The Reliability of Surface Mounted Solder Joints Under PWB Cyclic Mechanical Stresses

The advent of Surface Mounting Technology (SMT) has led to concern about the long term reliability of solder joints during environmental thermal cycling, power cycling and PWB mechanical flexure. This paper describes the results of testing PWB assemblies of both through hole and SM components in a mechanical flexure jig in which the amplitude and frequency of cycling were varied. The apparent degradation of through hole solder joints (particularly to DIL devices) was often found to be worse than the SMT joints. Lower flexure cycle frequencies were much more damaging to all solder joints than higher frequencies. The results indicate that most SMT solder joints survive realistic flexure cycling very well but that poorly formed joints, which may appear visually to be acceptable, can fail prematurely. Solder joints to leadless ceramic chip carriers are the most susceptible to flexure stresses and special precautions should be taken in their application to substrates which are not entirely rigid, such as PWBs.