Definition
Solder joint fatigue is the gradual mechanical failure of a soldered connection under repeated cyclic strain. Whereas a brittle fracture happens in one event, fatigue accumulates damage over thousands of cycles: micro-cracks nucleate at a stress concentration, grow a little with each cycle, and eventually sever enough of the joint that resistance climbs and the connection becomes intermittent or fully open. It is one of the most common reasons aged boards develop flaky, hard-to-trace faults.
Thermal versus mechanical cycling
The cyclic strain has two main sources. Thermal cycling drives strain through coefficient-of-thermal-expansion mismatch every time the assembly heats and cools. Mechanical cycling comes from vibration, fan-induced resonance, board flex during handling, or repeated connector mating. In both cases the solder, a relatively soft and creep-prone material, deforms plastically each cycle, and that plastic work is what consumes joint life.
Diagnosis and prevention
Fatigued joints often look dull, ringed, or visibly cracked under magnification, and a fault that comes and goes with temperature or light pressure is a classic symptom. Reworking the joint with fresh solder restores it, but the better defense is reducing the strain: dampen vibration, mechanically support heavy components and connectors, avoid unnecessary thermal cycling, and keep operating temperatures stable so the joint is not constantly working.
For the underlying driver of thermal-cycle damage, see our entry on thermal fatigue; for a defect present from assembly, see dry joint.
Trace intermittent faults with the diode & voltage reference.
In Simple Terms
Solder joint fatigue is the gradual mechanical failure of a soldered connection under repeated cyclic strain. Whereas a brittle fracture happens in one event, fatigue…