Thermomechanical behaviour of environmentally benign Pb-free solders

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Electronic packaging is a critical part of products such as computers, cellular phones, automotive components and other electronic devices. The package must be tailored to incorporate as many input/output interconnects as possible, in a limited amount of space. Until recently, most solder balls were made of a eutectic Pb-Sn alloy, because of its low melting point, excellent wetting characteristics and adequate creep and thermal fatigue strength. The potential health hazards associated with the toxicity of lead are significant. Given the widespread use of Pb-Sn solder in the manufacture and assembly of circuit boards, the development and reliability of new Pb-free solders is crucial for the successful substitution of these materials in the electronics industry. Pb-free solder alloys are complex materials with various important microstructural attributes. These include the nanoscale precipitates of Ag₃Sn in Sn-Ag-Cu or Sn-Ag alloys, as well as Cu₆Sn₅ intermetallic formed at the interface between the solder and Cu metallisation. The mechanical behaviour of solder alloys is extremely important because solder joints must retain their mechanical integrity under a myriad of conditions such as creep, thermal fatigue, and mechanical shock and drop resistance. A significant amount of work has been carried out on the monotonic shear, creep and thermal fatigue resistance of these materials. An important new area of research is the mechanical shock and vibration fatigue behaviour of Pb-free solders. The developments in all these areas are critically examined in this paper.