Mechanical fatigue characteristics of Sn-3.5Ag-X (X=Bi, Cu, Zn and In) solder alloys

In our previous study, the fatigue life of Sn-3.5Ag-Bi alloy was found to be dominated by the fracture ductility of the alloy and to obey a modified Coffin-Manson’s law: (Δε_p/2D)· N _f ^α , where Δε_p is plastic strain range, N_f is fatigue life, and α and=C are nondimensional constants. In this study, copper, zinc, and indium are selected as the third element, and the effect of these elements on the isothermal fatigue properties of Sn-3.5%Ag alloy has been investigated. The relationship between fatigue life and crack propagation rate estimated from load drop curve during fatigue test is also discussed. The addition of copper, indium, and zinc up to 2% slightly decreases the fatigue life of Sn-3.5Ag alloy due to the loss of ductility, while the life still remains higher than that of tin-lead eutectic alloy. The modified Coffin-Manson’s equation can also be applied to ternary Sn-3.5Ag-X. It is found that both ductility and fatigue life are significantly responsible for the load drop rate of the alloy, which reflects the extent of crack propagation. The fatigue life of Sn-3.5Ag-X alloy is therefore dominated not by the kinds and amount of third element but by the ductility of each alloy.     

Effects of Co addition in eutectic Sn-3.5Ag solder on shear strength and microstructural development

In this study, the approach of composite solder using eutectic Sn-3.5Ag solder and Co was tried. Co particles and Sn-3.5Ag solder paste were mechanically mixed at Co weight fractions from 0.1% to 2.0%. For the Co-mixed Sn-3.5Ag solder pastes, their melting temperatures and spreading areas were measured. The solder pastes were stencil printed on test substrates and reflowed to form solder bumps. Ball shear test was performed to examine shear strength of Co-reinforced Sn-3.5Ag solder bumps. As a result, Co addition up to 2 wt.% did not alter the melting temperature under heating but reduced undercooling. The maximum shear strength of Co-reinforced Sn-3.5Ag solder bumps increased by 28% compared to normal ones. The increase in shear strength can be attributed to the (Cu,Co)₃Sn₂ intermetallic compounds.     

Effect of cooling rate on microstructure and shear strength of pure Sn,Sn-0.7Cu,Sn-3.5Ag,and Sn-37Pb solders

The microstructure and shear strength characteristics of pure Sn and the eutectic compositions of Sn-37Pb, Sn-0.7Cu, and Sn-3.5Ag prepared under identical reflow conditions but subjected to two different cooling conditions were evaluated at room temperature. For the four solders, the ultimate shear strength increased with increasing strain rate from 10⁻⁵ s⁻¹ to 10⁻¹ s⁻¹. Decreasing the cooling rate tended to decrease the ultimate shear strength for both the Sn-0.7Cu and Sn-3.5Ag solders. The effects of work hardening resulting from increased strain rate were more prevalent in quench-cooled (QC) samples.     

Effects of cooling rate on the microstructure and tensile behavior of a Sn-3.5wt.%Ag solder

The tensile behavior and microstructure of bulk, Sn-3.5Ag solders as a function of cooling rate were studied. Cooling rate is an important processing parameter that affects the microstructure of the solder and, therefore, significantly influences mechanical behavior. Controlled cooling rates were obtained by cooling specimens in different media: water, air, and furnace. Cooling rate significantly affected secondary dendrite-arm size and spacing of the Sn-rich phase, as well as the aspect ratio of Ag₃Sn. The Sn-rich dendrite-arm size and spacing were smaller for water-cooled specimens than for air-cooled specimens. Furnace cooling yielded a nearly eutectic microstructure because the cooling rate approached equilibrium cooling. The morphology of Ag₃Sn also changed from spherical, at a fast cooling rate, to a needlelike morphology for slower cooling. The changes in the microstructure induced by the cooling rate significantly affected the mechanical behavior of the solder. Yield strength was found to increase with increasing cooling rate, although ultimate tensile strength and strain-to-failure seemed unaffected by cooling rate. Cooling rate did not seem to affect Young’s modulus, although a clear coorelation between modulus and porosity was obtained. The mechanical behavior was correlated with the observed microstructure, and fractographic analysis was employed to elucidate the underlying damage mechanisms.     

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.     

