Progress in the design of new lead-free solder alloys

Various alloy design approaches have been employed to develop new lead-free solder alloys that can not only substitute for the lead-tin solders, but also offer significantly improved mechanical properties. Three new alloys are described in this article. In Sn-3.5Ag-1Zn (melting point ~217°C), the solidification structure and the eutectic precipitate morphology are6 refined by the addition of zinc. As a result, a high-strength, high-ductility solder with significantly improved creep resistance is obtained. In Bi-43Sn+2.5Fe, a eutectic alloy (melting point ~137°C), dispersion hardening by magnetically distributed iron particles retards both high-temperature deformation and microstructural coarsening, thus widening the useful service range of Bi-Sn eutectic alloys to much higher homologous temperatures than are typical for the Sn-Pb eutectic alloy. Lastly, Sn-Zn-In based alloys (melting point ~185°C) have been developed for consideration as a drop-in replacement for the neareutectic Sn-Pb alloy(melting point ~183°C).     

High temperature lead-free solder for microelectronics

This paper reports results of a four-year industrial consortium effort to develop lead-free solders for high-temperature applications (up to 160°C). Work included preliminary evaluations of 32 tin-based alloys, a screening of the thermomechanical fatigue performance of 13 promising alloys, and a full manufacturability and fatigue testing of the seven most promising of those alloys, namely Sn-3.5Ag, Sn-4Ag-1Cu, Sn-4Ag-0.5Cu, Sn-2.5Ag-0.8Cu-0.5Sb, Sn-4.6Ag-1.6Cu-1Sb-1Bi, Sn-3.3Ag-1Cu-3.3Bi, and Sn-3.5Ag-1.5In (compositions in weight percent). Eight different components were used on the reliability test vehicle, and the alloys were compared through Weibull analysis. In addition, the same seven experimental alloys were tested with ball grid array packages cycled up to 100°C or 125°C. All the lead-free alloys performed well, but those containing bismuth showed especially outstanding performance. In general, the ternary and higher alloys performed as well or better than the industry standard tin-silver eutectic, suggesting that solders other than the tin-silver eutectic should be considered for high-reliability, high-temperature applications. Frank W. Gayle is with NIST, Stop 8555, 100 Bureau Drive, Gaithersburg, MD 20899, frank.gayle@nist.gov.     

低银SnAgCuBiNi无铅钎料润湿及溶解行为分析

对Sn0.8Ag0.5Cu2.0Bi0.05Ni（SACBN）和Sn3.0Ag0.5Cu（SAC305）无铅钎料的显微组织、润湿性能和溶解行为进行了比较研究.结果表明,SACBN钎料显微组织由β-Sn＋共晶体组成,共晶组织为在β-Sn基体上均匀分布细针状的Ag3Sn、粒状的（Cu,Ni）6Sn5和细小粒状的铋.在相同温...     

Sn-Zn系钎料专用助焊剂

采用铺展试验法研究了Sn-9Zn钎料配合自制Sn—Zn系钎料专用助焊剂、NH4Cl-ZnCl2助焊剂、树脂型助焊剂以及水溶性助焊剂在铜板上的铺展能力。结果表明,使用自制助焊剂A。匹配Sn-9Zn钎料铺展性能相比其它助焊剂铺展面积明显增大．自制助焊剂不含卤素,钎料的铺展面积最大为65．7mm2,相比NH4C1-ZnCl2助焊剂、树脂型助焊剂、水溶性助焊剂分别提高了16．1％,116．1％,85．1％．此外,复配助焊剂能进一步促进钎料在铜板上的铺展,最大铺展面积分别达到76．5,72．5mm2,控制磺酸亚锡的含量为20％（质量分数）,二乙醇胺,丁二酸的最佳添加量依次为8％,10％（质量分数）．     

无铅钎焊材料的研究

介绍了无铅钎料开发和应用上存在的一些问题和基本要求,通过分析国内外Sn-Ag、Sn-Bi、Sn-Zn、Sn-In几大合金系无铅钎料的合金元素、微观组织、润湿性、熔点、腐蚀行为等方面,逐一阐述了近几年来无铅钎料的研究动态和发展趋势,以期为今后无铅钎料开发和应用提供一些有益指导.     

电子组装用无铅焊料发展现状

在电子组装早期用得最多的焊料主要是锡和铅两种元素,尤其是质量百分比为w(Sn)63%和w(Pb)37%的共晶铅锡合金为多数电子产品所采用.大量使用的Sn/Pb合金焊料正是造成污染的重要来源之一.在制造和使用Sn/Pb焊料的过程中,由于熔化温度较高,有大量的铅蒸气逸出,将严重影响操作人员的身体健康.同时焊接互联的常见热疲劳失效与富铅相有关,期望在设计锡基无铅焊料中不含有铅相,从而改善力学性能,强化焊料.介绍了无铅焊料的基本要求、无铅焊料的研究现状、无铅焊料面临的问题:指出无铅焊料及焊接可靠性仍是工业界和学术界研究的热点问题.     

