Controlling tin nucleation and grain orientations in Pb-free solders

Lead-free solder joints made with Sn-Ag-Cu or Sn-Ag solders usually contain only a single βSn grain or three twinned βSn grains which are oriented differently in every joint. Due to the anisotropy of βSn every joint therefore has unique thermomechanical properties and, in an array of numerous joints, it is likely that some will be poorly oriented and could cause early failure of a component. The problems of few grains with variable orientation can be attributed to the nucleation of βSn. This thesis explores catalyzing βSn nucleation using heterogeneous nucleation and develops methods to control βSn microstructures and orientations in Sn-3Ag-0.5Cu/Cu solder joints. The developed droplet nucleation technique in this study provides a new ‘motorway’ for heterogeneous nucleation study, and the introduced orientation control method paves the way to solve chronic problems, such as electromigration, thermomechanical fatigue, and shear fatigue in the electronic packaging industry. It is found that Co additions and Co substrates are effective at catalyzing βSn nucleation. This is demonstrated to be due to heterogeneous nucleation on αCoSn3 crystals. By using a ‘droplet nucleation technique’, in which Sn droplets are solidified directly on an intermetallic compound (IMC) particle, it is proved that αCoSn3 forms a reproducible orientation relationship (OR) with βSn that has a good lattice match. Strong grain refinement occurred in 60g Co-microalloyed Sn-3.0Ag-0.5Cu samples but only weak grain refinement occurred in 550μm solder balls and joints. When soldering Sn-3.0Ag-0.5Cu on Co substrates, βSn is always observed to grow from the interfacial αCoSn3 layer with an orientation inherited from the αCoSn3 layer texture. However, it is shown that nucleation on the αCoSn3 layer does not give useful βSn orientation control. The droplet nucleation technique was then applied to gain a deeper understanding of βSn nucleation on a range of IMC phases. A family of transition metal stannides, PtSn4, PdSn4, and βIrSn4 that have similar crystal structures to αCoSn3, were identified as potent nucleants for βSn. The common solder IMCs, Cu6Sn5, Ag3Sn, Ni3Sn4, were also investigated. It was found that reproducible ORs formed on all IMCs studied and the nucleation mechanisms were explored by combining nucleation undercooling measurements with measured ORs. The nucleation potency of all studied intermetallics is: αCoSn3>βIrSn4 >PdSn4>PtSn4 >Ni3Sn4> Ag3Sn, Cu6Sn5. The droplet nucleation technique also generated new insights into solidification twinning in solder joints. It was found that cyclic twins formed in droplets when the undercooling was sufficiently high and the liquid contained Ag, Cu and/or Ni. Complex interrelated cyclic twins were found in droplets on Cu6Sn5, Ag3Sn, and Ni3Sn4 where up to five rings of cyclic twins formed each related by a common . The twinning mechanisms in these cases were explored and discussed. The thesis then applies the new understanding developed in the previous chapters to develop a technique to reliably control the orientation of βSn in solder joints. Ball grid array (BGA) joints were fabricated reproducibly by introducing an extra step into the manufacturing process: bonding a nucleant IMC ‘seed crystal’ onto each Cu pad so as to control the nucleation location, nucleation undercooling and crystallographic orientation of βSn at the moment of nucleation. Each joint made by this technique had a uniform single-grain microstructure with the c-axis of βSn parallel with the substrate plane. This orientation is reported in the literature to give the best resistance to electromigration and shear fatigue.Open Acces

Fatigue assessment on local components of a semi-submersible platform subjected to wind and wave loads

The objective in this work is to assess fatigue damages on local components of a semi-submersible platform under combined actions of wind and wave loads in time domain. Some improvements are provided in the present study to improve the efficiency and accuracy of the whole evaluating process. Firstly, a combined wind and wave relationship as well as an innovative mixture simulation method are used to generate time series of random wind and waves. Moreover, an m-block division method is proposed to compress the number of the whole short-term sea states in the wind-wave scatter diagram. Then, with an improved multiple interpolation sub-model method, the structural stress responses of the local structural components are calculated as is in the whole model analysis. Finally, a modified rain-flow counting method is provided and validated to count the stress cycles efficiently and accurately. Thus, the short- and long-term fatigue damages are computed based on the S-N curve approach and the cumulative fatigue damage rule. In relative agreement with the numerical results by the traditional time-domain method and existing experimental data, these proposed improved methods are demonstrated to be applicable and efficient methods for fatigue damage analyses. All the fatigue damages on local components satisfy the specification requirements and the minimum value appears under the up wind-wave state, which is the proper working condition for a semi-submersible platform

Histomorphological study of the spinal growth plates from the convex side and the concave side in adolescent idiopathic scoliosis

