The creep and strain rate sensitivity of a high Pb content solder with comparisons to 60Sn/40Pb solder

Creep plays an important role in the mechanical behavior of solder alloys. This paper presents creep and strain rate sensitivity data for a Pb rich solder (92.5Pb, 2.5Ag, 5Sn-Indalloy 151) and compares it to the behavior of near eutectic 60Sn/40Pb solder. The high Pb alloy is used for exposures to higher temperatures than can be withstood by eutectic Sn/Pb solders. The Pb rich solder tested here is less strain rate sensitive than 60Sn/40Pb. There are also differences in the creep behavior.     

叠层芯片封装元件热应力分析及焊点寿命预测

研究了温度循环载荷下叠层芯片封装元件(SCSP)的热应力分布情况,建立了SCSP的有限元模型。采用修正后的Coffin-Masson公式,计算了SCSP焊点的热疲劳寿命。结果表明:多层芯片间存在热应力差异。其中顶部与底部芯片的热应力高于中间的隔离芯片。并且由于环氧模塑封材料、芯片之间的热膨胀系数失配,芯片热应力集中区域有发生脱层开裂的可能性。SCSP的焊点热疲劳寿命模拟值为1 052个循环周,低于单芯片封装元件的焊点热疲劳寿命(2 656个循环周)。     

Effects of load and thermal conditions on Pb-free solder joint reliability

Reliability of lead-free solder joints has been a hot topic widely debated in the electronic industry. Most published data indicate that a change to lead-free soldering has the potential benefit of more reliable solder joints than the current Sn-Pb eutectic solder joints. However, in reality many mechanical, metallurgical, thermal, and environmental factors affect the service reliability of solder joints. This paper tries to shed some light on the effects of mechanical loading and thermal conditions on solder joint reliability. These conditions are determined not only by external environments but also by the solder alloy itself and the joint geometry. Analyses with first principles are carried out on solder joints of both areal array and peripheral packages. Effects on fatigue life of solder joint geometry, thermal and mechanical characteristics of components and substrate materials, and application conditions are discussed. The analysis helps explain why lead-free solder joints may not be more reliable in certain application conditions than the current Sn-Pb eutectic solder joints.     

Effects of cooling rate on mechanical properties of near-eutectic tin-lead solder joints

This paper reports the results of a study on the effect of the cooling rate during solidification on the shear creep and low cycle shear fatigue behavior of 60 Sn/40 Pb solder joints, and on bulk solder tensile properties. Solder joints were made with three different initial microstructures by quenching, air-cooling and furnace-cooling. They have similar steady-state strain rates under creep at relatively high shear stresses (i.e. in the matrix creep region) but creep at quite different strain rates at lower shear stresses (i.e. in the grain boundary creep region). These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate. Tensile tests on bulk solders that were cold-worked, quenched and furnace-cooled show that a faster cooling rate decreases the ultimate strength and increases the ductility at low strain rates. The fatigue life of quenched solder joints is shown to be longer than that of the furnace-cooled joints.     

Time-independent mechanical and physical properties of the ternary 95.5Sn-3.9Ag-0.6Cu solder

The development of a constitutive model for predicting the thermal-mechanical fatigue (TMF) of 95.5Sn-3.9Ag-0.6Cu (wt.%) Pb-free solder interconnects requires the measurement of time-independent mechanical and physical properties. Yield stress was measured over the temperature range of −25–160°C using strain rates of 4.2 × 10⁻⁵ s⁻¹ and 8.3 × 10⁻⁴ s⁻¹. The yield-stress values ranged from approximately 40 MPa at −25°C to 10 MPa at 160°C for tests performed at 4.2 × 10⁻⁵ s⁻¹. The faster strain rate and specimen aging had a limited impact on the yield stress. The true stress/true strain curves indicated that dynamic-recovery and dynamic-recrystallization processes took place in as-cast samples exposed to temperatures of 125°C and 160°C, respectively, while tested at a strain rate of 4.2 × 10⁻⁵ s⁻¹. Aging the sample prior to testing, as well as a faster strain rate, mitigated both phenomena. Dynamic Young’s modulus values ranged from 55 GPa at −50°C to 35 GPa at 200°C, while the coefficient of thermal expansion (CTE) increased from approximately 12 × 10⁻⁶°C⁻¹ to 24 × 10⁻⁶°C⁻¹ for the same temperature range. The aging treatment had little effect on either Young’s modulus or the CTE.     