Effect of cerium on microstructure and mechanical properties of Sn-Ag-Cu system lead-free solder alloys

The melting point, spreading property, mechanical properties and microstructures of Sn-3.0Ag-2.8Cu solder alloys added with micro-variable-Ce were studied by means of optical microscopy, scanning electron microscopy（SEM） and energy dispersive X-ray（EDX）. The results indicate that the melting point of Sn-3.0Ag-2.8Cu solder is enhanced by Ce addition; a small amount of Ce will remarkably prolong the creep-rupture life of Sn-3.0Ag-2.8Cu solder joint at room temperature, especially when the content of Ce is 0.1%, the creep-rupture life will be 9 times or more than that of the solder joint without Ce addition; the elongation of Sn-3.0Ag-2.SCu solder is also obviously improved even up to 15.7%. In sum, the optimum content of Ce is within 0.05%-0.1%.     

SnAgCuEr系稀土无铅钎料的显微组织

系统研究了添加微量稀土Er对Sn3.8Ag0.7Cu无铅钎料合金显微组织和性能的影响.试验发现,微量Er的添加,使Sn3.8Ag0.7Cu钎料合金的熔化温度稍有降低,铺展面积有所增加,抗剪强度有所提高.通过显微组织观察,加Er后网状共晶物体积百分比增大,初晶金属间化合物(IMC)的尺寸变小.微量Er抑制了时效过程钎料与铜基体界面IMC层(IML)的增厚,时效400 h后差别更明显,有利于提高钎料接头的可靠性.指出含微量稀土的SnAgCuEr合金是性能优良的无铅钎料合金.     

新型无铅钎料的优化设计

利用正交设计得出27组试验钎料配方,并测试了钎料的熔化温度、润湿性和搭接接头强度.选取熔化温度、润湿面积、抗剪强度的试验数据作为参数,根据回归设计理论,通过计算机辅助计算建立起关于五个合金元素(Cu、Ag、Sh、Bi、Re)与性能指标之间的函数方程,即二次回归方程.在满足回归方程的基础上,依据最优化原理,通过计算机编程...     

Sn-6Bi-2Ag(Cu, Sb)无铅钎料合金微观组织分析

利用差示扫描量热计 (DSC)测定了Sn 6Bi 2Ag ,Sn 6Bi 2Ag 0 .5Cu ,Sn 6Bi 2Ag 2 .5Sb三种新无铅钎料合金的熔化温度。结果表明 ,少量Cu的加入能降低Sn Bi Ag系无铅钎料合金的熔化温度 ,而Sb的加入使合金的熔化温度升高。利用光学显微镜 (OM )、扫描电子显微镜 (SEM )、能谱分析 (EDX)对合金的微观组织进行了分析与比较 ,钎料合金的微观组织与冷却条件和合金元素的含量有关 ,Sb的加入使析出相的尺寸细化。硬度测定表明Sn Bi Ag(Cu ,Sb)无铅钎料合金的硬度远大于纯Sn的硬度 ,加入少量的Cu(0 .5 % ) ,Sb(2 .5 % )对Sn Bi Ag系钎料合金的硬度影响较小     

低温免清洗无铅焊膏用活化剂的优化

锡铋合金是一种比较理想的低温无铅焊料,但是铋的氧化会使焊料润湿性变差,严重阻碍其应用。活化剂能除去焊料表面氧化物,提高焊料的润湿性。以锡铋焊料的铺展性能和焊点形貌作为评价的主要指标,通过焊接实验研究活化剂用量、活化剂配比对锡铋焊料助焊剂的物理性能及锡铋焊料铺展性能的影响。结果表明:活化剂质量分数为25%,活化剂柠檬酸、水杨酸、丁二酸的质量之比为2∶3∶4时,助焊剂不挥发物含量低于5%,稳定性好,不粘性合格;且得到的焊膏焊点外观规则,光亮饱满,焊接头光滑,焊料铺展率达到79.6%。     

Sn-Zn-Ga-xPr钎料组织与性能

研究了添加合金元素Ga及稀土元素Pr对Sn-Zn钎料组织与性能的影响.适量添加镨,可以提高钎料的润湿性能;但当镨含量超过0.12%(质量分数)时,镨会在钎料表面形成氧化渣,恶化钎料的润湿性能.镨的添加可以改善焊点的力学性能,抗剪强度在镨含量为0.08%时增加至最大.镨加入后细化了钎料基体中的富锌相,当镨含量不超过0.08%时,界面层平整且厚度变化不大,但若稀土添加过量,则界面层会明显增厚.镨在Sn9Zn0.5Ga钎料中的最佳添加量为0.08%左右.     

