Development of a low-cycle fatigue life curve for 80In15Pb5Ag

The purpose of this study is to develop a methodology to predict the low-cycle (large strain—from 0.1 to 0.35 strain) fatigue life of solders subject to thermal cycling. Solders are commonly used in electronic assemblies. Using thermal fatigue data measured for 80In15Pb5Ag, a low-cycle fatigue curve for 80In15Pb5Ag solder subject to thermal cycling was developed. Specifically a Coffin-Manson relationship was derived for the solder, with a high degree of correlation (see Table I), for four different failure criteria, defined in the body of the paper. This relationship, together with calculated strains in the solder joint, allows the low-cycle fatigue life of the solder joint to be predicted.     

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     

Thermal cycling aging effects on microstructural and mechanicalproperties of a single PBGA solder joint specimen

This paper reports on an experimental study on how thermal cycling aging exposure changes the solder joint microstructure, intermetallic layer thickness and the residual shear strength and fatigue life in a single plastic ball grid array (PBGA) solder joint specimen. The single BGA solder joint specimen was specially designed to evaluate the microstructure and mechanical properties of three different batches of solder joint after subjected to 0, 500, 1000, and 2000 cycles of thermal cycling aging (-40°C to 125°C). It is important to relate the effects of thermal cycling aging on the changes of the microstructural and intermetallic layer thickness to the residual shear strength and fatigue life of solder joints subjected to thermal cycling aging exposure. The results of this study shows that the microstructural and intermetallic development due to thermal cycling aging has a major impact on the residual mechanical and fatigue strength of the solder joint. It was noted that the solder joint shear strength and residual fatigue life degrades with exposure to thermal cycling aging     

Experimental determination of fatigue behavior of lead free solder joints in microelectronic packaging subjected to isothermal aging

The effects of aging on the cyclic shear stress–strain and fatigue behavior of lead-free solders have been explored experimentally and have been presented in this paper. An experimental procedure has been developed for preparing Iosipescu shear specimens of SAC105 (Sn–1.0Ag–0.5Cu) lead-free solder, and the resulting solder joint specimens have been subjected to cyclic shear stress/strain loading at different aging conditions. A combination of four-parameter hyperbolic tangent empirical models has been used for the empirical fit of the entire cyclic stress strain curve. The fatigue life data were then fit using popular empirical failure criteria such as the strain-based Coffin–Manson model and the energy-based Morrow model. Evolution of shear hysteresis loop of SAC 105 with aging has been studied. Degradation of isothermal fatigue life due to aging has also been studied in this paper. A comparison between uniaxial fatigue data and shear fatigue data is shown and a good qualitative agreement has been found. Subsequent microstructure analysis has also been presented in the paper in support of isothermal aging effects.     

基于稳健设计的PBGA器件焊点热机械疲劳可靠性的优化设计

基于稳健设计与有限元法,研究了加速热循环测试条件下塑封球栅阵列(PBGA)焊点的热机械疲劳可靠性.考虑PCB大小(A)、基板厚度(D)、硅片热膨胀系数(G)、焊点热膨胀系数(H)等八个控制因素,使用L₁₈(2¹×3⁷)混合正交表,以对焊点热机械疲劳寿命的考核为目标,对PBGA焊点进行了优化设计.结果表明,影响焊点可靠性的显著性因素依次是基板热膨胀系数、焊点的热膨胀系数、基板厚度、芯片的热膨胀系数;最优方案组合为A1B2C3D1E2F1G3H1.进一步的验证试验结果表明,与原始方案相比,该优化方案的最大等效应变降低了66%,信噪比提高了22.4％.     

