Dispersion strengthening for dimensional stability in low-melting-point solders

An improved solder structure with an ultrafine grain size of ∼200–500 nm and significantly enhanced mechanical properties has been created by incorporating nanosized, nonreacting, noncoarsening oxide dispersoids into solder alloys. These solders display up to three orders of magnitude reduction in the steady-state creep rate, 4–5 times higher tensile strength at low strain rates, and improved ductility under highstrain-rate deformation. With a dispersion of TiO₂ particles, the Pb-Sn eutectic solder with a low-melting point of 183°C can be made more creep resistant than the Au-20Sn eutectic solder with a much higher melting point of 278°C. This technique can be extended to other solder systems, such as the emerging lead-free solder alloys, and used to achieve enhanced dimensional stability. For more information, contact H. Mavoori, Bell Laboratories, Lucent Technologies, 1A-102 Bell Labs, 600 Mountain Avenue, Murray Hill, New Jersey 07974; e-mail hareesh@lucent.com.     

Understanding the Influence of Copper Nanoparticles on Thermal Characteristics and Microstructural Development of a Tin-Silver Solder

This paper presents and discusses issues relevant to solidification of a chosen lead-free solder, the eutectic Sn-3.5%Ag, and its composite counterparts. Direct temperature recordings for the no-clean solder paste during the simulated reflow process revealed a significant amount of undercooling to occur prior to the initiation of solidification of the eutectic Sn-3.5%Ag solder, which is 6.5 °C, and for the composite counterparts, it is dependent on the percentage of copper nanopowder. Temperature recordings revealed the same temperature level of 221 °C for both melting (from solid to liquid) and final solidification (after recalescence) of the Sn-3.5%Ag solder. Addition of copper nanoparticles was observed to have no appreciable influence on melting temperature of the composite solder. However, it does influence solidification of the composite solder. The addition of 0.5 wt.% copper nanoparticles lowered the solidification temperature to 219.5 °C, while addition of 1.0 wt.% copper nanoparticles lowered the solidification temperature to 217.5 °C, which is close to the melting point of the ternary eutectic Sn-Ag-Cu solder alloy, Sn-3.7Ag-0.9Cu. This indicates the copper nanoparticles are completely dissolved in the eutectic Sn-3.5%Ag solder and precipitate as the Cu₆Sn₅, which reinforces the eutectic solder. Optical microscopy observations revealed the addition of 1.0 wt.% of copper nanoparticles to the Sn-3.5%Ag solder results in the formation and presence of the intermetallic compound Cu₆Sn_5. These particles are polygonal in morphology and dispersed randomly through the solder matrix. Addition of microsized copper particles cannot completely dissolve in the eutectic solder and projects a sunflower morphology with the solid copper particle surrounded by the Cu₆Sn₅ intermetallic compound coupled with residual porosity present in the solder sample. Microhardness measurements revealed the addition of copper nanopowder to the eutectic Sn-3.5%Ag solder resulted in higher hardness.     

The Effect of Wetting Gravity Regime on Shear Strength of SAC and Sn-Pb Solder Lap Joints

The failure of solder joints due to imposed stresses in an electronic assembly is governed by shear bond strength. In the present study, the effect of wetting gravity regime on single-lap shear strength of Sn-0.3Ag-0.7Cu and Sn-2.5Ag-0.5Cu solder alloys reflowed between bare copper substrates as well as Ni-coated Cu substrates was investigated. Samples were reflowed for 10 s, T _gz (time corresponding to the end of gravity regime) and 100 s individually and tested for single-lap shear strength. The single-lap shear test was also carried out on eutectic Sn-Pb/Cu- and Sn-Pb/Ni-coated Cu specimens to compare the shear strength values obtained with those of lead-free alloys. The eutectic Sn-Pb showed significantly higher ultimate shear strength on bare Cu substrates when compared to Sn-Ag-Cu alloys. However, SAC alloys reflowed on nickel-coated copper substrate exhibited higher shear strength when compared to eutectic Sn-Pb/Ni-coated Cu specimens. All the substrate/solder/substrate lap joint specimens that were reflowed for the time corresponding to the end of gravity regime exhibited maximum ultimate shear strength.     

