Reliability of solder joint and SMT assembly

In SMT the reliability of the soldered joint is significantly influenced by its mechanical stress, which appears as result of varying coefficient of thermal expansion of the main board of PCB and the components as both are heated during assembly, repairs, or operation. During the through-hole assembly the mechanical stress does not appear as the leads are sufficiently flexible which prevents change of size to cause any stress in the soldered joint. There is almost no flexibility in SMD components especially chip components and any types of packages for IC. Appearing forces are significant and can cause breaking of the joint, the body of component or tearing off the metal land from PCB.