The Effect of Micro-Alloying of Sn Plating on Mitigation of Sn Whisker Growth

Tin (Sn) is a key industrial material in coatings on various components in the electronics industry. However, Sn is prone to the development of filament-like whiskers, which is the leading cause of many types of damage to electronics reported in the last several decades. Due to its properties, a tin-lead (Sn-Pb) alloy coating can mitigate Sn whisker growth. However, the demand for Pb-free surface finishes has rekindled interest in the Sn whisker phenomenon. In order to achieve properties similar to those naturally developed in a Sn-Pb alloy coating, we carried out a study on deposited films with other Sn alloys, such as tin-bismuth (Sn-Bi), tin-zinc (Sn-Zn), and tin-copper (Sn-Cu), electrodeposited onto a brass substrate by utilizing a pulse plating technique. The results indicated that the Sn alloy films modified the columnar grain structure of pure Sn into an equiaxed grain structure and increased the incubation period of Sn whisker growth. The primary conclusions were based on analysis of the topography and microstructural characteristics in each case, as well as the stress distribution in the plated films computed by x-ray diffraction, and the?amount of Sn whisker growth in each case, over 6 months under various environmental influences.     

Mitigation of Sn Whisker Growth by Composite Ni/Sn Plating

Tin (Sn) is a key industrial material in coatings on various components in the electronics industry. However, Sn is prone to the development of filament-like whiskers, which is the leading cause of many types of damage to electronics reported in the last several decades. Due to its properties, a tin-lead (Sn-Pb) alloy coating can mitigate Sn whisker growth. However, the demand for Pb-free surface finishes has rekindled interest in the Sn whisker phenomenon. In order to achieve properties similar to those naturally developed in a Sn-Pb alloy coating, we carried out a study on deposited films with other Sn alloys, such as tin-bismuth (Sn-Bi), tin-zinc (Sn-Zn), and tin-copper (Sn-Cu), electrodeposited onto a brass substrate by utilizing a pulse plating technique. The results indicated that the Sn alloy films modified the columnar grain structure of pure Sn into an equiaxed grain structure and increased the incubation period of Sn whisker growth. The primary conclusions were based on analysis of the topography and microstructural characteristics in each case, as well as the stress distribution in the plated films computed by x-ray diffraction, and the␣amount of Sn whisker growth in each case, over 6 months under various environmental influences.     

Whisker Growth Behavior of Sn and Sn Alloy Lead-Free Finishes

Sn whisker growth behavior, over periods of time up to 10,080 h at room temperature, was examined for Sn and Sn-Cu, Sn-Ag, Sn-Bi, and Sn-Pb coatings electroplated on copper in 2 μm and 5 μm thicknesses to understand the effects of the alloying elements on whisker formation. Sn-Ag and Sn-Bi coatings were found to significantly suppress Sn whisker formation compared with the pure Sn coatings, whereas whisker growth was enhanced by Sn-Cu coatings. In addition, annealed Sn and Sn-Pb coatings were found to suppress Sn whisker formation, as is well known. Compared with the 2-μm-thick coatings, the 5-μm-thick coatings had high whisker resistance, except for the Sn-Cu coating. Whisker growth was correlated with coating crystal texture and its stability during storage, crystal grain microstructure, and the formation of intermetallic compounds at Sn grain boundaries and substrate–coating interfaces.     

Kinetics of interface reaction in 40Sn-Bi/Cu and 40Sn-Bi-2Ag/Cu systems during aging in solid state

The characteristics of thickening kinetics of the intermetallic compounds (IMC) (Cu/sub 6/Sn/sub 5/+Cu/sub 3/Sn) during aging at 120/spl deg/C and 90/spl deg/C in solid state were studied by quantitative metallographic analysis for three solder alloy systems: 40Sn-Bi/Cu, 40Sn-Bi-2Ag/Cu and Sn-37Pb/Cu. The diffusion couples were prepared by hot immersion in the molten solder bath. The results showed that the rate of the IMC thickening increases in the order Sn-37Pb/Cu<40Sn-Bi/2-Ag/Cu<40Sn-Bi/Cu. The IMC thickening rate in the Sn-Bi eutectic system is one order of magnitude faster than that in Sn-Pb eutectic system. The time exponents are different from each other. The relationship between the increase of IMC and aging time follows a parabolic function in the Sn-Pb system, and a linear relation in the Sn-Bi system. Addition of Ag in Sn-Bi inhibits the IMC thickening process.     

