Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

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.     

Developing a lead-free solder alloy Sn-Bi-Ag-Cu by mechanical alloying

A new lead free alloy, Sn-6Bi-2Ag-0.5Cu, has been developed by mechanical alloying and has great potential as a lead-free solder system. Initial trials on the manufacture of solder joints with this alloy revealed that a high quality bond with copper could be formed. Its melting range of 193.87°C to 209.88°C is slightly higher than that of eutectic tin-lead solder. Examination of the microstructure of the as-soldered joints revealed that it mainly consists of small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ particles. The Cu-Sn intermetallic compound (IMC) layer formed at solder-copper interface is the η-Cu₆Sn₅ phase with grain size of 2 μm. The shear strength of the solder joint is higher than that of Sn-37Pb or Sn-3.5Ag. Under shear loading, fracture occurred at IMC layer-solder interface as well as in the bulk of solder.     

Role of shape-memory alloy reinforcements on strain evolution in lead-free solder joints

Microelectronic solder joints are exposed to aggressive thermomechanical cycling (TMC) during service, resulting in strain localization near solder/bond-pad interfaces, which eventually leads to low-cycle fatigue (LCF) failure of the joint. In order to mitigate these strain concentrations, a “smart solder” reinforced with a martensitic NiTi-based shape-memory alloy (SMA) has been proposed before. In the present work, the role of NiTi particles on strain evolution in composite solders was studied using a combination of experimental and numerical means. Finite element modeling showed that NiTi pariculate reinforcements can reduce inelastic strain levels in the solder via shape recovery associated with the B19′ → B2 transformation. In situ TMC studies in the scanning electron microscope (SEM), in conjunction with strain analysis via digital image correlation (DIC), showed evidence of reverse deformation in the solder commensurate with the NiTi phase transformation, demonstrating the conceptual viability of the smart solder approach. The SEM-DIC experiments also suggested that the presence of particulates mitigates shear localization, which is commonly observed in monolithic solder joints close to joint/bond-pad interfaces. Finally, TMC experiments on monolithic solder and NiTi/solder single-fiber composite joints highlighted the beneficial effect of shape-memory transformation in reducing inelastic strain range of solders.     

SMT无铅电子焊膏活性剂的研究

选择腐蚀性较低的有机酸作为制备无铅电子焊膏活性剂的原料。对五种有机酸进行筛选和复合选取,并进行回流焊接模拟。按国家标准(GB/T9491—2002)对其铺展率、腐蚀性进行了测试。结果表明,选用w(己二酸)为60%和w(丁二酸)为40%的混合酸作为活性剂,具有良好的润湿性和助焊活性,铺展率在87%左右。焊后残留少,腐蚀性弱,焊点饱满光亮,焊层薄而明晰。     

Reliability testing of WLCSP lead-free solder joints

The interfacial reactions of solder joints between Sn-4Ag-0.5Cu solder ball and a couple of presoldered pastes (Sn-7Zn-Al(30ppm) and Sn-3Ag-0.5Cu) were investigated in wafer-level chip-scale package (WLCSP). After appropriate surface mount technology reflow processes on printed circuit boards with a Cu/OSP (organic solderability preservative) surface finish, samples were subjected to 150°C high-temperature storage (HTS) for 1,000 h of aging or 1,000 cycles of a thermal cycling test (TCT). Sequentially, cross-section analysis is scrutinized by scanning electron microscopy/energy dispersive spectrometry and energy probe microanalysis to observe metallurgical evolution in the interface and solder buck itself. It was found that the degradation of the joint shear strength after TCT is more pronounced than that of the shear strength after HTS. Fracture surface analyses of the shear tests show that the degradation of the joint strength for HTS is solely due to the influence of the interfacial IMC grain growth, while the shear strength degradation for TCT is mainly due to the coefficient thermal expansion mismatch from the thermal cycling at the chip-solder interface and can lead to the occurrence of the crack.     

无铅焊点在热疲劳和电迁移作用下的组织演变

研究了Cu/Sn-58Bi/Cu焊点接头在室温和55℃下通电过程中阴极和阳极界面处微观组织的演变,电流密度均采用104 A/cm2.结果表明,室温条件下通电达到25 d,Bi原子由阴极向阳极发生了扩散迁移,在阳极界面处形成了厚度约22.4μm的均匀Bi层,而阴极出现了Sn的聚集,加载55℃通电2d后,焊点发生了熔融,阴极界面处形成了厚度为34.3 μm的扇贝状IMC,而阳极界面IMC的厚度仅为9.7 μm.在IMC层和钎料基体之间形成了厚度约7.5μm的Bi层,它的形成阻碍了Sn原子向阳极界面的扩散迁移,进而阻碍了阳极界面IMC的生长,导致了异常极化效应的出现.     