Influence of Intermetallic Properties on Reliability of Lead-Free Flip-Chip Solder Joints

Electroplated pure tin bumping as a lead-free alternative for ultra fine pitch applications is a relatively easy process and has provided us with comparable results to eutectic Sn/Pb for thermal cycling reliability. Experimentally, it has been reported that a significantly higher (~40%) thermal cycle fatigue life is seen with the use of cobalt under bump metallization (UBM) instead of copper UBM for a flip-chip device assembled on an alumina substrate. In the current approaches used to estimate fatigue life of solder joints, the solder joint is treated as a homogenous material and modeled as such. However, the smaller joint sizes and higher reactivity of Sn implies that a larger amount of intermetallics are formed as a percentage of bump volume. The existing approach cannot account for the influence on the fatigue behavior of these intermetallic layers within the solder joint. In order to investigate if a simplified engineering approach can provide some insight into this issue, we have attempted to explicitly model the intermetallics as a continuous but separate part of the solder joint. The main damage parameter investigated is the accumulated inelastic strain in a single thermal cycle. From the results, it is clear that the Young's modulus of the intermetallic layer plays an important role, more so when the ratio of intermetallic thickness to the solder joint standoff increases. Thickness of the intermetallic layer also influences the overall strain accumulation in the same manner. The CTE of the intermetallic layer has a relatively lesser influence on the strain accumulation. Both the experimental and FE results suggest that changing the UBM from copper to cobalt can improve the fatigue life by 20%-30%.     

Finite element based fatigue life estimation of the solder joints with effect of intermetallic compound growth

This paper develops an analysis procedure to study the effects of intermetallic compound (IMC) growth on the fatigue life of 63Sn-37Pb (lead-rich)/96.5Sn-3.5Ag (lead-free) solder balls for flip-chip plastic ball grid array packages under thermal cycling test conditions. In this analysis procedure, the thickness of the IMC increased with the number of thermal cycles, and was determined using the growth rate equation. A series of non-linear finite element analyses was conducted to simulate the stress/strain history at the critical locations of the solder balls with various IMC thicknesses in thermal cycling tests. The simulated stress/strain results were then employed in a fatigue life prediction model to determine the relationship between the predicted fatigue life of the solder ball and the IMC thickness. Based on the concept of continuous damage accumulation and incorporated with the linear damage rule, this study defines the damage of each thermal cycle as the reciprocal of the predicted fatigue life of the solder joints with the corresponding IMC thickness. The final fatigue failure of the solder ball was determined as the number of cycles corresponding to the cumulative damage equal to unity. Results show that the solder joint fatigue life decreased as the IMC thickness increased. Moreover, the predicted thermal fatigue life of lead-rich solders based on the effects of IMC growth is apparently smaller than that without considering the IMC growth in the reliability analysis. Results also show that the influence of the IMC thickness on the fatigue life prediction of the lead-free solder joint can be ignored.     

大功率半导体激光器高可靠烧结技术研究

近几年大功率半导体激光器的应用领域越来越广,许多应用领域都要求半导体激光器能够高可靠性工作.工作焊接质量直接影响着大功率半导体激光器的可靠性,焊接缺陷会导致激光器迅速退化.目前国内普遍采用的铟焊料和锡铅焊料都是软焊料,焊层有形成晶须和热疲劳等可靠性问题.为提高烧结可靠性,采用了金锡焊料烧结激光器新技术.金锡焊料是硬焊料,焊接强度高,抗疲劳性好,对金层无浸蚀现象.通过实验研究掌握了金锡焊料的制备和烧结技术,并与铟焊料、锡铅焊料进行了对比实验.实验结果显示采用金锡焊料烧结激光器可获得更好的性能,是提高半导体激光器可靠性的有效途径.     

