Anisotropic Crystal Plasticity Finite Element Modeling of the Effect of Crystal Orientation and Solder Joint Geometry on Deformation after Temperature Change

The crystal orientation of the tin phase in a Pb-free Sn solder joint has a significant effect on the stress state, and hence on the reliability of the solder joint. A set of crystal plasticity analyses was used to evaluate stress and strain resulting from a 165°C temperature change in a single-crystal joint using two simplified geometries used in practical solder joints. Phenomenological flow models for ten slip systems were estimated based upon semiquantitative information available in the literature, along with known anisotropic elastic property information. The results show that the internal energy of the system is a strong function of the tin crystal orientation and geometry of the solder joint. The internal energy (and presumably the likelihood of damage) is highest when the crystal c-axis lies in the plane of the substrate, leading to significant plastic deformation. When the a-axis is in the plane of the interface, deformation due to a 165°C temperature change is predominantly elastic. The texture of the copper substrate using isotropic Cu elastic properties, or anisotropic elastic properties with [001] substrate normal direction, does␣not have a significant effect on the stress or strain in the Sn phase of the joint.     

Crystal Plasticity Finite-Element Analysis of Deformation Behavior in Multiple-Grained Lead-Free Solder Joints

The elastic and plastic anisotropy of the tin phase in a Pb-free tin-based solder joint has a very important effect on the reliability of solder joints. The crystal plasticity finite-element (CPFE) method takes into account the effect of anisotropy, and it can be used to solve crystal mechanical deformation problems under complicated external and internal boundary conditions imposed by inter- and intragrain micromechanical interactions. In this study, experimental lap-shear test results from the literature are used to calibrate the CPFE model. The spatial neighbor orientation relationships of the crystals were assessed by studying four different sets of orientations using a very simple model to establish a basis for further development of the model. Average shear strain and Schmid factor analyses were applied to study the activity of slip systems. Further optimization of model parameters using comparisons with experiments will be needed to identify more suitable rules for stress evolution among the 10 slip systems in Sn. By suppression of some of the slip systems the CPFE model is able to simulate heterogeneous deformation phenomena that are similar to those observed in experiments. This work establishes a basis for an incremental model development strategy based upon experiments, modeling, and comparative analysis to establish model parameters that could predict the slip processes that lead to damage evolution in lead-free solder joints.     

Analysis of Slip Behavior in a Single Shear Lap Lead-Free Solder Joint During Simple Shear at 25°C and 0.1/s

The relative activity of the potential slip systems in Sn is examined by comparing an experiment of a single shear lap deformation with simulations using the viscoplastic self-consistent crystal plasticity model developed by Lebensohn and Tomé. In a single shear lap specimen made using eutectic Sn-Ag solder on copper pull tabs, the initially polished side was characterized using orientation mapping before and after 0.8 shear deformation at 25°C at a shear strain rate of 0.1/s. The critical resolved shear stress of potential slip systems and the rate sensitivity was altered by trial and error until good agreement between experimentally observed and computed texture was obtained. This result indicated that slip on {101) and {211) planes is much more difficult to activate than on other slip systems for the grain orientations present in this sample. This particular sample showed much activity on the {010)〈101] slip system, but the activation of this slip system may be related to the initial dominant orientation in the specimen. This result is compared with literature and related experiments on ball grid arrays in a companion paper in this volume that show similar trends. As lead-free solder joints are commonly single crystals or multicrystals, this particular result is not indicative of lead-free solder joints as a whole, but must be interpreted in the context of a larger data set.     

The Role of Elastic and Plastic Anisotropy of Sn in Recrystallization and Damage Evolution During Thermal Cycling in SAC305 Solder Joints

Because failures in lead-free solder joints occur at locations other than the most highly shear-strained regions, reliability prediction is challenging. To gain physical understanding of this phenomenon, physically based understanding of how elastic and plastic deformation anisotropy affect microstructural evolution during thermomechanical cycling is necessary. Upon solidification, SAC305 (Sn-3.0Ag-0.5Cu) solder joints are usually single or tricrystals. The evolution of microstructures and properties is characterized statistically using optical and orientation imaging microscopy. In situ synchrotron x-ray measurements during thermal cycling are used to examine how crystal orientation and thermal cycling history change strain history. Extensive characterization of a low-stress plastic ball grid array (PBGA) package design at different stages of cycling history is compared with preliminary experiments using higher-stress package designs. With time and thermal history, microstructural evolution occurs mostly from continuous recrystallization and particle coarsening that is unique to each joint, because of the specific interaction between local thermal and displacement boundary conditions and the strong anisotropic elastic, plastic, expansion, and diffusional properties of Sn crystals. The rate of development of recrystallized microstructures is a strong function of strain and aging. Cracks form at recrystallized (random) boundaries, and then percolate through recrystallized regions. Complications arising from electromigration and corrosion are also considered.     

