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.     

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.     

A Study on the Thermal Fatigue Behavior of Solder Joints Under Power Cycling Conditions

Failure mechanisms exposed by environmental accelerating testing methods such as thermal cycling or thermal shock test, may differ from those at service operating conditions. While the device is heated up or cooled down evenly on its external surface during environmental testing, real operating powered devices experience temperature gradients caused by internal local heating, components' different heat dissipation capability, and ambient temperature variation, etc. In this study, a power cycling technique is introduced to better approximate the field operating conditions so as to activate the field failure modes. Power cycling thermal fatigue test is performed with different ball grid array solder joints, that is, lead contained [Sn/37 Pb (SP)] and lead free [Sn/4.0Ag/0.5 Cu (SAC)], and the result is compared. In order to account for the thermal fatigue life behavior discrepancy for different solder joint composition, real time Moire interferometry is applied to measure the global/local thermo-mechanical behavior during power cycling excursion. Effective damage parameter, the total average shear strain, is extracted from the experiment and applied to account for the difference in fatigue life result of two different solders. In addition, amount of experimentally measured total average shear strain is mutually verified with finite element method analysis. It is clear that total average shear strain of a solder joint can be an effective damage parameter to predict thermo-mechanical fatigue life. A physical mechanism in terms of thermal material property of solder joints' is proposed to offer some thoughts to abnormal shear strain behavior that leads to discrepancies in fatigue life of two solders. An importance of power cycling testing method is emphasized for certain package designs.     

Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads

The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints.     

基于统计的高密度CCGA封装热循环寿命预测方程

利用ANSYS有限元软件建立了CCGA-1144全阵列三维1/8模型,分析了焊柱在热循环条件下的应力应变情况,提出疲劳寿命预测模型.并对CCGA结构进行可靠性正交试验设计,利用有限元方法预测了正交试验各方案的疲劳寿命,并利用统计学方法分析了不同结构参数变化对疲劳寿命的影响,建立了寿命预测回归方程.     

Reliability of Lead-Free Solder Interconnections in Thermal and Power Cycling Tests

Lead-free solder interconnection reliability of thin fine-pitch ball grid array (BGA) lead-free packages has been studied experimentally as well as with finite-element (FE) simulations. The reliability tests were composed of the thermal shock test, the local thermal cycling test (resistors embedded in the board around the package), and the power cycling test (heat generation in the die). A 3-D board-level finite-element analysis (FEA) with local models was carried out to estimate the reliability of the solder interconnections under various test conditions. Due to the transient nature of the local thermal cycling test and the power cycling test, a sequential thermal-structural coupling analysis was employed to simulate the transient temperature distribution as well as the mechanical responses. Darveaux's approach was used to predict the life time of the solder interconnections. Furthermore, the numerical results validated by the experimental results indicated that the diagonal solder interconnections beneath the die edge were the most critical ones of all the tests studied here. It has been found that the fatigue life in the power cycling test was much longer than that in the other two tests. Detailed discussions about the failure mechanism of solder interconnections as well as the microstructural observations of the primary cracks are reported in this paper.      

CSP封装Sn-3.5Ag焊点的热疲劳寿命预测

对芯片尺寸封装(CSP)中Sn-3.5Ag无铅焊点在热循环加速载荷下的热疲劳寿命进行了预测.首先利用ANSYS软件建立CSP封装的三维有限元对称模型,运用Anand本构模型描述Sn-3.5Ag无铅焊点的粘塑性材料特性;通过有限元模拟的方法分析了封装结构在热循环载荷下的变形及焊点的应力应变行为,并结合Darveaux疲劳寿命模型预测了无铅焊点的热疲劳寿命.     

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.     

A weighted least squares solution for space intersection ofspaceborne stereo SAR data

The use of stereoscopic SAR images offers an alternative to interferometric SAR for the generation of digital elevation models (DEMs). The stereo radargrammetric method is robust and can generate DEMs of sufficient accuracy to geocode SAR images. Previous work has shown that ground coordinates with accuracy of four times the resolution cell can be obtained from ERS data without using any ground control points (GCPs), where the high accuracy of the orbit and satellite position of the order of metres introduce insignificant errors into the intersection procedure. The orbit data for RADARSAT is not as accurate as that for ERS, and the perpendicular relationship between the resultant velocity vector and the resultant range vector is uncertain in terms of image geometry. Hence, it is necessary to refine the method to allow for possible errors. This paper introduces a weighted space intersection algorithm based on an analysis of the predicted errors. A radargrammetric error model for observation errors is also formulated to predict the accuracy of the algorithm. The revised method can be used without any GCPs, but this can lead to systematic errors due to less accurate orbit data, and it has been found that the use of two GCPs provides a reasonable solution. The method is insensitive to the spatial distribution of GCPs, which is often critical in traditional methods. The error statistics of the results generated from 32 independent check points, distributed through the entire SAR image, approach the predicted errors and give positional accuracy of 38 m in three dimensions     