Optimization of thermo-mechanical reliability of solder joints in crystalline silicon solar cell assembly

A robust solder joint in crystalline silicon solar cell assembly is necessary to ensure its thermo-mechanical reliability. The solder joint formed using optimal parameter setting accumulates minimal creep strain energy density which leads to longer fatigue life. In this study, thermo-mechanical reliability of solder joint in crystalline silicon solar cell assembly is evaluated using finite element modelling (FEM) and Taguchi method. Geometric models of the crystalline silicon solar cell assembly are built and subjected to accelerated thermal cycling utilizing IEC 61215 standard for photovoltaic panels. In order to obtain the model with minimum accumulated creep strain energy density, the L₉ (3³) orthogonal array was applied to Taguchi design of experiments (DOE) to investigate the effects of IMC thickness (IMC_T), solder joint width (SJ_W) and solder joint thickness (SJ_T) on the thermo-mechanical reliability of solder joints. The solder material used in this study is Sn3.8Ag0.7Cu and its non-linear creep deformation is simulated using Garofalo-Arrhenius creep model. The results obtained indicate that solder joint thickness has the most significant effect on the thermo-mechanical reliability of solder joints. Analysis of results selected towards thermo-mechanical reliability improvement shows the design with optimal parameter setting to be: solder joint thickness — 20 μm, solder joint width — 1000 μm, and IMC thickness — 2.5 μm. Furthermore, the optimized model has the least damage in the solder joint and shows a reduction of 47.96% in accumulated creep strain energy density per cycle compared to the worst case original model. Moreover, the optimized model has 16,264 cycles to failure compared with the expected 13,688 cycles to failure of a PV module designed to last for 25 years.     

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

The thermal fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver context (x=1 and 2) had a greater failure rate compared to those with higher silver content (x=3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x=3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag₃Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.     

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。     

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.     

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.     

Mechanical response of solder joints in flip-chip type structures

The mechanical response of PbSn solder joints of two different solder alloys (37 wt.% Pb - 63 wt.% Sn and 95 wt.% Pb - 5 wt.% Sn) used as flip-chip type interconnects is measured through mechanical testing (in tension and in shear). The influence of solder pad composition (Au and Ni) upon the behaviour of the solder joints is examined. Fatigue testing performed upon flipchip samples demonstrates the difference in mechanical comportment between Pb37Sn63 and Pb95Sn5 solders. A model for predicting fatigue life is put forward.     

Board level solder joint reliability modeling and testing of TFBGA packages for telecommunication applications

For thin-profile fine-pitch BGA (TFBGA) packages, board level solder joint reliability during the thermal cycling test is a critical issue. In this paper, both global and local parametric 3D FEA fatigue models are established for TFBGA on board with considerations of detailed pad design, realistic shape of solder joint, and nonlinear material properties. They have the capability to predict the fatigue life of solder joint during the thermal cycling test within ±13% error. The fatigue model applied is based on a modified Darveaux’s approach with nonlinear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during the thermal cycling test. For the test vehicles studied, the maximum SED is observed at the top corner of outermost diagonal solder ball. The modeling predicted fatigue life is first correlated to the thermal cycling test results using modified correlation constants, curve-fitted from in-house BGA thermal cycling test data. Subsequently, design analysis is performed to study the effects of 14 key package dimensions, material properties, and thermal cycling test condition. In general, smaller die size, higher solder ball standoff, smaller maximum solder ball diameter, bigger solder mask opening, thinner board, higher mold compound CTE, smaller thermal cycling temperature range, and depopulated array type of ball layout pattern contribute to longer fatigue life.     

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.     

Comprehensive board-level solder joint reliability modeling and testing of QFN and PowerQFN packages

For quad flat non-lead (QFN) packages, board-level solder joint reliability during thermal cycling test is a critical issue. In this paper, a parametric 3D FEA sliced model is established for QFN on board with considerations of detailed pad design, realistic shape of solder joint and solder fillet, and non-linear material properties. It has the capability to predict the fatigue life of solder joint during thermal cycling test within ±34% error. The fatigue model applied is based on a modified Darveaux’s approach with non-linear viscoplastic analysis of solder joints. A solder joint damage model is used to establish a connection between the strain energy density (SED) per cycle obtained from the FEA model and the actual characteristic life during thermal cycling test. For the test vehicles studied, the maximum SED is observed mostly at the top corner of peripheral solder joint. The modeling predicted fatigue life is first correlated to thermal cycling test results using modified correlation constants, curve-fitted from in-house QFN thermal cycling test data. Subsequently, design analysis is performed to study the effects of 17 key package dimensions, material properties, and thermal cycling test condition. Generally, smaller package size, smaller die size, bigger pad size, thinner PCB, higher mold compound CTE, higher solder standoff, and extra soldering at the center pad help to enhance the fatigue life. Comparisons are made with thermal cycling test results to confirm the relative trends of certain effects. Another enhanced QFN design with better solder joint reliability, PowerQFN, is also studied and compared with QFN of the same package size.     

