Effect of strain waveform on creep-fatigue life for Sn–8Zn–3Bi solder

This paper describes the creep‐fatigue life of Sn–8Zn–3Bi under push–pull loading. Creep‐fatigue tests were carried out using Sn–8Zn–3Bi specimens in fast–fast, fast–slow, slow–fast, slow–slow and hold–time strain waveforms. Creep‐fatigue lives in the slow–slow and hold‐time waveforms showed a small reduction from the fast–fast lives but those in the slow–fast and fast–slow waveforms showed a significant reduction from the fast–fast lives. Conventional creep‐fatigue life prediction methods were applied to the experimental data and the applicability of the methods was discussed. Creep‐fatigue characteristics of Sn–8Zn–3Bi were compared with those of Sn–3.5Ag and Sn–37Pb.     

<Emphasis Type="Italic">In-situ</Emphasis> SEM observation on fracture behaviors of Sn-based solder alloys

The effects of displacement rate on fracture behaviors of 100Sn solder and 63Sn37Pb solder alloys have been investigated by SEM in-situ testing. It was found that for the 100Sn solder, grain boundary sliding was the dominant deformation mechanism at lower tensile rate regime, while a large area of slip lines crossing grains were detected at the surface of specimens at higher tensile rate regime. For the 63Sn37Pb eutectic alloy, however, because of existence of the second phase, fracture behavior depended on the growth and linkage of cavities ahead of crack tip, and the crack paths changed from intergranular to transgranular with the increasing of loading rate.     

Enhancement of melt-spun process Bi–Ag lead-free solder for high temperature applications

The enhancement of lead free solders has emerged as one of the most important issues in the electronics assembling industries. Bi–Ag hypo and eutectic alloys have been considerd as one of the lead free solders that can replace the Pb–Sn solder for high temperature applications. This study investigates the effect of various compositions of silver on the mechanical, thermal and electrical properties of the melt-spun process Bi–Ag hypo and eutectic alloys was directionally solidified using copper wheel technology.The microhardness (Hv), creep resistance were measured from solidified ribbons.The results showed that presence of α-Bi phase, Ag_99.5Bi_0.5 (with a cubic structure Fm-3m) and Ag_0.97Bi_0.03 (with a cubic structure Fm-3m) embedded in Bi matrix. Furthermore, the eutectic composition Bi—2.5 wt% Ag shows the more regular microstructure than hypo eutectic composition. It is also concluded that the solderability of solder is significantly imported due to the surface tension of molten Bi–Ag lead-free solder decreases with the Ag content. The obtained results show a surprising desirable decrease in melting temperature with increasing Ag from 269 to 261 °C. Also, it was found that the eutectic composition Bi—2.5 Ag exhibit good mechanical properties superior to hypo-eutectic composition.     

Impression creep behaviour of tin based lead free solders

Abstract

Growing concern about the toxic effects of lead in conventional solders has prompted the development of lead free solders. Creep owing to heating in service is one of the causes of solder joint failures in electronic packages. The present study deals with the impression creep behaviour of eutectic Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb alloys in the temperature range 303 – 393 K and stress range 2˙6 – 180 MPa. Power law creep with stress exponent n varying from 2 to 6˙3 is observed. All the alloys reveal a strong stress dependence of activation enthalpy with values 155, 120 and 112 kJ mol^-1 for Sn – 58Bi, Sn – 57Bi – 1˙3Zn and Sn – 38Pb, respectively, which are well above those for self-diffusion. The steady state impression velocity varies linearly with punch diameter for all three alloys. It is concluded that a mechanism such as forest intersection involving attractive junctions controls the creep flow in these alloys.     

Thermal properties of Sn-based solder alloys

During last few decades, emerging environmental regulations worldwide, more notably in Europe and Japan, have targeted the elimination of Pb usage in electronic assemblies due to the inherent toxicity of this element. This situation drives to the replacement of the Sn–Pb solder alloy of eutectic composition commonly used as joining material to suitable lead-free solders for microelectronic assembly. Sn-based alloys containing Ag, Cu, Bi, and Zn are potential lead-free solders, usually close to the binary or ternary eutectic composition. For this reason a great effort was directed to establish reliable thermophysical data fundamental to interpret the solidification process and fluidity of alloys belonging to these systems. In this work, an analysis of the solidification process of pure Sn, binary Sn–Ag, Sn–Cu, Sn–Bi, Sn–Zn, Sn–Pb and ternary Sn–Ag–Cu eutectic alloys was carried out using computer aided-cooling curve analysis and differential scanning calorimetry.     

