Microstructural Evolution And Mechanical Characteristics Of Full Intermetallics Structured Cu/Sn/Cu(Ni) Microjoints

Soldering interconnection has played an essential role in the electronic manufacturing industry, ranging from the board-level packaging to the miniaturized 3D IC integration. Particularly, the continuously decreasing size and pitch of solder joints in the 3D IC stacking interconnection contributes to an increasing possibility of total consumption of Sn-solder for the interfacial reaction during the soldering process and service stage. As a result, the full intermetallic compounds(IMCs) microjoints are produced and the properties of IMCs interlayer would be critical to the interconnecting reliability. Therefore, this thesis has conducted focusing on the microstructural evolutionof IMCs interlayer and its properties, the main content and conclusions are shown as followings.The IMCs planar growth behaviour in microjoints was studied, which would induce some potential deficiency for the fine-pitch interconnection. The initial sandwich structures were prepared through electroplating Sn and Cu layer sequently on rolled-Cu and rolledNi substrate, as Cu/Sn/Cu and Ni/Sn/Cu structures, and the plated Sn layer acting as solder was only 2.5~5 ?m thick. After soldering under 260 °C for 30 mins, the IMCs planar growth phenomenon was observed evidently on the cross-section of microjoints. Two IMCs layers grew planarly from the up and down Cu/Sn interfaces above the cross-section, and touched each other around the center-line of microjoints. Meanwhile, two-layer structure appeared in each IMCs layer as some fine-grain Cu3 Sn underneath the cobblestone-shape-like Cu6Sn5 grains. In the soldered Cu/Sn(2.5 ?m)/Cu structure, there were some Cu-rich whiskers emerged around the Cu/Sn interface and the defect within IMCs interlayer. In the Cu/Sn/Cu micro-pillars made by focus ion beam(FIB), the massive migration of interconnected material outwards to the peripheral happened, driving by the concertration gradient and chemical reaction, then leaving some big holes inside. Moreover, for the reaction rates are different on Cu/Sn and Ni/Sn interfaces, the planar growth of IMCs in the Ni/Sn/Cu structure turned to be asymmetric.Regarding to the perpendicular growth of IMCs(be perpendicular to the Cu/interlayer interface), the Cu/Sn(2.5 ?m)/Cu structure was made by plating Sn and Cu layers on rolled-Cu and plated-Cu substrates seperately, and then the influence of soldering temperature/ time and the types of substrates on the microstructure of IMCs interlayer were investigated. The result shows that the rapid solidification of molten Sn layer suppressed the growth of Cu5Sn6 grains in scalloped-shape, then the compact Cu6Sn5 IMC microjoint was made. With the elevated soldering temperature or the prolonged dwelling time, the thickness of Cu6Sn5 layer in IMCs interlayer decreased while the thickness of Cu3 Sn layer inceased, however, this progress was constrained by the types of substrates. When the temperature rised from 240 °C to 290 °C, the growth factor of Cu3 Sn on the plated Cu substrate minished from 0.48 to 0.34. In contract, the value of Cu3 Sn growth factor increased from 0.29 to 0.36 on the rolled Cu substrate. Accordingly, the growth rate of Cu3 Sn was limited by the diffusion rate of Cu atoms from substrate into interlayer at lower temperature, while that was slowed down for the thick Cu3 Sn layer at higher temperature. And Kirkendall voids were only distributed in the Cu3 Sn layer adjacent to the plated Cu substrate. After soldering at 290 °C for 15 mins, the Cu/Sn(2.5 ?m)/Cu structure became the porous Cu3 Sn microjoint, which still had a good performance in the shear test.In particular, the homogenizing process of Cu3 Sn microjoints was analyzed by Electron Back-Scattered Diffraction(EBSD) technology. The cross-section morphologies of IMCs microjoints were observed, which formed with different dwelling time(10/30/50/70 mins) under 260 °C. The result reveals that the middle Cu6Sn5 layer disappeared through the phase transformation into Cu3 Sn in IMCs interlayer, and then the IMCs mixture formed as the coarsened Cu3 Sn grains surrounding by many ultra-fine Cu6Sn5 grains. With the dwell time prolonged, the ratio of Cu6Sn5 phase in IMCs interlayer decreased, while the growth and ripening of Cu3 Sn grains occurred simutanousely under the control of Cu atoms diffusion. As a result, the Cu3 Sn grains trended to be columnar grains perpendicular to the Cu/IMCs interface.The microstructural evolution process and mechanism of Ni/Sn(1.5 ?m)/Cu structure were also studied. Under 240 °C, the interlayer in these IMCs microjoints was constituted with three parts, such as the Cu-rich area on plated Cu substrate, to be(Cu,Ni)3Sn layer with the growth factor of 0.36; the middle Sn-rich area with(Cu,Ni)6Sn5 mainly, and the ultra-thin Ni-rich area on the Ni substrate. And the boundaries between different phases became blurred with the rising of soldering temperature. When the most of(Cu,Ni)6Sn5 phase transformed into(Cu,Ni)3Sn phase in the microjoints under 290 °C, some Ni-rich segments appeared around the center-line of IMCs interlayer, as well as the ones on Ni/IMCs interface with Ni up to 20 at.%. Overall, the result from this part shows that the diffusion behaviour of Cu and Ni in IMCs interlayer was significant to the microstructural evlution of microjoints and an obvious cross-interaction of interfacial reactions was observed in Ni/IMCs/Cu structure. Moreover, with the usage of nanoindentation machine, the Young's modulus and hardness of(Cu,Ni)6Sn5/(Cu,Ni)3Sn IMCs were measured as 160.6±3.1 GPa/ 7.34±0.14 GPa and 183.7±4.0 GPa/ 7.38±0.46 GPa respectively, which were higher than the velues of Cu6Sn5 and Cu3 Sn. The creep properties of them under 2000 ?N load were in the range of 3.5~5 nm at room temperature. Besides, some IMCs microcantilevers were designed and fabricated by FIB to proceed the in-situ micro/ nanocompression and bending tests. Both inter-and trans-granular brittle fracture happened in these IMCs interlayers which formed under 290 °C with different dwell time. And the compression fracture strength of them was around 2.46 GPa, the ultimate tensile strength was 2.3±0.7 GPa which was not sensitive with the change of loading rate and IMCs microstructure.