Effects of strain ratio and tensile hold time on low-cycle fatigue of lead-free Sn-3.5Ag-0.5Cu solder

Low-cycle fatigue (LCF) behavior of a lead-free Sn-3.5Ag-0.5Cu solder alloy was investigated at various combinations of strain ratio (R = −1, 0, and 0.5) and tensile hold time (0, 10, and 100 sec). Results showed that the LCF life of the given solder, at each given combination of testing conditions, could be individually described by a Coffin-Manson relationship. An increase of strain ratio from R=−1 to 0 and to 0.5 would cause a significant reduction of LCF life due to a mean strain effect instead of mean stress effect. LCF life was also markedly reduced when the hold time at tensile peak strain was increased from 0 to 100 sec, as a result of additional creep damage generated during LCF loading. With consideration of the effects of strain ratio and tensile hold time, a unified LCF lifetime model was proposed and did an excellent job in describing the LCF lives for all given testing conditions.     

Influence of initial morphology and thickness of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn intermetallics on growth and evolution during thermal aging of Sn-Ag solder/Cu joints

Intermetallic-layer formation and growth in Pb-free solder joints, during solder reflow or subsequent aging, has a significant effect on the thermal and mechanical behavior of solder joints. In this study, the influence of initial intermetallic morphology on growth rate, and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu. The initial morphology of the intermetallic was tailered by cooling in water, air, or furnace conditions. Solder aging was conducted at 100°C, 140°C, and 175°C and aged for 0–1,000 h. Cooling rate, aging temperature, and aging time played an important role on microstructure evolution and growth kinetics of Cu₆Sn₅ (η) and Cu₃Sn (ɛ) intermetallic layers. Prior to aging, faster cooling rates resulted in a relatively planar Cu₆Sn₅ layer, while a nodular Cu₆Sn₅ morphology was present for slower cooling. Intermetallic-growth rate measurements after aging at various times, indicated a mixed growth mechanism of grain-boundary and bulk diffusion. These mechanisms are discussed in terms of the initial intermetallic thickness and morphology controlled by cooling rate, diffusion kinetics, and the competition between Cu₆Sn₅ and Cu₃Sn growth.     

Low-cycle fatigue behavior and mechanisms of a lead-free solder 96.5Sn/3.5Ag

Low-cycle fatigue tests of as-cast Sn-Ag eutectic solder (96.5Sn/3.5Ag) were performed using a noncontact strain controlled system at 20°C. The fatigue behavior followed the Coffin-Manson equation with a fatigue-ductility exponent of 0.76. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, crack initiation and propagation behavior was observed on the specimen surface using a replication technique. After failure, the longitudinal cross sections were also examined using scanning electron microscopy (SEM). Microcracks initiated from steps at the boundary between the Sn-dendrite and the Sn-Ag eutectic structure and cavities along the boundaries especially around the Ag₃Sn particles. Stage II crack propagated in mixed manner with intergranular cracks along the Sn-dendrite boundaries and transgranular cracks through the Sn-dendrites and the Sn-Ag eutectic structure. Propagation of stage II cracks could be expressed by the relation of da_c/dN = 4.7 × 10⁻¹¹[ΔJ]^1.5, where a_c is the average crack length and ΔJ is the J-integral range. After fatigue tests, small grains were observed in Sn-dendrites near the fracture surface.     

Sn-3.0Ag-0.5Cu/Ni/Cu微焊点剪切强度与断口的研究

用直径为200~500μm的Sn-3.0Ag-0.5Cu无铅焊球分别在Ni和Cu焊盘上制作焊点,并对焊后和时效200h后的焊点进行剪切测试,并采用SEM观察剪切断口形貌。结果表明,焊后和时效200 h后焊点接头的剪切强度都随焊球尺寸增大而减小。焊后断口处韧窝形状为抛物线型,断裂方式为韧性断裂;随着焊球尺寸的增大,剪切断口处的韧窝数量增多,韧窝的变小变浅。时效200 h后,韧窝变浅,趋于平坦,韧窝数量也明显减少,材料的韧性下降,脆性增加,断裂方式由韧性向脆性发生转变。     

Microstructural effect on the creep strength of a Sn-3.5%Ag solder alloy

The effect of microstructure obtained by rapid or slow solidification and cooling of a Sn-3.5%Ag lead-free solder alloy on the creep strength has been investigated. The rapidly cooled alloy showed that the microstructure consisted of the primarily crystallized Sn phase and the quasi-eutectic phase, where fine Ag₃Sn particles dispersed in the Sn matrix. In the slowly cooled alloy, large platelets of Ag₃Sn were formed sparsely in the Sn matrix. A difference of about 2.5 orders of magnitude in the cooling rate translates to about 1.5 orders of magnitude in the creep-rupture time. Accordingly, fine particle dispersion of Ag₃Sn is considered to be very beneficial for the restraining of creep deformation, that is, for the decreasing of creep rate of the Sn-3.5%Ag alloy, compared with the effect of large platelets of Ag₃Sn sparsely formed in the Sn matrix.     