Residual shear strength of Sn-Ag and Sn-Bi lead-free SMT joints after thermal shock

In this study, two lead-free solder alloys, namely 50 tin-50 bismuth (Sn-Bi) and 96.5 tin-3.5 silver (Sn-Ag), were studied for their use in surface mount solder joints. They have been considered as potential replacements for 63 tin-37 lead (Sn-Pb) solder. All joints were subjected to various cycles of thermal shock with temperature ranging from -25 to 125/spl deg/C. Shear tests were conducted on joints with and without thermal shock treatment. Another thermal shock cycle (-25 to 85/spl deg/C) was carried out on Sn-Bi solder joints for comparison. Their performance against thermal shock was compared with eutectic Sn-Pb solder by evaluating their residual shear strength and studying their microstructural change. For the Sn-Ag solder, a fine rod-like Ag/sub 3/Sn intermetallic was formed in the solder matrix after the thermal shock. On the other hand, Bi-rich and Sn-rich phases appeared in the Sn-Bi solder after the -25 to 125/spl deg/C thermal shock. Moreover, fine cracks were observed along the Bi-rich grain-like phase boundary. These were not observed in the Sn-Bi solder with the -25 to 85/spl deg/C thermal shock treatment. Voids and cracks were also observed in the joint of Sn-Bi solder alloy after 1000 thermal shock cycles. In addition, the thickness of intermetallic compound (IMC) of three solder alloys gradually grew with the number of thermal shock cycles. These defects reduced the strength of solder joint and led to thermal fatigue failure. In general, the shear strength is found to decrease with increasing number of thermal shock cycles. The Sn-Ag solder was better than the Sn-Bi solder in terms of residual thermal shock shear strength. Sn-Bi solder showed good properties when it was treated with the -25 to 85/spl deg/C thermal shock. It has a strong potential to replace Sn-Pb solder in low temperature applications such as consumer electronics. The Sn-Ag solder is suitable for high temperature applications.     

Stress Relaxation in Sn-Based Films: Effects of Pb Alloying, Grain Size, and Microstructure

Stress is believed to provide the driving force for growth of Sn whiskers, so stress relaxation in the Sn layer plays a key role in their formation. To understand and enhance stress relaxation in Sn-based films, the effects of Pb alloying and microstructure on their mechanical properties have been studied by observing the relaxation of thermal expansion-induced strain. The relaxation rate is found to increase with film thickness and grain size in pure Sn films, and it depends on the microstructure in Pb-alloyed Sn films. Measurements of multilayered structures (Sn on Pb-Sn and Pb-Sn on Sn) show that changing the surface layer alone is not sufficient to enhance the relaxation, indicating that the Pb enhances relaxation in the bulk of the film and not by surface modification. Implications of our results for whisker mitigation strategies are discussed.     

The Role of Silver in Mitigation of Whisker Formation on Thin Tin Films

The mitigating effect of alloying Sn thin films with Ag on the formation of Sn whiskers was investigated by time-resolved investigations employing x-ray diffraction for phase and stress analyses and focused ion beam microscopy for morphological characterization of the surfaces and cross-sections of the specimens. The investigated Sn-6 wt.%Ag thin films were prepared by galvanic co-deposition. The results are compared with those obtained from investigation of pure Sn films and discussed with regard to current whisker-growth models. The simultaneous deposition of Sn and Ag leads to a fine-grained microstructure consisting of columnar and equiaxed grains, i.e. an imperfect columnar Sn film microstructure. Isolated Ag3Sn grains are present at the Sn grain boundaries in the as-deposited films. Pronounced grain growth was observed during aging at room temperature, which provides a global stress relaxation mechanism that prevents Sn whisker growth.     

