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.     

Constitutive modeling on creep deformation for a SnPb-based composite solder reinforced with microsized Cu particles

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the creep strain tests, the constitutive modeling on the steady-state creep rate is determined for the Cu particle-reinforced Sn37Pb-based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of Sn37Pb solder joint. In addition, the stress exponent of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of the Sn37Pb solder joint. It is expected that the creep resistance of the Cu particle-reinforced Sn37Pb-based composite solder joint is superior to that of the Sn37Pb solder. Finally, the creep deformation mechanisms of the solder joint are discussed.     

Constitutive and Aging Behavior of Sn3.0Ag0.5Cu Solder Alloy

We describe double-lap shear experiments on Sn3.0Ag0.5Cu solder alloy, from which fits to Anand's viscoplastic constitutive model, power-law creep model, and to time-hardening primary-secondary creep model are derived. Results of monotonic tests for strain rates ranging from 4.02E-6 to 2.40E-3 s^-1, and creep response at stress levels ranging from 19.5 to 45.6 MPa are reported. Both types of tests were conducted at temperatures of 25degC, 75degC , and 125degC. Following an earlier study where Anand model and time hardening creep parameters for Sn3.8Ag0.7Cu and Sn1.0Ag0.5Cu solder alloys were reported, here we report power law model parameters so as to enable a comparison between all three alloys. Primary creep in Sn3.0Ag0.5Cu solder alloy is shown to be significant and are considered in addition to secondary creep and monotonic behavior. Aging influence on behavior is also shown to be significant. On the basis of experimental data, the following four aspects are discussed: 1) difference between testing on bulk versus joint specimen; 2) consistency between the creep and monotonic behaviors; 3) comparison against behaviors of Sn1.0Ag0.5Cu and Sn3.8Ag0.7Cu alloys as well as aganist Sn40Pb, 62Sn36Pb2Ag and 96.5Sn3.5Ag alloys; and 4) comparison of Sn3.0Ag0.5Cu and Sn3.8Ag0.7Cu relative to their aging response.     

The deformation behavior of Sn62Pb36Ag2 and itsimplications on the design of thermal cycling tests for electronicassemblies

The introduction of new packages as well as the ongoing miniaturization in SMT make the evaluation of the reliability of solder joints a permanent task. Since solder joints fail due to low cycle fatigue caused by cyclic thermomechanical stress passive thermal cycling is an important test to evaluate the lifetime of solder joints. Since a reliability prediction with the thermal cycle encountered in reality would take years to complete the tests, methods to accelerate the test cycle are to be used. However, due to the viscoplastical deformation behavior of tin-lead solder it is mandatory to take the metallurgical behavior of the solder into account when designing accelerated tests. Two different deformation mechanisms occur, depending on the temperatures of the test as well as the temperature gradient: grain boundary sliding (GBS) and dislocation climb (DC) each one having its own influence on the damage occurring in the solder. Therefore, one is not free in choosing the parameters of a test cycle. In this paper the deformation behavior of tin lead solder is explained, A constitutive equation extracted from experiments is presented, describing the deformation behavior of Sn62Pb36Ag2. Results of simulations of the deformation behavior of Sn62Pb36Ag2 on the base of the constitutive equation are shown. Suggestions for the design of accelerated lifetime tests for solder joints are given     

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.     

Characterization of 63Sn37Pb and 80Au2OSn solder sealed opticalfiber feedthroughs subjected to repetitive thermal cycling

A highly accurate prediction of hermeticity lifetime is made for eutectic 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber-Kovar ^TM nosetube feedthroughs subjected to repetitive thermal cycling. Thermal fatigue fracture of the Sn-Pb solder/Kovar^TM interface develops when cracks, initially generated from creep deformation of the solder, propagate gradually through the junction in the axial direction. A nonlinear axisymmetric finite element analysis of the 63Sn37Pb fiber feedthrough seal is performed using a thermo-elastic creep constitutive equation, and solder joint fatigue based on accumulated strain energy associated with solder creep imposed by temperature cycling is analyzed. Additionally, thermal effective stress and plastic strain is studied for alternative 80Au20Sn solder by the finite element method with results indicating significant increase in useful life as compared to 63Sn37Pb. SEM/EDX metallurgical analysis of the solder/Ni-Au plated Kovar^TM nosetube interface indicates that AuSn₄ intermetallic formed during soldering with 63Sn37Pb also contributes to joint weakening, whereas no brittle intermetallic is observed for 80Au20Sn. Hermetic carbon coated optical fibers metallized with Ni,P-Ni underplate and electrolytic Au overplating exhibit correspondingly similar metallurgy at the solder/fiber interface. Combined hermeticity testing and metallurgical analysis carried out on 63Sn37Pb and 80Au20Sn alloy solder sealed optical fiber feedthroughs after repetitive temperature cycling between -65 and +150°C, and -40 and +125°C validated the analytical approach     

