Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

Sn-Ag系电子无铅软钎料的超电势研究

随着微电子表面组装技术（SMT）的迅速发展和公众环境意识的增强，无铅软钎料成为近年来研究的焦点。主要研究了Sn-Ag系合金钎料的超电势电化学性能，并且与传统的Sn-Pb近共晶合金进行了对比。试验证明在酸、碱两种不同的溶液环境中，无铅软钎料在Sn-Pb合金的超电势有明显的差异。并指出含铋的钎料的超电势随着铋的含量的增加而降低，从而提出了在无铅软钎料研究中应当考虑因超电势变化而引起的问题。     

The Effect of Bi Contamination on the Solidification Behavior of Sn-Pb Solders

This paper presents experimental results and theoretical calculations that evaluate the effects of Bi contamination on the solidification behavior of Sn-Pb alloys. The pasty (mushy) range, the type of solidification path, and the microstructure of the solidified alloys are described. The experimental results are obtained from thermal analysis and metallography, and the solidification calculations are performed using the lever rule and Scheil assumptions. The experimental results show that the solidification behavior of the contaminated solder at cooling rates of 5°C/min and 23°C/min is closer to the predictions of the lever rule calculations than those of the Scheil calculations. Although the freezing range of Bi-contaminated Sn-Pb solders is increased, formation of a ternary eutectic reaction at 95°C is not observed for contamination levels below the Bi mass fraction of 6%.     

Use of thermodynamic data to calculate surface tension and viscosity of Sn-based soldering alloy systems

A thermodynamic database for the Pb-free soldering alloy systems, which include Sn, Ag, Cu, Bi, and In, has been made using the CALPHAD method. The resulting thermodynamic properties of the Sn-based binary alloy systems were used to determine the surface tensions and viscosities. The surface tensions were calculated using Butler’s monolayer model and the viscosities by Hirai’s and Seetharaman’s models. Butler’s model was also used to determine the surface active element. The results for binary systems were extended to the Sn-based ternary systems (Sn-Ag-Cu, Sn-Ag-Bi). The surface tensions of commercial eutectic Sn-Pb and Sn-Pb-Ag solder alloys were measured by the sessile drop method. The measured values and other researchers’ results were compared with the calculated data.     

Sn-Ag和Sn-Zn及Sn-Bi系无铅焊料

随着微电子组装技术的发展，研制新型的和实用的无铅焊料替代传统的Sn-Pb焊料已成为近年的研究热点。介绍了目前最常见的Sn-Ag、Sn-Zn和Sn-Bi为基体的无铅焊料并与传统的Sn-Pb焊料的性能进行了比较。     

电子产品中的无铅焊料及其应用与发展

由于传统焊接技术使用的Sn-Pb焊料中的铅会对环境造成污染而被禁止使用,近年来无铅焊料成为了研究热点。文中介绍了运用于电子产品中的无铅焊料的发展背景、特点及要求。根据应用温度不同,无铅焊料可以分为低温、中温和高温无铅焊料。文章综述了它们各自的应用特点、场合及存在的问题和发展前景。     

Sn-Zn-Bi无铅焊料表面张力及润湿性研究

采用气泡最大压力法对Sn-9Zn-XBi焊料进行了表面张力测试,用平衡法测试焊料的润湿性。结果表明:Bi的添加大大降低了Sn-Zn系焊料熔体的表面张力;然而焊料暴露在空气环境下1min后,表面形成ZnO导致其表面张力增大;Bi的增加提高了焊料在铜片上的润湿力,缩短了润湿时间;Sn-9Zn-XBi焊料润湿力仍低于Sn-40Pb,其原因是焊料–铜界面能偏高。     

Lead (Pb)-free solders for electronic packaging

The harmful effects of lead on the environment and human health, coupled with the threat of legislation, have prompted a serious search for lead-free solders for electronic packaging applications. At present, Sn-Pb eutectic and other Pb-containing solders find widespread use in printed circuit board assembly. Several Pb-free solder alloys which appear to have the potential for replacing Sn-Pb solders are receiving increased attention from the electronic assembly community. Recently, numerous studies have been published detailing the wetting characteristics, tensile and shear strength, and creep and low-cycle fatigue properties of these alloys. It is the purpose of this paper to review these results and assess the suitability of the solders for electronic packaging from the viewpoints of process technology and reliability.     

