Factors influencing current density distribution and current crowding in flip chip solder joints

Abstract

Three-dimensional simulations were carried out to investigate the current density distribution in the flip chip solder joints. There were two kinds of factors influencing the current density distributions in the flip chip solder joints, i.e. the structural geometry and the material's properties. It is found that increasing the dimensions of trace and under bump metallisation (UBM) and/or decreasing the resistivity of trace, UBM and solder bump are beneficial to relieve the current crowding. The experimental results also show that microvoids were formed at the cathode side of Cu/Sn–37Pb/Cu and narrower Cu trace Cu/Sn–3.0Ag–0.5Cu/Cu structures, but not at that structure with a wider Cu trace under the same test conditions. However, increasing the dimensions of the solder joint unrestrictedly departs from the miniaturisation trend of integrated circuit. But this difficulty can be overcome by optimising the above factors synthetically with finite element modelling method. The results also partly account for a better electromigration (EM) resistant ability of Sn–Ag–Cu than Sn–Pb solder.

Keywords:

• ELECTROMIGRATION
• CURRENT CROWDING
• FEM
• SOLDERING
• INTERFACIAL STRUCTURE