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu₆Sn₅ (η phase) adjacent to the solder and the Cu₃Sn (ε phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni₃Sn₄, and a P-rich Ni (Ni₃P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni₃Sn₄ IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni₃P was planar, and the time exponent for the Ni₃P layer growth was also 0.5. The Ni₃P layer thickness reached about 2.5 μm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu₆Sn₅ + Cu₃Sn) and the Ni₃Sn₄ IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.     

Effect of phosphorus content on Cu/Ni-P/Sn-3.5Ag solder joint strength after multiple reflows

Electroless Ni-P layers with three different P contents (6.1 wt.%, 8.8 wt.%, and 12.3wt.%) were deposited on copper (Cu) substrates. Multilayered samples of Sn-3.5Ag/Ni-P/Cu stack were prepared and subjected to multiple reflows at 250°C. A tensile test was performed to investigate the effect of P content on the solder joint strength. The low P samples exhibited the highest joint strength after multiple reflows, while the strength of medium and high P samples decreased more rapidly. From interfacial analysis, the Ni₃Sn₄ intermetallic compound (IMC) formed at the interface of low P sample was found to be more stable, while the one of medium and high P samples spalled into the molten solder. The IMC spallation sped up the consumption of electroless Ni-P, leading to the large formation of Cu-Sn IMCs. Fractographic and microstructural analyses showed that the degradation in solder joint strength was due to the formation of layers of voids and growth of Cu-Sn IMCs between the solder and the Cu substrate.     

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.     

热时效和焊点尺寸对SnAgCu微焊点强度的影响

针对高度为100～300μm的无铅钎料Sn-3.0Ag-0.5Cu微焊点,研究了等温热时效和焊点尺寸对其在100℃下拉伸强度的影响。结果表明,保持焊点直径不变时,高度为100,200和300μm微焊点未经热时效的平均拉伸强度分别为53.75,46.59和44.38MPa;热时效时间延长使微焊点内钎料合金显微组织明显粗化,导致焊点拉伸强度降低,前述三种高度的微焊点96h热时效后平均拉伸强度分别为44.13,38.38和33.48MPa,但96h热时效对IMC厚度无明显影响。     

电迁移致SnAgCu微焊点强度退化及尺寸效应研究

在100℃温度下,对直径为300μm、高度为100,200和300μm的Sn-3.0Ag-0.5Cu钎料微焊点施加密度为1×104A/cm2的直流电流。通电48h后,利用DMA对电迁移作用后微焊点拉伸强度的变化进行了研究。结果表明,电迁移作用导致微焊点拉伸强度明显退化,且退化程度随微焊点高度的减小而减弱。其中,高度为300μm的微焊点的平均拉伸强度由初始态的44.4MPa降至27.8MPa,下降了37.4%。电迁移还导致微焊点的断裂由延性变为延性与脆性相结合的模式。     

The effect of soldering process variables on the microstructure and mechanical properties of eutectic Sn-Ag/Cu solder joints

Fundamental understanding of the relationship among process, microstructure, and mechanical properties is essential to solder alloy design, soldering process development, and joint reliability prediction and optimization. This research focused on the process-structure-property relationship in eutectic Sn-Ag/Cu solder joints. As a Pb-free alternative, eutectic Sn-Ag solder offers enhanced mechanical properties, good wettability on Cu and Cu alloys, and the potential for a broader range of application compared to eutectic Sn-Pb solder. The relationship between soldering process parameters (soldering temperature, reflow time, and cooling rate) and joint microstructure was studied systemati-cally. Microhardness, tensile shear strength, and shear creep strength were measured and the relationship between the joint microstructures and mechani-cal properties was determined. Based on these results, low soldering tempera-tures, fast cooling rates, and short reflow times are suggested for producing joints with the best shear strength, ductility, and creep resistance.     