Methodology for Analyzing Strain States During <Emphasis Type="Italic">In Situ</Emphasis> Thermomechanical Cycling in Individual Lead-Free Solder Joints Using Synchrotron Radiation

To examine how a lead-free solder joint deforms in a thermal cycling environment, both the elastic and plastic stress and strain behavior must be understood. Methods to identify evolution of the internal strain (stress) state during thermal cycling are described. A slice of a package containing a single row of solder joints was thermally cycled from 0°C to 100°C with a period of about 1 h with concurrent acquisition of transmission Laue patterns using synchrotron radiation. These results indicated that most joints are single crystals, with some being multicrystals with no more than a few Sn grain orientations. Laue patterns were analyzed to estimate local strains in different crystal directions at different temperatures during a thermal cycle. While the strains perpendicular to various crystal planes all vary in a similar way, the magnitude of strain varies. The specimens were subsequently given several hundred additional thermal cycles and measured again to assess changes in the crystal orientations. These results show that modest changes in crystal orientations occur during thermal cycling.     

An evaluation of the lap-shear test for Sn-rich solder/Cu couples: Experiments and simulation

The lap-shear technique is commonly used to evaluate the shear, creep, and thermal fatigue behavior of solder joints. We have conducted a parametric experimental and modeling study, on the effect of testing and geometrical parameters on solder/copper joint response in lap-shear. It was shown that the farfield applied strain is quite different from the actual solder strain (measured optically). Subtraction of the deformation of the Cu substrate provides a reasonable approximation of the solder strain in the elastic regime, but not in the plastic regime. Solder joint thickness has a profound effect on joint response. The solder response moves progressively closer to “true” shear response with increasing joint thickness. Numerical modeling using finite-element analyses were performed to rationalize the experimental findings. The same lap-shear configuration was used in the simulation. The input response for solder was based on the experimental tensile test result on bulk specimens. The calculated shear response, using both the commonly adopted far-field measure and the actual shear strain in solder, was found to be consistent with the trends observed in the lap-shear experiments. The geometric features were further explored to provide physical insight into the problem. Deformation of the substrate was found to greatly influence the shear behavior of the solder.     

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.     

Thermally induced deformation of solder joints in real packages: Measurement and analysis

The paper presents a hybrid experimental and analytical approach to track the deformation of solder joints in an electronic package subject to a thermal process. The solder joint strain is directly measured using a computer vision technique. The strain measurement is analyzed following an approach that is devised based on an established solder constitutive relation. The analysis leads to the determination of the solder joint stress and in turn, to the separation of the elastic, plastic and creep strain from the measured total strain. The creep strain rate and stress–strain hysteresis loop are also obtained. Two case studies are presented to illustrate the applications and to show the viability of the approach. Each case involves a resistor package with SAC (Sn95.5Ag3.8Cu0.7) solder joints, which is subjected to a temperature variation between ambient and 120 °C. The results confirm that shear is a dominant strain component in such solder joints. The shear strain varies nearly in phase with the temperature whereas the shear stress exhibits a different trend of variation due to stress relaxation. The peak shear stress of around 10 MPa to 15 MPa are found, which occur at near 70 °C in both cases, when the temperature ramps up at approximately 3 °C/min. The creep shear strain goes up to 0.02 and accounts for over 80% of the total shear strain. The creep strain rate is in the order of magnitude of 10⁻⁵ s⁻¹. Responding to the temperature cycling with such moderate rate, the creep strain shows modest ratcheting while the stress–strain hysteresis stabilizes in two cycles.     

A Study of the Shear Response of a Lead-Free Composite Solder by Experimental and Homogenization Techniques

The current study proposes a combined experimental and modeling approach to characterize the mechanical response of composite lead-free solders. The influence of the reinforcement volume fraction on the shear response of the solder material in the joint is assessed. A novel optimized geometry for single lap shear specimens is proposed. This design minimizes the effect of plastic strain localization, leading to a significant improvement of the quality of experimental data. The constitutive model of the solder material is numerically identified from the load–displacement response of the joint by using inverse finite element identification. Experimental results for a composite solder with 0.13 reinforcement volume fraction indicate that the presence of the reinforcement leads to a 23% increase of the ultimate stress and a 50% decrease of the ultimate strain. To interpret experimental data and predict the elastoplastic response of the composite solder for varying particle volume fraction, a three-dimensional (3D) homogenization model is employed. The agreement between experiments and homogenization results leads to the conclusion that the increase in the ultimate strength and the decrease in ductility are to be attributed to load sharing between matrix material and particles with the development of a significant triaxial stress state which restricts plastic flow in the matrix.     

