Nanoindentation measurements and mechanical testing of as-soldered and aged Sn-0.7Cu lead-free miniature joints

Nanoindentation testing has been used to analyze local mechanical parameter changes across interfaces formed between the solder and the Cu substrate and also in the eutectic solder, in miniature lead-free soldered joints. Effects of plastic deformation and aging were investigated. The lead-free solder alloy used in this study is commercially available and has nominal composition by weight of 99.3% Sn and 0.7% Cu. During aging, the joints were exposed to 150 degrees C for 1000 h in an inert atmosphere. Tensile testing of as-soldered miniature joints shows strong positive dependence of the plastic flow stresses on strain rate, meaning that the stresses increase with strain rate. Similar behavior was observed for aged miniature soldered joints with similar to 10% decrease in the level of maximum stresses, compared to as-soldered joints. Indentation hardness and modulus were measured in soldered joint components, in the as-soldered, aged conditions, and tensile tested with strain rates in the range of 1.8 x 10(-3)-1.8 x 10(-1) s(-1). Scanning electron and optical microscopy were employed to analyze the fracture paths and microstructure of the as-soldered and aged miniature joints, as well as the location and shape of the indentations. The measured indentation hardness and modulus agreed well with previous studies on similar alloys. The tested modulus of the intermetallic phase in the eutectic area exhibited a considerable reduction as compared to the intermetallic phase at the interface between the solder and the Cu substrate. Strain rate strongly influences local mechanics: for both as-soldered and deformed miniature joints; close to the fracture face in the eutectic solder, the indentation modulus values were 53% higher and hardness more than 100% higher in joints exposed to the highest strain rate relative to the smallest rate. (C) 2010 Elsevier B.V. All rights reserved.