Tin Whiskers Formation on an Electronic Product: A Case Study

During qualification testing, a printed circuit board (PCB) of an electronic device for a drilling tool failed. The circuit board was exposed to a 120 h aging cycle at 180°C followed by 10 thermal cycles between −40 and 180°C before a failure was noticed. During inspection numerous white whiskers were observed over a lead-free solder surface. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) were used for microscopic examination and material characterization of the whiskers, end-cap metallization, and the solder materials. The tin whisker formation was attributed to the compressive stress in the tin solder material, which was caused by diffusion of the end cap metallization, formation of intermetallics, and thermal cycling of the soldered components. Recommendations are given to mitigate/control whisker formation on the lead-free solder materials.     

Interfacial reactions of Sn-3.5% Ag and Sn-3.5% Ag-0.5% Cu solder with electroless Ni/Au metallization during multiple reflow cycles

The increasing industry awareness of lead-free activities has prompted original equipment manufacturers and suppliers to investigate lead-free solder systems in detail. The reliability of lead-free solders has been studied a lot recently, but the knowledge of it is still incomplete and many issues related to them are under heavy debate. In this study, the interfacial reactions of Sn-3.5Ag and Sn-3.5Ag-0.5Cu (wt.%) solders with Cu/Ni(P)/Au ball grid array (BGA) pad metallization were systematically investigated after multiple reflows. The peak reflow temperature was fixed at 260°C. It was found that relatively high consumption of Ni(P) was observed in the case of Sn-3.5%Ag solder alloys during multiple reflow cycles. A white layer of P rich Ni-Sn compound was observed above the dark Ni₃P layer for Sn-3.5%Ag solder after several reflows. It was noticed that the mean thickness of the intermetallics and the dark P-rich Ni layer at the interface was decreased just by adding 0.5% Cu in Sn-3.5%Ag solder alloy with less overall interfacial reaction at the solder joint.     

纳米结构强化无铅焊点的力学性能

新型的无铅钎料不仅要具备含铅钎料的工艺性能,更重要的是要有更高的力学性能,特别是焊接接头的抗蠕变能力。将纳米级多面齐聚倍半硅氧烷(Polyhedral oligomeric silsesquioxanes,POSS)颗粒作为增强相添加到基体钎料中,能够有效地改善Sn-3. 5Ag基复合钎料的性能。研究了不同种类POSS增强颗粒对Sn-3. 5Ag钎料显微组织和力学性能的影响,确定出POSS增强颗粒复合钎料的最佳配比,并对最佳配比复合钎料在不同温度不同载荷条件下的蠕变寿命进行了研究。结果表明：POSS颗粒质量分数小于2%时,可以抑制基板界面处初晶金属间化合物的生长;复合钎料的抗剪切强度明显提高;低温时,最大蠕变寿命明显改善。     

An Optimization Analysis of UBM Thicknesses and Solder Geometry on A Wafer Level Chip Scale Package Using Robust Methods

Wafer level chip scale package (WLCSP) has been recognized providing clear advantages over traditional wire-bond package in relaxing the need of underfill while offering high density of I/O interconnects. Without the underfill, the solder joint reliability becomes more critical. Adding to the reliability concerns is the safety demand trend toward "green' products on which unleaded material, e.g. lead-free solders, is required. The requirement of lead-free solders on the packages results in a higher reflow temperature profile in the package manufacturing process, in turn, complicating the reliability issue. This paper presents an optimization study, considering the fatigue reliability, for a wafer level chip scale IC package in which a Ti/Cu/Ni UBM is involved. A finite element model is developed for the package. The model employs Sn3.8Ag0.7Cu lead-free solders built on build-up layers with micro-vias. Finite element analyses are performed to study the mechanical behaviors of the package elements in which the solder as well as the UBM is of interest. Firstly, a Surface Evolver program is used to construct the solder based non-solder mask defined (NSMD) pad. Then, multi-purpose finite element software, ANSYS^tm, is used to create a double symmetric 3-D numerical model to investigate the mechanical behaviors including deformation, stress-strain relation as well as hysteresis loops for temperature cycles. The Garofalo-Arrhenius Creep Model is employed. A modified Coffin-Manson formula is also employed to estimate the fatigue life for the package. Finally, the Taguchi robust analysis is adopted for optimization analysis of UBM thicknesses and solder geometry. Our results show that thicker UBM layers tend to increase the fatigue life while a small solder pad will prolong the fatigue life and as volume increases so does the fatigue life. From the results of Taguchi robust analysis, it is shown that among the factors of UBM layer thickness, solder pad radius and solder volume, the solder volume is the most dominating factor on the fatigue life of the package. The optimal combination of UBM thickness set at 0.0066 mm (level 3), solder pad radius set at 0.10 mm (Level 1), and solder volume set at 0.020 mm³ (Level 3) contributes the greatest fatigue life of 1229 cycles which is 448% gained over our reference package model.     