Reliability evaluations for board-level chip-scale packages under coupled power and thermal cycling test conditions

To evaluate conjointly the effects of ambient temperature fluctuation and operation bias on the reliability of board-level electronic packages, a coupled power and thermal cycling test has been proposed. In this study, the sequential thermal–mechanical coupling analysis, which solves in turn the transient temperature field and subsequent thermomechanical deformations, is performed to investigate thermal characteristics along with fatigue reliability of board-level thin-profile fine-pitch ball grid array chip-scale packages under coupled power and thermal cycling test conditions. Effects of different power cycling durations are studied. A pure thermal cycling condition is also examined and compared. Numerical results indicate that, for the coupled power and thermal cycling test, a shorter power cycling duration in general leads to a shorter fatigue life. However, the temperature compensation effect elongates the fatigue life under certain power cycling durations.     

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.     

Fatigue analysis of high-speed photodiode submodule by using FEM

Higher frequencies, super high-speed, and low-cost demands in wireless communication devices have lead to high density packaging technologies such as flip chip interconnections and multichip modules, as substitutes for wire bonding interconnection. Solder is widely used to connect chips to their packaging substrates in flip chip technology and surface mount technology. We constructed global full 3-D FE models for one photodiode on a submount to predict the fatigue life of solder interconnects during an accelerated thermal cycling testing. The 3-D FE models applied is based on the Darveaux approach does this approach have a non-linear viscoplastic analysis. In the bump structural photodiode submodule, the shortest fatigue life of 233 cycles was obtained at the thermal cycling testing condition from −65 to 150 °C. The bump material, rather than submount material, affected and varied the fatigue life. Also, The fatigue life is decreased with increase in creep strain energy density.     

Assessment of acceleration models used for BGA solder joint reliability studies

Solder joint reliability was one of the top priorities when evaluating the reliability of electronic packages. In general, an acceleration model would be used to predict solder joint fatigue life in the use conditions. However, the accuracy of the model was difficult to validate. As a result, the fatigue life of the solder joints could be over-designed with added cost or time, or under-estimated with a compromised reliability performance. It was an important goal for engineers to use valid and accurate life models to predict the field life of the solder joints and reduce development cost and time.Many empirical models including Norris-Landzberg model and its modifications usually considered the effects of temperature range, the cycle frequency, and the maximum temperature. No matter what the package structures were or the materials were used, engineers had been using the same model parameters for many years. Moreover, little was done to validate the models for modern packages structures and materials.In this article, a variety of package was studied and the failure data was analyzed through a reliability engineering approach. The results showed that the available model parameters were not suitable to predict the solder joint life of test samples exclusively. A new set of model parameters might be required for certain cases. Also, the acceleration factor models would depend on the solder joint materials and microstructures. The solder joint fatigue life performance was too complicated to be assumed as a fixed empirical model. One of the reasons was there were too many factors affecting the strain which the solder joints would endure.In the future study, critical factors such as materials or structures could be integrated into the current model format. Additionally, the ramp rate could be a concern especially when dealing with cases under thermal shock conditions. The methodology to develop an acceleration factor model and the demonstration of their weakness would help achieve reliable solder connections in the future.     

On thermo-mechanical reliability of plated-through-hole (PTH)

The mechanical properties of plated copper in plated-through-hole (PTH) were investigated experimentally by a thermo-mechanical analyzer, a nano-indenter, and an acoustic emission instrument. Coefficients of fatigue life prediction models for plated copper have been determined by different failure criteria. Afterwards thermal fatigue test of PTH at three different diameters were performed, and Weibull statistics was employed to evaluate the fatigue life of samples under different failure criteria. Finally, the strain variation of PTH during thermal cycle has been simulated by the finite element analysis (FEA) so as to predict its fatigue life by the selected models. From the comparison of the estimated fatigue life and its experimental results, it is found that the error can be minimized to be within 100%, provided that the drifting of electrical resistance by 20% is used as failure criteria and total strain amplitude is used as control variable in fatigue life prediction model. Based on these findings, design of PTH in printed circuit board (PCB) can be optimized by FEA. It is concluded that fatigue life of PTH will increase with lesser PCB layers, smaller depth-to-diameter ratio, higher PTH density and thicker plated copper.     

