Effects of Rare-Earth Addition on Properties and Microstructure of Lead-Free Solder Balls

In the present work, lead-free solder balls were manufactured by a uniform droplet spray (UDS) method. The solder used to produce solder balls was based on the Sn-3.8Ag-0.7Cu alloy. Different amounts of cerium-based mixed rare-earth (RE) elements were added into the Sn-3.8Ag-0.7Cu solder alloy in order to examine the effects of small amounts of RE additions on the physical properties, microstructure, and surface smoothness of the solder balls. Results show that a small amount of RE addition has no obvious effect on the melting temperature, but it decreases the nucleation undercooling degree. Moreover, a small amount of RE addition (<0.25 wt.%) can improve the surface smoothness of the solder balls. However, when the RE was added up to 0.5 wt.%, the surface smoothness of the solder balls was deteriorated. From observations of the microstructure of the solder balls, it is obvious that the RE addition affects solidification behavior, and as a result, the surface smoothness of the solder balls.     

Local and Global Properties of a Lead-Free Solder

Elastic and viscous properties including Young’s modulus, hardness, creep rate sensitivity, and fatigue resistance of Sn-1.2Ag-0.5Cu-0.05Ni lead-free solder have been investigated. The properties of bulk specimens and in situ solder balls are compared. Experiments show good correlations of Young’s modulus and creep rate sensitivity between conventional measurements and nanoindentation results on bulk specimens. Further mechanical properties of the beach-ball microstructure in solder balls are characterized by nanoindentation. The load–partial unload technique has been used to determine the variation in mechanical properties with increasing depth of penetration into the intermetallic inclusions in the in situ solder. The fatigue resistances of the bulk specimens and solder balls are compared by using the novel nanoimpact method. In comparison with bulk specimens, it is found that in situ solder has higher Young’s modulus, lower creep strain rate sensitivity, and better fatigue resistance. The effects of soldering and the scale differences strongly affect the mechanical and fatigue properties of in situ solder.     

Modeling Material Properties of Lead-Free Solder Alloys

A full set of physical and thermophysical properties for lead-free solder (LFS) alloys have been calculated, including liquidus/solidus temperatures, fraction solid, density, coefficient of thermal expansion, thermal conductivity, Young’s modulus, viscosity, and liquid surface tension, all as a function of composition and temperature (extending into the liquid state). The results have been extensively validated against data available in the literature. A detailed comparison of the properties of two LFS alloys Sn-20In-2.8Ag and Sn-5.5Zn-4.5In-3.5Bi with Sn-37Pb has been made to show the utility and need for calculations that cover a wide range of properties, including the need to consider the effect of nonequilibrium cooling. The modeling of many of these properties follows well-established procedures previously used in JMatPro software for a range of structural alloys. This paper describes an additional procedure for the calculation of the liquid surface tension for multicomponent systems, based on the Butler equation. Future software developments are reviewed, including the addition of mechanical properties, but the present calculations can already make a useful contribution to the selection of appropriate new LFS alloys.     

Al对Sn-58Bi无铅钎料组织及性能的影响

研究了在Sn-58Bi合金中添加少量Al后对合金性能、恒温时效焊点组织稳定性及接头界面处金属间化合物层生长的影响.研究结果表明:在80℃时Sn-58Bi合金具有稳定的组织结构,但超过100℃Bi相会异常粗化,导致合金性能变差.在100℃恒温时效试验中,Al能有效抑制Bi相的粗化,接头界面处IMC层的生长呈减缓趋势.随着Al含量的增加,Sn-58Bi-Al系合金的拉伸强度增大,铺展性有所降低.     

Thermal Simulation of the Solidification of Lead-Free Solder Interconnections

Two thermal models of different level, a component model and an interconnection model, were established to simulate the solidification of lead-free solder interconnections of a chip-scale packaged component during reflow soldering. The thermal properties of the interconnections were derived with the help of thermodynamic calculations relevant to the phase transformations occurring during melting and solidification. Experimental measurements were carried out and the data were used to determine some parameters so that the model is more realistic. Although the results of the component model agreed with the experimental measurements in the faster cooling of the component than the board, the interconnection model suggested that the temperature gradients over the interconnections were unlikely to be of significance until the invariant eutectic reaction commenced. The findings imply that solder/metallization interfaces on printed wiring board and component sides are equally likely sites for initiating the solidification of interconnections. On the basis of the simulated temperature distribution, the growth conditions of the primary Sn are evaluated and an explanation for the sequence of solidification steps has also been given.     

