In search of new lead-free electronic solders

Tin-lead solder has been widely used in the electronics industry for many years. However, increasing environmental pressures, together with some technological issues, have now prompted great interest in finding a viable nonleaded substitute. This paper first summarizes some of the background issues, specifically availability, cost, and toxicity. It then outlines some of the work undertaken by Cookson Group plc on behalf of Alpha-Fry in searching for alloys with a narrow melting range between 135 and 183°C.     

Rare-earth-enabled universal solders for microelectromechanical systems and optical packaging

In packaging of microelectromechanical systems (MEMS), optical, and electronic devices, there is a need to directly bond a wide variety of inorganic materials, such as oxides, nitrides, and semiconductors. Such applications involve hermetic-sealing components, three-dimensional MEMS assembly components as well as active semiconductor or optical components, dielectric layers, diffusion barriers, waveguides, and heat sinks. These materials are known to be very difficult to wet and bond with low melting-point solders. New Sn-Ag- or Au-Sn-based universal solders doped with a small amount of rare-earth (RE) elements have been developed, which now allow direct and powerful bonding onto the surfaces of various MEMS, optical, or electronic device materials. The microstructure, interface properties, and mechanical behavior of the bonds as well as the potential packaging applications of these new solder materials for MEMS and optical fiber devices are described. Various packaging-related structural, thermal, or electrical issues in MEMS are also discussed.     

Creep phenomena in lead-free solders

A critical review of data on microstructure and creep process activation energy values for a number of lead-free solder alloys like Sn-Ag; Sn-Bi; Sn-In was conducted. The review revealed a scatter in experimental data, which could not be explained by the dislocation creep mechanism only, even after the published data was corrected for Young’s modulus temperature dependence. An analysis of the data implies that possible origin of such a scatter is nucleation, accumulation and further growth of such internal defects as pores and microcracks during creep. It is shown that these processes may affect the measured steady-state creep rates, and may be one of the major reasons for the observed scatter in experimental data, and, therefore, must be taken into consideration in lead-free solder alloys’ creep studies.     

Microstructure characterization of SnAgCu solder bearing Ce for electronic packaging

The creep-rupture lives of Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce lead-free solder joints for electronic packaging were investigated, respectively. And the relationship between creep behavior and intermetallic compound (IMC: Ag₃Sn, Cu₆Sn₅, CeSn₃) particles in SnAgCu/SnAgCuCe solder joints has been obtained. Meanwhile, rare earth Ce concentration gradient and retardation effect of Ce on the IMC layer have been observed at the solder/Cu interface. Moreover, aging reaction of Sn and Cu, and the effect mechanism of rare earth Ce on two IMCs (Cu₆Sn₅ and Cu₃Sn) are reported.     

Reactions of lead-free solders with CuNi metallizations

We have done experimental research on the dissolution rate and intermetallic growth on Cu, Ni, and CuNi-alloy substrates as a function of time and Cu/Ni ratio of the substrate. Reactions that occur when CuNi metallizations are soldered with lead-free solders were investigated. The experiments were performed using Sn-3.5Ag and Sn-3.8Ag-0.7Cu solders and different CuNi alloys. To determine the rate of dissolution of the substrate material into the solder, CuNi foils of different concentrations were immersed in Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder baths for soldering times ranging from 15 sec to 5 min at 250°C. In addition, reflows of solder balls were made on top of bulk substrates to study the reaction when there is a practically infinite amount of CuNi available compared to the amount of solder. Thin film experiments were also done, where Ni containing under bump metallizations (UBMs) were fabricated and reflowed with eutectic SnAg solder balls. The nickel slows down the dissolution of the UBM into the solder and the formation of intermetallics during reflow compared to Cu metallizations. The solder/UBM interfaces were analyzed with SEM to find out how Ni concentration affects the reaction, and how much Ni is needed to obtain a sufficiently slow reaction rate.     