Effects of palladium and solder aging on mechanical and fatigue properties of tin-lead eutectic solder

The effects of (a) 0.5 wt.% of Pd addition, and (b) aging on mechanical and fatigue properties of eutectic solder (63Sn37Pb) were investigated. The creep rate of eutectic solder at room temperature is not affected by Pd addition. However, at 80°C, solder containing Pd creeps slower than Sn-Pb eutectic. Strain rate dramatically affects yield and tensile stress of eutectic solder with Pd as it does for the binary solder. Isothermal fatigue life of solder at 25°C is essentially not changed by Pd addition. The microstructure of Pd-containing solder consisted of polyhedral grains of (Pb), (Sn), and a dispersion of PdSn₄ intermetallic. Significant microstructural changes and interphase interface phenomena take place during creep deformation at 25 and 80°C. Ambient aging for seven years leads to solder softening and to moderate increase in isothermal fatigue life.     

Evaluation of the mechanical properties of a ternary Sn-20In-2.8Ag solder

The Sn-20In-2.8Ag solder alloy is a potential lead-free solder for replacing the traditional Sn-Pb solders. In this study, the mechanical properties of the bulk material are reported by tensile test at various strain rates and temperatures. The Sn-20In-2.8Ag solder possessed a solidus and liquidus between 170.8°C and 195.5°C. The ultimate tensile strength (UTS) and elongation were 59.3 MPa and 50.2% at a strain rate of 10⁻³ s⁻¹ at room temperature. Moreover, the UTS of this alloy decreased, but its elongation increased, with increasing testing temperature. Stress exponents of Sn-20In-2.8Ag alloy varied from 6.5 at room temperature to 4 at 100°C, and the activation energy for creep was 51.0 kJ/mol at the higher temperature range from 50°C to 100°C. The typical intergranular creep fracture mode was observed in Sn-20In-2.8Ag solder during tensile creep deformation.     

The microstructures and mechanical properties of the Sn-Zn-Ag-Al-Ga solder alloys—the effect of Ag

The microstructures and mechanical properties of Sn-8.55Zn-xAg-0.45 Al-0.5Ga (wt.%) lead-free solders were investigated. The x content of the solders investigated were 0.5–3.0 wt.%. The results indicate that Ag plays an important role not only in the structure but also in the mechanically properties. The mechanical properties and differential scanning calorimetry (DSC) behavior has been compared with that of 63Sn-37Pb solder. Small additions of Ag decreased the melting point of the Sn-8.55Zn-xAg-0.45Al-0.5Ga solders while maintaining the same strength and ductility as the 63Sn-37Pb solder.     

Microstructure, creep properties, and failure mechanism of SnAgCu solder joints

The effect of microstructure on the creep properties and the failure mechanism of SnAgCu solder joints was studied. Single overlap shear specimens made of FR-4 printed circuit boards (PCBs) with organic solderability preservative (OSP), NiAu, and immersion Sn surface finish were reflow-soldered with hypoeutectic, eutectic, and hypereutectic SnAgCu solder paste. Creep tests of the solder joints were performed at 85°C and 105°C under constant load. The effect of microstructure on the creep behavior of the joints was studied by examining the fracture surfaces and cross-sectional samples of the tested joints. Results show that the intermetallic compound at the interface between the PCB and solder affects the fracture behavior of SnAgCu solder joints, thus creating a significant difference in the creep properties of solder joints on different surface finishes. Composition of SnAgCu solder was also found to affect the creep properties of the joints.     

