The monoclinic lattice distortion of η′-Cu6Sn5

Abstract

In commonly used Sn-based solder systems the η′-Cu6Sn5 phase, which develops during field operation, is of huge interest concerning solder joint reliability in electronic components. For all mentioned results in this paper, the η′-Cu6Sn5 was produced via solid state interdiffusion by employing an industrial like set up. The specimens were characterized via SEM, EDS and XRD measurements. The main focus was set on getting really precise lattice parameters, which gave rise to the problem of partially poor refinements during XRD evaluations. Strongly overlapping fundamental reflections of different occurring phases lead to the danger of getting trapped into false, local minima during refinement. For that reason an unusual refinement method was applied, which only considers the superstructure reflections of the η′-Cu6Sn5 phase. Thereby, the obtained precise lattice parameters could be used for calculating the spontaneous strain tensors of the η′-Cu6Sn5. The concept of the spontaneous strain initially introduced for ferroelastics was applied to quantify the metric distortion of the η′-Cu6Sn5 phase. The calculated eigenvectors of the corresponding strain tensors reveal the direction of the most prominent distortion, which concludes referred to the NiAs/Ni2In structure in a contraction in the [110]h and an expansion in a corresponding, perpendicular [11¯0]h direction. Therefore, the corresponding eigenvalues (−0.00088 and 0.00080) of η′-Cu6Sn5 are 5–6 times smaller than the ones from the comparable structure Ni3Sn2. Overall, a refinement strategy for evaluating lattice parameters based on XRD pattern of η′-Cu6Sn5 was worked out, which is also applicable for other problematic superstructures. Hence, the obtained really precise lattice parameters could be used to characterize for the first time the exact monoclinic metric and therefore the monoclinic distortion of the η′-Cu6Sn5 phase.