SnCuNi-xPr焊点组织和性能分析

分析了不同稀土元素Pr的添加量对Sn0.7Cu0.05Ni无铅钎料润湿性能、焊点力学性能以及钎料显微组织的影响,并对焊点内部组织的演化规律与其性能之间的内在联系进行了初步探讨.研究发现,稀土元素Pr的适宜添加量为0.025%～0.075%,且存在一最佳添加量0.05%.微量稀土元素Pr的加入可显著降低液态钎料的表面张力...     

SnAgCu无铅钎料焊点结晶裂纹

针对印刷电路板(PCB)上无铅钎料SnAgCu微小焊点的结晶裂纹,利用设计的试件重现无铅钎料钎焊过程中产生的结晶裂纹,对无铅钎料结晶裂纹进行模拟,同时研究了微量元素的添加对SnAgCu合金焊点结晶裂纹形成的影响。结果表明,在尺寸很小的焊点上仍然存在明显的结晶裂纹。用焊点结晶裂纹的总长度定量地评价无铅钎料结晶裂纹的敏感倾向,添加Ni和Ce元素能够降低无铅钎料结晶裂纹的形成,而P元素的添加却加剧了结晶裂纹的形成,明显增加了焊点处的结晶裂纹。     

Physical metallurgy in lead-free electronic solder development

Classical physical metallurgy principles play significant roles in the pursuit of suitable substitutes for traditional lead-based solders in the electronic industry. Phase diagrams, alloy development, solidification, diffusion, wetting, aging, precipitation of second-phase particles, microstructural coarsening, temperature effects, thermomechanical behavior, and creep are among the issues to be considered. This article focuses on the importance of physical metallurgy in these developments. For more information, contact K.N. Subramanian, Michigan State University, Department of Chemical Engineering and Materials Science, East Lansing, MI 48824-1226; e-mail: subraman@egr.msu.edu.     

The creep properties of lead-free solder joints

This paper describes the creep behavior of three tin-rich solders that have become candidates for use in lead-free solder joints: Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-0.7Cu. The three solders show the same general behavior when tested in thin joints between copper and Ni/Au metallized pads at temperatures between 60‡C and 130°C. Their steady-state creep rates are separated into two regimes with different stress exponents. The low-stress exponents range from ∼3–6, while the high-stress exponents are anomalously high (7–12). Strikingly, the high-stress exponent has a strong temperature dependence near room temperature, increasing significantly as the temperature drops from 95°C to 60°C. The anomalous creep behavior of the solders appears to be due to the dominant tin constituent. Research on creep in bulk samples of pure tin suggests that the anomalous temperature dependence of the stress exponent may show a change in the dominant mechanism of creep. Whatever its source, it has the consequence that conventional constitutive relations for steady-state creep must be used with caution in treating tin-rich solder joints, and qualification tests that are intended to verify performance should be carefully designed.     

无铅波峰焊Sn-Bi-Ag-Cu钎料焊点剥离机制

对波峰焊常用的Sn-Bi-Ag-Cu无铅钎料进行了通孔波峰焊焊点剥离现象的模拟实验。分析表明,凝固延迟及铋元素偏析导致焊盘拐角附近钎料区在结晶后期残存液相最终成为缩孔的聚集区,结晶后期该区的低塑性使收缩应变容易超过材料的塑性而发生开裂,且发生机制与结晶裂纹发生的机制相同,强偏析元素铋的存在导致剥离概率极高。通孔拐角附近是应力应变的集中区,沿界面至焊盘外缘,应力应变逐渐降低。Sn-Bi-Ag-Cu系钎料焊点剥离的发生机制是由于Bi元素在焊盘/钎料界面富集使拐角附近钎料的液相线温度降低,结晶后期该区因固液共存而成为相对低塑性区,在应力作用下开裂并向焊盘外缘扩展。     

Sn-Ag-Cu-Ce无铅钎料合金体系的热力学计算及预测

利用周模型对Sn—Ag—Cu—Ce无铅钎料合金体系进行了热力学计算预测。热力学计算结果表明，Ag、Cu含量(质量分数)分别为0．5％-4．5％时，当Ce的含量(质量分数)超过0．05％时，体系达到化学平衡状态；当Ce的含量(质量分数)达到0．6％左右时，Sn、Ag、Cu分别都出现了“等活度系数”现象。这一研究结果可为无铅钎料合金的成分设计提供理论指导。