Resonant vibration behavior of lead-free solders

This study investigated the resonant vibration-fatigue characteristics of some potential lead-free solders, including Sn-Zn, Sn-Ag Sn-Cu, and Sn-Bi alloys. Results show that, under a fixed vibration force, the damping capacity and vibration-fracture resistance of Sn-Cu and Sn-Ag eutectic alloys with an off-eutectic structure are higher than Sn-Pb and are also higher than Sn-Bi and Sn-Zn. This is closely related to the vibration-deformed structure and crack-propagation morphology associated with the microstructural features of the materials. Also, the striated deformation in the Sn-rich phase can be regarded as an effective mechanism in absorbing vibration energy. Moreover, microstructural modification of the Sn-Zn eutectic alloy can be achieved through Ag addition, and thus, the damping capacity and vibration-fracture resistance can be significantly improved.     

Stress Relaxation Mechanisms of Sn and SnPb Coatings Electrodeposited on Cu: Avoidance of Whiskering

The interrelations of microstructural evolution, phase formation, residual stress development, and whiskering behavior were investigated for the systems of Sn coating on Cu and SnPb coating on Cu during aging at room temperature. It was shown that the whisker-preventing effect of Pb addition to pure Sn can be attributed to a Pb-induced change of the stress relaxation mechanism in the coating: Pure Sn coatings, with a columnar grain morphology, relax mechanical stress via localized, unidirectional grain growth from the surface of the coating (i.e., whisker formation occurs), whereas SnPb coatings, with an equiaxed grain morphology, relax mechanical stress via uniform grain coarsening without whisker formation. It can thus be suggested that tuning of the Sn grain morphology (i.e., establishing an equiaxed grain morphology) is a straightforward method of microstructural control to suppress whisker formation at room temperature. Experimental results obtained in this project validate this conclusion.     

Effects of strain rates and biaxial stress conditions on plastic yielding and flow stress of solder alloys

The rate-dependent mechanical properties of Sn3.8Ag0.7Cu (SAC387) Pb-free alloy and Sn-Pb eutectic alloy were investigated in this study under pure shearing and biaxial stress conditions with thin-walled specimens using a servo-controlled tension-torsion material testing system. The pure shearing tests were conducted at strain rates between 6.7 × 10⁻⁷ and 1.3 × 10⁻¹/sec. In addition, axial tensile stresses were superimposed onto the shearing samples to examine the effects of biaxial stress conditions on the yielding and on post-yielding plastic flow of the solder alloys. Strain hardening is observed for the Pb-free alloy at all the tested strain rates, while strain softening happens with the Sn-Pb eutectic solder at low strain rates. Special tests were also conducted for sudden strain-rates changes and stress relaxation for the purpose to develop a viscoplastic model to simulate time-dependent multiaxial deformation and to assess damage and fatigue life of general solder interconnections.     

Diffusivities and Atomic Mobilities of Sn-Bi and Sn-Pb Melts

The recently developed Arrhenius formula of the modified Sutherland equation was first employed to calculate the self- and impurity diffusivities in liquid Sn, Bi, and Pb. The reliability of the calculated results was validated in comparison with the critically reviewed literature data. Based on the reliable tracer and chemical diffusivities available in the literature, the atomic mobility parameters in both Sn-Bi and Sn-Pb melts were then evaluated by the DICTRA (DIffusion-Controlled TRAnsformations) software package with the aid of thermodynamic parameters. Comprehensive comparisons show that most of the measured and theoretical diffusivities in Sn-Bi and Sn-Pb melts can be reasonably reproduced by the currently obtained atomic mobilities. Moreover, the atomic mobilities were further verified by comparing the model-predicted concentration profiles and the measured ones in various liquid Sn-Bi and Sn-Pb diffusion couples.     

