Effects of Ce Addition on the Microstructure and Mechanical Properties of Sn-58Bi Solder Joints

The effects of a rare-earth element on the microstructure, mechanical properties, and whisker growth of Sn-58Bi alloys and solder joints in ball grid array (BGA) packages with Ag/Cu pads have been investigated. Mechanical testing indicated that the elongation of Sn-58Bi alloys doped with Ce increased significantly, and the tensile strength decreased slightly, in compar- ison with undoped Sn-58Bi. In addition, the growth of both fiber- and hillock-shaped tin whiskers on the surface of Sn-58Bi-0.5Ce was retarded in the case of Sn-3Ag-0.5Cu-0.5Ce alloys. The growth of interfacial intermetallic compounds (IMC) in Sn-58Bi-0.5Ce solder joints was slower than that in Sn-58Bi because the activity of Ce atoms at the interface of the Cu₆Sn₅ IMC/solder was reduced. The reflowed Sn-58Bi and Sn-58Bi-0.5Ce BGA packages with Ag/Cu pads had a ball shear strength of 7.91 N and 7.64 N, which decreased to about 7.13 N and 6.87 N after aging at 100°C for 1000 h, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics after ball shear tests.     

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.     

Sn-58Bi在镀镍层表面的焊接工艺

针对镀镍盒体表面易被氧化等原因导致可焊性下降,且可选无铅焊料Sn-58Bi焊膏的情况下,通过在空气中直接加热焊接、借助真空炉在无氧环境中焊接等方式,确认空洞率高与空气中氧气的存在无关。最终通过在焊接过程中另加自配的助焊剂,将焊接后的空洞率控制在5％以内,满足了焊接要求。     

Effect of Bi Segregation on the Asymmetrical Growth of Cu-Sn Intermetallic Compounds in Cu/Sn-58Bi/Cu Sandwich Solder Joints During Isothermal Aging

An asymmetrical interfacial microstructure was observed at both top and bottom interfaces of Cu/Sn-58Bi/Cu solder joints after isothermal aging at 120°C for different times. The asymmetrical interfacial microstructure resulted from asymmetrical Bi segregation, which was attributed to the density difference between Bi and Sn atoms. Bi atoms were driven to the bottom solder/Cu interface by gravity during the liquid soldering procedure since Bi atoms are more massive than Sn atoms. With increasing aging time, Bi accumulated at the bottom Cu₃Sn/Cu interface and the Bi segregation enhanced Cu₆Sn₅ intermetallic compound growth, blocked Sn transport to the Cu₃Sn intermetallic compound, and facilitated growth of the Cu₆Sn₅, based on the measured thicknesses of intermetallic compounds (including Cu₆Sn₅ and Cu₃Sn) at both bottom and top interfaces for Cu/Sn-58Bi/Cu sandwich joints under the same aging process.     

Effects of Ce and La Additions on the Microstructure and Mechanical Properties of Sn-9Zn Solder Joints

The effects of rare-earth elements on the microstructure and mechanical properties of Sn-9Zn alloys and solder joints in ball grid array packages with Ni/Au(ENIG) surface finishes have been investigated. Metallographic observations showed that (Ce_0.8Zn_0.2)Sn₃ and (La_0.9Zn_0.1)Sn₃ intermetallic compounds appeared in the solder matrix of Sn-9Zn-0.5Ce and Sn-9Zn-0.5La alloys, respectively. Both fiber- and hillock-shaped tin whiskers were inhibited in the Sn-9Zn-0.5Ce solder, while tin fibers were still observed on the surface of oxidized (La_0.9Zn_0.1)Sn₃ intermetallics in Sn-9Zn-0.5La after air exposure at room temperature. Mechanical testing indicated that the tensile strength of Sn-9Zn alloys doped with Ce and La increased significantly, and the elongation decreased, in comparison with the undoped Sn-9Zn. The bonding strengths of the as-reflowed Sn-9Zn-0.5Ce and Sn-9Zn-0.5La solder joints were also improved. However, aging treatment at 100°C and 150°C caused degradation of ball shear strength in all specimens. During the reflowing and aging processes, AuZn₈ intermetallic phases appeared at the interfaces of all solder joints. In addition, Zn-rich phases were observed to migrate from the solder matrix to the solder/pad interfaces of the aged specimens.     