Reliability of Lead-Free Solder Interconnections in Thermal and Power Cycling Tests

Lead-free solder interconnection reliability of thin fine-pitch ball grid array (BGA) lead-free packages has been studied experimentally as well as with finite-element (FE) simulations. The reliability tests were composed of the thermal shock test, the local thermal cycling test (resistors embedded in the board around the package), and the power cycling test (heat generation in the die). A 3-D board-level finite-element analysis (FEA) with local models was carried out to estimate the reliability of the solder interconnections under various test conditions. Due to the transient nature of the local thermal cycling test and the power cycling test, a sequential thermal-structural coupling analysis was employed to simulate the transient temperature distribution as well as the mechanical responses. Darveaux's approach was used to predict the life time of the solder interconnections. Furthermore, the numerical results validated by the experimental results indicated that the diagonal solder interconnections beneath the die edge were the most critical ones of all the tests studied here. It has been found that the fatigue life in the power cycling test was much longer than that in the other two tests. Detailed discussions about the failure mechanism of solder interconnections as well as the microstructural observations of the primary cracks are reported in this paper.      

Sn-Bi-Sb无铅焊料微观结构及性能

研究了Sn-（1．3～1．5）Bi-（0．4～0．6）sb无铅焊料的制备工艺和微观组织,并测试了钎料的相关物理、力学性能,阐述了焊料的力学性能与微观结构特征间的关系。试验测试结果表明：该焊料具有较高的强度和塑性,具有良好的润湿铺展性和机械加工性能。焊料微观结构由（Sn）、B（SbSn）第二相和（Bi）所构成,其抗拉强度为55．4MPa,延伸率为35．9％,扩展率为80．6％,熔点为226．9℃～234．4℃。     

Wetting Behavior and Mechanical Properties of Sn-Zn and Sn-Pb Solder Alloys

A comparative experimental study of the main features of the eutectic Sn-Pb alloy and Sn-Zn alloys was carried out with a view to application of the latter alloys as alternative solder materials. The resulting microstructures, mechanical properties (ultimate tensile strength and elongation), and wettability behavior (spreading area and contact angle) of a hypoeutectic Sn-Zn (Sn-4wt.%Zn), a hypereutectic Sn-Zn (Sn-12wt.%Zn), and the eutectic Sn-9wt.%Zn alloy were examined and compared with the corresponding results of the conventional Sn-40wt.%Pb solder alloy. It was found that, of the Sn-Zn alloys examined, the eutectic Sn-9wt.%Zn alloy offers a compromise between lower wettability and higher mechanical strength.     

Influence of Intermetallic Properties on Reliability of Lead-Free Flip-Chip Solder Joints

Electroplated pure tin bumping as a lead-free alternative for ultra fine pitch applications is a relatively easy process and has provided us with comparable results to eutectic Sn/Pb for thermal cycling reliability. Experimentally, it has been reported that a significantly higher (~40%) thermal cycle fatigue life is seen with the use of cobalt under bump metallization (UBM) instead of copper UBM for a flip-chip device assembled on an alumina substrate. In the current approaches used to estimate fatigue life of solder joints, the solder joint is treated as a homogenous material and modeled as such. However, the smaller joint sizes and higher reactivity of Sn implies that a larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behavior of these intermetallic layers within the solder joint. In order to investigate if a simplified engineering approach can provide some insight into this issue, we have attempted to explicitly model the intermetallics as a continuous but separate part of the solder joint. The main damage parameter investigated is the accumulated inelastic strain in a single thermal cycle. From the results, it is clear that the Young's modulus of the intermetallic layer plays an important role, more so when the ratio of intermetallic thickness to the solder joint standoff increases. Thickness of the intermetallic layer also influences the overall strain accumulation in the same manner. The CTE of the intermetallic layer has a relatively lesser influence on the strain accumulation. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20%-30%.     