Chip Scale Package (CSP) solder joint reliability and modeling

A viscoplastic constitutive model was used to analyze the thermally induced plastic and creep deformation and low cycle fatigue behavior of the solder joints in Chip Scale Packages (CSP) mounted on Printed Circuit Boards (PCB). The time-dependent and time-independent viscoplastic strain rate and plastic hardening work factors of solder material were used in 2-D plane strain finite element models. The viscoplastic strain rate data was fitted to the viscoplastic flow equation. The plastic hardening factors were considered in the evolution equation. A viscoelastic constitutive model was used for molding compound. Finite element models, incorporating the viscoplastic flow and evolution equations for solder and the viscoelastic equations for molding compound, were verified by temperature cycling tests on assembled CSPs. The effect of the cyclic frequency, dwell time, and temperature ramp rate on the response of the viscoplastic deformation was studied for a tapeless Lead-on-Chip (LOC) CSP and a flexible substrate CSP. The ramp rate significantly affects the equivalent stress range in solder joints while a dwell time in excess of 10 min per half cycle does not result in increased strain range. The failure data from the experiments was fitted to the Weibull failure distribution and the Weibull parameters were extracted. After satisfactory correlation between the experiment and the model was observed, the effect of material properties and package design variables on the fatigue life of solder joints in CSPs was investigated and the primary factors affecting solder fatique life were subsequently presented. Furthermore, a simplified model was proposed to predict the solder fatigue life in CSPs.     

A microcellular ray-tracing propagation model and evaluation of itsnarrow-band and wide-band predictions

Due to the site specific nature of microcellular operational environments, propagation models are required to take into account the exact position, orientation and electrical properties of individual buildings, and hence, ray-tracing techniques have emerged as the dominant methods to predict propagation in such environments. A novel hybrid three-dimensional (3-D) ray-tracing algorithm which can evaluate scenarios incorporating many thousands of objects by utilising the concept of “illumination zones,” is presented. In order to evaluate the accuracy of the presented model, comparisons of narrow-band and wide-band predictions with measurements are performed for a variety of scenarios. First, power comparisons show that very accurate predictions can be achieved (RMS errors less than 3.7 dB). Then, wide-band analysis shows that since the RMS delay spread for systems with finite bandwidth is a function of the multipath phase, only average measured and predicted RMS delay spread values can be compared and as a result, limited averaging can produce large RMS errors. With sufficient averaging the achieved wide-band accuracy in terms of the predicted RMS delay spread, is adequate for most planning purposes     

Boundary element analysis of thermal fatigue effects on high power IGBT modules

The technology of high power IGBT modules has been significantly improved these last years against thermal fatigue. The most frequently observed failure modes, due to thermal fatigue, are the solder cracks between the copper base plate and the direct copper bonding (DCB) substrate and bond wire lift-off. Specific simulation tools are needed to carry out reliability researches and to develop device lifetime models. In other respects, accurate temperature and flux distributions are essential when computing thermo-mechanical stresses in order to assess the lifetime of high power modules in real operating conditions. This study presents an analysis method based on the boundary element method (BEM) to investigate thermal behavior of high power semiconductor packages subjected to power cycling loads. The paper describes the boundary integral equation which has been solved using the BEM and applied to the case of a high power IGBT module package (3.3 kV–1.2 kA). A validation of the numerical tool is presented by comparison with experimental measurements. Finally, the paper points out the effect on the thermal stress of the IGBT chips position on the DCB substrate. In particular, a light shifting of the silicon chips may be sufficient to delay significantly the initiation and the propagation of the cracks, allowing a higher device lifetime of the studied module.     

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.     

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.     

An accurate error analysis model for fast Fourier transform

An error propagation model is proposed for the in-place decimation-in-time version of the radix-2 FFT algorithm. With the model, an accurate error expression and error variance for the computation of FFT are derived. This correspondence deals with fixed-point and block floating-point arithmetic. Simulation results agree closely with the theoretical predicted ones. We find that some roundoff errors at different stages correlate with each other. The density of correlations is closely associated with the round-off approach used in butterfly calculations