Joining of Cu,Ni, and Ti Using Au-Ge-Based High-Temperature Solder Alloys

Au-Ge-based solder alloys are promising alternatives to lead containing solders due to the fact that they offer a combination of interesting properties such as good thermal and electrical conductivity and high corrosion resistance in addition to a relatively low melting temperature (361 °C for eutectic Au-28Ge at.%). By adding a third element to the eutectic Au-28Ge alloy not only the Au content could be reduced but also the melting temperatures could be further decreased. In this study, in addition to the eutectic Au-28Ge (at.%) two ternary alloys were chosen from the Au-Ge-Sb and Au-Ge-Sn system, respectively. The soldering behavior of these alloys in combination with the frequently used metals Cu, Ni, and Ti was investigated. The interface reactions and microstructures of the joints were characterized in detail by SEM and EDX analysis. For the determination of the mechanical properties, shear tests were conducted. Mean shear strength values up to 104 MPa could be achieved.     

Metallic materials for structural applications beyond nickel-based superalloys

This paper reviews our current research activities on developing new multiphase metallic materials for structural applications with a temperature capability beyond 1,200°C. Two promising material systems have been chosen: first, alloys in the system Mo-Si-B which have demonstrated potential due to their high melting point of around 2,000°C and due to the formation of a protecting borosilicate glass layer on the surface at temperatures exceeding 900°C; and second, novel Co-Re-based alloys which have been chosen as a model system for complete miscibility between the elements cobalt and rhenium, offering the possibility of continuous increases of the melting point of the alloy through rhenium additions.     

Wetting of Cu and Al by Sn-Zn and Zn-Al Eutectic Alloys

Wetting properties of Sn-Zn and Zn-Al alloys on Cu and Al substrates were studied. Spreading tests were carried out for 3 min, in air and under protective atmosphere of nitrogen, with the use of fluxes. In the case of Zn-Al eutectic, spreading tests were carried out at 460, 480, 500, and 520 °C, and in the case of Sn-Zn eutectic at 250, 300, 350, 400, 450, and 500 °C, respectively. Solidified solder/substrate couples were cross-sectioned and subjected to microstructure examination. The spreading tests indicated that the wetting properties of eutectic Sn-Zn alloys, on copper pads do not depend on temperature (up to 400 °C), but in the lack of protective atmosphere, the solder does not wet the pads. Wettability studies of Zn-Al eutectic on aluminum and copper substrates have shown a negative effect of the protective nitrogen atmosphere on the wetting properties, especially for the copper pads. Furthermore, it was noted that with increasing temperature the solder wettability is improved. In addition, densities of liquid solders were studied by means of dilatometric technique.     

Microstructural characteristics of Nb–Si based ultrahigh temperature alloys with B and Hf additions

Four Nb–Si based ultrahigh temperature alloys with compositions of Nb-22Ti-16Si-5Cr-3Al-mHf-nB ((m, n) = (0, 0), (0, 2), (4, 0) and (4, 2), respectively) (at.%) were prepared by vacuum non-consumable arc melting and then heat-treated at 1450 °C for 50 h. The effects of B and Hf additions on the phase selection, phase stability, microstructure and microhardness of these alloys under both as-cast and heat-treated conditions have been investigated. The results show that the microstructures of all the four alloys are composed of primary silicide blocks, Nbss dendrites together with one or two types of eutectic colonies around. However, the crystal structures of silicides, types of eutectic and amounts of constituent phases have obviously varied with B or Hf addition. Moreover, a low melting point three-phase eutectic is also observed in Hf-containing alloys. After 1450 °C/50 h heat-treatment, the microstructural uniformity of the alloys has been significantly ameliorated as well as their equilibrium phases have been basically obtained, and also some phase transformation reactions have occurred. The microhardness of the constituent phases present in the alloys is dependent on their types or crystal structures.     

Influence of rapid solidification on Sn–8Zn–3Bi alloy characteristics and microstructural evolution of solder/Cu joints during elevated temperature aging

The effects of rapid solidification on the microstructure and melting behavior of the Sn–8Zn–3Bi alloy were studied. The evolution of the microstructural characteristics of the solder/Cu joint after an isothermal aging at 150 °C was also analyzed to evaluate the interconnect reliability. Results showed that the Bi in Sn–8Zn–3Bi solder alloy completely dissolved in the Sn matrix with a dendritic structure after rapid solidification. Compared with as-solidified Sn–8Zn–3Bi solder alloy, the melting temperature of the rapid solidified alloy rose to close to that of the Sn–Zn eutectic alloy due to the extreme dissolution of Bi in Sn matrix. Meanwhile, the adverse effect on melting behavior due to Bi addition was decreased significantly. The interfacial intermetallic compound (IMC) layer of the solder/Cu joint was more compact and uniform. Rapid solidification process obviously depressed the formation and growth of the interfacial IMC during the high-temperature aging and improved the high-temperature stability of the Sn–8Zn–3Bi solder/Cu joint.     