Equi-Axed Grain Formation in Electrodeposited Sn-Bi

Sn is widely used as a coating in the electronics industry because it provides excellent solderability, ductility, electrical conductivity, and corrosion resistance. However, Sn whiskers have been observed to grow spontaneously from Sn electrodeposits and are known to cause short circuits in fine-pitched pre-tinned electrical components. We report here a deposition strategy that produces an equi-axed and size-tunable grain structure in Sn-Bi alloys electrodeposited from a commercial bright Sn electrolyte. An equi-axed grain structure should allow a more uniform creep to relieve compressive stress with no localized surface disturbance. The standard potential for Bi is about 0.45 V more positive than Sn. Pulsed deposition can selectively turn on and off the Sn deposition reaction. During the off cycle, a displacement reaction between metallic Sn on the electrode surface and Bi³⁺ in solution selectively dissolves Sn and deposits Bi, effectively terminating the growth from the previous cycle and forcing the Sn to nucleate a new grain on the Bi-enriched surface. The grain size is tunable by varying the pulsing conditions, and an equi-axed structure can be obtained with as little as 3 at.% Bi. This surface enrichment of Bi by potential modulation is similar to that which occurs naturally in Sn-Pb, and provides an avenue for breaking up the columnar grain structure inherent to pure Sn, thus providing an additional diffusion path for Sn that may prevent whisker growth.     

微量元素添加对多元Sn-Bi系焊料合金组织与性能的影响

传统焊料合金由于熔点温度高,不能满足部分有机基板、温度敏感器件以及3D封装等多层封装形式的低温封装要求。以Sn-Bi合金为基体,通过添加微量Ag、Cu、Co和Ni元素形成新型多元合金,对多元合金的熔化性能、润湿性能、微观组织和力学性能进行研究。结果表明:微量元素的添加(质量分数0~1%)对多元Sn-Bi系合金的固相线温度影响很小,降低了SnBi57AgCuCo合金的液相线温度和熔程;添加微量元素降低了合金的表面能,提高了润湿性;多元Sn-Bi系合金的微观组织由SnBi共晶组织、β-Sn相和块状富Bi相组成,微量元素的添加细化了β-Sn相中的Bi相颗粒。组织中的金属间化合物颗粒提高了多元Sn-Bi系合金抗拉强度和延伸率;断口形貌表明合金主要沿Bi相晶界断裂,Bi相的细化可改善焊料合金的力学性能。     

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.     

Characterization of ternary Cu-Sn-Au bulk alloys and thin films

The variation of the microstructure and corresponding properties of several Cu-based Au alloys with differing Sn contents was investigated. Both bulk alloys and thin films of the same composition were studied to determine whether characterization of the bulk alloys could be used to predict the optimal thin film composition. For bulk alloys with less than 10% Sn content, the electrical resistivity and hardness were found to be only slightly increased over those of a Cu-Au alloy with no added Sn ; however they increase rapidly with further increases in Sn content. A microstructural study indicated that these changes in properties are primarily attributable to the formation of a hard ε-Cu₃Sn phase. The characteristic needle shape of these preciptated particles, as well as their volume fraction, seems to contribute to the embrittlement of the alloy. The orientation relationship between α-Cu and the ε-Cu₃Sn precipitate was found to be {fx191-1} and {fx191-2} The relative variation in resistivity as a function of Sn concentration was the same for the thin film and the bulk alloy, although their absolute values were different because of surface oxidation and small grain size in thin films. Given the observed microstructural similarity, the similar composition depen dence leads to the conclusion that the properties of bulk alloys may be used to make reasonable predictions about the behavior of thin films in systems in which the thin film phase composition is the equilibrium one by heat treatment.     

Trends and issues in Pb-free soldering for electronic packaging

A variety of Pb-free solder alloys have been proposed for use as interconnects for electronic packaging including Sn?Ag, Sn?Cu, Sn?Ag?Cu, Sn?Ag?Bi, and Sn?Sb, among others. This paper presents a review of the behavior of promising Pb-free solder alloys as related to their microstructure. Recommendations of optimal alloy composition as a function of performance requirements are given. For surface mount applications, eutectic Sn?Ag?Cu is recommended as the optimal alloy. For flip chip interconnects, the eutectic Sn-Cu alloy has the best performance. The materials and process trends of Pb-free packaging are summarized with optimal conditions identified.     