电子焊料的工艺性能及影响因素

通过分析焊料在钎焊过程中的行为,提出了电子焊料的工艺性能主要包括熔化/固化温度、抗氧化性、润湿性和漫流性。给出了各工艺性能的定义。分析表明:相关的影响因素主要包括合金的组成、纯度和化学均匀性;母材的成分、性质和表面的洁净度;液态焊料的表面张力;钎焊温度、气氛、助焊剂的活性;液态焊料表面膜的组成、结构和性能等。     

Development of new no-flow underfill materials for both eutectic Sn-Pb solder and a high temperature melting lead-free solder

In recent years, no-flow underfill technology has drawn more attention due to its potential cost-savings advantages over conventional underfill technology, and as a result several no-flow underfill materials have been developed and reported. However, most of these materials are not suitable for lead-free solder, such as Sn/Ag (m.p. 225/spl deg/C), Sn/Ag/Cu (m.p. 217/spl deg/C), applications that usually have higher melting temperatures than the eutectic Sn-Pb solder (m.p. 183/spl deg/C). Due to the increasing environmental concern, the demand for friendly lead-free solders has become an apparent trend. This paper demonstrates a study on two new formulas of no-flow underfill developed for lead-free solders with a melting point around 220/spl deg/C. As compared to the G25, a no-flow underfill material developed in our research group, which uses a solid metal chelate curing catalyst to match the reflow profile of eutectic Sn-Pb solder, these novel formulas employ a liquid curing catalyst thus provides ease in preparation of the no-flow underfill materials. In this study, curing kinetics, glass transition temperature (Tg), thermal expansion coefficient (TCE), storage modulus (E') and loss modulus (E') of these materials were studied with a differential scanning calorimetry (DSC), a thermo-mechanical analysis (TMA), and a dynamic-mechanical analysis (DMA), respectively. The pot-life in terms of viscosity of these materials was characterized with a stress rheometer. The adhesive strength of the materials on the surface of silicon chips were studied with a die-shear instrument. The influences of fluxing agents on the materials curing kinetics were studied with a DSC. The materials compatibility to the solder penetration and wetting on copper clad during solder reflow was investigated with both eutectic Sn-Pb and 95.9Sn/3.4Ag/0.7Cu solders on copper laminated FR-4 organic boards.     

Effects of POSS-Silanol Addition on Whisker Formation in Sn-Based Pb-Free Electronic Solders

Previous studies have indicated that silanol in the form of polyhedral oligomeric silsesquioxane (POSS) trisilanol could form strong bonds with solder matrix without agglomeration, and inhibit diffusion of metal atoms when subjected to high ambient temperature and/or high current density. Addition of POSS-trisilanol has also been shown to improve the comprehensive performance of Sn-based Pb-free solders, such as shear strength, resistance to electromigration, as well as thermal fatigue. The current study investigated the whisker formation/growth behaviors of Sn-based Pb-free solders (eutectic Sn-Bi) modified with 3 wt.% POSS-trisilanol. Solder films on Cu substrates were aged at ambient temperature of 125°C to accelerate whisker growth. The microstructural evolution of the solder films’ central and edge areas was examined periodically using scanning electron microscopy. Bi whiskers were observed to extrude from the surface due to stress/strain relief during growth of Sn-Cu intermetallic compounds (IMCs). Addition of POSS-trisilanol was shown to retard the growth of Bi whiskers. The IMCs formed between POSS-modified solders and the Cu substrate showed smoother surface morphology and slower thickness growth rate during reflow and aging. It was indicated that POSS particles located at the phase boundaries inhibited diffusion of Sn atoms at elevated temperatures, and thus limited the formation and growth of IMCs, which resulted in the observed inhibition of Bi whisker growth in POSS-modified solders.     

