Thermal fatigue is defined as thermal expansion of a section or the entirety of parts bundled together leading to degradation or cracking of materials caused by repeated heating and cooling. Thermal cycling during the testing or normal working life of boards can cause thermal fatigue of solder joints. Thermal fatigue causes lowering the life of lead-free solder joints.

If thermal fatigue is severe, it can damage or crack to lead-free solder joints. Thermal fatigue causes serious reliability issues in case of solder joints of surface mount components.

Many researchers found that thermal fatigue is more in many lead free solder joints compared to that of conventional Tin/Lead (SnPb) solder joints.

In case of Aerospace/Defense applications, it is advised to use Lead (Pb) based solder alloys to mitigate the risk of thermal fatigue.

If RoHS/lead-free compliance is to be strictly followed (for applications other than Aerospace/Defense), the lead free solder alloy shall be carefully selected from which have very good thermal properties, such as Tin-Silver-Copper (Sn3.5Ag0.7Cu).