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.     

Effects of reflow on wettability, microstructure and mechanical properties in lead-free solders

Solder joints used in electronic applications undergo reflow operations. Such operations can affect the solderability, interface intermetallic layer formation and the resultant solder joint microstructure. These in turn can affect the overall mechanical behavior of such joints. In this study the effects of reflow on solderability and mechanical properties were studied. Nanoindentation testing (NIT) was used to obtain mechanical properties from the non-reflow (as-melted) and multiple reflowed solder materials. These studies were carried out with eutectic Sn-3.5Ag solders, with or without mechanically added Cu or Ag reinforcements, using Cu substrates. Microstructural analysis was carried out on solder joints made with the same solders using copper substrate.     

Influence of flux on wetting behavior of lead-free solder balls during the infrared-reflow process

The effects of two different fluxes (A6 and B6) on the wetting performance of Sn-3.5Ag-0.5Cu lead-free solder balls were investigated during the reflow process. Solder ball wetting behavior in real time via an optical microscope coupled with a video recorder during the reflow process was studied. The lead-free solder balls started to melt and wet at 210°C by using A6, which is 8°C lower than the melting point (218°C) of the solder material used. The wetting performance of the lead-free solder ball was dramatically enhanced by using A6. The wettability test indicated that the height of the solder ball after the reflow process with flux A6 was significantly lower than that with B6. It was found that strong fluxing capability caused these phenomena.     

Low-cycle fatigue prediction model for pb-free solder 96.5Sn-3.5Ag

Low-cycle fatigue (LCF) data of Sn-Ag eutectic solder (96.5Sn-3.5Ag) under various temperatures and frequencies has been described using three different prediction models, i.e., Coffin-Manson model, Smith-Watson-Topper (SWT) model, and Morrow energy model. The LCF behavior represented by the present prediction models showed temperature and frequency dependences, i.e., the fatigue ductility coefficient increased with increasing frequency and decreasing temperature. In order to better correlate the LCF data, a flow stress and/or frequency-dependent modifications were introduced to the Coffin-Manson and Morrow energy models. The frequency-modified Coffin-Manson model could not describe the influence of temperature on LCF behavior, while the flow stress-modified frequency-modified Morrow energy model, into which the metallurgical response (flow stress and frequency) was introduced to account for the effect of temperature and frequency on LCF behavior, gave reasonable predictions of LCF data under various temperatures and frequencies.     

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.     

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimen

Creep and rupture behavior of Cu wire/lead-free solder-alloy joint specimens have been investigated using Sn-3.5% Ag and Sn-0.5% Cu alloys. A Sn-37% Pb solder alloy is also used as a reference material. The present authors have fabricated a creep-rupture testing machine for Cu wire/solder-alloy joint specimens, performed creep and rupture tests at 303 K and 403 K, analyzed the characteristics of the creep and rupture behavior, and compared these to test specimens cut from the same alloy ingots. It is also found that the rupture strength of the joint specimens is related to the rupture strength of the alloys.     

Low-cycle fatigue characteristics of Sn-based solder joints

Low-cycle, lap-shear fatigue behavior of Sn-based, Pb-free solder alloys, Sn-3.5Ag, Sn-3.5Ag-Cu, Sn-3.5Ag-Bi, and Sn-0.7Cu, were studied at room temperature using specimens with printed circuit board (PCB)/solder/PCB structure under total displacement of ±10 μm, 12 μm, 15 μm, and 20 μm. The fatigue lives of various solder joint materials, defined as 50% load drop, were correlated with the fracture paths and analyzed using the Coffin-Manson relation, Morrow’s plastic-energy dissipation model, and Solomon’s load-drop parameter. The Sn-3.5Ag, Sn-0.7Cu eutectics, and Sn-3.5Ag-Cu ternary alloys showed the same level of fatigue resistance, while Bi-containing alloys showed substantially worse fatigue properties. Cross-sectional fractography revealed cracks initiated at the solder wedge near the solder mask and subsequently propagated into the solder matrix in the former group of alloys, in contrast with the crack propagation along the solder/under bump metallurgy (UBM) interfaces in the Sn-3.5Ag-Bi alloys. Inferior fatigue resistance of Bi-containing alloys was ascribed to high matrix hardness, high stiffness, possible Bi segregation to the interface, and high residual stress in the interfacial area.     