Sn-3.5Ag-0.5Cu无铅焊料力学性能与组织特征研究

研究了Sn-3.5Ag-0.5Cu无铅焊料的力学性能与组织特征,结果表明焊料的应力应变关系表现出明显的温度和应变速率相关性,当温度升高,应变速率降低时,焊料的强度极限均会减小,但强度极限与温度和应变速率之间呈非线性相关;焊料在拉断前后,内部晶粒会与排列方式发生变化,且变形大小与加载的应变速率相关.     

Assessment of Tertiary Dendritic Growth and Its Effects on Mechanical Properties of Directionally Solidified Sn-0.7Cu-xAg Solder Alloys

Although commercial SAC107/207/307 alloys are being used as alternatives to traditional Sn-Pb solder alloys, there is a lack of studies emphasizing some metallurgical aspects, for instance, the different morphologies of Cu₆Sn₅ and Ag₃Sn intermetallic compounds (IMCs) as well as conditions for the launch of tertiary dendritic β-Sn branches and their effects on localized mechanical properties. A wide range of cooling rates during solidification is normally associated with quite different microstructural length parameters. Hence, Sn-0.7 wt.%Cu-x wt.%Ag (where x = 1.0, 2.0, and 3.0) alloys were directionally solidified under transient heat flow conditions, undergoing cooling rates varying from 0.1 K/s to 32.0 K/s. This experimental study encompasses: primary, secondary, and tertiary dendrite arm spacings (λ ₁, λ ₂, and λ ₃) associated with the evolution of the tip cooling rate ( $ \dot{T} $ ) during solidification; start and growth of tertiary dendrite branches; yield (σ _y) and ultimate tensile strengths (σ _u); elongation to fracture (δ); and the morphology of IMCs embedded in the Sn-rich phase. A single ratio between the cooling rate ( $ \dot{T} $ ) and the alloy silver content (C _0-Ag) of 0.45 seems to be the parametric factor associated with the beginning of the growth of tertiary dendritic branches. SAC307 is shown to be the only alloy examined having λ ₃ along the entire casting length. Despite the presence of some tertiary branches in part of the SAC207 alloy casting, for both the SAC107 and SAC207 alloys, σ _u and σ _y are shown to increase with decreasing λ ₂, while the opposite trend is exhibited by the SAC307 alloy. It seems that the well-defined array of tertiary branches in such alloy, and consequently the more complex dendritic network, allowed the strength to increase despite the associated increase in λ ₂ and the change in the morphology of the Ag₃Sn IMC from spheroidal to fibrous.     

稀土改性的Sn-58Bi低温无铅钎料

研究了微量稀土对Sn-58Bi低温钎料的改性作用.试验添加质量分数为0.1 ?组混合稀土的无铅材料,并对比Sn-58Bi和Sn-58Bi0.5Ag合金.观察了钎料显微组织的变化并做了定量分析,采用DSC测试了钎料的熔化温度,同时测量了钎料的润湿性能、接头强度与硬度.结果表明,微量稀土添加细化了Sn-58Bi钎料合金的显微组织,对钎料的熔化温度几乎没有影响,能显著改善Sn-58Bi钎料的润湿性能和接头剪切强度,而且改善的程度优于添加微量Ag对Sn-58Bi钎料的作用.     

Ce对SnAgCu系无铅焊锡力学性能的影响

通过向Sn-3Ag-2.8Cu钎料合金中添加微量稀土Ce,利用扫描电子显微镜(SEM)研究了不同稀土含量对Sn-3Ag-2.8Cu合金的力学性能的影响,同时对显微组织进行了分析.实验结果表明,微量的Ce稀土可以显著提高Sn-3Ag-2.8Cu钎料的延伸率、延长其焊接接头在室温下的蠕变断裂寿命,尤其是当稀土的质量分数为0.1%时,其蠕变断裂寿命可以达到Sn-3Ag-2.8Cu钎料的9倍以上,当稀土的质量分数超过0.1%时,接头的蠕变断裂寿命呈下降趋势.综合考虑,最佳的稀土质量分数为0.05%～0.10%.     

