Influence of solder ball volume on mechanical shock reliability of right-angle solder interconnects

Abstract

With rapid development of modern electronic devices and products towards miniaturization and portability, different types of solder interconnects with distinct configurations are often used in electronic packages. Meanwhile, the volume of solder interconnects gets smaller and smaller. The right-angle solder interconnect, which exhibits asymmetric configuration, has been widely used for soldering temperature-sensitive components with the three dimensional (3D) configuration. In our previous work, the essential factors, such as the thicker tin oxide layer on the surface of solder balls and surface contamination on Au bonding pads which may influence the wettability of the molten solder ball on Au bonding pads, the burning failure of right-angle solder interconnects resulted by the reflection characteristic of the laser beam, were studied comprehensively. However, other factors, which resulted in the reliability deterioration even failure of right-angle solder interconnects, have not yet been investigated sufficiently, for instance, the fracture of solder interconnects induced by the mechanical shock during the transportation or under conditions such as accidental dropping. In this study, from the viewpoint of different diameters of solder balls used in the laser jet solder ball bonding (SBB), the key factors resulting in the above mentioned fracture failures were evaluated comparatively by the dropping test. The fracture characteristics were determined by means of scanning electron microscopy to reveal the mechanism. The results show that right-angle interconnects fabricated by Sn–3.0Ag–0.5Cu (SAC305) solder balls with a diameter of 60 μm has poorer mechanical shock resistance than that of large solder balls with a diameter of 70 μm after reflow soldering. Under the same test condition, the former fractures easier than the latter. Further, back-scattered electrons (BSE) images of the cross-sectional microstructure of the joints reveal that the interfacial intermetallic compound (IMC) occupies a relatively large area in the former than that in the latter. Due to brittleness of the IMC phase, the smaller (60 μm) ductile solder in solder interconnects can neither effectively absorb the mechanical shock energy nor alleviate the mechanical shock stress during the dropping test. Therefore, the right-angle solder interconnects fabricated by 60 μm SAC305 solder balls may fracture easily and finally induce damage of the solder interconnects and failure of the electronic products.