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     

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.     

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.     

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.     

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.     

Rate dependent constitutive relations based on Anand model for92.5Pb5Sn2.5Ag solder

A rate dependent constitutive model, the Anand model, was applied to represent the inelastic deformation behavior for a Pb-rich solder 92.5Pb5Sn2.5Ag used in electronic packaging and surface mount technology. This rate dependent model is a unified viscoplastic constitutive model using an internal state variable, the deformation resistance, to describe the averaged isotropic resistance to macroscopic plastic flow. In order to obtain the acquired data for the fitting of the material parameters of this unified model for 92.5Pb5Sn2.5Ag solder, a series of experiments of constant strain rate test and constant load creep test were conducted under isothermal conditions at different temperatures ranged from -65°C to 250°C. A procedure for the determination of material parameters was proposed in this paper. Model simulations and verifications revealed that there are good agreements between model predictions and experimental data. Moreover, some discussions on using this rate dependent model in the finite element simulation of stress/strain responses of solder joints under thermal fatigue loading were presented     

Superplastic creep of low melting point solder joints

In prior work, we showed that eutectic Sn-Pb solder joints exhibit superplastic behavior after rapid solidification. Further examples of superplasticity in nominally air-cooled solder joints are reported in this study of three low-melting point alloys: 40In-40Sn-20Pb (wt. %), eutectic 52In-48Sn, and 43Sn-43Pb-14Bi, which were creep-tested in shear at 20°, 65°, and 90° C. The test results indicate that above 65° C, the indium-containing solders have stress exponents between 2.4 to 2.9, a possible overall shear strains of 500%, and an absence of primary creep; at 90° C, 43Sn-43Pb-14Bi solder has a stress exponent close to 2.3. Optical microstructures of the three solders are presented; they help to explain the superplastic behavior.     

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.     

Fatigue crack growth behavior in 63Sn-37Pb and 95Pb-5Sn solder materials

Fatigue crack growth behavior of two typical solders 63Sn-37Pb and 95Pb-5Sn has been investigated under room temperature (20°C) and frequency of 10 Hz. Fatigue crack growth behavior of the 63Sn-37Pb specimen tested at stress ratios up to 0.5 and the 95Pb-5Sn specimen tested at stress ratios up to 0.7 were dominantly cyclic dependent. Intergranular crack growth was observed for the 63Sn-37Pb specimen tested at stress ratio of 0.7, which indicates that a crack grows under influence of creep. The difference of effect of stress ratio on crack growth behavior between two solders may results from the difference of homologous temperature. Fatigue crack growth mechanisms for solder materials are summarized as follows: A crack propagates in a transgranular manner at low homologous temperature and low stress ratio, while it propagates along eutectic grain boundaries at high homologous temperature and high stress ratio under influence of creep. At the intermediate homologous temperatures, a crack propagates in a transgranular manner with some region of intergranular fracture.     

Impact of the number of chips on the reliability of the solder balls for wire-bonded stacked-chip ball grid array packages

This investigation examines how the number of chips affects the reliability of solder balls for wire-bonded stacked-chip ball grid array packages under thermal cycling tests. The studied objects were packages with one, two, three and four stacked chips. Three-dimensional finite element analysis was utilized to simulate the stress/strain behavior of all studied packages. Two kinds of properties of 63Sn/37Pb eutectic solder were employed individually in the finite element analyses. One property of the solder was assumed to exhibit the elastic–plastic–creep behavior. Temperature-dependent stress/strain curves and Norton’s steady creep equation were used in the analysis. Another property of the solder governed by the Anand’s viscoplastic model was also employed to describe the behavior of solder balls. The simulation results in the elastic–plastic–creep analyses and viscoplastic analyses reveal that the von Mises stress, the non-linear strain, and the inelastic strain energy density of the critical solder balls increase with the number of stacked chips, but the increments become gradually stable as the number of chips increases. Three fatigue life prediction models—Darveaux’s model, the modified Coffin–Manson model and the creep-fatigue model—were applied to evaluate the fatigue life of the studied packages. Prediction results indicate that the fatigue life of the solder balls decreases as the number of stacked chips increases, and the decrease in predicted life shows stable behavior as the number of chips increases. The stable trend is consistent with experimental observation in the thermal cycling tests. By comparing with the experimental data, it is shown that the Darveaux’s model gives better prediction than the other two models.     

