Microstructural considerations for ultrafine lead free solder joints

As solder joints become increasingly miniaturised to meet the challenging demands of future electronic packaging, it is vitally important to consider whether the solder joint size and geometry could become a reliability issue and thereby affect the implementation of the Pb-free solders. In this study, different bumping techniques, e.g., solder dipping, stencil printing followed by solder reflow, and electroplating of solders and subsequent reflow, were used to investigate the microstructure and interfacial interactions of molten Sn and Sn-based Pb-free solders on different metallizations, e.g., copper and electroless nickel immersion gold (ENIG). The resultant microstructures from a variety of pad sizes, ranging from 1 mm down to 25 μm, and representing different solder bump geometries have been investigated. In addition, thermodynamic and combined thermodynamic-kinetic modelling has been used in order to understand the microstructure of Pb-free solders, the kinetics of dissolution of the metallizations and the formation of interfacial intermetallic compounds (IMCs). Both the experimental results and theoretical predictions suggest that the solder bump size and geometry can influence the as-soldered microstructure. In the light of the increasing importance of the microstructural features of the ultrafine solder joint in determining its long term reliability, a novel computational interface between software for thermodynamic calculations, high-level scientific computing and multiphysics modelling, is introduced. This modelling methodology provides a potential platform for microstructure-based Finite Element (FE) reliability modelling of ultrafine interconnects for future microelectronic products.