The effects of underfill and its material models onthermomechanical behaviors of a flip chip package

In this paper, the effects of underfill on thermomechanical behavior of two types of flip chip packages with different bumping size and stand-off height were investigated under thermal cycling both experimentally and two-dimensional (2-D) finite element simulation. The materials inelasticity, i.e., viscoelasticity of underfill U8437-3 and viscoplasticity of 60 Sn40 Pb solder, were considered in the simulations. The results show that the use of underfill encapsulant increases tremendously (~20 times) the thermal fatigue lifetime of SnPb solder joint, weakens the effects of stand-off height on the reliability, and changes the deformation mode of the package. It was found that the thermal fatigue crack occurs in the region with maximum plastic strain range, and the Coffin-Manson type equation could then be used for both packages with and without underfill. Solder joint crack initiation occurred before delamination when using underfill with good adhesion (75 MPa) and the underfill delamination may not be a dominant failure mode in the present study. The interfacial stresses at the underfill/chip interface were calculated to analyze delamination sites, which agree with the results from acoustic image. Moreover, the effects of material models of underfill, i.e., constant elasticity (EC) and temperature dependent elasticity (ET) as well as the viscoelasticity (VE), on the thermomechanical behaviors of flip chip package were also studied in the simulation. The VE model gives comparatively large plastic strain range and large displacements in the shear direction, as well as decreased solders joint lifetime. The ET model gives similar results as the VE model and could be used instead of VE in simulations for the purpose of simplicity     

Solder joint reliability of TFBGA assemblies with fresh and reworked solder balls

In this article, the solder joint reliability of thin and fine-pitch BGA (TFBGA) with fresh and reworked solder balls is investigated. Both package and board level reliability tests are conducted to compare the solder joint performance of test vehicle with fresh and reworked solder balls. For package level reliability test, ball shear test is performed to evaluate the joint strength of fresh and reworked solder balls. The results show that solder balls with rework process exhibit higher shear strength than the ones without any rework process. The results also exhibit that the different intermetallic compound (IMC) formation at solder joints of fresh and reworked solder balls is the key to degradation of shear strength. For board level reliability tests, temperature cycling and bending cyclic tests are both applied to investigate the fatigue life of solder joint with fresh and reworked solder balls. It is observed that package with reworked solder ball has better fatigue life than the one with fresh solder ball after temperature cyclic test. As for bending cyclic test, in addition to test on as-assembled packages, reworked and fresh samples are subjected to heat treatment at 150 °C for 100 h prior to the bending cyclic test. The purpose is to let Au–Ni–Sn IMC resettle at solder joints of fresh solder ball and examine the influence of Au–Ni–Sn IMC on the fatigue life of solder joints (Au embrittlement effect). The final results confirm that reworked solder balls have better reliability performance than fresh one since Au embrittlement dose exist at fresh solder ball.     

Analysis of ring and plug shear strengths for comparison of lead-free solders

The drive to replace the use of toxic lead metal and its alloys has spurred the development of many new lead-free solder alloys. Moreover, current leaded solders lack shear strength, resistance to creep and to thermal-mechanical fatigue. Solder that exhibits enhancements of these properties and retains solderability is crucial in applications where the solder joints are subjected to thermal cycling, severe vibrations, and temperatures of up to 125°C. Modified ring and plug joints were made with 18 selected lead-free solders and three well characterized lead-containing solders. Analysis of the results provides a guide for the design of additional testing.