Influence of solder joint constitution and aging process duration on reliability of lead-free solder joints under vibrations combined with thermal cycling

Mechanical resistance of a solder joint to vibrations and thermal cycling strongly depends on its structure. Defects such as cracks, empty spaces, voids, non-uniform layers of intermetallic compounds (IMCs) can decrease mechanical strength of a solder joint significantly. Constitution of IMC layers depends on joined materials. Formation of an IMC layer can go on after a soldering process. Storage or working temperature have a great impact on thickness of formed IMC layers. Stress accumulated in solder joints during reflow process or vibrations or temperature cycles used to be relieved by cracks. Solder joints crack usually along the boundary between IMC layers and a bulk of solder. Within the frame of the study several commercial solder alloys and pad coatings were evaluated. Tested solder joints were subjected to microscopic analysis (SEM) as well as to accelerated reliability tests. During the study an influence of pad coating constitution and aging process duration on durability of tested solder joints was evaluated.