The effect of model building on the accuracy of fatigue life predictions in electronic packages

Empirical fatigue life models such as the Coffin–Manson rule and its variants are commonly used at the present time to predict the reliability of microelectronic packages. While there have been reports of substantial error in empirical correlations relative to the experiments, this has not been accompanied by a rigorous understanding of the sources of the error. In this paper we systematically explore the various modeling errors in the fatigue life prediction. These errors include those in geometry representation, material behavior, load history and boundary condition application, and in the numerical solution procedure. As part of the study, experimentally validated correlations between temperature cycling and power cycling are developed for a TI 144 chip-scale package. The accuracy of the predicted life under power cycling conditions compared to the experimentally determined life is used as the basis for judging the model accuracy. The criticality of spatial refinement, temporal refinement, and accurate boundary conditions, including the often ignored natural convection boundary conditions, and their effect on predicted life is described in detail. It is shown that model errors can be a significant part of both the constitutive life models and the application models that use the constitutive life models to predict the fatigue life under a given environmental condition. It is also shown that with careful model building, solutions accurate to within 3% can be obtained.     

Impact life prediction modeling of TFBGA packages under board level drop test

Reliability performance of IC packages during drop impact is critical, especially for handheld electronic products. Currently, there is no model that provides good correlation with experimental measurements of acceleration and impact life. In this paper, detailed drop tests and simulations are performed on TFBGA (thin-profile fine-pitch BGA) and VFBGA (very-thin-profile fine-pitch BGA) packages at board level using testing procedures developed in-house. The packages are susceptible to solder joint failures, induced by a combination of PCB bending and mechanical shock during impact. The critical solder ball is observed to occur at the outermost corner solder joint, and fails along the solder and PCB pad interface. Various testing parameters are studied experimentally and analytically, to understand the effects of drop height, drop orientation, number of PCB mounting screws to fixture, position of component on board, PCB bending, solder material, etc. Drop height, felt thickness, and contact conditions are used to fine-tune the shape and level of shock pulse required. Board level drop test can be better controlled, compared with system or product level test such as impact of mobile phone, which sometimes has rather unpredictable results due to higher complexity and variations in drop orientation. At the same time, dynamic simulation is performed to compare with experimental results. The model established has close values of peak acceleration and impact duration as measured in actual drop test. The failure mode and critical solder ball location predicted by modeling correlate well with testing. For the first time, an accurate life prediction model is proposed for board level drop test to estimate the number of drops to failure for a package. For the correlation cases studied, the maximum normal peeling stresses of critical solder joints correlate well with the mean impact lives measured during the drop test. The uncertainty of impact life prediction is within ±4 drops, for a typical test of 50 drops. With this new model, a failure-free state can be determined, and drop test performance of new package design can be quantified, and further enhanced through modeling. This quantitative approach is different from traditional qualitative modeling, as it provides both accurate relative and absolute impact life prediction. The relative performance of package may be different under board level drop test and thermal cycling test. Different design guidelines should be considered, depending on application and area of concern.     

Experimental study and life prediction on high cycle vibration fatigue in BGA packages

A technique and loading apparatus have been developed which allow ball grid array (BGA) packages to be visually inspected during high cycle vibration testing. This system provides controls for varying the cycling frequency and magnitude of the applied load. The failures of solder interconnects in BGA specimens were recorded by a direct visual monitoring method. Stroboscopic video was employed to freeze the motion of the vibrating solder interconnects while showing the real-time evolution of failure. In all test cases, BGA interconnect failure was observed to be the result of crack initiation and propagation along the nickel/solder interface. A primary crack developed at one edge of the interconnect and progressed stably until a secondary crack initiated from the opposite edge. The crack growth accelerated until these cracks coalesced, resulting in complete separation of the interconnect. The percentages of time spent in crack initiation, stable propagation and accelerated propagation are, on the order of 15%, 60% and 25%, respectively. Vibration tests at frequencies ranging from 50 to 100 Hz were performed and the number of cycles to failure was found to be frequency-independent in this range.Several commonly used damage mechanics and fracture mechanics fatigue life-prediction models are examined based on failure parameters computed from a nonlinear finite element analysis. It was found that while the damage models examined show large discrepancies between predicted and actual cycles-to-failure, the fracture model correlates with the test data within a factor of 1.5.     

