Sn-rich alloys, such as SnCu0.7, SnAg3.5 and SnAg3.4-4.1Cu0.45-0.9, are leading candidates to replace SnPb solders in electronic packaging. In these alloys, the properties of tin dominate the solder behavior because of the high content of tin (more than 95% weight percent). So it is important to understand the properties of tin. At the same time, in order to meet the multi-functional and high density packaging of electronic products, the feature size of solder joints has reached 50μm, comparable to the typical grain size of as-solidified tin alloys. At this scale, it is necessary to consider the influence of deformation behavior of single crystal and crystal orientations on the performance of solder joints.
In this study, [110] and [112] orientated β-Sn samples were prepared by Bridgman method and investigated in tensile experiments under different conditions. Major differences in deformation behavior were observed between these two oriented samples. The strain rate sensitivity exponent of [110] oriented sample is 0.13, which is higher than 0.11 of the [112] oriented sample. The work hardening exponent of [110] oriented sample is 0.54, also higher than that of the [112] oriented sample which is 0.46. Slip was found during deformation of [110] oriented sample, while both slip and twin were observed during deformation of [112] oriented sample. For the [110] oriented crystal, the primary slip system is {010}/<100>. From the value of the activation energy and slip geometry, it was concluded that the double cross slip of dislocations occurred during the deformation.
The linear relations between creep rate and stress were found through stress relaxation of tin single crystal. The stress exponents ranged from 8 to 12 and decreased as temperature or prestrain increased. The relaxation rates increased with the prestrain. The activation energy for the stress relaxation process was similar to that for self-diffusion along the c axis.
Extensive primary hardening stage was not found in tensile deformation of single crystal solder joints. The hardening resulted in a much higher strength and tensile ductility for the joint sample than those of the bulk crystal. The stress relaxation of the joint sample was slower than that of the bulk crystal. A much higher stress exponent was found in the joint sample as compared to the bulk crystal during stress relaxation. Stress exponents increased with prestrain and prestrain rate.
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