Interrelation of wettability–microstructure–tensile strength of lead-free Sn–Ag and Sn–Bi solder alloys

A comparative investigation on the wettability and tensile strength of a Sn–2Ag, a Sn–40Bi and the traditional eutectic Sn–Pb solder alloys was carried out. The wettability is represented by thickness of covered layer (TCL) and spread area (SA) while the mechanical behaviour by the ultimate tensile strength (UTS). It is shown that the TCL of studied alloys decreased with the increase in the dipping temperature. It is also shown that TCL and SA have opposite behaviour with respect to the cooling rate. The Sn–Bi solder alloy has lower SA when compared with those of the Sn–Ag solder when similar cooling rates are considered. The Sn–Bi solder exhibits the best UTS/SA combination for dendritic spacings between 25 and 27?µm, associated with cooling rates ～2°C?s^?1, 2× lower than those of the Sn–Ag alloy. Besides, the Sn–Bi alloy has shown SA >70～80% associated with higher UTS (～80?MPa) as compared with the other alloys examined.     

高温合金差示扫描量热分析(DSC)的影响因素研究III：合金状态和升降温速率

摘 要：对固溶强化型镍基高温合金625进行升、降温差示扫描量热分析（DSC）试验,研究了同一合金不同状态（粉末态、粉末+热等静压态和铸态）以及升/降温速率（5-10℃/min）对相变温度的影响。采用场发射扫描电镜（FESEM）、电子探针（EPMA）对不同状态625合金的微观组织和元素分布进行表征。结果表明：（1）铸态比粉末态合金的枝晶间距大2个数量级,而热等静压态合金为无枝晶偏析的细等轴晶结构。（2）升、降温速率对DSC曲线中加热时基体开始熔化（等于固溶强化型合金的初熔温度）和冷却时开始凝固温度（偏离基线的拐点）无影响,但对合金加热熔化结束、冷却时大量凝固析出温度（峰位）和终凝温度（拐点）有明显影响。采用加热、冷却曲线相应相变温度平均值的方法可减少DSC试验和样品条件的影响,获得相对固定且更具可比性的合金相变温度。（3）合金状态对初熔温度和DSC升温曲线固相线温度附近的圆弧段有明显影响。根据DSC加热曲线固相线温度附近的圆弧大小可以判断合金的偏析倾向,弱偏析倾向的粉末态和热等静压态PM625合金DSC加热曲线固相线附近区域拐点尖锐,表现为合金开始熔化温度（偏离基线的拐点）与名义固相线温度（切线交点）差异很小,分别仅为5℃和6℃;偏析倾向较大的铸态IN625合金的DSC加热曲线中固相线温度附近区域为较大圆弧,开始熔化温度与名义固相线温度差异可达52℃。铸态625合金的初熔温度比热等静压态和粉末态分别低45℃和40℃,在实际热处理和热等静压等热工艺参数选择时应注意圆弧段较大的合金降低初熔温度的影响。在所有DSC冷却曲线中,由于完全熔化重新凝固消除了合金原始显微组织特征,不同状态625合金固相线温度区域附近曲线形态相似,均为较大的圆弧。     

Design of new face-centered cubic high entropy alloys by thermodynamic calculation

A new face-centered cubic (fcc) high entropy alloy system with non-equiatomic compositions has been designed by utilizing a CALculation of PHAse Diagram (CALPHAD) - type thermodynamic calculation technique. The new alloy system is based on the representative fcc high entropy alloy, the Cantor alloy which is an equiatomic Co- Cr-Fe-Mn-Ni five-component alloy, but fully or partly replace the cobalt by vanadium and is of non-equiatomic compositions. Alloy compositions expected to have an fcc single-phase structure between 700 °C and melting temperatures are proposed. All the proposed alloys are experimentally confirmed to have the fcc single-phase during materials processes (> 800 °C), through an X-ray diffraction analysis. It is shown that there are more chances to find fcc single-phase high entropy alloys if paying attention to non-equiatomic composition regions and that the CALPHAD thermodynamic calculation can be an efficient tool for it. An alloy design technique based on thermodynamic calculation is demonstrated and the applicability and limitation of the approach as a design tool for high entropy alloys is discussed.     