Effect of Ni layer thickness and soldering time on intermetallic compound formation at the interface between molten Sn-3.5Ag and Ni/Cu substrate

The binary eutectic Sn-3.5wt.%Ag alloy was soldered on the Ni/Cu plate at 250°C, the thickness of the Ni layer changing from 0 through 2 and 4 μm to infinity, and soldering time changing from 30 to 120 s at intervals of 30 s. The infinite thickness was equivalent to the bare Ni plate. The morphology, composition and phase identification of the intermetallic compound (IMC, hereafter) formed at the interface were examined. Depending on the initial Ni thickness, different IMC phases were observed at 30 s: Cu₆Sn₅ on bare Cu, metastable NiSn₃ + Ni₃Sn₄ on Ni(2 μm)/Cu, Ni₃Sn₄ on Ni(4 μm)/Cu, and Ni₃Sn + Ni₃Sn₄ on bare Ni. With increased soldering time, a Cu-Sn-based η-(Cu₆Sn₅)_1−xNi_x phase formed under the pre-formed Ni-Sn IMC layer both at 60 s in the Ni(2 μm)/Cu plate and at 90 s in the Ni(4 μm)/Cu plate. The two-layer IMC pattern remained thereafter. The wetting behavior of each joint was different and it may have resulted from the type of IMC formed on each plate. The thickness of the protective Ni layer over the Cu plate was found to be an important factor in determining the interfacial reaction and the wetting behavior.     

Mechanical and Electrical Properties of Cu/Sn-3.5Ag/Cu Ball Grid Array (BGA) Solder Joints after Multiple Reflows

The microstructural evolution, die shear strength, and electrical resistivity of Cu/Sn-3.5Ag (wt.%)/Cu ball grid array (BGA) solder joints were investigated after 1 to 10 reflows using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), bonding testing, and a four-point probe station. A Cu₆Sn₅ intermetallic compound (IMC) was formed at both the upper and lower interfaces after one reflow. The IMC thickness increased at the lower interface with increasing reflow number, whereas the IMC morphology and thickness remained virtually unchanged at the upper interface, irrespective of the reflow number. The amount of Cu₆Sn₅ IMC contained in the solder ball increased with increasing reflow number. These microstructural evolutions with increasing reflow number strongly affected the mechanical and electrical properties of the solder joint.     

Effect of aging on the microstructure and shear strength of SnPbAg/Ni-P/Cu and SnAg/Ni-P/Cu solder joints

The effect of aging on the microstructure and shear strength of 62Sn36Pb2Ag/Ni-P/Cu and SnAg/Ni-P/Cu surface mount solder joints was investigated. An intermetallic (IMC) layer of Ni₃Sn₄ forms at the interface between both solders and the Ni-P barrier layer and it thickens with aging time, with a decrease in the thickness of remaining Ni-P layer. The SnAg solder joint initially has a greater shear force than that of SnPbAg, but it drops dramatically after 250 h aging, and fracture occurs at the Ni-P/Cu interface afterwards, although it initiates in the solder in the initial stage of aging. The fracture in SnAg solder joint may arise from the excessive depletion of Ni characterized by a rapid accumulation of P in the remaining Ni-P layer, which results in a poor adhesion between the Ni-P layer and the Cu substrate. However, for the SnPbAg solder joint, the shear force initially decreases rapidly then asymptotically approaches a minimum, and fracture occurs from inside solder toward the solder/Ni-P interface. SnPbAg solder joint keeps relatively higher shear strength compared to SnAg solder joint after long term aging even though it decreases with aging time.     

Effect of reaction time on mechanical strength of the interface formed between the Sn-Zn(-Bi) solder and the Au/Ni/Cu bond pad

In this work, the shear strengths and the interfacial reactions of Sn-9Zn, Sn-8Zn-1Bi, and Sn-8Zn-3Bi (wt.%) solders with Au/Ni/Cu ball grid array (BGA) pad metallization were systematically investigated after extended reflows. Zn-containing Pb-free solder alloys were kept in molten condition (240°C) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 min. to 60 min. to render the ultimate interfacial reaction and to observe the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with an energy dispersive x-ray spectrometer. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. The solder ball shear load for all the solders during extended reflow increased with the increase of reflow time up to a certain stage and then decreased. It was found that the formation of thick Ni-Zn intermetallic compound (IMC) layers at the solder interface of the Au/electrolytic Ni/Cu bond pad with Sn-Zn(-Bi) alloys deteriorated the mechanical strength of the joints. It was also noticed that the Ni-Zn IMC layer was larger in the Sn-Zn solder system than that in the other two Bi-containing solder systems.