Superplastic creep of eutectic tinlead solder joints

This paper presents experimental evidence that as-solidified eutectic Pb-Sn solder joints can exhibit superplastic behavior in shear creep loading. Stepped load creep tests of as-solidified joints show a change in the stress exponent from a high value typical of con-ventional creep at high stress and strain rate to a superplastic value near 2 at lower stress and strain rates. In addition, the change in stress exponent is accompanied by a change in the activation energy for creep from a value near that for bulk self-diffusion (20 kcal/mol) to a value near that for grain boundary diffusion (12 kcal/mol). The total shear deformation of joints in stress-rupture tests performed at 65° C are found to ex-ceed 150%. The concomitant observation that quenched solder joints creep faster than air-cooled ones is attributed to a grain, or phase, size dependence of the strain rate. The source of superplastic behavior is a fine, equiaxed microstructure. It is not yet clear whether the superplastic microstructure is present in the as-solidified joint, or develops during the early stages of plastic deformation.     

Viscoplastic Creep Response and Microstructure of As-Fabricated Microscale Sn-3.0Ag-0.5Cu Solder Interconnects

The viscoplastic behavior of as-fabricated, undamaged, microscale Sn-3.0 Ag-0.5Cu (SAC305) Pb-free solder is investigated and compared with that of eutectic Sn-37Pb solder and near-eutectic Sn-3.8Ag-0.7Cu (SAC387) solder from prior studies. Creep measurements of microscale SAC305 solder shear specimens show significant piece-to-piece variability under identical loading. Orientation imaging microscopy reveals that these specimens contain only a few, highly anisotropic Sn grains across the entire joint. For the studied loads, the coarse-grained Sn microstructure has a more significant impact on the scatter in primary creep compared to that in the secondary creep. The observed lack of statistical homogeneity (microstructure) and joint-dependent mechanical behavior of microscale SAC305 joints are consistent with those observed for functional microelectronics interconnects. Compared with SAC305 joints, microscale Sn-37Pb shear specimens exhibit more homogenous behavior and microstructure with a large number of small Sn (and Pb) grains. Creep damage in the Pb-free joint is predominantly concentrated at highly misoriented Sn grain boundaries. The coarse-grained Sn microstructure recrystallizes into new grains with high misorientation angles under creep loading. In spite of the observed joint-dependent behavior, as-fabricated SAC305 is significantly more creep resistant than Sn-37Pb solder and slightly less creep resistant than near-eutectic SAC387 solder. Average model constants for primary and secondary creep of SAC305 are presented. Since the viscoplastic measurements are averaged over a wide range of grain configurations, the creep model constants represent the effective continuum behavior in an average sense. The average secondary creep behavior suggests that the dominant creep mechanism is dislocation climb assisted by dislocation pipe diffusion.     

Isothermal aging effects on the microstructure and solder bump shear strength of eutectic Sn37Pb and Sn3.5Ag solders

The reliability of the eutectic Sn37Pb (63%Sn37%Pb) and Sn3.5Ag (96.5%Sn3.5%Ag) solder bumps with an under bump metallization (UBM) consisting of an electroless Ni(P) plus a thin layer of Au was evaluated following isothermal aging at 150 °C. All the solder bumps remained intact after 1500 h aging at 150 °C. Solder bump microstructure evolution and interface structure change during isothermal aging were observed and correlated with the solder bump shear strength and failure modes. Cohesive solder failure was the only failure mode for the eutectic Sn37Pb solder bump, while partial cohesive solder failure and partial Ni(P) UBM/Al metallization interfacial delamination was the main failure mode for eutectic Sn3.5Ag solder bump.     

