Effect of Rare-Earth (La,Ce, and Y) Additions on the Microstructure and Mechanical Behavior of Sn-3.9Ag-0.7Cu Solder Alloy

In this article, we report on the microstructure and mechanical properties of Ce- and Y-containing Sn-3.9Ag-0.7Cu solders. The microstructures of both as-processed solder and solder joints containing rare-earth (RE) elements (up to 0.5 wt pct) are more refined compared to conventional Sn-3.9Ag-0.7Cu, with decreases in secondary Sn dendrite size and spacing and a thinner Cu₆Sn₅ intermetallic layer at the Cu/solder interface. These results agree well with similar observations seen in La-containing solders reported previously. The monotonic shear behavior of reflowed Sn-3.9Ag-0.7Cu-X(Ce, Y)/Cu lap shear joints was studied as well as the creep behavior at 368 K (95 °C). The data were compared with results obtained for Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-XLa alloys. All RE-containing alloys exhibited creep behavior similar to Sn-3.9Ag-0.7Cu. Alloys with Ce additions exhibited a small decrease in ultimate shear strength but higher elongations compared with Sn-Ag-Cu. Similar observations were seen in La-containing solders. The influence of the RE-containing intermetallics (CeSn₃ and YSn₃) that form in these alloys on the microstructural refinement, solidification behavior, and mechanical performance of these novel materials is discussed.     

添加微量稀土元素对Sn-Ag-Cu系无铅焊料性能的影响

以Sn-3．0Ag-0．5Cu（质量分数，％）焊料为母合金，探讨了微量稀土元素Ce、Er、Y和Sc对Sn—Ag—Cu合金物理性能、润湿性能以及力学性能的影响。试验结果表明：稀土元素对焊料的性能有不同的影响，添加微量Ce元素可以更好地改善焊料综合性能。     

Influence of the Substrate on the Creep of SN Solder Joints

The creep rate of Sn solder joints is noticeably affected by joint metallization. Cu|Sn|Cu joints have significantly higher creep rates than Ni|Sn|Cu joints, which, in turn, have higher creep rates than Ni|Sn|Ni joints. Replacing Ni by Cu on both substrates increases the creep rate at 333 K (60 °C) by roughly an order of magnitude. The increased creep rate appears with no apparent change in the dominant creep mechanism; the change in the constitutive equation for creep (the Dorn equation) is in the pre-exponential factor. The decreased creep rate on substituting Ni is accompanied by an increase in the hardness of the polygranular solder but a decrease in the nanohardness of the grain interiors. The source of the strong influence of the Ni substrate appears to be the introduction of an array of Ni₃Sn₄ intermetallic precipitates along the grain boundaries. These precipitates inhibit grain boundary sliding, boundary reconfiguration, and grain growth during creep. The intermediate creep rate of the asymmetric Ni|Sn|Cu joint has two causes: a decrease in grain boundary mobility due to precipitate decoration and a restriction in the free volume of the joint due to rapid intermetallic growth from the substrate on the Ni side. The sources of this anomalous intermetallic growth are discussed.     

Effect of Lanthanum on Driving Force for Cu6Sn5 Growth and Improvement of Solder Joint Reliability

By means of adding low content of rare earth element La into Sn6(bPb40 solder alloy, the growth of Cu6Sn5 intermetallic compound at the interface of solder joint is hindered, and the thermal fatigue life of solder joint is increased by 2 times. The results of thermodynamic calculation based on diffusion kinetics show that, the driving force for Cu6Sn5 growth is lowered by adding small content of La in Sn60-Pb40 solder alloy. Meanwhile, there is aneffective local mole fraction range of La, in which, 0.18% is the limited value and 0.08% is the best value.     

