Elastic properties, hardness, and indentation fracture toughness of intermetallics relevant to electronic packaging

摘要

Many intermetallics, such as Ag_3Sn, AuSn_4, Cu_3Sn, Cu_6Sn_5 (η and η′)> Ni_3Sn_4, and γ-Cu_5Zn_8 are present in modern solder interconnects as a result of solder chemistry and/or due to the interfacial reaction between solder and metallization scheme. Coarse-grained, single-phase intermetallics are produced by conventional casting followed by annealing for long time. Ambient temperature isotropic elastic moduli (bulk, Young's, shear, and Poisson's ratio) and selected plastic properties (hardness and indentation fracture toughness) of these intermetallics are presented. The isotropic elastic moduli of these intermetallics are determined by the pulse-echo technique. The measured bulk, Young's and shear moduli lie in the range of 6.3 to 11.4 x 10~(10) N/m~2, 7.1 to 12.3 x 10~(10) N/m~2 and 2.7 to 4.5 x 10~(10) N/m~2, respectively. The hardness and fracture toughness are determined by an indentation method. The loads used for indentation experiments were: 100-10,000 g for Ag_3Sn and γ-Cu_5Zn_8, 10-50 g for AuSn_4, 200-1000 g for Cu_3Sn, 50-100 g for Cu_6Sn_5, and 100-200 g for Ni_3Sn_4. The measured Vickers hardness lies in the range of 50 to 470 Kg/mm~2, and the measured indentation fracture toughness lies in the range of 2.5 to 5.7 MPa m~(1/2). Due to coarse grain size of the specimens, the indentation cracks were contained within one grain. In Cu_3Sn, Cu_6Sn_5 (η and η′) and Ni_3Sn_4 intermetallics, the indentation cracks were found to be nearly straight and run along the indent diagonal. However, the cracks in AuSn_4 showed significant zig-zag and branching phenomena, and they seemed to propagate along particular cleavage plane(s). The presence of slip bands are also observed in AuSn_4, Ag_3Sn, Cu_3Sn, γ-Cu_5Zn_8, and Ni_3Sn_4. In the case of Ag_3Sn and γ-Cu_5Zn_8, indentation cracks cannot be induced by applying loads up to 10 kg. Rather, extensive plastic deformation occurs resulting in the formation of a large number of shear/kink bands, and possibly twins, that spread across several grains. At a load of 5000 g or higher, Ag_3Sn exhibits grain boundary decohesion near the indents. Among the intermetallics studied, Ag_3Sn is shown to be the most ductile.