Effect of thermal exposure on microstructure and mechanical properties of Al−Si−Cu−Ni−Mg alloy produced by different casting technologies

The effect of thermal exposure at 350 °C for 200 h on microstructure and mechanical properties was investigated for Al−Si−Cu−Ni−Mg alloy, which was produced by permanent mold casting (PMC) and high pressure die casting (HPDC). The SEM and IPP software were used to characterize the morphology of Si phase in the studied alloys. The results show that the thermal exposure provokes spheroidization and coarsening of eutectic Si particles. The ultimate tensile strength of the HPDC alloy after thermal exposure is higher than that of the PMC alloy at room temperature. However, the TEPMC and TEHPDC alloys have similar tensile strength around 67 MPa at 350 °C. Due to the coarsening of eutectic Si, the TEPMC alloy exhibits better creep resistance than the TEHPDC alloy under studied creep conditions. Therefore, the alloys with small size of eutectic Si are not suitably used at 350 °C.     

Formulation and technology research of new silver solder with flux covering

In their eutectic compositions, Au–Si alloys have a melting point of 369°C, and a colour similar to that of high-grade gold. Results of the manufacture of gold alloys with 2.5%, 2.8% and 3.0% silicon, by means of melting in a plasma furnace with an inert argon atmosphere, are presented in this study. Chemical composition characterisation was carried out by energy dispersion spectroscopy and wave dispersion spectrometry, and metallographic and microstructural analysis by optical microscopy, SEM and DRX, Vickers hardness and mircohardness testing, melting point evaluation by DSC and determination of the SCIELab colorimetric coordinates. The results of the hardness and micro-hardness testing gave values around 110HV, presenting an important increase in the mechanical properties with respect to the traditional high purity gold alloys. The melting temperature for the alloys was around 374°C as well as being a heat very close to the values of pure gold.     

微量Ni对Sn0.7Cu钎料物理性能及钎焊工艺性能影响

SnCu共晶钎料是公认的SnPb钎料最具潜力替代品,尤其在波峰焊上,但与其他无铅钎料相比,该钎料物理性能及铺展性能差,严重影响其广泛应用.本文通过在Sn0.7Cu合金基础上添加微量Ni来改善合金性能.结果表明:随Ni含量(w(Ni)＜0.7％)增加,Sn0.7Cu钎料熔点逐渐增加,但变化不大,当w(Ni)0.7％时,钎...     

新型无铅焊料合金Sn-Zn-Ga的研究

以Sn-Zn合金为母合金，添加Ga元素，得到了新型的无铅焊料合金。测量了其熔点、硬度、剪切强度和可焊性等性能。研究发现，Ga元素的添加使焊料的熔点降低，熔程增大。焊料的硬度和剪切强度有所降低。焊料的铺展率增大，浸润角减小，提高了焊料的可焊性。通过实验研究确定了具有较好综合性能的焊料的成分范围。     

Back Matter

The present work was performed on twenty-one alloys containing Al-11.5 wt% Si, with magnesium (Mg) in the range of 0.1–0.4 wt%, and copper (Cu) in the range of 1.0–3.0 wt%. Fluidity measurements and thermal analysis for each of these alloy melts were carried out. The alloys were cast in the form of tensile test bars. The test bars were solution heat treated at a temperature of ~500°C for 8h, then quenched in hot water (60°C), followed by artificial ageing at 155°C for 5 h, and then cooling in air. The effects of Mg and Cu additions on the tensile properties, depression in the Al-Si eutectic temperature, and microstructural characteristics (Si and Cu-phase particle characteristics and morphology) have been discussed in detail. The results show that the addition of Mg decreases the fluidity and the eutectic Si temperature. While addition of Cu also decreases the eutectic temperature, the fluidity, however, is increased. The presence of Mg and Cu decreases the modifying effect of Sr on the Si particles due to an increase in the solidification time, as well as the Sr, Mg, Cu interactions that occurs as a result of these additions. Mg additions in the range of 0.1–0.4 wt% increase YS (from 22% up to 94%) and UTS (from 7% up to 52%) and decrease the percent elongation (40%) depending on the Cu content of the alloy, i.e., the higher the Cu content, the lower the increase in strength. Addition of Cu has a similar effect on YS and UTS at alloy Mg levels of 0.1 wt% only, with no effect at higher Mg values, while elongation continuously decreases. The volume fraction of Al₂Cu phase increases by approximately 0.76% for every 1 wt% increase in Cu. This observation is important in the selection of the appropriate solution heat treatment regime in order to avoid incipient melting.     