Effects of Forming Processes on the Microstructure and Solderability of Sn-3.5Ag Eutectic Solder Ribbons as well as the Mechanical Properties of Solder Joints

Two kinds of Sn-3.5Ag eutectic solder ribbons of 0.13 mm thickness were prepared by a casting–rolling process and a rapid solidification process. The microstructure, phase constitution, melting characteristics, wetting behavior and soldering strength were compared using optical microscopy, scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, differential scanning calorimetry and a MTS ceramic testing system. The results show that the microstructure of rapidly solidified solder is finer and more uniform, and the eutectic structure has a higher solid solubility and more homogeneous distribution of Ag in a Sn matrix. The solidus and liquidus temperature decreased, resulting in a 3.3% reduction of pasty range. In addition, the wettability and shear strength of the solder joints increased by 13.2% and 7.9%, respectively.     

Sn-3Ag-3Bi焊点剪切强度的研究

提出一种可有效降低测试误差的剪切强度测试方法。用该方法对Sn-Ag-Bi无铅焊料分别在Cu基板和Ni-P基板上的焊点,经不同的热时效后的剪切强度,进行了测量并对断裂面的微观结构进行了研究。结果显示,焊点的剪切强度及断裂位置与焊接界面金属间化合物(IMC)的组成和厚度有关。     

Ni对Sn-0.7Cu焊料微观组织和力学性能的影响

研究了添加微量Ni元素对Sn-0.7Cu焊料的力学性能、微观组织和断裂特性的影响.结果表明,微量的Ni可已细化焊料合金的微观组织,显著提高焊料的塑性,从而提高焊料的综合力学性能;但Ni含量太高,焊料的塑性反而受到弱化,合适的Ni添加量为0.133%,此时焊料的拉伸强度为35.7 MPa,延伸率为50%.     

Effects of Thermal Aging on Microstructure and Microhardness of Sn-3.7Ag-0.9Zn-1In Solder

The structural stability of Sn-3.7wt.%Ag-0.9wt.%Zn-1wt.%In solder was explored by thermal aging at 473 K. According to the microstructural observations, the high cooling rate (about 10² K/s) applied to the water-cooled specimens prompted the formation of β-Sn dendrites and inhibited the formation of the AgZn phase. After annealing for 20 h and 50 h, the corresponding microstructure changed significantly, especially the morphology of the intermetallic compounds (IMCs) in the slowly cooled solder. IMC particles grew in order to minimize the system energy. In addition, β-Sn dendrites grew coarser as the IMCs segregated along their grain boundaries in the water-cooled solder. Furthermore, the Ag-Zn IMCs were suppressed in the water-cooled Sn-Ag-Zn-In solder. It is suggested that coarsening of the microstructure led to significant softening during annealing of the investigated Sn-Ag-Zn-In alloys.     

Effects of Electromigration on the Creep and Thermal Fatigue Behavior of Sn58Bi Solder Joints

Electromigration (EM), creep, and thermal fatigue (TF) are the most important aspects of the reliability of electronic solder joints, the failure mechanisms of which used to be investigated separately. However, current, mechanical loading, and temperature fluctuation usually co-exist under real service conditions, especially as the magnitude of current density is increasing with joint miniaturization. The importance of EM can no longer be simply ignored when analyzing the creep and TF behavior of a solder joint. The published literature reports that current density substantially changes creep rate, but the intrinsic mechanism is still unclear. Hence, the purpose of this study was to investigate the effects of EM on the creep and TF behavior of Sn58Bi solder joints by analyzing the evolution of electrical resistance and microstructure. The results indicated that EM shortens the lifetime of creep or TF of Sn58Bi solder joints. During creep, EM delays or suppresses the cracking and deforming process, so fracture occurs at the cathode interface. During TF, EM suppresses the cracking process and changes the interfacial structure.     

无铅钎料Sn-0.7Cu机械合金化研究

利用扫描电子显微镜和差热分析仪,研究了Sn-0.7Cu粉末的机械合金化过程;同时将机械合金化获得的粉末制成焊膏,进行铺展实验,并对金相组织进行了分析.研究结果表明,机械合金化是一种制备钎料粉体的方法;粉末的最终颗粒尺寸可通过合适的工艺参数来控制;用机械合金化获得的钎料粉体制成的焊膏,其铺展面积及润湿角方面与传统的熔炼法获得的钎料差别不大,铺展试样的金相组织由富Sn相、Cu6Sn5和少量Cu3Sn相组成.