The role of stress state and stress triaxiality in lifetime prediction of solder joints in different packages utilized in automotive electronics

This work presents an overview on the role of the stress state and stress Triaxiality Factor (TF, see Eq. (1)) in lifetime prediction of solder connections. According to various literature sources, the TF is one of the most important factors influencing initiation of ductile fracture (Bao and Wierzbicki, 2004; Davis and Connelly, 1959). It is widely reported that lifetime of the ductile materials decreases under hydrostatic tension when combined with high TF-values. Recent investigations report that the compressive hydrostatic stress state combined with a high shearing load and low (or even zero) TF-values also contributes to failure (Bao and Wierzbicki, 2005; Kweon, 2002; Nielsen and Tvergaard, 2011).Two package types, the Loss Free Packaging (LFPAK) and the Plastic Ball Grid Array (PBGA), were investigated by means of FE-simulation on Board- and System-Level, and presented damage prediction was compared with an experimental data. In the LFPAK and BGA solder joints the regimes of hydrostatic tension and compression during temperature cycles are evaluated and compared with distribution of equivalent von Mises stress, stress intensity (maximum shear stress) and triaxiality factor. The multiaxial effects were included in both, lifetime prediction and fracture location: the damage related variables, inelastic strain and energy density, were modified in an APDL post-processing routine based on the state of hydrostatic stress and corresponding TF for each time increment. Further, using a simplified simulation approach, the path of the crack propagation was calculated according to the distribution of the TF-modified and non-modified inelastic strain. It is shown that when including multiaxial effects by modification of damage related variables a better correlation between calculated and experimentally observed crack path is achieved.     

证券电子化立法的必要性与可行性

我国证券市场利用后发优势,在1999年证券发行和交易彻底实现了证券电子化。但是,我国相关的证券法律制度并没有及时跟上,导致了证券市场的发展与法律制度建设的脱节。现行的法律还停留在纸面化层面,很少涉及电子化证券。     

Lifetime prediction modeling of non-insulated TO-220AB packages with lead-based solder joints during power cycling

In this paper, a physical model is proposed to estimate the TRIAC solder joint fatigue during power cycling. The lifetime prediction is based on the following assumptions: the case temperature swing (ΔT_case) is the main acceleration factor, the solder joints are the weakest materials in the non-insulated TO-220AB TRIAC package and the plastic strain within the solder layer due to shearing is the failure cause.     

Microstructure and strength of bump joints in photodiode packages

The joint strength and fracture surfaces of Sn-Pb and Au stud bumps for photodiode packages after isothermal aging were studied experimentally. Aluminum/gold stud bumps and Cu/Sn-Pb solders were adopted and aged for up to 900 h to analyze the effect of intermetallic compound (IMC) formation. The joint strength decreased with aging time. The diffraction patterns of Cu₆Sn₅, scallop-shaped IMCs, and planar-shaped Cu₃Sn were characterized by transmission electron microscopy (TEM). The IMCs between Au stud bumps and Al pads was identified as AlAu₂. The formation of Kirkendall voids and the growth of IMCs at the solder joint were found to be a possible mechanism for joint strength reduction.     

Universal fatigue life prediction equation for ceramic ball grid array (CBGA) packages

A traditional approach to predicting solder joint fatigue life involves finite-element simulations in combination with experimental data to develop a Coffin–Manson type predictive equation. The finite-element simulations often require good understanding of finite-element modeling, physics-based failure models, and time-, temperature-, and direction-dependent material constitutive behavior. Also, such simulations are computationally expensive and time-consuming. Microelectronic package designers often do not have the time and the expertise to perform such simulations. The traditional solder joint fatigue predictive equations fall short of ideal because: (1) they are not applicable to others due to numerical modeling issues, (2) they require a mature understanding of mechanics, numerical modeling, and reliability theory, and (3) they are difficult to implement into the design process. This includes both design of an individual electronic component and selecting which type of existing component to include in an application.Therefore, this work develops universal predictive equations that are: (1) simple, quick, and accurate, (2) require only a basic understanding of reliability and mechanics, (3) require no special software; easy to implement in a spreadsheet or current reliability tools, (4) information rich in regards to design parameters, and (5) maximize available information from experimental tests and numerical models. Using experimental data and finite-element simulations as a basis, this work has developed a predictive equation for solder joint fatigue life in lead-containing ceramic ball grid array (CBGA) package. The developed equation has been validated with other experimental data with good success. Efforts are underway to develop similar equations for other packages and Pb-free CBGAs.     