Cu和Sb元素添加对Sn-Bi共晶合金性能的影响

向Sn-Bi共晶合金中同时添加Cu和Sb元素设计额定温度为142℃的易熔合金,并对合金的熔点、相组成、准静态拉伸性能、焊接接头力学性能进行了研究.结果表明,Cu与Sb元素的添加使合金的熔点上升,但是合金的过冷度和熔化潜热下降.添加Cu和Sb元素后,在合金基体内形成了块状的SnSb相和长条状的Cu6Sn5、Cu3Sn相,...     

Sn-Ag-Cu-In无铅钎料润湿性能及显微组织

在Sn-1.2Ag-0.6Cu合金中添加0～1.0%的In元素,研究了Sn-Ag-Cu-In无铅钎料的熔化温度、润湿性能及显微组织.结果表明,添加In元素对钎料的熔化温度有一定的降低作用,在相同的试验温度下,含铟钎料粘度更低,流动性更强,钎料的润湿性能随之改善.低银Sn-Ag-Cu钎料在空气中的润湿时间在260℃才达到...     

Effect of homogenization treatment on microstructure and hot workability of high strength 7B04 aluminium alloy

The effects of homogenization treatment on microstructure, overbumt temperature and hot rolling plasticity of high strength 7B04 aluminium alloy were investigated. Under the condition of homogenization at 470 ℃, the starting melting temperature of the primary eutectics in ingot of non-equilibium solidified 7B04 alloy is 478 ℃. Using two-step homogenization processing at ultra-high temperature which comprises heating the ingots to 470 ℃ at 10 ℃/h and holding for 64 h, and then heating to 500 ℃ at 1 ℃/h and holding for 10 h, the ingots of 7B04 aluminium alloy could safely pass the sensitive overbumt zone between 480 ℃ and 495 ℃, and the ordinary burnt phenomena of the ingots between 480 ℃ and 495 ℃ does not occur because the excess low-melting point eutectic phases in the as-cast alloy dissolve into the matrix during the two-step homogenization processing. Consequently, the hot rolling plasticity of ingot of 7B04 aluminium alloy is greatly improved.     

Observations of nucleation catalysis effects during solidification of SnAgCuX solder joints

While modification of a strong (high Cu) Sn-Ag-Cu (SAC) solder alloy with a substitutional alloy addition (X=Co, Fe, Zn, and Ni) for Cu has been demonstrated to enhance solder joint strength and ductility after aging at 150°C for 1,000 h, control of the as-solidified SAC+X solder joint microstructure is also needed to inhibit under cooling and nucleation of brittle pro-eutectic phases (e.g., Ag₃Sn). Bulk undercooling measurements of SAC+X alloys and microstructural analysis of SAC+X solder joints were used to rank the effectiveness and consistency of low-level (X < 0.15 wt.%) substitutional additions to a base SAC composition, Sn-3.5Ag-0.95Cu (wt.%). This SAC composition was selected to favor thermodynamically the nucleation of pro-eutectic Cu₆Sn₅ over that of Ag₃Sn and the formation of an enhanced ternary eutectic fraction in the joint microstructure, while retaining a pasty range that is only 3°C. Using differential scanning calorimetry with sample pans that serve as either inert (aluminum) or actively wetting (copper) substrates, reflow cycles were studied that simulated surface mount (1.5°C/s) and ball-grid array (0.17°C/s) cooling rates. Of the SAC+X solders tested with copper pans, X = Zn appeared to be most effective and consistent, providing catalytic enhancement of the nucleation temperature for even the minimum concentration (0.05 wt.%) and lowest cooling rate.