Advances in lead-free electronics soldering

Lead-free soldering has emerged as one of the key technologies for assembling in environmental-conscious electronics. Among several candidate alloys, the Sn–Ag–Cu alloy family is believed to be the first choice with the combination of other alloys such as Sn–Zn–Bi, Sn–Cu and Sn–Bi–Ag. Phase diagrams of lead-free alloy systems have been intensively examined by using careful thermal and microstructural analysis combined with the thermodynamic calculation such as the CLAPHAD method. The Cu₆Sn₅/Cu₃Sn layers are formed at most lead-free solder alloy/Cu interfaces, while Cu–Zn compound layers are formed in the Sn–Zn/Cu system. Growth kinetics of intermetallic layers both in solid-state and in soldering are also discussed. Creep and fatigue phenomena are also reviewed. In many aspects of lead-free soldering, much more work is required to establish a sound scientific basis to promote their applications.     

Microstructures formed from mixed solders on copper substrate

Abstract

With the development and use of a variety of Pb free solders, it is probable that some solder joints in electronic assemblies may be made with solders of two different compositions. To investigate possible microstructures resulting from such procedure, samples were prepared using small balls of four different Sn–Ag–Cu (SAC) Pb free solders, as well as Sn–Zn–Al solder, melted together with eutectic Pb–Sn solder paste and also various SAC solder pastes, on a copper substrate. It was observed that using eutectic Pb–Sn solder paste with an SAC solder ball introduced some Pb–Sn eutectic microstructure and changed the ternary eutectic present from Ag₃Sn–Cu₆Sn₅–Sn to Ag₃Sn–Pb–Sn. Use of an SAC solder paste with Sn–Zn–Al solder introduced an apparent Ag–Cu–Zn ternary compound, replacing Zn lamellae of the Sn–Zn eutectic. With eutectic Pb–Sn solder paste, the Pb–Sn–Zn ternary eutectic was formed. It was noted that use of a high Sn solder results in rapid dissolution of the copper substrate.     

Solderability of SnBi-nano Cu solder pastes and microstructure of the solder joints

The solderability of the Sn58Bi (SnBi)-nano Cu solder pastes and the microstructure of the solder joints were investigated. Experimental results indicated that the addition of the nano Cu particles in the SnBi solder paste shows limited effect on the solidus. The liquidus of the SnBi-3nano Cu solder paste was 1 °C higher than the SnBi solder paste. Solid Cu₆Sn₅ intermetallic particles formed in the SnBi-3nano Cu solder paste during the heating process. The Cu₆Sn₅ intermetallic particles decreased the mobility and wettability of the molten solder. Meanwhile, the Cu₆Sn₅ nano particles worked as nucleation sites for the formation of Bi grains and Sn–Bi eutectic phase during the cooling process and led to the grain refinement of the solder bulk. The SnBi-1nano Cu solder paste showed the smallest grain size in this research. Additionally, the SnBi-3nano Cu/Cu solder joint showed a eutectic microstructure of Sn–Bi system at the center of the solder bulk but a hypereutectic microstructure with polygon Bi grains near the margin in the solder bulk.     

Mechanical fatigue of Sn-rich Pb-free solder alloys

Recent fatigue studies of Sn-rich Pb-free solder alloys are reviewed to provide an overview of the current understanding of cyclic deformation, cyclic softening, fatigue crack initiation, fatigue crack growth, and fatigue life behavior in these alloys. Because of their low melting temperatures, these alloys demonstrated extensive cyclic creep deformation at room temperature. Limited amount of data have shown that the cyclic creep rate is strongly dependent on stress amplitude, peak stress, stress ratio and cyclic frequency. At constant cyclic strain amplitudes, most Sn-rich alloys exhibit cycle-dependent and cyclic softening. The softening is more pronounced at larger strain amplitudes and higher temperatures, and in fine grain structures. Characteristic of these alloys, fatigue cracks tend to initiate at grain and phase boundaries very early in the fatigue life, involving considerable amount of grain boundary cavitation and sliding. The growth of fatigue cracks in these alloys may follow both transgranular and intergranular paths, depending on the stress ratio and frequency of the cyclic loading. At low stress ratios and high frequencies, fatigue crack growth rate correlates well with the range of stress intensities or J-integrals but the time-dependent C* integral provides a better correlation with the crack velocity at high stress ratios and low frequencies. The fatigue life of the alloys is a strong function of the strain amplitude, cyclic frequency, temperature, and microstructure. While a few sets of fatigue life data are available, these data, when analyzed in terms of the Coffin–Mason equation, showed large variations, with the fatigue ductility exponent ranging from −0.43 to −1.14 and the fatigue ductility from 0.04 to 20.9. Several approaches have been suggested to explain the differences in the fatigue life behavior, including revision of the Coffin–Mason analysis and use of alternative fatigue life models.     