Asymmetrical growth of the vertebrae has been implicated as one possible etiologic factor in the pathogenesis of adolescent idiopathic scoliosis. The longitudinal vertebral growth derives from the endochondral ossification of the vertebral growth plate. In the present study, the growth plates from the convex and concave side of the vertebrae were characterized by the method of histology and immunohistochemistry to evaluate the growth activity, cell proliferation, and apoptosis. Normal zoned architectures were observed in the convex side of the growth plate and disorganized architectures in the concave side. The histological grades were significantly different between the convex and the concave side of the growth plate in the apex vertebrae (P < 0.05). The histological difference was also found significant statistically between end vertebrae and apex vertebrae in the concave side of vertebral growth plates (P < 0.05). The proliferative potential indexes and apoptosis indexes of chondrocytes in the proliferative and hypertrophic zone in the convex side were significantly higher than that in the concave side in the apex vertebral growth plate (P < 0.05). There was a significant difference of the proliferative potential index (proliferating cell nuclear antigen, PCNA index) between convex side and concave side at the upper end vertebra (P < 0.05). The difference of the proliferative potential index and apoptosis index were found significant statistically in the concave side of the vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). The same result was also found for the apoptosis index (terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate biotin nick end labeling assay, TUNEL index) in the convex side of vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). Some correlation were found between radiographic measurements and proliferation and apoptosis indexes. The difference in histological grades and cellular activity between the convex and concave side indicated that the bilateral growth plate of the vertebrae in AIS patients have different growth kinetics which may affect the curve progression

Visualizing Conformational Space of Functional Biomolecular Complexes by Deep Manifold Learning

The cellular functions are executed by biological macromolecular complexes in nonequilibrium dynamic processes, which exhibit a vast diversity of conformational states. Solving the conformational continuum of important biomolecular complexes at the atomic level is essential to understanding their functional mechanisms and guiding structure-based drug discovery. Here, we introduce a deep manifold learning framework, named AlphaCryo4D, which enables atomic-level cryogenic electron microscopy (cryo-EM) reconstructions that approximately visualize the conformational space of biomolecular complexes of interest. AlphaCryo4D integrates 3D deep residual learning with manifold embedding of pseudo-energy landscapes, which simultaneously improves 3D classification accuracy and reconstruction resolution via an energy-based particle-voting algorithm. In blind assessments using simulated heterogeneous datasets, AlphaCryo4D achieved 3D classification accuracy three times those of alternative methods and reconstructed continuous conformational changes of a 130-kDa protein at sub-3 &Aring; resolution. By applying this approach to analyze several experimental datasets of the proteasome, ribosome and spliceosome, we demonstrate its potential generality in exploring hidden conformational space or transient states of macromolecular complexes that remain hitherto invisible. Integration of this approach with time-resolved cryo-EM further allows visualization of conformational continuum in a nonequilibrium regime at the atomic level, thus potentially enabling therapeutic discovery against highly dynamic biomolecular targets

A quasi-Z-source boost DC-DC converter

For problems of traditional boost converter including limited boost ability and high capacitor voltage stress, a boost DC-DC converter was proposed which used two quasi-Z-source networks in cascade. Working process of the converter was analyzed, and expressions of step-up ratio and capacitor voltage stress were derived. Compared with traditional Z-source boost converter, the quasi-Z-source converter can realize larger step-up ratio with small duty cycle, and has lower capacitor voltage stress under the same step-up ratio. The simulation and experimental results verify validity of the theoretical analysis

The Release of Endogenous Nitrogen and Phosphorus in the Danjiangkou Reservoir: A Double-Membrane Diffusion Model Analysis

Endogenous contamination from the newly submerged sediment may have an impact on the water quality of the Danjiangkou Reservoir, the water source of the middle route of the South-to-North Water Diversion Project. In this study, in situ sediment samples were collected at different locations (Guojiashan (DJK1), Kongqueyuan (DJK2), Shijiagou (DJK3), Shiqiao Wharf (DJK4), and Songgang Wharf (DJK5) from the north to the south) of the Danjiangkou Reservoir and the upstream tributary using a columnar sediment sampler and statically cultured in laboratory, and the distribution and release of endogenous N and P from the sediment were investigated based on a double-membrane diffusion model. The results showed that the P contents in the main reservoir and the upstream tributary followed the order of inorganic phosphorus IP>organic phosphorus OP>calcium−phosphorus Ca−P>iron−phosphorus Fe−P>labile phosphorus LP. IP accounted for the largest proportion (47.10-59.70%) of total phosphorus (TP), while LP accounted for only a small proportion (0.10-0.90%) of TP. There is a significant difference in the spatial distribution of P at different sampling points, especially for the OP content (226.90 mg·kg-1) with a coefficient of variation of 26.90%. The contents of different forms of P and NH4+-N decreased from the upstream tributary to the main reservoir, which was mainly attributed to the land use type. In the vertical distribution, the contents of different forms of P in DJK4 and DJK5 decreased with the increase of sediment depth. The contents of NH4+-N and PO43--P in the sediment interstitial water of DJK4 and DJK5 were higher than those in the overlying water. In addition, the contents of NH4+-N and PO43--P were higher in DJK5 than that in DJK4. The static culture experiments showed that N and P were mainly released from the sediment-overlying water interface to the overlying water. The release rate of NH4+-N and PO43--P ranged from 13.08 mg·(m2·d)-1 to 21.39 mg·(m2·d)-1 and from 3.06 mg·(m2·d)-1 to 6.02 mg·(m2·d)-1, and the release amount calculated based on the double-membrane diffusion model was 1.17×103 t·a-1 and 0.53×103 t·a-1, respectively. Thus, endogenous contamination from the newly submerged land is an important factor affecting the water quality of the Danjiangkou Reservoir