Microstructure, tensile and creep properties of as-cast Mg-3.8Zn-2.2Ca-xCe（x=0, 0.5,1 and 2 wt.%） magnesium alloys

The microstructure and mechanical properties of as-cast Mg-3.8Zn-2.2Ca alloy with different Ce contents were investigated by both optical and electron microscopy, X-ray diffraction, differential scanning calorimetry analysis, tensile and creep tests. The results indi-cated that adding 0.41 wt.%-1.83 wt.% Ce could refine the grains of the alloy, and the grain size gradually decreased as the Ce content in-creased. Furthermore, addition of either 0.41 wt.% or 0.89 wt.% Ce caused the morphology of the Ca2Mg6Zn3 phase to change partially from semi-continuous block to discrete fine particles. However, after adding 1.83 wt.% Ce, portions of the Ca2Mg6Zn3 and Mg12Ce phases were mixed and this Ca2Mg6Zn3+Mg12Ce eutectic changed to a different coarse semi-continuous morphology. In addition, addition of 0.41 wt.%-1.83 wt.% Ce improved the tensile and creep properties of the alloy. Amongst these Ce-containing alloys, the alloy with 0.89 wt.% Ce exhibited the best ultimate tensile strength and elongation while the alloy with 1.83 wt.% Ce had the best yield strength and creep properties.     

On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu

When solder interconnects are fabricated, a Sn-based alloy is melted between two substrates with metallization layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu₆Sn₅ intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The morphology of the IMC layer greatly influences the mechanical behavior of the solder joint. Here, we report on the characterization of a novel, asymmetric growth behavior of IMC layers in Sn-3.9Ag-0.7Cu solder joints, based on gravity-induced spalling of the IMC.     

Performance of MWCNT-Reinforced SAC0307/Cu Solder Joint Under Multiple Reflow Cycles

The evolution of interfacial microstructure and its effect on shear strength under multiple reflow cycles for multi-walled carbon nanotubes (MWCNT)-reinforced Sn0.3Ag0.7Cu solder/copper joint was investigated. The melting characteristics, wettability and mechanical properties of the solder alloy were assessed. The addition of MWCNT in the range of 0.01–0.05 wt% improved the wettability, melting behaviour and mechanical strength of the SAC0307 solder alloy. The nanoparticles in small weight fraction (0.01–0.05 wt%) addition were more effective in retarding intermetallic compounds growth at the interface. Amongst all compositions studied, the SAC0307–0.05MWCNT nanocomposite showed significant improvement in the performance of SAC0307/Cu solder joint under multiple reflow condition. The nanoparticles’ reinforcement above 0.1 wt% of the solder alloy was ineffective in improving the solder performance due to increased clustering in the matrix.     

Dissolution and interface reactions between the 95.5Sn-3.9Ag-0.6Cu, 99.3Sn-0.7Cu, and 63Sn-37Pb solders on silver base metal

Dissolution and intermetallic compound (IMC) layer development were examined for couples formed between 99.9 silver (Ag) and molten 95.5Sn-3.9Ag-0.6Cu (wt pct), 99.3Sn-0.7Cu, and 63Sn-37Pb solders, using a range of solder temperatures and exposure times. The interface reactions that controlled Ag dissolution were sensitive to the solder composition. The Ag₃Sn IMC layer thickness and interface microstructure as a whole exhibited nonmonotonic trends and were controlled primarily by the near-interface solder composition. The kinetics of IMC layer growth were weakly dependent upon the solder composition. The processes of Ag dissolution and IMC layer growth were independent of one another.     

Effect of La_2O_3 on microstructure and high-temperature wear property of hot-press sintering FeAl intermetallic compound

FeAl intermetallic compound with different contents of rare earth oxide La2O3 addition was prepared by hot pressing the mechanically alloyed powders.Effect of La2O3 on microstructure and high-temperature wear property of the sintered FeAl samples was investigated in this paper.The results showed that 1 wt.% La2O3 addition could refine the microstructure and increase the density of the FeAl intermetallic compound,and correspondingly improved the high-temperature wear resistance.SEM and EDS analyses of the wo...     