Demonstration and Characterization of Sn-3.0Ag-0.5Cu/ Sn-57Bi-1Ag Combination Solder for 3-D Multistack Packaging

A combination solder of Sn-3.0Ag-0.5Cu (numbers are all in weight percent unless specified otherwise) wrapped by Sn-57Bi-1Ag was tested for application to three-dimensional (3-D) multistack packaging. The experimental variables controlled were the reflow peak temperatures (170, 185, 200, and 230°C), the reflow cycles (up to four times), and the mask which controls the amount of Sn-57Bi-1Ag solder paste (two sizes). We demonstrate and evaluate the combination solder structure, focusing on microstructural changes and the shear strength. The degree of mixing in the combination solder, which is enhanced by an increase in the reflow peak temperature, is independent of the number of reflow cycles. The ball shear strength and the lab shear strength both increased with increases in the reflow peak temperatures. This behavior is explained by the amount of the brittle Bi phase that constitutes the eutectic Sn-Bi phase.     

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.     

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。     

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 mechanical deformation and annealing on the microstructure and hardness of Pb-free solders

The microstructure property relations of several Pb-free solders are investigated to understand the microstructural changes during thermal and mechanical processes of Pb-free solders. The Pb-free solder alloys investigated include pure Sn, Sn-0.7% Cu, Sn-3.5% Ag, and Sn-3.8% Ag-0.7% Cu (in weight percent). To reproduce a typical microstructure observed in solder joints, the cooling rate, ingot size, and reflow conditions of cast alloys were carefully controlled. The cast-alloy pellets are subjected to compressive deformation up to 50% and annealing at 150°C for 48 h. The microstructure of Pb-free solders is evaluated as a function of alloy composition, plastic deformation, and annealing. The changes in mechanical property are measured by a microhardness test. The work hardening in Sn-based alloys is found to increase as the amount of alloying elements and/or deformation increases. The changes in microhardness upon deformation and annealing are correlated with the microstructural changes, such as recrystallization or grain growth, in Pb-free solder alloys.     

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.     

Effect of thermal cycles on the mechanical strength of quad flat pack leads/Sn-3.5Ag-X (X=Bi and Cu) solder joints

Quad Flat Pack (QFP) Leads/Sn-3.5Ag-X (X=Bi and Cu) joint was thermally cycled between 243 K and 403 K or 273 K and 373 K, and both metallographic examination and mechanical pull test were performed to evaluate thermal fatigue damage of the joint. The addition of bismuth drastically degrades the thermal fatigue resistance of Sn-3.5Ag solder. On the other hand, the pull strength of Sn-3.5Ag-Cu solder joints slightly decreased with increasing number of thermal cycles, though it still remains higher in comparison to that for conventional Sn-37Pb or bismuth containing solder joint. The behavior observed here reflects the isothermal fatigue properties of bulk solder, because thermal fatigue crack initiates at the surface of solder fillet and propagates within the fillet in an early stage of fatigue damage. Furthermore, the lead phases lying at the interface between lead-frame and bismuth containing solder joint may promote the crack propagation at the interface, resulting in the extremely low thermal fatigue resistance of the joint.     

Revealing the nucleation and growth mechanism of a novel solder developed from Sn-3.5Ag-0.5Cu nanoparticles by a chemical reduction method

Sn-3.5Ag-0.5Cu nanoparticles were synthesized by chemical precipitation with NaBH₄. By using x-ray diffraction and transmission and scanning electron microscopy, the microstructural characteristics of particle growth were evaluated. The results indicated that the primary particles after precipitation were (Ag,Cu)₄Sn, with a size of 4.9 nm. (Ag,Cu)₄Sn was transformed into (Ag,Cu)₃Sn, when the total amount of Sn contributed from both (Ag,Cu)₄Sn and Sn covering the (Ag,Cu)₄Sn overtook that of (Ag,Cu)₃Sn. The final particle size of polycrystalline particles was 42.1 nm owing to the depletion of Sn atoms in the solution. Nucleation and growth mechanisms of Sn-3.5Ag-0.5Cu nanoparticles are also discussed and proposed.