Effects of POSS on the interfacial reactions between Sn-3.5Ag solders and Cu substrates during soldering

More extensive application of electronic products lead electronic packaging trend to be miniaturization and multi-functional. With prohibiting of solder containing SnPb, lead-free solder now has been widely utilized. However, there are still a plenty of reliability issues exist in it such as high melting point, poor wettability, more apparent electromigration effect and heat transfer effect. Thus, it is necessary to find comprehensive approach to improve the overall performance of lead-free solders. Besides, the volumes of solders at the electrical interconnects have been limited to be microscopic level due to the miniaturisation of circuitry proportionally while the volume fraction of intermetallic compounds (IMCs) was increased in the microstructure of solder joints. Such as, the proportion of IMCs in a solder continually increases while the electronic packaging size decreased, which could deteriorate the solder reliability. Adding reinforced particle is one of effective ways to improve the performance of solder joints. POSS, a kind of reinforced particles with Si-O cage structure, could be a good connection with metal atoms, highly dispersed distribution in the solder, effectively hinder dislocation motion. However, the influence of POSS on IMCs structure was rarely considered. In this paper, the effects of POSS reinforcing particles on IMCs microstructure and crystal orientation evolution were inversitgated during the interfacial reactions between Sn-3.5Ag solders and Cu substrates. The growth and solidification are considered during the experiment by blocking the melting solder from natural cooling or controlling the melting parameters to distinguish the enhancement effect of POSS. To explore the true effect of POSS, no addition solder group is set with respective melting condition all the same but not adding reinforcing particles. As the result shows, adding reinforcing particle like POSS into Sn3.5Ag makes scalloped Cu6Sn5 achieve planarization, which results from that the average thickness of the intermetallic compounds was significantly suppressed by the reinforcing particles. During the growth behavior, as soldering takes longer time, Cu diffuses more inside of solder and reacted more entirely with Sn, resulting in interfacial IMC size becomes larger. POSS reinforcing particles inhibit Ag from diffusing, resulting in the inhibition of Ag3Sn formulation. POSS inhibits Cu from diffusing inside of solder, blocking Cu6Sn5 growing into solder, and POSS makes scalloped Cu more level and smooth, decreasing interfacial energy of Cu6Sn5. Cu6Sn5 is growing in the process of solidification, and once solder added POSS its Cu6Sn5 will be limited from growing into the insider of solder, which decreases the interfacial surface energy of Cu6Sn5. Besides, POSS makes interfacial IMC more level and smooth, which increases the strength of solder and Cu connections and reduce the possibility of crack at interfacial surface in solder, improving the performance of solder.