Solidification Condition Effects on Microstructures and Creep Resistance of Sn-3.8Ag-0.7Cu Lead-Free Solder

Metallurgical, mechanical, and environmental factors all affect service reliability of lead-free solder joints and are under extensive study for preparation of the transition from Sn-Pb eutectic soldering to lead-free soldering in the electronic industry. However, there is a general lack of understanding about the effects of solidification conditions on the microstructures and mechanical behavior of lead-free solder alloys, particularly on the long-term reliability. This study attempts to examine the creep resistance of the Sn-Ag-Cu eutectic alloy (Sn-3.8Ag-0.7Cu, SAC387) with a variety of solidification conditions with cooling rates ranging from 0.3 °C/s to 17 °C/s. Results indicate that solidification conditions have a major influence on the creep resistance of SAC387 alloy; up to two orders of magnitude change in the steady-state creep rates were observed at low stress levels. An understanding of the mechanical property change with microstructures, which are determined by the solidification conditions, should shed some light on the fundamental deformation and fracture mechanisms of lead-free solder alloys and can provide valuable information for long-term reliability assessment of lead-free solder interconnections. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

Effect of misch metal on elevated temperature tensile ductility of the Cu-Zn-Bi alloy

Cu-Zn-Bi alloy has been regarded as the most promising substitute for free-cutting leaded brass (Cu-Zn-Pb), which contains over 0.5 wt pct of hazardous Pb for the machinability of Cu. The Bi in Cu-Zn-Bi tertiary alloys, however, embrittles Cu because of its tendency to form a Bi film along phase boundaries, prohibiting the hot rolling over 271.4 °C, which is the melting temperature of BI. Misch (Ms) metal, which is a mixture of Ce, La, Nd, etc., can form a variety of stoichiometric compounds with Bi, and may reduce the Bi film formation along α and β phase boundaries. In the present study, the effect of Ms metal on tensile behavior of Cu-Zn-Bi alloys was examined at room temperature and 300 °C to quantify the host rollability. It was found that elevated temperature tensile elongation of Cu-Zn-Bi alloy was significantly improved, while the Bi film along phase boundaries was greatly inhibited, with the addition of 0.1 wt pct Ms metal. It was also noted that the Bi film along phase boundaries was greatly inhibited probably due to the formation of intermetallic compound between Ms metal and Bi. The addition of 0.3 wt pct Ms metal was, however, not beneficial to the tensile ductility of Cu-Zn-Bi alloy at 300 °C. Since the solubility of Ms metal in Cu is very low, the Ms metal, added ove r0.1 wt. pct, may therefore remain as impurities. These impurities would eventually reduce the tensile elongation of Cu-Zn-Bi alloy at room and particularly high temperatures.     

Influence of reflow and thermal aging on the shear strength and fracture behavior of Sn-3.5Ag solder/Cu joints

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 μm, brittle fracture between Cu₆Sn₅ and Cu₃Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.     

The effect of cerium on high temperature tensile and creep behavior of a superalloy

The presence of trace impurities such as S and O can cause embrittlement during elevated and high temperature deformation of iron or nickel-base alloys. In this study various amounts of Ce, ranging from 0 to 0.24 wt pct, have been added to an iron-nickel base superalloy, Udimet 901, in order to study its role in the refining process of S and O in the melt, hot workability, creep and stress rupture, and microstructure. It is found that Ce addition decreases the 0 and S content in the melt and improves both the hot workability and creep ductility. An optimum residual Ce content of 30 ppm was found for which ductilities are maximum. Higher residual cerium contents result in deleterious hot embrittlement. SEM as well as TEM/STEM microscopy combined with X-ray EDS spectroscopy were used to determine the inclusion content present in the alloy as well as the fine spatial microchemistries, especially at grain boundaries.     

Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints

This work presents an investigation on the influence of the solder/under bump metallization (UBM) interfacial reaction to the tensile strength and fracture behavior of Sn-3.5Ag/Ni-P solder joints under different thermal aging conditions. The tensile strength of Sn-3.5Ag/Ni-P solder joints decreases with aging temperature and duration. Four types of failure modes have been identified. The failure modes shift from the bulk solder failure mode in the as-soldered condition toward the interfacial failure modes. Kirkendall voids do not appear to affect the tensile strength of the joint. The volume change of Ni-P phase transformation during the thermal aging process generates high tensile stress inside the Ni-P layer; this stress causes mudflat cracks on the remaining Ni-P coating and also leads to its delamination from the underlying Ni substrate. In general, interfacial reaction and the subsequent growth of Ni₃Sn₄ intermetallic compound (IMC) layer during solid-state reaction are the main reasons for the decrease of tensile strength of the solder joints. The current study finds there is an empirical linear relation between the solder joint strength and the Ni₃Sn₄ intermetallic compound (IMC) thickness. Therefore, the IMC thickness may be used as an indication of the joint strength.     

Behaviors of Lanthanum and Cerium on Grain Boundaries in Carbon Manganese Clean Steel

The behaviors of La and Ce on gram boundaries in carbon manganese clean steel were investigated by high-reso- lution transmission elecetron microscope （HRTEM）, scanning elecetron microscopy（SEM ）, energy dispersive spectrometer （EDS） and X-ray diffraction（XRD） analysis. The existing forms of rare earths （RE） in clean steel were as follows： dissolved in sohd solution, forming inclusion or second phase containing RE （RE-Fe-P, La-P, Fe-La eutectic and Fe-Ce phase）. The dissolved La or Ce segregated at grain boundaries. The segregation of both S and P at gram boundaries was reduced with suitable RE content. The impact toughness of the steel was improved obviously. La and Ce had effecets on purifying molten steel and modifying inclusions in clean steel, whereas with excessive La or Ce, La-Fe-P, La-P and Fe-La eutecetic phase or Ce-Fe-P and Fe-Ce intermetallic compound would form along grain boundaries, causing the impact energy to decrease significantly.     

Comparison of effects of cerium, yttrium and gadolinium additions on as-cast microstructure and mechanical properties of Mg-3Sn-1Mn magnesium alloy

The effects of Ce, Y and Gd additions on the as-cast microstructure and mechanical properties of Mg-3Sn-1Mn alloy were investigated and compared by using optical and electron microscopies, X-ray diffraction analysis, and tensile and creep tests. The results indicated that the microstructure of the Mg-3Sn-1Mn alloy with the addition of 0.87 wt.%Ce was refined while the microstructures of the alloys with the additions of 0.79 wt.%Y or 0.84 wt.%Gd were coarsened. Furthermore, adding 0.87 wt.%Ce to the Mg-3Sn-1...     

The growth of Cu-Sn intermetallics at a pretinned copper-solder interface

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu₆Sn₅) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and ε phase (Cu₃Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to ε phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when the Sn is consumed; ε phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases. A.J. SUNWOO, Formerly with the Lawrence Berkeley Laboratory, Berkeley, CA,     

Effect of cerium content on the deformation behavior of rapidly solidified Al-Fe-Ce alloys

The deformation behavior of a rapidly solidified Al-8.9Fe-6.9Ce (wt pct) alloy was studied in the temperature range of 250 °C to 350 °C and stress range of 20 to 175 MPa. The stress exponents and activation energies suggest that the alloy exhibits a pronounced diffusional creep regime with a transition to power law creep behavior at stresses beyond 60 MPa. Comparing these data with those obtained earlier for an Al-8.8Fe-3.7Ce alloy, it was found that in the diffusional creep regime, the Ce content had no effect on the creep rate. However, in the power law creep regime, a strong dependence on the precipitate spacing, as predicted by the structureinvariant creep law,^[5] was observed. The higher volume fraction of precipitates in the Al-8.9Fe6.9Ce alloy causes a decrease in the power law creep rates by a factor of 5. Formerly Graduate Student. Formerly Assistant Professor, University of Illinois at Urbana-Champaign.