Intermetallic compounds in 3D integrated circuits technology: a brief review

Abstract

The high performance and downsizing technology of three-dimensional integrated circuits (3D-ICs) for mobile consumer electronic products have gained much attention in the microelectronics industry. This has been driven by the utilization of chip stacking by through-Si-via and solder microbumps. Pb-free solder microbumps are intended to replace conventional Pb-containing solder joints due to the rising awareness of environmental preservation. The use of low-volume solder microbumps has led to crucial constraints that cause several reliability issues, including excessive intermetallic compounds (IMCs) formation and solder microbump embrittlement due to IMCs growth. This article reviews technologies related to 3D-ICs, IMCs formation mechanisms and reliability issues concerning IMCs with Pb-free solder microbumps. Finally, future outlook on the potential growth of research in this area is discussed.     

Magnetic nanoparticle-based solder composites for electronic packaging applications

Sn–Ag–Cu (SAC) alloys are regarded as the most promising alternative for traditional Pb–Sn solders used in electronic packaging applications. However, the higher reflow temperature requirement, possible intermetallic formation, and reliability issues of SAC alloys generate several key challenges for successful adoption of Pb-free solder for next generation electronic packaging needs. Localized heating in interconnects can alleviate thermal stresses by preventing subjection of entire package to the higher reflow temperatures associated with the SAC solders. It had been demonstrated that SAC solder–FeCo magnetic nanoparticles (MNPs) composite paste can be reflowed locally with AC magnetic fields, enabling interconnect formation in area array packages while minimizing eddy current heating in the printed circuit board.Solder/magnetic nanocomposite pastes with varying MNP concentration were reflowed using AC magnetic fields. Differential scanning calorimetry results show a reduced undercooling of the composite pastes with the addition of MNPs. TEM results show that the FeCo MNPs are distributed in Sn matrix of the reflowed solder composites. Optical and SEM micrographs show a decrease in Sn dendrite regions as well as smaller and more homogeneous dispersed Ag₃Sn with the addition of MNPs. The MNPs promote Sn solidification by providing more heterogeneous nucleation sites at relatively low undercoolings. The mechanical properties were measured by nanoindentation. The modulus, hardness, and creep resistance, increase with the MNP concentration. The enhanced mechanical properties are attributed to grain boundary and dispersion strengthening.The reflow of solder composites have been modeled based on eddy current power loss in the substrate and magnetic power losses in the solder bumps. Induction reflow of pure solder bumps (<300 μm) in an area array package using 500 Oe magnetic field at 300 kHz requires excessive eddy current power loss in the substrate, resulting in extreme temperatures that lead to blistering and delamination of the substrate. Solder–MNP composites with modest MNP loading showed temperature increases sufficient to achieve solder reflow when subjected to the same AC magnetic fields. Thermomechanical behavior of a solder joint was also modeled under cyclic temperature variations. The stress and strain are highly localized at the interface between solder and substrate. Plastic work accumulated per cycle can be used for lifetime prediction.In this article we review lead-containing and lead-free solder systems, and the electronic packaging technologies pertinent to soldering process. Recent research on the effects of MNPs on localized heating, microstructure evolution, mechanical properties, and thermomechanical reliability are summarized.     

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.     

跌落碰撞下SMT焊点可靠性研究进展

针对在跌落碰撞条件下便携式电子产品中SMT连接焊点的可靠性问题,对该领域相关的理论和实验研究成果进行了综述。介绍了跌落碰撞环境下SMT焊点在跌落试验、数值模拟、寿命预测模型以及无铅焊点的可靠性研究的现状和进展,并对跌落碰撞下SMT焊点的可靠性研究进行了展望。     

Failure mechanisms of solder interconnects under current stressing in advanced electronic packages

The pursuit of greater performance in microelectronic devices has led to a shrinkage of bump size and a significant increase in electrical current. This has resulted in a high current density and accompanying Joule heating in solder interconnects, which places great challenges on the reliability of advanced electronic packages. A review of current stressing-induced failures of solder interconnects is thus timely. This review is devoted to five types of physical failure mechanisms occurring in high current density applications, which include electromigration (EM), Joule heating-induced failures, interfacial reactions, stress-related damage, and thermomigration (TM). In practice, some of these failure mechanisms are mixed together so that the real root cause cannot be easily detected and understood. Reliability designers need to be well informed to evaluate the electrical characteristics, thermal characteristics and mechanical strength for solder interconnects in advance. This review summarizes recent progress and presents a critical overview of the basis of atomic transport, diffusion kinetics, morphological evolution, and numerical simulation. Special emphasis is on the understanding of the interactions of EM with other failure mechanisms. Aside from the review of the current status of knowledge, the remaining challenges as well as future directions are also discussed.     