锡铅稀土钎料合金高温蠕变性能试验研究

文中对比分析了Ｓｎ６０Ｐｂ４０和Ｓｎ－Ｐｂ－０．０５Ｒｅ两种钎料合金在多轴应力状态下的蠕变断裂规律,初步探讨了主控两种钎料复合金为断裂的力学因素,并从金属学角度初步探讨了两种钎料合金的蠕变断裂方式,着重分析了混合稀土对锡铅秆料合金蠕变性能的影响。     

激光熔覆Al2CrFeCoCuNixTi高熵合金涂层的组织及耐蚀性能

采用激光熔覆工艺在Q235钢表面制备了Al2CrFeCoCuNixTi高熵合金涂层,分析了Al2CrFeCoCuNixTi高熵合金涂层的组织结构,测试了Al2CrFeCoCuNixTi高熵合金涂层在0.5 mol/L HNO3溶液及0.5 mol/L HCl溶液中的耐蚀性能。结果表明:Al2CrFeCoCuNixTi高熵合金涂层主要分为熔覆区、结合区、热影响区,熔覆区组织主要由等轴晶组成,等轴晶上分布有微米尺度的粒子;合金相结构简单,由体心立方（BCC）及面心立方（FCC）结构组成;Cr元素和Ni元素的钝化作用及由Al元素形成Al2O3或Al2O3H2O膜使得Al2CrFeCoCuNixTi高熵合金涂层在0.5 mol/L HNO3溶液及0.5 mol/L HCl溶液中具有较好的耐蚀性能,自腐蚀电流密度与基体Q235钢相比降低一两个数量级;0.5 mol/L HCl溶液中的Cl-会穿透Ni0.5高熵合金涂层表面形成的钝化膜,出现轻微小孔腐蚀。     

Interfacial Reactions in the Sn-Bi/Te Couples

Sn-Bi alloys are promising Pb-free solders and Te is the primary constituent element of the bismuth telluride thermoelectric materials. Interfacial reactions in the Sn-Bi/Te couples are examined herein. Very unusual cruciform patterns have been observed in the Sn-Bi/Te couples reacted at 250°C. The reaction product is the SnTe phase. The reaction layer thickness is very uniform along the Te substrate edges, and there are no reaction products at the substrate corners. The cruciform patterns of the reaction layers gradually disappear in the Sn-Bi/Te reaction couples with higher Bi contents, and the critical composition is about Sn-36.25at.%Bi (Sn-50wt.%Bi). The reaction product is also the SnTe phase in the Sn/Te couple reacted at 210°C but without the cruciform pattern. At 210°C, the SnTe phase layer in the Sn/Te couple has a duplex structure, and its growth rate follows a parabolic law.     

Microstructure and elevated-temperature shear strength of Zn–4Al–3Mg–xSn high-temperature lead-free solders

The microstructure and shear strength of the high-temperature Zn–4Al–3Mg, Zn–4Al–3Mg–7Sn, and Zn–4Al–3Mg–13Sn solder alloys were investigated in the temperature range of 25–200 °C. The results revealed that the shear yield stress (SYS) and ultimate shear strength (USS) of all three alloys decrease with increasing test temperature. The ternary base alloy showed higher strength levels up to 145 °C, above which all alloys behave similarly. The superiority of the ternary alloy is ascribed to the higher volume fraction of the fine α−η eutectic and eutectoid structures and the hard MgZn₂ and Mg₂Zn₁₁ particles. Introduction of Sn into the base alloy, however, resulted in substantial decrease in the strength, due to the presence of the soft Sn that reduces the volume fraction of the eutectic structure and the hard second phase particles. Despite the weakening effects of Sn, the strength of quaternary alloys is still higher than those of the Zn–Sn and Pb–Sn high-temperature solders.