Creep deformation behavior in eutectic Sn-Ag solder joints using a novel mapping technique

Creep deformation behavior was measured for 60–100 μm thick solder joints. The solder joints investigated consisted of: (a) non-composite solder joints made with eutectic Sn-Ag solder, and (b) composite solder joints with eutectic Sn-Ag solder containing 20 vol. %, 5 μm diameter in-situ Cu₆Sn₅ intermetallic reinforcements. All creep testing in this study was carried out at room temperature. Qualitative and quantitative assessment of creep deformation was characterized on the solder joints. Creep deformation was analyzed using a novel mapping technique where a geometrical-regular line pattern was etched over the entire solder joint using excimer laser ablation. During creep, the laser-ablation (LA) pattern becomes distorted due to deformation in the solder joint. By imaging the distortion of laser-ablation patterns using the SEM, actual deformation mapping for the entire solder joint is revealed. The technique involves sequential optical/digital imaging of the deformation versus time history during creep. By tracing and recording the deformation of the LA patterns on the solder over intervals of time, local creep data are obtained in many locations in the joint. This analysis enables global and localized creep shear strains and strain rate to be determined.     

A Comprehensive Shear-Testing Facility for Joint-Scale Solder Samples

This paper describes the design and commissioning of a comprehensive shear testing apparatus for joint-scale solder samples. It allows for conducting most testing methods that are usually performed on solder specimens at highest precision for different temperatures. in situ observation of the solder joint can also be performed for micro structural studies. The various design features and the specimen manufacturing process are explained in detail. Creep test results on Sn96.5Ag3.0Cu0.5 (SAC305) and eutectic Sn63Pb37 (SP) solder are presented. Secondary creep rate, saturated primary creep strain and transition times between creep phases are investigated to demonstrate the capabilities of the shear-testing facility.      

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.     

Study on Cu particles-enhanced SnPb composite solder

The Sn-Pb solder is widely used in the electronics industry. With the development of surface mount technology and miniaturization of elements, mechanical properties of the solder are critical. Creep resistance and size stability of soldered joints are important for optical electronics. In the present work, Cu particles with a size of about 8 μm were added to the eutectic 63Sn-37Pb solder to improve the creep property of the soldering alloy. The contents of the Cu particles are 5 vol.% and 10 vol.% separately. The solder matrix is 63Sn37Pb particles with a normal size of 43 μm. The composite solder pastes are manufactured from a mixture of these particles with no-clean flux. Under reflow soldering, the metal particles were uniformly dispersed in the Sn-Pb alloy, and very thin intermetallic compounds were formed between the particles and matrix. To simulate practical soldering of printed circuit boards, a specially designed mini specimen with lap joint is used for the creep-rupture test. For the condition of ambient temperature, the creep-rupture lifetime of the soldered joint can be increased by one order quantitatively using the composite solder compared to the 63Sn37Pb eutectic solder. Other mechanical properties are measured also. In addition, the wetting property of the enhanced solder is good through the wettability test.     

Analysis of thermomechanical interactions in a miniature solder system under cyclic fatigue loading

This paper discusses the possible thermomechanical interaction (coupling) phenomena of a miniature solder system in electronic packaging application similar to those which have been identified for some metallic material systems in aerospace and nuclear structures under cyclic fatigue loads at different frequencies. The main objective is to investigate the heat generated by the viscoplastic deformations, and vice versa, especially on the thermal transient and the gradient induced viscoplastic ratchetting response of cyclic creep. A literature review was conducted to focus on the temperature-dependent, strain rate-sensitive stress-strain response of the eutectic or near-eutectic lead-tin (Pb37-Sn63 or Pb40-Sn60) solder alloys. The result was used to develop and apply a simple overstress constitutive theory for modeling the coupled, isotropic thermoviscoplasticity of the eutectic lead-tin solder alloy. A fully coupled heat transfer and mechanical finite element model is used to simulate possible thermal-mechanical interactions of temperature rise and viscoplastic ratchetting of the miniature solder systems in a C4/BGA chip scale package (CSP) under cyclic fatigue loads at different frequencies. The results of analysis are discussed to compare between a coupled thermomechanical model and that of a pure mechanical model.     

Stress relaxation behavior of eutectic tin-lead solder

The creep behavior of eutectic tin-lead solder was investigated using stress relaxation techniques. Stress relaxation experiments were performed on cast tensile specimens of commercial eutectic tin-lead solder, SN63. The sample casting conditions were controlled to produce microstructures similar to those found in typical solder joints on electronic assemblies. The stress relaxation data was analyzed to extract constitutive relations for creep. The strain rate during relaxation was found to follow two power law expressions, one with n = 3.2 at low stress levels and the other with n = 6.2 at higher stress levels. The apparent activation energy for creep and the power law exponent are discussed with relation to the published data for this alloy.     

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.