Effect of Sn addition on microstructure and dry sliding wear behaviors of hypereutectic aluminum–silicon alloy A390

In this study, the effect of Sn addition on the microstructure and dry sliding wear behaviors of as-cast and heat-treated hypereutectic A390 alloys was investigated. The microstructural features of the alloys were characterized by means of optical microscope, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy techniques and their wear characteristics were evaluated at different loads. The worn morphologies of the wear surface were examined by SEM. The results show that the β-Sn in as-cast A390 alloy precipitates mainly in the form of particles within the Al₂Cu network on the interface of the eutectic silicon and α-Al phases and the grain boundaries of α-Al phase. The addition of Sn promotes the disintegrating and spheroidizing of both the eutectic and primary silicon of the A390 alloy during solid solution-aging treatment and β-Sn phase grains coalesces and grows, and some of them form the structure of Sn wrapping Si. The wear rates and friction factors of the as-cast and heat-treated A390 alloys with Sn are lower than those without Sn. At lower load, the addition of Sn changes the wear mechanism of as-cast A390 alloy from the combination of abrasive and adhesive wear without Sn into a single mild abrasion wear with Sn; at higher load, the wear of as-cast A390 alloy without Sn includes abrasion, adhesive, and fatigue one, while the addition of Sn effectively restrains the net-like cracks on the worn surface of the alloy and avoids the fatigue wear emerged.     

多元低熔点共晶合金Sn16Bi52Pb32和In21Sn12Bi49Pb18凝固组织和相组成研究

随着环境保护的需要,具有较低Pb含量或不含Pb元素的多元低熔点合金越来越受到工业界的重视和青睐,但是该类合金的组织结构形态、相组成和基本的物理化学数据比较缺乏,不利于其工业化应用。鉴于此,文章对两种具有较低Pb含量的Sn_(16)Bi_(52)Pb_(32)(质量分数)合金和In_(21)Sn_(12)Bi_(49)Pb_(18)(质量分数)合金进行了凝固组织、相组成以及物化性能方面的研究。扫描电镜(SEM)、差示扫描量热法(DSC)、X射线衍射(XRD)等分析测试结果表明:Sn_(16)-Bi_(52)Pb_(32)合金由Bi-(Pb)固溶体相、Sn-(Bi,Pb)固溶体相以及Pb_7Bi_3化合物相组成,具有准规则共晶凝固组织结构。而In21Sn12Bi49Pb18合金由InBi化合物相、PbBi化合物相以及Sn-(Bi,In)固溶体相组成,具有复杂规则的共晶组织形貌。     

Interfacial microstructure and mechanical properties of In–Bi–Sn lead-free solder

The interfacial microstructure and mechanical properties of a low melting temperature lead-free solder of In-18.75Bi-22.15Sn (in at.%) (In–Bi–Sn) were investigated. The microstructure analysis of bulk In–Bi–Sn revealed that irregular lamellar γ-Sn phases distributed in the In₂Bi matrix. There was only a single endothermic peak with an onset temperature of 62 °C on the DSC curve, indicating that In–Bi–Sn is close to a ternary eutectic solder. The ultimate tensile strength of the bulk In–Bi–Sn was 21.76 MP at a strain rate of 10^?2s^?1 at 25 °C. The elongation of the bulk In–Bi–Sn solder reached 87 %, indicating an excellent ductility of the In–Bi–Sn solder. Two intermetallic compounds (IMCs), needle-like Cu(In,Sn)₂ and laminar Cu₆(In,Sn)₅, formed at the In–Bi–Sn/Cu interface. An IMC layer of polyhedral crystallites of InNi formed at the In–Bi–Sn/Ni interface. The shear strength of Cu/In–Bi–Sn/Cu solder joints was 21.15 MP, and the shear fractograph showed that the ductile fracture with dimples appearance occurred in the solder.     