Impact of Thermal Aging on the Microstructure Evolution and Mechanical Properties of Lanthanum-Doped Tin-Silver-Copper Lead-Free Solders

An extensive study is made to analyze the impact of pure lanthanum on the microstructure and mechanical properties of Sn-Ag-Cu (SAC) alloys at high temperatures. Different compositions are tested; the temperature applied for the isothermal aging is 150°C, and aging times of 10 h, 25 h, 50 h, 100 h, and 200 h are studied. Optical microscopy with cross-polarized light is used to follow the grain size, which is refined from 8 mm to 1 mm for as-cast samples and is maintained during thermal aging. Intermetallic compounds (IMCs) present inside the bulk Sn matrix affect the mechanical properties of the SAC alloys. Due to high-temperature exposure, these IMCs grow and hence their impact on mechanical properties becomes more significant. This growth is followed by scanning electron microscopy, and energy-dispersive spectroscopy is used for elemental mapping of each phase. A significant refinement in the average size of IMCs of up to 40% is identified for the as-cast samples, and the coarsening rate of these IMCs is slowed by up to 70% with no change in the interparticle spacing. Yield stress and tensile strength are determined through tensile testing at 20°C for as-cast samples and after thermal aging at 150°C for 100 h and 200 h. Both yield stress and tensile strength are increased by up to 20% by minute lanthanum doping.     

Mechanical Properties of Lead-Free Solder Joints Under High-Speed Shear Impact Loading

In this study we expanded on recently reported research by using a modified miniature Charpy impact-testing system to investigate the shear deformation behavior of Sn–3.0Ag–0.5Cu lead-free solder joints at high strain rates ranging from 1.1 × 10³ s^?1 to 5.5 × 10³ s^?1. The experimental results revealed that the maximum shear strength of the solder joint decreased with increasing load speed in the ranges tested in this study. For solder joints tested at a shear speed exceeding 1 m/s, corresponding to an approximate strain rate that exceeds 1950 s^?1, the brittle fracture mode is the main failure mode, whereas lower strain rates result in a ductile-to-brittle transition in the fracture surfaces of solder joints. In addition, the mode II stress intensity factor (K _II) used to evaluate the fracture toughness (K _C) of an interfacial intermetallic compound layer between Sn–3.0Ag–0.5Cu solder and the toughness of copper substrate was found to decrease from 1.63 MPa m^0.5 to 0.97 MPa m^0.5 in the speed range tested here.     

Sn0．45Ag0．68Cu无铅焊点热冲击性能的研究

采用Sn0．45A g0．68Cu亚共晶无铅钎料通过热浸焊获得铜接头,在－45～125℃的温度循环区间内对焊接接头进行200、400、600、800、1000周期高低温热冲击循环实验,分析了焊点的剪切强度变化,组织演变及界面IM C的生长规律。结果表明：焊点组织中弥散分布的Cu6Sn5相内部晶粒逐渐粗化长大,最后转变为圆形或者椭圆形;焊点界面IM C层厚度明显增厚,且由最初的细小扇贝状转变为大的波浪状,最终趋于平缓;焊点的剪切强度随热冲击循环周期的增加而急剧下降,经400周期的热冲击循环之后,焊点的剪切强度已下降了约22．5％,在400周期的热冲击循环后开始变得平缓,最后趋于稳定。     

Characterization of Recrystallization and Microstructure Evolution in Lead-Free Solder Joints Using EBSD and 3D-XRD

Development of vulnerable high-angle grain boundaries (and cracks) from low-angle boundaries during thermal cycling by means of continuous recrystallization was examined in fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) (SAC305) lead-free solder joints. Electron backscatter diffraction (EBSD) and differential-aperture x-ray microscopy (DAXM or 3D-XRD) were used for surface and subsurface characterization. A large number of subgrain boundaries were observed in the parent orientation using both techniques. However, unlike studies of anisotropic deformation in noncubic metals at much lower homologous temperatures, no streaked diffraction peaks were observed in DAXM Laue patterns within each 1 μm³ voxel after thermal cycling, suggesting that geometrically necessary dislocations (GNDs) are effectively absorbed by the preexisting subgrain boundaries. Storage at room temperature (0.6T _m) prior to DAXM measurement may also facilitate recovery processes to reduce local GND contents. Heterogeneous residual elastic strains were found near the interface between a precipitated Cu₆Sn₅ particle and the Sn grain, as well as near particular subgrain boundaries in the parent orientation. Grain boundary migration associated with recrystallization resulted in regions without internal strains, subgrain boundaries, or orientation gradients. Development of new grain orientations by continuous recrystallization and subsequent primary recrystallization and grain growth occurred in the regions where the cracks developed. Orientation gradients and subgrain structure were observed within newly formed recrystallized grains that could be correlated with slip systems having high Schmid factors.     