Effect of solder creep on the reliability of large area die attachment

High temperature solders have been widely used for power device die attachment. One typical solder is Pb92.5In5Ag2.5, which is a ternary eutectic alloy with a eutectic temperature of 310°C. Such a Pb-based solder has a low Young's modulus, a low yield strength, and a high strain prior to failure. So it can be used to attach large size silicon die to mismatched substrates. In this paper, stresses and strains have been studied on a large size power MOSFET attachment using the Pb92.5In5Ag2.5 solders. A commercial finite element analysis software is employed as the simulation tool. Three types of substrates, pure copper, copper–tungsten composite, and pure molybdenum are used in the study, where molybdenum has the closest coefficient of thermal expansion to silicon. In addition to the plastic deformation simulation of the solder, a creep model of the solder was incorporated due to the low melting temperature of the solder alloy. Firstly, stresses and strains are calculated during the cooling cycle after attachment. It is found that the creep strain is the dominant plastic strain at low cooling rate (10°C/min). Also, the maximum Von Mises stress in the Si chip is decreased from 174 to 62.7 MPa after adding creep strain. As expected, the maximum creep strain happens to the die-to-copper substrate attach. Simulation on temperature cycling is done from −55°C to +150°C. The peak Von Mises stress occurs at the low temperature extreme and holds steadily during the soaking period, indicating insignificant contribution from creep. The Von Mises stress at the high temperature extreme is much lower and decreases with holding time. Significant plastic deformation of the solder layer is observed in cooling cycles. For silicon to copper substrate attach, its plastic deformation increases with each cycle. For all three substrates used, considerable solder creep is observed at heating cycles. The creep strain is much larger than the rate-independent plastic strain in the solder alloy for all three types of substrates. It is concluded that solder creep is the dominant factor affecting long term reliability of power semiconductor die attachment.     

Metallurgical reactions in composite 90Pb10Sn/lead-free solder joints and their effect on reliability of LTCC/PWB assembly

Use of 90Pb10Sn solder as a noncollapsible sphere material with 95.5Sn 4Ag0.5Cu and SnInAgCu lead-free solders is investigated. Practical reflow conditions led to strong Pb dissolution into liquid solder, resulting in >20 at.% Pb content in the original lead-free solders. The failure mechanism of the test joints is solder cracking due to thermal fatigue, but the characteristic lifetime of 90Pb10Sn/SnInAgCu joints is almost double that of 90Pb10Sn/95.5Sn4Ag0.5Cu in a thermal cycling test (TCT) over the temperature range from −40°C to 125°C. It is predicted that this is mainly a consequence of the better fatigue resistance of the SnPbInAgCu alloy compared with the SnPbAgCu alloy. Indium accelerates the growth of the intermetallic compound (IMC) layer at the low temperature co-fired ceramic (LTCC) metallization/solder interface and causes coarsening of IMC particles during the TCT, but these phenomena do not have a major effect on the creep/fatigue endurance of the test joints.     

Partitioned viscoplastic-constitutive properties of the Pb-free Sn3.9Ag0.6Cu solder

The partitioned viscoplastic-constitutive properties of the Sn3.9Ag0.6Cu Pb-free alloy are presented and compared with baseline data from the eutectic Sn63Pb37 solder. Steady-state creep models are obtained from creep and monotonic tests at three different temperatures for both solders. Based on steady-state creep results and creep-test data, a transient creep model is developed for both Pb-free and Sb37Pb solders. A one-dimensional (1-D), incremental analytic model of the test setup is developed to simulate constant-load creep and monotonic and isothermal cyclic-mechanical tests performed over various temperatures and strain rates and stresses using a thermome-chanical-microscale (TMM) test system developed by the authors. By fitting simulation results to monotonic testing data, plastic models are also achieved. The comparison between the two solders shows that Sn3.9Ag0.6Cu has much better creep resistance than Sn37Pb at low and medium stresses. The obtained, partitioned viscoplastic-constitutive properties of the Sn3.9Ag0.6Cu Pb-free alloy can be used in commercial finite-element model software.     