无铅组装技术浅析

从无铅组装的角度出发,详细分析了无铅组装技术涉及的无铅焊料和导电胶组装技术,阐述了传统的锡铅焊接与无铅焊接的工艺差异,提出了该技术要推广应用所面临的技术难点。     

A constitutive model for creep of lead-free solders undergoing strain-enhanced microstructural coarsening: A first report

Lead-free solder joints in microelectronic applications frequently have microstructures comprising a dispersion of intermetallic particles in a Sn matrix. During thermomechanical cycling (TMC) of the solder joint, these particles undergo strain-enhanced coarsening, resulting in a continuously evolving, creep behavior. Because the extent of coarsening is dependent on the stress/strain state, which is dependent on the location within a joint, it is important that creep models used in joint-life prediction incorporate these effects. Here, an approach for incorporating the effect of in-situ second-phase particle coarsening in a dislocation-creep model applicable to lead-free solder alloys is proposed. The formulation, which can be expressed in a closed analytic form following some simplifications, incorporates the effects of both static- and strain-enhanced coarsening and accounts for the effects of inelastic-strain history and hydrostatic constraint. Predictions of coarsening based on the model agreed reasonably well with experimentally observed trends. Because of its simplicity, the microstructurally adaptive creep model proposed here can be easily incorporated in current finite-element codes for joint behavior simulation.     

低银无铅焊膏板级封装工艺和焊点可靠性试验

针对低Ag无铅焊膏的市场需求,研究开发了一种适用于99.0Sn0.3Ag0.7Cu低Ag无铅焊膏用松香型无卤素助焊剂(WTO—LF3000),配制了相应的无铅焊膏(WTO—LF3000—SAC0307),并对其板级封装工艺适应性及焊点可靠性进行了考察,用测试后样品的电气可靠性作为接头可靠性评价条件。结果表明:所开发的低Ag无铅焊膏熔点和润湿性符合产品实际要求。配制的焊膏印刷质量良好,焊点切片观察其孔隙率<25%,满足行业标准IPC—A—610D之要求。样品分别经跌落、震动和温度循环试验后,无焊点脱落等现象,电气功能正常。     

Intermetallic growth between lead-free solders and palladium

Intermetallic growth between Pd and the lead-free solders Sn-Ag and Sn-Ag-Cu has been studied. Diffusion couples were prepared by reflowing the solders on Pd and then aging the couples at 156°C, 175°C, 195°C, and 210°C. At the higher temperatures of 175°C, 195°C, and 210°C, PdSn₄ made up most of the layer that grew between the solders and the Pd, although small regions of second phases were always found in the PdSn₄ matrix, and it was sometimes possible to identify discontinuous regions of PdSn₃ next to the Pd. The thickness of the intermetallic layer increased with the square root of time, consistent with diffusion-controlled growth. In couples annealed at 156°C, the morphology of the PdSn₃ phase and growth kinetics differed depending on the composition of the solder.     

微电子封装无铅钎焊的可靠性研究

本文阐述了微电子封装聚用无铅钎料的必要性。概述了无钎铅料的研究现状,最后着重分析讨论了微电子封装无铅钎焊的可靠性问题。     

Materials for high-density electronic packaging and interconnections in the higher packaging levels

This paper concentrates on the materials requirements anticipated for future packaging strategies of electronic components beyond the 1st level package. This includes Wafer Scale Integration (Level 0), hybrid assembly of bare chips in multichip modules (Level 1.5), printed wiring board (PWB) assembly, including surface mount (Level 2), and higher levels (connectors, mother boards, and cables). Furthermore, high-performance digital VLSI logic packaging only is addressed, to the exclusion of memory, analog, and power circuitry (except power supplies).     