Thermodynamic database for phase diagrams in micro-soldering alloys

A thermodynamic database for the calculation of phase diagrams in micro-soldering alloy systems, which include the elements Pb, Bi, Sn, Sb, Cu, Ag, and Zn has been developed using the CALPHAD method. The various thermodynamic parameters for describing the Gibbs energies of the different constituent phases have been evaluated by optimizing experimental data on phase boundary compositions and thermochemical properties such as activity, heat of mixing and enthalpy of formation. The resulting database provides the means whereby the liquidus and solidus surfaces, isothermal and vertical section diagrams, phase percentages and the mole fraction of the phase constitutions etc., in multi-component soldering alloys can be readily calculated. Related information such as the surface tension and viscosity of the liquid phase can also be predicted, thus rendering the database as a valuable tool for developing leadbearing and lead-free solders.     

Ag对Sn-Pb电子钎料合金性能的影响

研究了Ag的合金化对Sn-Pb钎料合金材料性能的影响。并从钎料的润湿性能，机械性能及抗腐蚀性能等方面讨论了Ag的有利作用，研究表明，在一些特定条件下的电子元件的焊接，Sn-Pb-Ag钎料可部分地或全部地替代昂贵的遗金属钎料合金。     

Comparative study of wetting behavior and mechanical properties (microhardness) of Sn-Zn and Sn-Pb solders

In this paper the solder balling, wetting, spreading, slumping and microhardness testing of the Sn-Zn based solders have been compared with the Sn-Pb solder. Two types of solders (Sn-9Zn and Sn-8Zn-3Bi) have been investigated along with Sn-37Pb solder for reference. The variation of these tests has been done as a function of reflow temperature from 220-250 °C. Solder balls of these three solder pastes after 15 min heating at 230 °C show no ball formation surrounding the central ball. Spread test shows that above 240 °C Sn-9Zn is very good and can be comparable to Sn-37Pb. The wetting angle of Sn-9Zn (39°) at 250 °C is even lower than the Sn-37Pb solder (41°). In case of Sn-8Zn-3Bi, the wetting angle is very high (77°) at 220 °C, which is unacceptable but it drops down to 48° at 250 °C. Line profiles of slump test show that after preheating at 160 °C, Sn-9Zn behaves similar to Sn-37Pb with better distinction in the finer pitch (120 μm). Microhardness shows two different characteristics for eutectic and non-eutectic solder pastes. Hardness of Sn-37Pb and Sn-9Zn (eutectic) decreases with increasing reflow temperature while the microhardness of Sn-8Zn-3Bi (non-eutectic) increases with increasing reflow temperature. Microstructural characterization at 220 and 250 °C shows grain coarsening in Sn-37Pb and Sn-9Zn solders, which cause the hardness to drop a little. For Sn-8Zn-3Bi, with increasing temperature the amount of hard Bi segregation increases which is the main cause of the rise in hardness. SEM images show the formation of Pb rich islands in Sn-37Pb, formation of Zn rod from spheroids in Sn-9Zn and precipitation of Bi-rich phase in Sn-8Zn-3Bi are the important features that contribute to different hardness nature.     

Microstructure and mechanical properties of Pb-free solder alloys for low-cost electronic assembly: A review

Lead-free solders, including Sn-58Bi, Sn-52In, and Sn-3.5Ag, are potential replacements for Sn-37Pb solder in low-cost electronic assembly. This paper reviews the literature on the microstructure and mechanical properties of these alloys. Because of the processing and testing conditions, many of the data are not predictive for electronic assembly applications. However, eutectic Sn-Bi seems to have properties approaching those of eutectic Sn-Pb under most conditions, while eutectic Sn-In seems far inferior in most respects. Eutectic Sn-Ag has many promising characteristics, but its relatively high melting temperature may preclude its use for this type of application.     

