Bi-Sn熔体在高熵合金上的润湿行为及界面特征

采用传统座滴法研究了低熔点合金(Bi-Sn)和高熵合金(AlCoFeNiCr和CuCoFeNiCr)之间的润湿行为及界面特征。借助扫描电子显微镜(SEM)和能谱分析(EDS)分析了Bi-Sn/AlCoFeNiCr和Bi-Sn/CuCoFeNiCr界面微观结构。结果表明:AlCoFeNiCr和CuCoFeNiCr高熵合金都是结构单一的固溶体,但Bi-Sn熔体在CuCoFeNiCr高熵合金基体上的润湿性明显地优于Bi-Sn熔体在AlCoFeNiCr高熵合金基体上的润湿性;Bi-Sn/CuCoFeNiCr界面发生剧烈的化学反应,有大量的界面反应物生成,Bi-Sn熔体中的原子Sn主要是沿着CuCoFeNiCr高熵合金中的富铜相扩散,而Bi-Sn/AlCoFeNiCr界面平直,且随着润湿温度的升高,Bi-Sn熔体中的原子向AlCoFeNiCr高熵合金基体的扩散程度加强并伴随化学反应,出现类似"皮下潜流"现象;由于CuCoFeNiCr高熵合金中富铜相的存在,为Bi-Sn在CuCoFeNiCr高熵合金基体上的铺展提供了"润湿通道"。     

Microstructural development of eutectic Bi-Sn and eutectic In-Sn during high temperature deformation

Eutectic Bi-Sn and In-Sn solder joints were subjected to high temperature deformation in shear in order to determine whether microstructural instabilities are generated during testing. Dynamic recrystallization had previously been observed in Sn-Pb solder joints during creep and fatigue in shear. The current study shows that Bi-Sn can recrystallize during deformation in creep or at constant strain rate, whereas no microstructural changes are observed in In-Sn. Recrystallization of Bi-Sn is concentrated in a narrow band along the length of the sample, parallel to the direction of shear strain, similar to behavior in Sn-Pb. The recrystallization appears to proceed by migration of interphase boundaries rather than by a nucleation and growth mechanism. A minimum total strain is required to induce obvious recrystallization in Bi-Sn, independent of applied stress or strain rate. This value of strain is much higher than the strain at initiation of tertiary creep or at the maximum shear stress. Onset of tertiary creep and strain softening occur as a result of nonuniform deformation in the samples that is independent of the microstructural instabilities. The creep behavior of In-Sn is relatively straightforward, with a single creep mechanism operating at all temperatures tested. The creep behavior of Bi-Sn is temperature-dependent. Two mechanisms operate at lower temperatures, but there is still some question as to whether one or both of these, or a third mechanism, operates at higher temperatures.     

Intermetallic compound layer growth by solid state reactions between 58Bi-42Sn solder and copper

Solid state intermetallic compound layer growth was examined following ther-mal aging of the 58Bi-42Sn/Cu couple for a temperature range of 55 to 120°C and time periods of from 1 to 400 days. The intermetallic compound layer was comprised of sublayers that included the traditional Cu₆Sn₅ stoichiometry as well as one or more complex Cu-Sn-Bi chemistries. The number of sublayers increased with aging temperature and time. Time-dependent layer thickness computations based upon the empirical expression, Atⁿ + B, revealed a time exponent, n, that decreased with increasing temperature from a maximum of 0.551 at 70°C to 0.417 at 120°C. The apparent activation energy for growth (at 100 days) was 55± 7 kJ/mol. The Bi-Sn/Cu data, together with that from the other solder/copper systems, suggested that at a given homologous temperature, the quantity of Sn in the solder field determines the intermetallic compound layer thickness as a function of time.     

Surface tension measurements of the Bi-Sn and Sn-Bi-Ag liquid alloys

The maximum bubble pressure method has been used to measure the surface tension of pure Bi, surface tension and density of liquid binary Bi-Sn alloys (X_Bi = 0.2, 0.4, 0.6, and 0.8 molar fractions) at the temperature range from about 500 K to 1150 K. Similarly, there were investigated ternary alloys adding to the eutectic (3.8/at.%Ag-Sn) 0.03, 0.06, 0.09, and 0.12 molar fractions of Bi. The linear dependencies of densities and surface tensions on temperature were observed and they were described by straight-line equation. It has been confirmed that the additions of Bi to liquid Sn and to the eutectic alloy (3.8at.%Ag-Sn) markedly reduce the surface tension. Experimental data of the surface tension of liquid Bi-Sn were compared with modeling based on Butler’s method and a reasonable agreement was observed.     

The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 μm)/Ni/Ti UBM, and it was confirmed to be the (Au_1−x−yBi_xNi_y)Sn₂ phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 μm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.     

Fabrication of bismuth oxide-tin oxide nanowires by direct thermal oxidation of Bi-Sn eutectic nanowires

Bismuth oxide-tin oxide (BiO_x-SnO_x) heterostructure nanowires with a diameter of 70 nm were fabricated by directly annealing Bi-Sn eutectic nanowires synthesized by the vacuum hydraulic pressure injection process. After removal of AAO (Anodic Aluminum Oxide) template with an etching solution, a spontaneous oxide was formed on nanowires to enclose the Bi-Sn eutectic alloys. While these nanowires went through the annealing process with the proper heating rate of 50 °C/min, the well-annealed oxide nanowires remain solid, straight and segmental. The results of cathodoluminescence (CL) spectrum and photoresponse proved that the products consisted of bismuth oxide and tin oxide. This fabrication methodology provides a simple way to produce one-dimensional oxide nanomaterials.