Microstructural development of Sn-Ag-Cu solder joints

The effects of Cu as pad material and of the metallization of pad (with Sn) and component (with Ni) on the evolving microstructure of lead-free solder joints were studied. A solder paste with composition 95.5wt.%Sn-4.0wt.%Ag-0.5wt.%Cu was used. Partial dissolution of the Cu substrate led to a change in the overall composition of the solder, which caused a precipitation morphology different from the one expected regarding the initial composition. Kinetics of growth of the Cu₆Sn₅ phase, as particles in the bulk of the solder and as a reaction layer adjacent to the Cu pad, was studied in the temperature range 125–175°C.     

Intermetallic growth studies on Sn-Ag-Cu lead-free solder joints

Solid-state intermetallic compound (IMC) growth behavior plays an important role in solder joint reliability of electronic packaging assemblies. The morphology and growth of interfacial IMC compounds between 95.5Sn-3.8Ag-0.7Cu Pb-free solders and nickel/gold (Ni/Au) surface finish on BGA solder joint specimen is reported. Digital imaging techniques were employed in the measurement of the average IMC growth thickness. The IMC growth behavior subjected to isothermal aging exposure at 125°C, thermal cycling (TC), and thermal shock (TS) with upper soak temperatures of 125°C are compared. An equivalent isothermal aging time is proposed for comparison of IMC layer growth data. It was noted that IMC layer growth under thermal cycling and thermal shock aging gives an acceleration factor of 1.4 and 2.3 based on the equivalent isothermal aging time.     

Nanoindentation on SnAgCu lead-free solder joints and analysis

The lead-free SnAgCu (SAC) solder joint on copper pad with organic solderability preservative (Cu-OSP) and electroless nickel and immersion gold (ENIG) subjected to thermal testing leads to intermetallic growth. It causes corresponding reliability concerns at the interface. Nanoindentation characterization on SnAgCu solder alloy, intermetallic compounds (IMCs), and the substrates subjected to thermal aging is reported. The modulus and hardness of thin IMC layers were measured by nanoindentation continuous stiffness measurement (CSM) from planar IMC surface. When SAC/Ni(Au) solder joints were subject to thermal aging, the Young’s modulus of the NiCuSn IMC at the SAC/ENIG specimen changed from 207 GPa to 146 GPa with different aging times up to 500 h. The hardness decreased from 10.0 GPa to 7.3 GPa. For the SAC/Cu-OSP reaction couple, the Young’s modulus of Cu₆Sn₅ stayed constant at 97.0 GPa and hardness about 5.7 GPa. Electron-probe microanalysis (EPMA) was used to thermal aging. The creep effect on the measured result was analyzed when measuring SnAgCu solder; it was found that the indentation penetration, and thus the hardness, is loading rate dependent. With the proposed constant P/P experiment, a constant indentation strain rate h/h and hardness could be achieved. The log-log plot of indentation strain rate versus hardness for the data from the constant P/P experiments yields a slope of 7.52. With the optimized test method and CSM Technique, the Modulus of SAC387 solder alloy and all the layers in a solder joint were investigated.     

低银无铅微互连焊点的振动疲劳行为研究

通过采用一系列与集成电路BGA(球栅阵列)、Flip Chip(倒装焊芯片)真实焊点体积接近的不同尺寸的典型“三明治”结构Sn0.3Ag0.7Cu低银无铅微互连焊点,基于动态力学分析的精密振动疲劳试验与微焊点疲劳断口形貌观察相结合的方法,研究了微焊点振动疲劳变形曲线的形成机制、裂纹萌生扩展与断裂机理、温度对振动疲劳行为的影响及微焊点振动疲劳行为的尺寸效应问题。结果表明,保持焊点直径恒定,随着焊点高度的减小,焊点的疲劳寿命增加,而疲劳断裂应变降低,同时焊点的疲劳断裂模式由韧性断裂转变为脆性断裂。     