湿热环境下倒装焊无铅焊点的可靠性

对板上倒装芯片底充胶进行吸湿实验,并结合有限元分析软件研究了底充胶在湿敏感元件实验标准MSL—1条件下吸湿和热循环阶段的解吸附过程,测定了湿热环境对Sn3.8Ag0.7Cu焊料焊点可靠性的影响,并用蠕变变形预测了无铅焊点的疲劳寿命。结果表明:在湿热环境下,底充胶材料内部残留的湿气提高了焊点的应力水平。当分别采用累积蠕变应变和累积蠕变应变能量密度寿命预测模型时,无铅焊点的寿命只有1740和1866次循环周期。     

Sn-Ag-Cu Solder Joint Microstructure and Orientation Evolution as a Function of Position and Thermal Cycles in Ball Grid Arrays Using Orientation Imaging Microscopy

Thermally cycled plastic ball grid array (PBGA) packages with full arrays of 196 solder joints after various preconditions were examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain a plan-view cross-section of every solder joint, and characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). By OIM observations, distribution maps were obtained based on Sn crystal c-axis orientations. Each precondition showed a characteristic distribution related to the combined thermal aging and thermal cycling history. This study on Sn grain orientation using OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling, and the impact of isothermal aging as a precondition.     

Three-Dimensional Finite Element Analysis of Multiple-Grained Lead-Free Solder Interconnects

A combination of various experimental techniques was coupled with three-dimensional numerical simulation to study the strain distribution in anisotropic, heterogeneous lead (Pb)-free solder ball grid array interconnects used in electronic packages. An in situ full-field deformation map on the cross section of the joint showed a nonuniform strain distribution when the package was subjected to thermal loading. This nonuniformity was correlated with the locations of various grains on the cross section as obtained by orientation imaging microscopy (OIM) and optical microscopy. The solder interconnect was progressively sectioned and imaged under cross polarizers to discern the␣three-dimensional shapes of various grains in the solder interconnect. A methodology to replicate the three-dimensional shapes and orientations of the various grains and grain boundaries in a microstructure-based finite element model was developed. The numerical results were compared with the displacement and strain distributions obtained experimentally. The demonstrated strain localization along the grain boundaries in the case of multigrain joints and along the pad-solder interfaces in the case of the single-grain joints matched very well with the locations of plastic damage accumulation when the same interconnect was subjected to several thermal cycles.     

QFP焊点可靠性研究

随着人们对健康和环境的要求越来越高,无铅焊料的研究倍受封装业的重视。塑性应变是影响电子封装焊点可靠性的主要因素,文章采用在多次温度循环条件下进行有限元数值模拟的方法,针对由不同元素(Sn,Pb,Ag,Cu)及配比构成的焊料,计算QFP焊点的塑性应变,定量评估其可靠性。给出焊料各参数对焊点可靠性的影响程度,仿真表明焊料激活能与气体常数的比值的变化对焊点可靠性影响最大,相应的焊点Y向塑性应变均值仅为优化前的11%。所得的结果可为今后QFP封装时的焊料选择提供新的理论依据。     

Reliability and failure analysis of SAC 105 and SAC 1205N lead-free solder alloys during drop test events

The purpose of this study is to establish a predictive fatigue life model for SAC 105 (Sn-1.0Ag-0.5Cu) and SAC 1205N (Sn-1.2Ag-0.5Cu with nickel) lead-free solder alloys. A simulation model approach was developed to investigate the stress and strain of the solder joint during drop tests. A Joint Electronic Device Engineering Council (JEDEC) Condition B drop test was simulated. This test is characterized by a 1500g peak acceleration for an impulse duration of 0.5 ms. At the point of impact during the drop test, the deformation of the printed circuit board (PCB) via bending and mechanical shocks can cause joint cracks in the solder. To establish a predictive model for the 10% fatigue life of the lead-free solder joint under drop test conditions, the study was conducted in three main phases: material analysis of the lead-free solder alloy, the drop test model, and the 10% fatigue life analysis. Tensile tests of SAC 105 and SAC 1205N were used to examine the elastic and plastic behavior of the solder alloy mechanism. Simulations and drop tests were performed to investigate the failure of the microelectronic package resulting from the drop test. The predictive fatigue life models of SAC 105 and SAC 1205N were validated by the experimental results with satisfactory accuracy.     