Life expectancies of Pb-free SAC solder interconnects in electronic hardware

The transition from lead (Pb) bearing solder to Pb-free solder has arisen in response to government restrictions on the use of lead (Pb) by the European Union. As a result, electronic manufacturers have sought a material comparable to the conventional 63Sn37Pb solder that has been traditionally used to assemble electronic hardware. Based on extensive review of various solder combination, the majority of electronic manufacturers appear to be adopting a tin–silver–copper (SAC) solder as a popular Pb-free solder replacement. Significant investments have been made by many researchers to characterize the material behavior and durability of this solder system. While the exact composition of the SAC solder is still in question, it now appears that the 96.5Sn3.0Ag0.5Cu (SAC305) solder is gaining wider acceptance as the favored Pb-free replacement, for surface mount assemblies that are going to be subjected predominantly to cyclic thermal environments. This paper presents a review of our current understanding of the life expectancy of Pb-free SAC solder interconnects for electronic hardware. To this end, the paper focuses on material characterization of SAC solder, as well as its temperature cycling and vibration fatigue reliability. From this review, SAC solder interconnects are shown to be suitable for providing adequate life expectancies for temperature cycling in electronic hardware. However, it is clear that there are differences between SAC and the conventional Sn37Pb solder, that need to be understood in order to design reliable electronic hardware.     

感应加热重熔无铅钎料凸台的界面反应及剪切性能

感应自发热重熔（ISHR）技术在电子互连的应用中具有明显的三维选择性加热和快速加热等优点．该方法能够很好地解决由于无铅钎料的应用引起的日益严重的诸多问题,如球栅阵列中各钎料球受热不均匀和芯片基板与钎料球同时受热等．为此,采用ISHR进行了无铅钎料Sn3．5Ag在Au／Ni／Cu焊盘上的重熔实验、高温老化实验以及凸台剪切实验．由实验结果可知钎料凸台可以提供足够的剪切强度．文中讨论了界面反应和金属间化合物的演化．在老化期间界面处生长了连续的Ni3Sn4金属间化合物层,同时在钎料体内部生成了分散的（Aux,Ni1-x）Sn4化合物．金属间化合物的生长速度与老化时间的平方根成正比。由此可以判断金属间化合物的生长是一种扩散控制过程．     

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.     

Wetting Behaviors and Interfacial Reaction between Sn-10Sb-5Cu High Temperature Lead-free Solder and Cu Substrate

Sn-10Sb-5Cu lead-free solder was fabricated for high temperature application in electronic package. Wetting behaviors and interfacial reaction between such a high temperature lead-free solder and Cu substrate were investigated and compared with those of 95Pb-Sn solder. The results showed that the wetting properties of Sn-10Sb-5Cu solder are superior to those of 95Pb-Sn solder in maximum wetting force, wetting time and wetting angle in the temperature range of 340-400 ℃. However, the surface of the Sn-10Sb-5...     

Development of Sn–Zn lead-free solders bearing alloying elements

Sn–Zn solder alloys have been considered as one of the more attractive lead-free solders since it can easily replace Sn–Pb eutectic alloy without increasing the soldering temperature. However, there are still some problems to be resolved, such as the argument about the poor oxidation resistance and embrittlement behavior. In order to overcome these drawbacks, and further enhance the properties of Sn–Zn lead-free solder alloys, a small amount of alloying elements (rare earths, Bi, Ag, Al, Ga, In, Cr, Cu, Sb, Ni, Ge) added into Sn–Zn alloys were selected by many researchers. For example, a small amount of Al, P, Bi, Ga can improve the high-temperature oxidation resistance of Sn–Zn solders remarkably as well as Cr. This paper summarizes the effects of alloying elements on the wettability, oxidation resistance, melting behavior, mechanical properties, creep properties, microstructures and intermetallic compounds layer of Sn–Zn lead-free solders.     