Constitutive relations on creep for SnAgCuRE lead-free solder joints

Taking the most promising substitute of the Sn-3.8Ag-0.7Cu solder as the research base, investigations were made to explore the effect of rare earths (REs) on the creep performance of the Sn-3.8Ag-0.7Cu solder joints. The SnAgCu-0.1RE solder with the longest creep-rupture life was selected for subsequent research. Creep strain tests were conducted on Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints in the intermediate temperature range from 298 K to 398 K, corresponding to the homologous temperatures η=0.606, 0.687, 0.748, and 0.809 and η = 0.602, 0.683, 0.743, and 0.804, respectively, to acquire the relevant creep parameters, such as stress exponent and activation energy, which characterize the creep mechanisms. The final creep constitutive equations for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints were established, demonstrating the dependence of steady-state creep rate on stress and temperature. By correcting the apparent creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints from the experiments, the true creep-activation energy is obtained. Results indicated that at low stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the lattice self-diffusion activation energy, so the steady-state creep rates of these two solder joints are both dominated by the rate of lattice self-diffusion. While at high stress, the true creep-activation energy of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints is close to the dislocation-pipe diffusion activation energy, so the steady-state creep rates are dominated by the rate of dislocation-pipe diffusion. At low stress, the best-fit stress exponents n of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are 6.9 and 8.2, respectively, and the true creep-activation energy of them both is close to that of lattice self-diffusion. At high stress, it equals 11.6 and 14.6 for Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints, respectively, and the true creep-activation energy for both is close to that of the dislocation-pipe diffusion. Thus, under the condition of the experimental temperatures and stresses, the dislocation climbing mechanism serves as the controlling mechanism for creep deformation of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints. The creep values of Sn-3.8Ag-0.7Cu and SnAgCu-0.1RE solder joints are both controlled by dislocation climbing. Dislocation glide and climb both contribute to creep deformation, but the controlling mechanism is dislocation climb. At low stress, dislocation climbing is dominated by the lattice self-diffusion process in the Sn matrix and dominated by the dislocation-pipe diffusion process at high stress.     

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag₃Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag₃Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.     

Rate-dependent properties of Sn-Ag-Cu based lead-free solder joints for WLCSP

The increasing demand for portable electronics has led to the shrinking in size of electronic components and solder joint dimensions. The industry also made a transition towards the adoption of lead-free solder alloys, commonly based around the Sn-Ag-Cu alloys. As knowledge of the processes and operational reliability of these lead-free solder joints (used especially in advanced packages) is limited, it has become a major concern to characterise the mechanical performance of these interconnects amid the greater push for greener electronics by the European Union.In this study, bulk solder tensile tests were performed to characterise the mechanical properties of SAC 105 (Sn-1%wt Ag-0.5%wt Cu) and SAC 405 (Sn-4%wt Ag-0.5%wt Cu) at strain rates ranging from 0.0088 s⁻¹ to 57.0 s⁻¹. Solder joint array shear and tensile tests were also conducted on wafer-level chip scale package (WLCSP) specimens of different solder alloy materials under two test rates of 0.5 mm/s (2.27 s⁻¹) and 5 mm/s (22.73 s⁻¹). These WLCSP packages have an array of 12 × 12 solder bumps (300 μm in diameter); and double redistribution layers with a Ti/Cu/Ni/Au under-bump metallurgy (UBM) as their silicon-based interface structure.The bulk solder tensile tests show that Sn-Ag-Cu alloys exhibit higher mechanical strength (yield stress and ultimate tensile strength) with increasing strain rate. A rate-dependent model of yield stress and ultimate tensile strength (UTS) was developed based on the test results. Good mechanical performance of package pull-tests at high strain rates is often correlated to a higher percentage of bulk solder failures than interface failures in solder joints. The solder joint array tests show that for higher test rates and Ag content, there are less bulk solder failures and more interface failures. Correspondingly, the average solder joint strength, peak load and ductility also decrease under higher test rate and Ag content. The solder joint results relate closely to the higher rate sensitivity of SAC 405 in gaining material strength which might prove detrimental to solder joint interfaces that are less rate sensitive. In addition, specimens under shear yielded more bulk solder failures, higher average solder joint strength and ductility than specimens under tension.     