Constitutive models of creep for lead-free solders

The constitutive modeling of creep has been extensively studied due to the important of the creep failure mode in solder joints. However, there are very few studies that considered room temperature aging contributions in their creep modeling studies. This study investigated constitutive modeling of creep of solders by taking into account the possible contribution room temperature aging. Lead-free solder (Sn–4.0Ag–0.5Cu) was found to have a higher creep resistance than Sn–Pb solder at the same stress level and testing temperature. The higher creep resistance was contributed by the second phase intermetallic compounds, Ag₃Sn and Cu₆Sn₅. The precipitation of these intermetallic compounds can significantly block the movement of dislocations and increase the creep resistance of the material. Constitutive models of creep for both lead-free and Sn–Pb eutectic solders were constructed based on the experimental data. The activation energy for SAC405 is much higher than that of Sn–Pb, which also indicates that SAC405 possesses higher creep resistance. The constitutive models can be used in finite element analysis of actual electronic packages to predict solder joint failure. The creep mechanisms of both lead-free and Sn–Pb eutectic solders were also extensively discussed in this dissertation. Dislocation gliding and climb is believed to be the major failure mode at high stresses, while lattice diffusion and grain boundary diffusion is believed to be the major failure mode at low stress levels. Grain boundary sliding is believed to contribute to creep deformation at both high-stresses and low-stresses. For eutectic Sn–Pb, superplastic deformation is a major the creep mechanism at low-stresses and high-temperatures.     

Large‐Grain Tin‐Rich Perovskite Films for Efficient Solar Cells via Metal Alloying Technique

Fast research progress on lead halide perovskite solar cells has been achieved in the past a few years. However, the presence of lead (Pb) in perovskite composition as a toxic element still remains a major issue for large‐scale deployment. In this work, a novel and facile technique is presented to fabricate tin (Sn)‐rich perovskite film using metal precursors and an alloying technique. Herein, the perovskite films are formed as a result of the reaction between Sn/Pb binary alloy metal precursors and methylammonium iodide (MAI) vapor in a chemical vapor deposition process carried out at 185 °C. It is found that in this approach the Pb/Sn precursors are first converted to (Pb/Sn)I₂ and further reaction with MAI vapor leads to the formation of perovskite films. By using Pb–Sn eutectic alloy, perovskite films with large grain sizes up to 5 µm can be grown directly from liquid phase metal. Consequently, using an alloying technique and this unique growth mechanism, a less‐toxic and efficient perovskite solar cell with a power conversion efficiency (PCE) of 14.04% is demonstrated, while pure Sn and Pb perovskite solar cells prepared in this manner yield PCEs of 4.62% and 14.21%, respectively. It is found that this alloying technique can open up a new direction to further explore different alloy systems (binary or ternary alloys) with even lower melting point.     

Effect of addition of Al and Cu on the properties of Sn–20Bi solder alloy

This study investigates the effect of the composite addition of Al and Cu on the microstructure, physical properties, wettability, and corrosion properties of Sn–20Bi solder alloy. Scanning electron microscopy and X-ray diffraction were used to identify the microstructure morphology and composition. The spreading area and contact angle of the Sn–20Bi–x (x?=?0, 0.1 wt% Al, 0.5 wt% Cu, and 0.1 wt% Al–0.5 wt% Cu) alloys on Cu substrates were used to measure the wettability of solder alloys. The results indicate that the alloy with 0.1 wt% Al produces the largest dendrite and the composite addition of 0.1 wt% Al and 0.5 wt% Cu formed Cu₆Sn₅ and CuAl₂ intermetallic compounds in the alloy structure. And the electrical conductivity of Sn–20Bi–0.1Al is the best, which reaches 5.32 MS/m. The spread area of the solder alloy is reduced by the addition of 0.1 wt% Al and 0.5 wt% Cu, which is 80.7 mm². The corrosion products of Sn–20Bi–x solder alloys are mainly lamellar Sn₃O(OH)₂Cl₂ and the corrosion resistance of 0.1 wt% Al solder alloy alone is the best. The overall corrosion resistance of Sn–20Bi–0.1Al–0.5Cu is weakened and the corrosion of solder alloy is not uniform.

     

Low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy

The eutectic 80Au/20Sn solder alloy is widely used in high power electronics and optoelectronics packaging. In this study, low cycle fatigue behavior of a eutectic 80Au/20Sn solder alloy is reported. The 80Au/20Sn solder shows a quasi-static fracture characteristic at high strain rates, and then gradually transforms from a transgranular fracture (dominated by fatigue damage) to intergranular fracture (dominated by creep damage) at low strain rates with increasing temperature. Coffin-Manson and Morrow models are proposed to evaluate the low cycle fatigue behavior of the 80Au/20Sn solder. Besides, the 80Au/20Sn solder has enhanced fatigue resistance compared to the 63Sn/37Pb solder.     

