Single Crystal Plasticity Finite Element Analysis of Cu 6 Sn 5 Intermetallic
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microelectronic packaging, solder joints work as the mechanical and electrical interconnects. Sn-Cu-Ag (SAC) alloys are the primary type of solder material used in fabrication of microelectronics second level interconnects. SAC alloys are mostly Sn-rich alloys with slight addition of Ag and/or Cu. When using in microelectronics as solder ball/joint at the interface of copper plates, SAC alloy can contain Sn-rich dendrites surrounded by an eutectic mixture of Cu6Sn5, Cu3Sn, and Ag3Sn intermetallic (IMC) phases whose amount depends largely on reflow temperature, reflow time, cooling rate,[1] and thermal aging.[2] The presence of IMC layers (Cu6Sn5 and Cu3Sn) are necessary for good metallurgical bonding. However, excessive IMC layers can lower the reliability of solder joints due to their brittle nature. Continuous miniaturization of the packages and joints SOUD FARHAN CHOUDHURY, Graduate Student, and LEILA LADANI, Associate Professor, are with the Department of Mechanical Engineering, University of Connecticut, Storrs, CT 062693139. Contact e-mail: [email protected] Manuscript submitted July 22, 2014. Article published online December 9, 2014 1108—VOLUME 46A, MARCH 2015
has inevitably brought volume fraction of intermetallics to such an extent that now smaller solder joints could even completely transform into IMCs.[3] Not only the size of new joints has diminished thereby increasing the IMCs fraction, but also it was shown by Islam and Sharif that the volume fraction of IMC layers in smaller joints is much larger than that of the bigger joints.[4] Thus, characterization of Cu-Sn IMCs is necessary to determine mechanical behavior of these joints. Cu6Sn5 is the main IMC layer with an effective thickness. This material can be in two different forms of crystal structure, Monoclinic and Hexagonal Closed Pack. At room temperature, Cu6Sn5 is usually in HCP form.[5,6] Miniaturized solder joints may contain very few grains of Cu6Sn5 which could introduce the anisotropic behavior in system. Previous studies reported the changes in mechanical performance including elastic modulus and yield strength of Cu6Sn5 with respect to crystallographic orientation.[7,8] As the deformation mechanisms are modulated by small-scale heterogeneities, in order to understand the behavior completely and to optimally design such microstructures, it is important to develop analytical tools that assist their detailed understanding. A more detailed understanding of the inelastic deformation mechanisms in Cu6Sn5 is vital for METALLURGICAL AND MATERIALS TRANSACTIONS A
successful applications. Although due to the interactions between different deformation mechanisms that prevail in HCP crystal structure,[9] modeling mechanical response is a challenging task. HCP crystal structure generally contains 3 slip modes—basal {0001} 11 20 , prismatic 1010 11 20 ; and pyramidal hai 10 11 11 20 which provide deformation to hai direction only. Furthermore, 1st order pyramidal hc + ai {10 11} h11 23i and 2nd order pyramidal hc + ai {102
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