三种高Sn无铅钎料的表面张力和润湿性能研究

采用悬滴法测量了3种无铅钎料合金(Sn-3.0Ag-0.5Cu、Sn-0.7Cu与Sn-9.0Zn)在260℃时的表面张力,分别为525.5,534.8和595.4 mN/m;同时采用座滴法测量了其在260℃熔融状态下与Cu基板的接触角,分别为24.5°、28.0°和102.5°,并且与传统Sn-37.0Pb钎料进行了比较研究。结果表明,无铅钎料合金的表面张力与接触角均大于Sn-37.0Pb钎料。结合Young-Dupre公式讨论了钎料合金表面张力与其润湿性能的相关性,认为Sn基钎料合金在Cu基板上的润湿性能主要取决于其表面张力。     

无铅波峰焊设备的特点

相对于传统的Sn-Pb焊料,无铅焊料需要较高的焊接温度,而且润湿性差,另外免清洗助焊剂和水溶性助焊剂的固体含量低,活性温度高和活性区间窄。无铅焊料和助焊剂的特性决定了无铅波峰焊设备在结构和材料选用上有很大不同。通过对无铅波峰焊设备的各个部分的特点进行分析,为传统的旧波峰焊设备的改造提供参考。     

含铅焊料绿色化的途径

分析了铅在焊料中的作用和对环境的污染行为,以及焊料无铅化所带来的技术、经济和社会问题。基于含铅焊料的污染源于铅的化学活性,提出了使含铅焊料绿色化的研究途径。即:通过采用液态金属结构的优化设计和掺杂的方法,可以改变液态焊料表面铅原子的结合形态,能降低铅的化学活性,提高挥发相变的活化能,从而抑制铅对大气的污染;通过控制焊料从液态转变为固态的行为,可以使表面形成n个原子层的改性膜,能降低铅的溶解活性,从而达到抑制或避免铅对水源污染的目的。     

无铅波峰焊接设备特点

相对于传统的Sn—Pb焊料，无铅焊料需要较高的焊接温度，而且润湿性差，另外免清洗助焊剂和水溶性助焊剂的固体含量低，活性温度高和活性区间窄。无铅焊料和助焊剂的特性决定了无铅波峰焊设备在结构和材料选用上有很大不同。本文通过对无铅波峰焊设备的各个部分的特点进行分析，为传统的旧波峰焊设备的改造提供参考。     

Wetting and reaction of Sn-2.8Ag-0.5Cu-1.0Bi solder with Cu and Ni substrates

The wettability of newly developed Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu and Ni substrates was assessed through the wetting balance tests. The wettability assessment parameters such as contact angle (ϑ_c) and maximum wetting force (F_w) were documented for three solder bath temperatures with three commercial fluxes, namely, no-clean (NC), nonactivated (R), and water-soluble organic acid flux (WS). It was found that the lead-free Sn-2.8Ag-0.5Cu-1.0Bi solder exhibited less wetting force, i.e., poorer wettability, than the conventional Sn-37Pb solder for all flux types and solder bath temperatures. The wettability of Sn-2.8Ag-0.5Cu-1.0Bi lead-free solder on Cu substrate was much higher than that on Ni substrate. Nonwetting for Sn-2.8Ag-0.5Cu-1.0Bi and Sn-Pb solders on Ni substrate occurred when R-type flux was used. A model was built and simulations were performed for the wetting balance test. The simulation results were found very close to the experimental results. It was also observed that larger values of immersion depth resulted in a decrease of the wetting force and corresponding meniscus height, whereas the increase in substrate perimeter enhanced the wettability. The wetting reactions between the solder and Cu/Ni substrates were also investigated, and it was found that Cu atoms diffused into the solder through the intermetallic compounds (IMCs) much faster than did the Ni atoms. Rapid formation of IMCs inhibited the wettability of Sn-2.8Ag-0.5Cu-1.0Bi solder compared to the Sn-Pb solder.