Partially-constrained thermomechanical fatigue of eutectic tin-bismuth/copper solder joints

Small bimetallic load-frames with reference assembly stiffness, k′, and fully-constrained shear strain, γ_fc, were used to simulate the thermo-mechanical conditions experienced by eutectic Bi-42wt.%Sn-to-Cu solder joints. Shear stress and strain were induced in the solder joint by a 45-minute, 0 to 100°C temperature cycle and were calculated from the assembly temperature, joint configuration, and measured elastic strain in the load-frame. Early in cycling, a hysteresis loop representing the maximum stress range and minimum strain range was reached. As damage accumulated in the solder, the stress range decreased and the strain range increased. The TMF life of the joints, defined by the load range drop, Φ, as a function of k′ and γ_fc, can be determined, defining an effective plastic strain range which allows data for various stiffnesses and thermal expansion mismatches to be summarized on a single Coffin-Mansion plot. The effective plastic strain range also provides an important link to conventional low cycle fatigue (LCF) data taken from an infinitely stiff load-frame.     

Numerical study of ductile failure morphology in solder joints under fast loading conditions

A numerical study is undertaken to investigate solder joint failure under fast loading conditions. The finite element model assumes a lap-shear testing configuration, where the solder joint is bonded to two copper substrates. A progressive ductile damage model is incorporated into the rate-dependent elastic-viscoplastic response of the tin (Sn)–silver (Ag)–copper (Cu) solder alloy, resulting in the capability of simulating damage evolution leading to eventual failure through crack formation. Attention is devoted to deformation under relative high strain rates (1–100 s⁻¹), mimicking those frequently encountered in drop and impact loading of the solder points. The effects of applied strain rate and loading mode on the overall ductility and failure pattern are specifically investigated. It is found that, under shear loading, the solder joint can actually become more ductile as the applied strain rate increases, which is due to the alteration of the crack path. Failure of the solder is very sensitive to the deformation mode, with a superimposed tension or compression on shear easily changing the crack path and tending to reduce the solder joint ductility.     

Comparisons of experimental and computed crystal rotations caused by slip in crept and thermomechanically fatigued dual-shear eutectic Sn-Ag solder joints

Single shear-lap specimens having a solder joint area of 1 mm² and nominally 100-μm thickness on copper substrates were crept at 85°C and compared to dual shear-lap specimens with copper and nickel interfaces that were thermomechanically cycled from −15°C to 150°C to mimic the solder-joint deformation history of surface-mount components. Electron microscopy revealed surface cracks in some grain boundaries in the creep specimen and shear bands and other surface relief features on the originally polished surfaces after 100 cycles with short times at high temperature and long times at cold temperature. Orientation imaging microscopy (OIM) studies of various regions of these specimens were used to determine how the microstructure and crystallographic orientations evolved with creep or thermomechanical cycling. These results are compared to ascertain how strain path, the anisotropy of Sn, and various slip systems could account for crack nucleation and ultimate failure of the solder joint in surface-mount components.     

A viable tin-lead solder substitute: Sn-Ag-Cu

Rising concern over the use of lead in industry provides a driving force for the development of improved lead-free industrial materials. Therefore, a new lead-free base solder alloy Sn-4.7Agl.7Cu (wt.%) has been developed upon which a family of lead-free solders can be based. This solder alloy exhibits a ternary eutectic reaction at 216.8 ± 1°C (L ↠ η+ ϕ + β-Sn; η = Cu₆Sn₅, θ = Ag₃Sn). Preliminary tests of solderability demonstrate intermetallic phase formation on model solder joint interfaces and good wettability in a fluxed condition suggest technological viability and motivates much more extensive study of this solder alloy.     

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.     

Reliability study of new SnZnAl lead-free solders used in CSP packages

We have implemented a company-wide effort to progressively reduce the use of Pb(lead) and eventually eliminate this environmental pollutant from its products. As part of this effort, it has developed a new lead-free solder that consisting of Sn(tin), Zn(zinc), and Al(aluminum) and yet offers superior productivity and joint reliability. The new lead-free solder has a melting point equivalent to that of a SnPb eutectic solder, and enables devices to be packaged at a lower temperature than with the increasingly popular Sn(tin), Ag(silver), Cu(copper) solder. Thus, the new lead-free solder accelerates the elimination of Pb from products. we have already used printed circuit boards containing the new lead-free solder in some products, and plans to extend its use to other products.We further mounted SnZnAl solder balls onto Cu/Ni(nickel)/Au(gold) plated polyimide substrate at a joining temperature of 215 °C for CSP applications. It was confirmed that the joint interface between soldered ball and substrate at the initial stage was made of 2-layered compounds, i.e., AuZnSn on soldered side and ZnSnNi on substrate side. After 1000 h aging at 150 °C, the two layers compounds become one layer of ZnSnNiAu compound. No strength deterioration of Cu/Ni/Au/SnZnAl after the 1300 h shelf test was detected in the ball shear strength compared with the initial value. This paper describes the characteristics of the new lead-free solder and the results of a study on its solder ball CSP package.