Effect of bismuth on the isothermal fatigue properties of Sn-3.5mass%Ag solder alloy

Sn-3.5mass%Ag eutectic solder is selected as a candidate base alloy for replacing the eutectic Sn-Pb, and the effect of bismuth (2, 5, 10mass%) on the fatigue life of bulk Sn-3.5mass%Ag eutectic at room temperature has been studied over the total strain range from 0.3 to 3 percent in tension-tension mode. Fatigue life is defined as the number of cycles at which the load decreases to a half of the initial value. The fatigue life dramatically decreases with increasing contents of bismuth and adding this element over 2% makes fatigue life shorter than that of tin-lead eutectic alloy. Tensile strength of the alloy significantly increases with an increase in bismuth contents due to solid solution hardening (<5%Bi) or dispersion strengthning of fine bismuth particles, while ductility of this system dramatically decreases with increasing bismuth contents. Fatigue life of these alloys depends on ductility obtained by tensile test. The fatigue life of Bi containing Sn-3.5%Ag alloys can be described by, (Δε_p/2D)·N _f ^0.59 =0.66 where N_f is fatigue life defined by number of cycles to one-half load reduction, Δε_p is the plastic strain range for initial cycles, D is the ductility as measured by reduction in area.     

A quantitative evaluation of time-independent and time-dependent deformations of lead-free and lead-containing solder alloys

A method to separate plasticity and creep is discussed for a quantitative evaluation of the plastic, transient creep, and steady-state creep deformations of solder alloys. The method of separation employs an elasto-plastic-creep constitutive model comprised of the sum of the plastic, transient creep, and steady-state creep deformations. The plastic deformation is expressed by the Ramberg-Osgood law, the steady-state creep deformation by Garofalo’s creep law, and the transient creep deformation by a model proposed here. A method to estimate the material constants in the elasto-plastic-creep constitutive model is also proposed. The method of separation of the various deformations is applied to the deformation of the lead-free solder alloy Sn/3Ag/0.5Cu and the lead-containing solder alloy Sn/37Pb to compare the differences in the plastic, transient creep, and steady-state creep deformations. The method of separation provides a powerful tool to select the optimum lead-free solder alloys for solder joints of electronic devices.     

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104A/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2 d后,焊点发生了熔融,阴极界面处形成了厚度为34.3μm的扇贝状IMC,而阳极界面IMC的厚度仅为9.7μm。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。     

The properties of Sn-9Zn lead-free solder alloys doped with trace rare earth elements

The Sn-Zn alloys have been considered as lead-free solders. It is well known that their poor properties of wetting and oxidation resistance are the main problems to prevent them from becoming commercially viable solders. In this paper, trace rare earth (RE) elements of mainly Ce and La have been used as alloying elements into the Sn-9Zn alloy. The results indicated that with the RE addition the originally coarse β-Sn grains in the microstructure of the alloy were refined. The tensile strength significantly increased with only a slight decrease in ductility. The surface tension was decreased, resulting in great improvement in wetting properties with rosin-based active flux.     

SnAgCuRE系钎料的钎焊接头组织与力学性能

以Sn2.5Ag0.7Cu为基础,添加微量的稀土(RE)r(Ce︰La)为4︰1,研究了钎焊接头的显微组织与力学性能。结果表明:添加微量的RE后,钎料与Cu试样间的界面层厚度明显减小,且界面处的组织更加平滑,相应地其剪切强度随微量RE的添加而增大,并在RE含量(质量分数)为0.1%时达到最大值36MPa。     

The microstructures and mechanical properties of the Sn-Zn-Ag-Al-Ga solder alloys—The effect of Ga

