Investigation of interfacial behavior in miniaturized solder interconnects

In the present study the influence of intermetallic compound growth on mechanical and thermal response of miniaturized lead free solder joints was investigated. The focus of the study was the relationship between the solder gap size and thickness of the intermetallic compound (IMC) on tensile behavior of Cu/Sn3.5Ag0.75Cu/Cu solder joints with different ratios of IMC to the gap size and temperature boundary conditions. Independent of the IMC to solder gap thickness ratio a steady increase of tensile strength with decreasing gap size was observed. The reason for this behavior is the so called constraint effect which is well explained by plasticity models. The variation of the IMC size was realized by different reflow times or heat treatments. An increasing ratio of IMC thickness to the gap size results in a transition of the fracture mode from ductile to brittle and affects the strength of the solder joint. Thereby, embrittlement of the material seems to be a consequence of short diffusion paths introduced by defects and failures as confirmed by finite element analysis. Furthermore, thermal experiments revealed that the overall coefficient of thermal expansion of the joints decreased with increasing the IMC proportion of the joints. This study shows that the mechanical response and failure modes of thin solder joints are strongly dependent on the interaction between the proportional thickness, inherent defects and morphology of the IMC layer in addition to the constraint effect.