Uncertainty analysis of solder alloy material parameters estimation based on model calibration method

Quantification of the uncertainties in the material characterization of solder joint has been one of the major concerns in the microelectronics packaging industry to predict fatigue failure accurately. Therefore, in this study, a model calibration method based on Bayesian approach is proposed to quantify these uncertainties arising in the material parameter estimation of the solder alloy. A specimen is fabricated to this end, which closely simulates solder joint behavior of the actual package under a thermal cycle. Experiment is conducted to examine the deformation by using Moiré interferometry. Viscoplastic finite element analysis procedure is constructed for the specimen based on the Anand model. The uncertainties which include inherent experimental error and insufficient data of experiments are addressed by using the likelihood estimation. Two materials, one being conventional solder of Sn36Pb2Ag and the other the lead-free solder of Sn3.0Ag0.5Cu, are considered to illustrate the approach. As a result, material parameters are identified in the form of credible interval (CI), and the displacements and strains using these parameters are given by the predictive interval (PI). The results suggest that the proposed approach can be a useful tool in the probabilistic estimation of the unknown material parameters of solder joint by accounting for the uncertainties due to the experimental data.     

Estimation of the degradation rate of multi-crystalline silicon photovoltaic module under thermal cycling stress

The overall power of an outdoor-exposed photovoltaic (PV) module decreases as a result of thermal cycling (TC) stress, due to the formation of cracks between the solder and metal. In this study, the thermal fatigue life of solder (62Sn36Pb2Ag) interconnection between copper and silver metallization in PV module was studied. This paper describes in detail the degradation rate (R_D) prediction model of solder interconnection for crystalline PV module. The R_D prediction model is developed which based on published constitutive equations for solder and TC test results on actual PV module. The finite element method was employed to study the creep strain energy density of solder interconnections in TC conditions. Three types of accelerated tests were conducted to determine the prediction model parameters. R_D in benchmark condition is predicted and compared with those of TC conditions.     

凸点材料的选择对器件疲劳特性的影响

为了研究凸点材料对器件疲劳特性的影响,采用非线性有限元分析方法、统一型黏塑性本构方程和Coffin-Manson修正方程,对Sn3.0Ag0.5Cu,Sn63Pb37和Pb90Sn10三种凸点材料倒装焊器件的热疲劳特性进行了系统研究,对三种凸点的疲劳寿命进行了预测,并对Sn3.0Ag0.5Cu和Pb90Sn10两种凸点材料倒装焊器件进行了温度循环试验.结果表明,仿真结果与试验结果基本吻合.在热循环过程中,凸点阵列中距离器件中心最远的焊点,应力和应变变化最剧烈,需重点关注这些危险焊点的可靠性;含铅凸点的热疲劳特性较无铅凸点更好,更适合应用于高可靠的场合;而且随着铅含量的增加,凸点的热疲劳特性越好,疲劳寿命越长.     

Characterization of eutectic Sn-Bi solder joints

This report presents experimental results on 58Bi-42Sn solder joints, optical and SEM microstructures of their matrix and of their interface with copper, solidification behavior studied by differential scanning calorimetry, wettability to copper, creep, and low cycle fatigue. These results are discussed in comparison with 60Sn-40Pb solder, and with three low temperature solders, 52In-48Sn, 43Sn-43Pb-14Bi, and 40In-40Sn-20Pb. The 58Bi-42Sn solder paste with RMA flux wets Cu matrix with a wetting angle of 35° and had a 15° C undercooling during solidification. The constitutive equation of the steady state shear strain rate, and the Coffin-Manson relation constants for the low cycle shear fatigue life at 65° C have been determined. The test results show that this solder has the best creep resistance but the poorest fatigue strength compared with the other four solders.     

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T _m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.     

Prediction of stress-strain relationship with an improved Anand constitutive Model For lead-free solder Sn-3.5Ag

An improved Anand constitutive model is proposed to describe the inelastic deformation of lead-free solder Sn-3.5Ag used in solder joints of microelectronic packaging. The new model accurately predicted the overall trend of steady-state stress-strain behavior of the solder for the temperature range from 233 K to 398 K and the strain rate range from 0.005 s/sup -1/ to 0.1 s/sup -1/. h/sub 0,/ a constant in the original Anand model, was set to a function of temperature and strain rate in the proposed model. Comparison of the experimental results and simulated results verified that the improved Anand model with modifying h/sub 0/ to a function reasonably simulated the inelastic stress-strain relationships.     

球栅阵列封装中SnPb焊点的应力应变分析

基于SnPb焊料的统一粘塑性Anand本构模型,运用ANSYS有限元软件分析了球栅阵列封装中复合SnPb焊点在热循环过程中的应力、应变的分布,观察到SnPb焊料的蠕变行为和应力松弛现象,结果证明:外侧焊点经受的应力、应变范围比内侧焊点大;焊点的最高应力区域出现在Sn60Pb40焊料的最外缘处,最高应变区域出现在Pb90Sn10焊料与UBM层接触面的最上缘处.     

