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Title: Microstructural and mechanical characteristics of micro-scale intermetallic compounds interconnections
Author: Mo, Liping
ISNI:       0000 0004 6062 9740
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2016
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Following the continually increasing demand for high-density interconnection and multilayer packaging for chips, solder bump size has decreased significantly over the years, this has led to some challenges in the reliability of interconnects. This thesis presents research into the resulting effects of miniaturization on the interconnection with Sn-solder, especially focusing on the full intermetallics (IMCs) micro-joints which appear in the 3D IC stacking packaging. Thereby, systematic studies have been conducted to study the microstructural evolution and reliability issues of Cu-Sn and Cu-Sn-Ni IMCs micro-joints. (1) Phenomenon of IMCs planar growth: The planar IMCs interlayer was asymmetric and composed of (Cu,Ni)6Sn5 mainly in Ni/Sn (2.5~5 μm)/Cu interconnect. Meanwhile, it was symmetric two-layer structure in Cu/Sn (2.5~5 μm)/Cu interconnect with the Cu3Sn fine grains underneath Cu6Sn5 cobblestone-shape-like grains for each IMCs layer. Besides, it is worth noticing that the appearance of Cu-rich whiskers (the mixture of Cu/Cu2O/SnOx/Cu6Sn5) could potentially lead to short-circuit in the cases of ultra-fine ( < 10 μm pitch) interconnects for the miniaturization of electronics devices. (2) Microstructural evolution process of Cu-Sn IMCs micro-joint: The simultaneous solidification of IMCs interlayer supressed the scalloped growth of Cu6Sn5 grains in Cu/Sn (2.5 μm)/Cu interconnect during the transient liquid phase (TLP) soldering process. The growth factor of Cu3Sn was in the range of 0.29~0.48 in Cu-Cu6Sn5 diffusion couple at 240~290 °C, which was impacted significantly by the type of substrates. And the subsequent homogenization process of Cu3Sn grains was found to be consistent with the description of flux-driven ripening (FDR) theory. Moreover, Kirkendall voids appeared only in the Cu3Sn layer adjacent to Cu-plated substrate, and this porous Cu3Sn micro-joint was mechanically robust during the shear test. (3) Microstructural evolution of Cu-Sn-Ni IMCs micro-joint: There was obvious inter-reaction between the interfacial reactions in Ni/Sn (1.5 μm)/Cu interconnect. The growth factor of (Cu,Ni)3Sn on Cu side was about 0.36 at 240 °C, and the reaction product on Ni side was changed from Ni3Sn4 into (Cu,Ni)6Sn5 with the increase of soldering temperature. In particular, the segregation of Ni atoms occurred along with phase transformation at 290 °C and thereby stabilized the (Cu,Ni)6Sn5 phase for the high Ni content of 20 at.%. (4) Micro-mechanical characteristics of Cu-Sn-Ni IMCs micro-joint: The Young s modulus and hardness of Cu-Sn-Ni IMCs were measured by nanoindentation test, such as 160.6±3.1 GPa/ 7.34±0.14 GPa for (Cu,Ni)6Sn5 and 183.7±4.0 GPa/ 7.38±0.46 GPa for (Cu,Ni)3Sn, respectively. Besides, in-situ nano-compression tests have been conducted on IMCs micro-cantilevers, the fracture strength turns out to be 2.46 GPa. And also, the ultimate tensile stress was calculated to be 2.3±0.7 GPa from in-situ micro-bending tests, which is not sensitive with the microstructural change of IMCs after dwelling at 290 °C.
Supervisor: Not available Sponsor: 7th European Community Framework Program
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Intermetallics ; Microstructure ; Phase transformation ; Electron backscatter diffraction (EBSD) ; In-situ micro-mechanical test