无铅微焊点的热效应仿真及可靠性分析

采用Anand模型描述无铅焊点(SAC305)的力学性能,运用有限元法模拟球栅阵列封装在温度循环载荷下的应力应变响应并对其进行分析,着重对关键焊点的应变能进行了讨论。结果表明,关键焊点的关键区域出现在焊点的上表面边缘处,为最容易出现损坏的部位,并得到了实验的验证;在温度循环的过程中,升温阶段塑性应变产生速率远高于高温驻留阶段的塑性应变产生速率,极大地影响着焊点使用寿命。     

剪切蠕变下无铅焊点厚度的尺寸效应

利用自制的电子测试系统,测量分析了试样焊点厚度(0.05～0.50mm)对电阻应变的影响。结果表明:在剪切蠕变条件下,焊点厚度为0.25mm时,电阻应变最小,蠕变寿命最长。利用有限元软件ANSYS对焊点的蠕变应变进行仿真分析。结果显示:随着焊点厚度变化,焊点蠕变应变的变化趋势与实验结果一致。将相同厚度下的电阻应变与蠕变应变进行拟合,得到了电阻应变与蠕变应变之间的定量关系式。     

等温时效对SnSb4.5CuNi/Cu焊接接头力学性能的影响

SnSb4.5CuNi/Cu焊点在175℃进行等温时效,分析了不同时效时间的SnSb4.5CuNi/Cu焊点中金属间化合物(IMC)组织形貌演变,通过纳米压痕法测量SnSb4.5CuNi/Cu焊点界面IMC的硬度和弹性模量,对焊接接头进行拉伸强度和低周疲劳测试。结果表明,时效48 h的焊缝中Cu6Sn5呈曲率半径均匀的半圆扇贝状特征,IMC的弹性模量与铜基板很接近,在恒幅对称应变条件下焊点的抗低周疲劳的性能最佳,焊点的抗拉强度高;当时效时间大于48 h,焊接接口的抗疲劳性能和抗拉伸强度逐渐变差。     

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.     

Microstructural evolution in the Sn-Cu-Ni and Pb-Sn solder joints with Cu and Pt-Ag metallized Al2O3 substrate

The growth mechanism of intermetallics between solders and metallized substrates, after thermal aging, are investigated. The solders used in this study are unleaded Sn-Cu-Ni solder and eutectic Pb-Sn solder. The Pt-Ag/Al₂O₃, Cu block and the electroless Cu/Pt-Ag/Al₂O₃ are employed as the metallized substrates. Microstructure evolution of the interfacial morphology, elemental, and phase distribution are probed with the aid of electron-probe microanalyzer (EPMA) and x-ray diffractometry. Two kinds of intermetallics, Cu₃Sn and Cu₆Sn₅, are formed at the solder/Cu interface. However, for the solder/Pt-Ag system, only Ag₃Sn is observed at the interface. The thickness of Cu₃Sn, Cu₆Sn₅, and Ag₃Sn compound layers for all solder/metallized substrate systems shows at t^0.5 dependence at 100, 125, 150 and 170 C. According to the calculated activation energy and diffusion constant, the growth rate of Cu₃Sn and Cu₆Sn₅ intermetallics in the electroless Cumetallized substrate is relatively higher than that for Cu block one at the range of 100 C to 170 C. However, the growth rate of Cu₆Sn₅ and Ag₃Sn is reduced in the Sn-Cu-Ni solder with respect to the eutectic Pb-Sn solder. On the other hand, the Sn-Cu-Ni solder system exhibits a thicker Cu₃Sn intermetallic layer than the eutectic Pb-Sn solder after various aging times at 100 C. The thickness of Cu3Sn in the eutectic Pb-Sn solder is, however, thicker than that for Sn-Cu-Ni solder at 170 C.     

Microstructural evolution of ultrasonically bonded high purity Al wire during extended range thermal cycling

This paper concerns the reliability of ultrasonically bonded high purity thick aluminium wires at elevated temperature. To date, the evolution of the microstructure of wire bonds during thermomechanical exposure and its influence on reliability have not been fully characterised and understood, particularly as they pertain to thermal cycling regimes which exceed 125 °C. Shear testing, indentation hardness and fine-scale microstructural data are reported here which show that the rate of wear-out can be influenced not only by the thermal cycling range (ΔT), but more importantly by the maximum temperature and duration to which bonds are exposed. There is evidence that significant annealing occurs during thermal cycling regimes with high T_max values, which results in the removal of some of the damage accumulated and a reduction in the rate of crack propagation. The rate of bond degradation is also found to be faster for 99.99% (4 N) than 99.999% (5 N) pure Al wires. Analysis of the two wire compositions after thermal cycling suggests that this difference could be attributable to a difference in their creep resistance. In conclusion, our findings suggest that high purity Al wire bonds may be suitable for operation at temperatures which exceed 125 °C.