Reliability Analysis of Lead-Free Solder Castellations

In this paper, the reliability of lead-free solder castellations is considered. The newly developed stress-dependent Engelmaier's solder fatigue model is utilized in this task. Based on this model, it is possible to interpret the thermal cycling test results. A very good agreement between the test results and the lifetime predictions is obtained. Using the lifetime prediction model, optimal solder castellation shape is investigated. Based on the findings, the fatigue life can be improved by up to 30% simply by solder pad length optimization. Further increment in lifetime length can be expected if the solder joint shape is optimized with the help of modeling tools presented here. Understanding how the crack propagates in solder material is vital if optimal lifetime behavior is expected.     

无铅钎料发展现状

无铅钎料是绿色电子产品的一个重要组成部分。本文综述了近年来国际上无铅钎料的发展动态，并为国内企业如何适应这一趋势提出了建议。     

无铅化热风整平的焊料与选择

文章概述了无铅化热风整平的焊料及其选择。在Sn—Cu中加入少量的Ni（或Co）是无铅化热风整平的焊料的有利选择。     

无铅焊及焊接点的可靠性试验

随着电子装置的小型化的发展,欧盟(EU．)的WEEE和RoHS提出禁止使用Sn-Pb焊 锡。这将导致一系列的工业革新,如部件体积和重量的减少,各种各样无铅产品的出现,改变现有 的焊接生产线等。参照国际标准(IEC,ISO)和日本国家标准(JIS),并根据这些标准做了一系列的试 验,通过试验对无铅焊润湿性、强度、耐久性等可靠性的评价方法进行说明。     

绿色高性能无铅钎料的研究与发展

铅和铅的化合物破坏环境，有害人体健康，国际上无铅的呼声日益高涨，美国，欧洲、日本等发达国家纷纷立法，电子产品无铅化的安排已经提到议事日程，各大公司纷纷展开无铅钎料的研究，并相继试验推出无铅产品，电子组装无铅化是一个系统工程，需要各个环节和全社会的配合，中国在激烈的国际竞争中要赢得市场，必须拥有自主知识产权的无铅钎料及产品，这需要全社会特别是相关企业的重视和全力配合。     

Creep Properties of Sn-1.0Ag-0.5Cu Lead-Free Solder with Ni Addition

In this work, tensile creep tests for Sn-1.0Ag-0.5Cu-0.02Ni solder have been conducted at various temperatures and stress levels to determine its creep properties. The effects of stress level and temperature on creep strain rate were investigated. Creep constitutive models (such as the simple power-law model, hyperbolic sine model, double power-law model, and exponential model) have been reviewed, and the material constants of each model have been determined based on experimental results. The stress exponent and creep activation energy have been studied and compared with other researchers’ results. These four creep constitutive models established in this paper were then implemented into a user-defined subroutine in the ANSYS™ finite-element analysis software to investigate the creep behavior of Sn-1.0Ag-0.5Cu-0.02Ni solder joints of thin fine-pitch ball grid array (TFBGA) packages for the purpose of model comparison and application. Similar simulation results of creep strain and creep strain energy density were achieved when using the different creep constitutive models, indicating that the creep models are consistent and accurate.     

Creep Behavior of Bi-Containing Lead-Free Solder Alloys

The creep behavior of Sn-3.0Ag-0.5Cu (SAC305), Sn-3.4Ag-1.0Cu-3.3Bi (SAC-Bi), and Sn-3.4Ag-4.8Bi (SnAg-Bi, all wt.%) was studied in constant-stress creep tests from room temperature to 125°C. The alloys were tested in two microstructural conditions. As-cast alloys had a composite eutectic-primary Sn structure, while in aged alloys the eutectic regions were replaced by a continuous Sn matrix with coarsened intermetallic (Cu₆Sn₅ and Ag₃Sn) particles. After aging, Bi in SAC-Bi and SnAg-Bi was found as precipitates at grain boundaries and grain interiors. The creep resistance of of-cast SAC305 was higher than that of as-cast Bi-containing alloys, but after aging the SAC305 had the lowest creep resistance. The creep strain rates in SAC-Bi and SnAg-Bi were much less affected by aging. The apparent activation energy for creep was also changed more for SAC305 than for the other two alloys. The creep behavior of SAC-Bi and SnAg-Bi can be understood by considering the solubility of Bi in Sn. The difference in creep behavior between as-cast and aged SAC-Bi is greatly reduced when room-temperature test results are excluded from analysis. This suggests that the strongest influence on creep in these alloys is due to Bi solute interaction with moving dislocations during deformation.