Micromechanical Behaviour of Lead-free Solder Joints Developed for 3D-IC Packaging Applications

Abstract

The objective of this thesis is to understand the micromechanical behaviour of lead-free solders, and to that end the thesis covers everything from the conventional ASTM-standard techniques to the indentation characterization techniques. The tensile properties of SnAgCu and Mo-reinforced SnAgCu composite (hereafter referred to as composites) solders are investigated over a range of temperatures and strain rates, to obtain the multivariable thermo-mechanical models. These models can be used to predict the magnitude of the mechanical properties for given reliability conditions. This study also extends to understanding the impact of solder specimen size (down to 500 um) on the mechanical properties, using ultra-low load microtensile testing equipment. It was found that mechanical properties were reduced by 10?15% for pure Sn and Sn-5Pb solders, while for composite solders this variation was within experimental error. Nanoindentation experiments were conducted on these micron-sized tensile specimens at the equivalent strain rates (of the microtensile testing) to predict the tensile properties using analytical models. The yield strength was measured using the Tabor relationship, and microtensile test results were compared with nanoindentation analysis. As a further extension of this work, the nanoindentation technique was used to determine the mechanical properties of solder joints as small as 100um. Time-dependent deformation of the solder joints was studied using nanoindentation techniques. Indentation creep analysis based on Garofalo?s model was used to obtain the creep parameters and was compared with other analytical models. Different types of indenter geometries (the Berkovich and Cylindrical punch) over a range of indentation loads were investigated to confirm the creep properties measured using the indentation method. The mechanical properties of interfacial IMCs in the solder joints, from the ball grid array to the microbump-joint regime, were determined using the nanoindentation technique. Taper-sectioning methodology effectively demonstrates the determination of interfacial IMC layer thicknesses as low as 500nm. Substrate effects due to underlying metallic/intermetallic layers on the measured elastic modulus, and the hardness of interfacial IMCs, were separated and eliminated using Stiffness-indentation depth and suare of Stiffness-Load analysis.