migration. In addition to the physical contact damage, there appears also a coating of carbon and other matter. This degradation drastically limits the overall operating life of a relay or contactor to a range of perhaps 100,000 operations, a level representing 1% or less than the mechanical life expectancy of the same device.