Creep–fatigue life of Sn–8Zn–3Bi solder under multiaxial loading

This paper describes a creep–fatigue life of Sn–8Zn–3Bi solder under multiaxial loading. A push–pull and a reversed torsion tests were carried out using seven types of strain waveforms, which are a fast–fast, a fast–slow, a slow–fast and a slow–slow waveforms and three types trapezoidal strain waveforms with different strain holding times. The strain waveforms had a significant effect on creep–fatigue life and the shortest creep–fatigue life was found in the slow–fast strain waveform while the longest life in the slow–slow waveform in the push–pull and the reversed torsion tests. Creep–fatigue life in the reversed torsion test was approximately twice longer than that in the push–pull test at each strain waveform. Applicability of common used creep–fatigue damage models for life evaluation was discussed based on the obtained experimental results and only a grain boundary sliding model could evaluate the lives within a small scatter.     

Formation of microstructure during solidification of Bi–Pb–Sn ternary eutectic

Abstract

To investigate the nature of the Bi–Pb–Sn ternary eutectic, specimens were solidified unidirectionally at very low speeds and quenched to form a representative solid/liquid interface for subsequent study. Specimens made using the generally accepted composition, as reported by Ho et al., did not form all three solid phases from the start of freezing. Specimens produced using the composition reported by Sakurai, i.e., 54 wt-%Bi, 28 wt-%Pb, and 18 wt-%Sn, did give all three phases from the beginning of freezing, indicating that it is the correct eutectic composition. It was found that this eutectic is of the faceted (Bi) non-faceted (X phase) non-faceted (Sn) type. Under the freezing conditions used, a double binary microstructure was formed, with one component consisting of Sn fibres in the X phase and the other of a Bi–Sn complex regular microstructure. While the occurrence of a double binary microstructure was predicted by C. S. Smith for a lamellar ternary eutectic, the current observation shows that it can also occur in a system with one fibrous phase.     

Effects of Sn segregation and precipitates on creep response of Mg‐Sn alloys

Mg‐Sn alloys are promising for the development of new cheap creep resistant magnesium alloys. In the present paper, the creep behaviours of Mg‐Sn and Mg‐Sn‐Ca alloys were examined at the constant temperature and different stresses. The measurements of stress exponents indicate that the dislocation climbing is the dominant mechanism during the creep of Mg‐3Sn or Mg‐3Sn‐2Ca alloys. The poor creep resistance of the binary Mg‐3Sn alloy is caused by the easy movement of dislocation and the segregation of Sn at the boundaries. Both T4 and T6 heat treatments improve the creep resistance of Mg‐3Sn alloy due to the alleviation of Sn segregation at grain boundaries and the precipitation of Mg₂Sn particles, respectively. Ca is an effective alloying element to increase the creep resistance of Mg‐Sn alloys. The Ca addition leads to the formation of thermal stable phases Mg₂Ca and CaMgSn in Mg‐3Sn‐Ca alloys. These two phases effectively hinder the movement of dislocations and the sliding of grain boundaries. On the other hand, the addition of Ca alleviates the segregation of Sn by the interaction of Ca with Mg and Sn to form the phase CaMgSn.     

Fatigue performance of dual‐phase steels for automotive wheel application

Fatigue performance of ferrite–martensite (FM) and ferrite–bainite (FB) dual‐phase (DP) steels used in automotive wheels has been compared in terms of (i) high‐cycle fatigue performance and failure mechanisms and (b) low‐cycle fatigue performance (Δε_t/2 = 0.002 to 0.01) and associated deformation mechanisms. FBDP steel exhibits moderately better high‐cycle fatigue performance, owing to delay in microcrack initiation. In FBDP steel, microcracks initiate predominantly along ferrite grain boundaries, while that at FB interface is significantly delayed in comparison with FMDP steel, where few microcracks appear at FM interface even below the endurance limit. During low‐cycle fatigue, however, FMDP steel performs considerably better than FBDP steel till Δε_t/2 ≤ 0.005 attributed to initial cyclic hardening, followed by cyclically stable behaviour exhibited by FMDP steel. In sharp contrast, at all Δε_t/2 > 0.002, FBDP steel undergoes continuous cyclic softening. The latter may cause undesirable deformation of wheels in service.