Modifying the mechanical properties of lead-free solder by adding iron and indium and using a lap joint test

Eliminating lead in electronics is an environmental consciousness that is taken prior to manufacturing. Lead-free solder has recently been developed to advance that goal. One of the most common types of lead-free solder is Sn–Ag–Cu(SAC). Adding alloying elements can modify the properties of SAC. The present study is devoted to the research and development of SAC for microelectronic packaging applications. The effects of iron and indium addition to SAC were investigated. Four different samples were fabricated by casting: Sn–3.6Ag–0.9Cu, Sn–3.6Ag–0.9Cu–0.2Fe, Sn–3.6Ag–0.9Cu–0.6Fe, and SAC-InCe. Reliability tests were done on Cu and Ni–P substrate. The shear strength of the joint was improved by decreasing the intermetallic compound (IMC) thickness; so the IMCs thickness must be controlled, because the formation of IMC leads to joint embrittlement at the interface. In conclusion, the addition of In and Fe can improve mechanical properties, such as shear strength, but the addition of In appears to be more effective for increasing the fracture toughness. The addition of Fe lowers the wetting angle and it can effectively improve the solder reliability, this improvement in shear behavior for the samples, which were reflowed on Cu substrate, is enhanced compared with the Ni substrate, but 0.6% Fe addition for the Cu substrate illustrates an decrease in fracture strain, because of abnormal growth of Cu₆Sn₅Whiskers in this case.     

Creep of sintered porous micron-silver: nanoindentation experiment and theoretical analysis

Journal of Materials Science - Nano/micron-silver paste has been applied in high-temperature packaging of chips as a potential lead-free solder. At specific sintering temperature, the particle size...     

微电子封装领域焊膏回流曲线的优化研究

对电子封装工业生产中无铅焊料的回流曲线工艺进行了研究,焊料的回流曲线影响界面处的金属间化合物的生成,通过引入加热因子开展研究,同时也进行了焊料的剪切强度实验用于评价封装的可靠性,研究结果表明:较厚的金属间化合物会降低焊球的剪切强度,而回流曲线的优化对焊料焊接的质量,可靠性有很强的实践意义.     

Influence of nanoparticle addition on the formation and growth of intermetallic compounds (IMCs) in Cu/Sn–Ag–Cu/Cu solder joint during different thermal conditions

Abstract

Nanocomposite lead-free solders are gaining prominence as replacements for conventional lead-free solders such as Sn–Ag–Cu solder in the electronic packaging industry. They are fabricated by adding nanoparticles such as metallic and ceramic particles into conventional lead-free solder. It is reported that the addition of such nanoparticles could strengthen the solder matrix, refine the intermetallic compounds (IMCs) formed and suppress the growth of IMCs when the joint is subjected to different thermal conditions such as thermal aging and thermal cycling. In this paper, we first review the fundamental studies on the formation and growth of IMCs in lead-free solder joints. Subsequently, we discuss the effect of the addition of nanoparticles on IMC formation and their growth under several thermal conditions. Finally, an outlook on the future growth of research in the fabrication of nanocomposite solder is provided.     

Formation and behavior of Kirkendall voids within intermetallic layers of solder joints

The sub-micron void called “Kirkendall void” has been widely observed within intermetallic compound (IMC) layers in solder joints of semiconductor package interconnections that include both the first level interconnection for a silicon die to a substrate and the second level interconnection for the substrate to a PCB board. Based on many researches on Kirkendall void through a variety of variables, it has been demonstrated as a critical reliability risk within various binary and ternary IMC layers of solder joints in electronic packaging industry. Even, it is more crucial for fine pitch and high complexity in chip-scale electronic packaging. Hence, it is necessarily demanding to review the dependency and influence of critical variables for Kirkendall void formation and behavior in the basis of solid and solid–liquid state interdiffusion process, time and temperature-dependent kinetic process, and morphology and microstructure change of IMCs. Specifically, we reviewed the initial formation, growth and behaviors of Kirkendall void in: (1) short and long-term interfacial reaction by aging in different time and temperatures (2) multiple reflows with different peak temperature (3) annealing after reflow and (4) electromigration, within IMCs of solder joints. Probably, this study may serve as conceptually helpful references to the overall understanding of formation, growth and behavior of Kirkendall void in interfacial reaction of solder joints.     

Sn-Cu系无铅钎料微合金化研究进展

电子产品绿色化的需求促进了电子组装中钎料合金的无铅化发展。目前,Sn-Cu系钎料以优良的综合性能和较低的成本成为最具使用前景的无铅钎料之一。但是Sn-Cu系钎料的熔点较高,在Cu基上的铺展性和钎焊性也较Sn-Pb钎料差,这在很大程度上限制了其应用。通过添加多种合金元素可改善Sn-Cu合金的微观组织和焊接性能。本文首先系统地综述了合金元素对Sn-Cu系无铅钎料微观组织、润湿性、力学性能等的影响,然后指出Sn-Cu系无铅钎料存在的问题。最后,对Sn-Cu系无铅钎料的发展方向和前景进行了展望。