Interfacial reactions during soldering with lead-tin eutectic and lead (Pb)-free,tin-rich solders

The use of lead (Pb)-containing solders for the interconnections of microelectronic subsystem assembly and packaging has become an environmental issue. Extensive research and development activities for replacing Pb-containing solders with environmentally safe Pb-free solders are in progress in electronic industries, universities, and national laboratories. One key technical issue recognized with the Pb-free, Sn-rich solders is a need to develop a good barrier metallurgy to control the interfacial reactions, namely, dissolution of the base metal(s) and concurrent formation of intermetallics at the soldering interfaces. In this study, the interfacial reactions of Cu and Ni metallizaton with several Pb-free and Pb-containing solders are investigated. The dissolution kinetics of the base metal(s) as well as the growth kinetics of the intermetallics are discussed.     

Damage based fatigue criterion for solders in electronic packaging

A fatigue failure criterion based on physical damage mechanism is proposed for solders. A solder structure is defined as fatigued when the portion of its failed grains reaches a critical percolation threshold, since under such state the failed grains may form large cluster of cracks, leading to the structure's mechanical instability as well as electrical degradation. Mechanically the inception of such state corresponds to a turning point of load carry capability in a simple fixed-strain tension-tension test of a solder structure. Experimental data for both lead-rich solders such as 96.5Pb-3.5Sn solders and lead-free solders such as 96.5Sn-3.5Ag showed consistency with the proposed criterion. Experimental data also show that by adoption of this failure criterion a better prediction of mechanical fatigue for a lead rich solder can be achieved. The criterion is derived from a science based fatigue theory, therefore the criterion should be applicable to anisotropic as well as isotropic structures regardless of solder structure sizes, which is useful to the development and design of small-scale (micron scale or smaller) solder joints for current electronic packaging.     

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

电子组装用高温无铅钎料的研究进展

分析了国内外电子组装用高温无铅钎料的研究现状。指出目前常用的高温钎料仍然是高Pb焊料或80Au-20Sn钎料,导致焊料含Pb而污染环境,或者含质量分数为80%的Au而使焊料成本奇高。指明了Bi-Ag系钎料具有潜力替代高Pb焊料或80Au-20Sn钎料。未来的研究将在成分设计及可靠性等方面进行探索,以最终找到既经济又可替代传统高铅钎料的高温无铅钎料。     

A review: On the development of low melting temperature Pb-free solders

Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.     

表面镀覆层与焊料的界面反应对可靠性的影响

通过对SBGA器件与不同表面镀覆层(OSP,ENIG,HASL和Im-Ag)的PCB板互连形成的SnAgCu焊点在温度循环条件下的可靠性进行研究,对比SnPb焊膏,结合有限元仿真进行分析.结果表明:使用ENIG镀覆层的可靠性最好,SnPb样品的焊点失效率为0.98%,SnAgCu样品只有0.68%.温度循环后SnAgC...     

Morphology of intermetallic compounds formed between lead-free Sn-Zn based solders and Cu substrates

The morphologies of intermetallic compounds formed between Sn-Zn based solders and Cu substrates were investigated in this study. The investigated solders were Sn-9Zn, Sn-8.55Zn-0.45Al, and Sn-8.55Zn-0.45Al-0.5Ag. The experimental results indicated that the Sn-9Zn solder formed Cu₅Zn₈ and CuZn₅ compounds on the Cu substrate, while the Al-containing solders formed the Al_4.2Cu_3.2Zn_0.7 compound. The addition of Ag to the Sn-8.55Zn-0.45Al solder resulted in the formation of the AgZn₃ compound at the interface between the Al_4.2Cu_3.2Zn_0.7 compound and the solder. Furthermore, it was found that the cooling rate of the specimen after soldering had an effect on the quantity of AgZn₃ compound formed at the interface. The AgZn₃ compound formed with an air-cooling condition exhibited a rougher surface and larger size than with a water-quenched condition. It was believed that the formation of the AgZn₃ compound at the interface occurs through heterogenous nucleation during solidification.     

电子封装无铅化现状

近年来铅污染广泛地受到了人们的重视。2003年2月13日欧盟颁布WEEE/RoHS法案,在世界范围内引发了一场电子封装的无铅化革命。综述了电子封装技术的现状以及我国如何面对无铅化的问题。