Maximum entropy fracture model and its use for predicting cyclic hysteresis in Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys

Appropriate constitutive, damage accumulation and fracture models are critical to accurate life predictions. In this study, we utilize the maximum entropy fracture model (MEFM) to predict and validate cyclic hysteresis in Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys through a damage enhanced Anand viscoplasticity model. MEFM is a single-parameter, information theory inspired model that aims to provide the best estimate for accumulated damage at a material point in ductile solids in the absence of detailed microstructural information. Using the developed model, we predict the load drop during cyclic fatigue testing of the two chosen alloys. A custom-built microscale mechanical tester was utilized to carryout isothermal cyclic fatigue tests on specially designed assemblies. The resultant relationship between load drop and accumulated inelastic dissipation was used to extract the geometry and temperature-independent damage accumulation parameter of the maximum entropy fracture model for each alloy. The damage accumulation relationship is input into the Anand viscoplastic constitutive model, allowing prediction of the stress–strain hysteresis and cyclic load drop. The damage accumulation model is validated by comparing predicted and measured load drops after 55 and 85 cycles respectively for Sn3.8Ag0.7Cu and Sn3.0Ag0.5 solder alloys. The predictions agreed to within 10% and 20% of the experimental observations respectively for the two alloys. The damage enhanced Anand model developed in this study will enable the tracking of crack fronts during finite element simulations of fatigue crack initiation and propagation in complex solder joint geometries.     

Durability of Pb‐free solder between copper interconnect and silicon in photovoltaic cells

The thermal cycling durability of large‐area Pb‐free (Sn3.5Ag) solder between silicon semiconductor and copper interconnects in photovoltaic (PV) cells is assessed and compared to benchmark results from Pb‐based (Sn36Pb2Ag) PV cells. Accelerated thermal cycling tests have been conducted on PV cells of both solder compositions, and the increase in series resistance due to interconnect damage has been characterized using in situ dark I–V measurements. Both the Pb‐free and Pb‐based cells show a steep initial rise followed by a steady rate of increase in degradation histories, with the Pb‐free cells showing a more pronounced ‘knee’ in the degradation curves. Extrapolation of the degradation data for both solders suggests that Pb‐free cells are four times more durable than the Pb‐based cells at the test condition. This superior thermal cycling fatigue durability of Pb‐free cells was also confirmed with physics of failure (PoF) analysis, consisting of nonlinear finite element (FE) stress analysis and an energy‐partitioning (E‐P) solder fatigue model. FE models error‐seeded with manufacturing voids in the solder interconnect predicted a significant reduction in the thermal cycling durability with increasing solder void density. However, even the most voided Pb‐free cells modeled are predicted to be twice as durable as void‐free Pb‐based cells, under the accelerated temperature cycle used in the test. The acceleration factor (AF) predicted by the PoF analysis for a typical service environment is three times higher for Pb‐free cells than that for Pb‐based cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Scanning electron microscope in-situ investigation of fracture behavior in 96.5Sn3.5Ag lead-free solder

In-situ tensile tests of as-cast 96.5Sn3.5Ag eutectic solder were performed under the scanning electron microscope (SEM) using different strain rates at room temperature, and various crack initiation and propagation behavior was observed on the specimen surface. It was found that, due to the existence of Ag₃Sn intermetallic particles and the special microstructure of β-Sn phases in Sn3.5Ag solder, grain boundary sliding (GBS) was no longer the dominant mechanism for this Pb-free solder. In the lower strain rate regime, accompanied by partial intragranular cracks, intergranular fracture along the grain boundaries in Sn-Ag eutectic structure or along the interphase boundaries between Sn-rich dendrites and Sn-Ag eutectic phases occurred primarily for the Sn3.5Ag solder in the early tensile stage. However, significant plastic deformation was observed in large areas for the specimens tested at higher strain rates, and cracks propagated in a transgranular manner across the Sn dendrites and Sn-Ag eutectic structure.     

Uniaxial Ratchetting Behavior of Solder Alloys and Its Simulation by an Elasto-Plastic-Creep Constitutive Model

Ratchetting deformation occurring at solder joints in electronic packaging is a concern for electronic devices. Therefore, ratchetting deformation due to thermal cycling at solder joints should be simulated by structural analysis employing tools such as the finite-element method (FEM). However, simulation of ratchetting deformation is difficult, and little modeling to simulate ratchetting deformation accurately has been reported. This work experimentally examines uniaxial ratchetting deformation of Pb-free and Pb-containing solder alloys to elucidate the effect of rate on uniaxial ratchetting. An elasto-plastic-creep constitutive model is developed to simulate uniaxial ratchetting deformation. The constitutive model incorporates a method to determine the material constants simply from a small number of pure tensile tests and subsequent stress relaxation tests. Uniaxial ratchetting deformation of solder alloys was successfully simulated using this constitutive model and simple method for material constant determination.