The microstructures and mechanical properties of Sn-8.55Zn-0.5Ag-0.45Al-yGa (wt.%) lead-free solders were investigated. The y content of the solders investigated was 0.5–3.0 wt.%. The results indicate that Ga exhibits prominent influence in the microstructure as well as mechanical properties of the solders. By increasing Ga, the fraction of the Sn/Zn eutectic region decreases and the Sn-matrix region increases. An increase in the Ga content from 0.5 wt.% to 2.0 wt.% enhances the tensile strength while degrading the ductility. The mechanical properties and differential scanning calorimetry (DSC) behavior have been compared with that of the 63Sn-37Pb solder. Gallium lowers the melting point of the Sn-8.55Zn-0.5Ag-0.45Al-yGa solders. The Sn-8.55Zn-0.5Ag-0.45Al-0.5Ga solders exhibit greater tensile strength and better ductility than the 63Sn-37Pb solder.     

纳米压痕法分析Sn-9Zn/Cu焊点力学性能

通过对Sn-9Zn/Cu焊点进行纳米压痕实验发现:在保载阶段,体钎料产生了明显的蠕变特征,蠕变深度随着加载速率的增加而增加.基于压痕做功概念确定了Sn-9Zn/Cu焊点中体钎料的蠕变应力指数n约为5.128.硬度和弹性模量测试结果表明:金属间化合物Cu5Zn8的压痕硬度(5.152±0.224) GPa约是体钎料压痕硬...     

Parameters affecting thermal fatigue behavior of 60Sn-40Pb solder joints

Solder joints in electronic packages experience cyclical thermally induced strain when temperature fluctuations are encountered in service. This study investigates three parameters that affect the microstructure and therefore the thermal fatigue behavior of 60Sn-40Pb solder joints. These parameters are: 1) the effect of a tensile component in thermal fatigue, 2) solder joint thickness variations, and 3) hold time variations at the elevated temperature portion of the thermal cycle. Solder joints were thermally fatigued in a tension/compression deformation mode. Cracks developed both in the interfacial intermetallic layer (early in thermal fatigue) and in the coarsened regions of the microstructure of the solder joint (after many more cycles). The effect of joint thickness on solder joints thermally fatigued in shear was also explored. Solder joint thickness was found not to significantly affect fatigue lifetimes. The effect of an increase in the hold time at the elevated temperature portion of the thermal fatigue cycle was also investigated. It was found that time spent at the high temperature end of the fatigue cycle does not determine solder joint lifetime, rather it is the combination of the amount of deformation induced during thermal fatigue in concert with the elevated temperature.     

Microstructure and mechanical properties of new lead-free Sn-Cu-RE solder alloys

The Sn-0.7%Cu alloy has been considered as a lead-free alternative to lead-tin alloys. In this work, various small amounts of rare earth (RE) elements, which are mainly Ce and La, have been added to the Sn-0.7%Cu alloy to form new solder alloys. It was found that the new alloys exhibit mechanical properties superior to that of the Sn-0.7%Cu alloy. In particular, the addition of up to 0.5% of RE elements is found to refine the effective grain size and provide a fine and uniform distribution of Cu₆Sn₅ in the solidified microstructure. Tensile, creep, and microhardness tests were conducted on the solder alloys. It was found that significant improvements of the tensile strength, microhardness, and creep resistance were obtained with RE element addition. Upon aging at 150°C for 20 h, the microstructure of Sn-Cu-RE is more stable than that of the Sn-Cu alloy.     

Effects of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys

The materials used in the present research are pure Sn metal and Sn-0.5% Cu, Sn-3.5%Ag, Sn-0.3%Sb, and Sn-3.5%Ag-0.5%Cu alloys. Effects of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys have been investigated. Creep tests are performed at the stress and temperature range of 3 to 12 MPa and 378 to 403 K, respectively. A 3.5% addition of Ag had the largest contribution to the creep-rupture strength of Sn metal among the single addition of 0.5%Cu, 3.5%Ag, and 0.3%Sb. The combined addition of 3.5%Ag and 0.5%Cu makes the largest creep-rupture strength. The effects of these elements on the microstructure of the lead-free alloys are also investigated with optical microscope (OM) and transmission electron microscope (TEM) observations.