Failure mechanism of lead-free solder joints in flip chip packages

The failure mechanisms of SnAgCu solder on Al/Ni(V)/Cu thin-film, underbump metallurgy (UBM) were investigated after multiple reflows and high-temperature storage using a ball shear test, fracture-surface analysis, and cross-sectional microstructure examination. The results were also compared with those of eutectic SnPb solder. The Al/Ni (V)/Cu thin-film UBM was found to be robust enough to resist multiple reflows and thermal aging at conditions used for normal production purposes in both SnAgCu and eutectic SnPb systems. It was found that, in the SnAgCu system, the failure mode changed with the number of reflows, relating to the consumption of the thin-film UBM because of the severe interfacial reaction between the solder and the UBM layer. After high-temperature storage, the solder joints failed inside the solder ball in a ductile manner in both SnAgCu and SnPb systems. Very fine Ag₃Sn particles were formed during multiple reflows in the SnAgCu system. They were found to be able to strengthen the bulk solder. The dispersion-strengthening effect of Ag₃Sn was lost after a short period of thermal aging, caused by the rapid coarsening of these fine particles.     

Modeling and transient simulation of planes in electronic packages

This paper presents a modeling and simulation approach for ground/power planes in high speed packages. A plane pair structure is first characterized in terms of its impedance (Z) matrix at arbitrary port locations in the frequency domain. This solution is then extended for multiple plane pairs under the assumption that skin effect is prominent at higher frequencies causing isolation between the layers. Since the solutions are in analytical form, the frequency and transient response can be computed efficiently requiring small computational time. To develop spice models, equivalent circuits are constructed using resonator models with passive elements using model order reduction methods. This paper also discusses a method for incorporating decoupling capacitors into the plane models. The simulation results show good correlation with measured data     

振动载荷下特种设备中电路板级焊点疲劳寿命预测

基于内聚力模型,提出一种用于振动载荷下特种设备中电路板级焊点疲劳寿命预测的介观尺度模型。将单调载荷与振动周期载荷相结合,建立焊点累积损伤参数来表征焊点的剩余疲劳寿命,利用焊点损伤累积率来表征焊点损伤演化规律。以Sn3.0Ag0.5Cu(SAC305)细间距无铅焊点为例分析了模型参数确定方法,并通过振幅为5 mm与10 mm的定频振动试验对模型的预测精度进行了验证,寿命预测结果误差小于10%。由于模型参数为焊点钎料累积塑性应变的固有函数,与焊点尺寸与几何形态无关,通过小样本试验,数据一经确定即可应用于同种钎料不同尺寸焊点在不同振动等级下的疲劳寿命预测,节约了时间与试验成本,该模型能为特种设备中的电子组件提供一种评估板级焊点疲劳寿命的简洁、实用方法。     

Finite element modeling on electromigration of solder joints in wafer level packages

This work studies the electromigration of solder joints in an encapsulated copper post wafer level package (WLP) by finite element modeling. Experimental data showed that the electromigration failure occurs in solder joints on the printed circuit board (PCB) side due to the current crowding. In order to improve the electromigration performance on the PCB side with a copper post WLP, two new line-to-bump geometry designs are proposed. Coupled electro-thermal finite element modeling is performed to obtain the electrical and thermal fields simultaneously. The ionic flux from electron wind and thermal response is calculated based on finite element solutions. The divergence of the total flux, which is the sum of the divergence of electromigration and thermomigration, is extracted at the critical locations in solder joints. Results show that the new proposed design structures can reduce the maximum current density by 19%, and the divergence of the total ionic flux by 42%. Thermal gradient is very small in solder joints, therefore, the main driving force for electromigration failures comes from the electron wind. The finite element results on mesh dependency are discussed in this paper.     

Peridynamic wetness approach for moisture concentration analysis in electronic packages

Within the finite element framework, a commonly accepted indirect approach employs the concept of normalized concentration to compute moisture concentration. It is referred to as “wetness” approach. If the saturated concentration value is not dependent on temperature or time, the wetness equation is analogous to the standard diffusion equation whose solution can be constructed by using any commercial finite element analysis software such as ANSYS. However, the time dependency of saturated concentration requires special treatment under temperature dependent environmental conditions such as reflow process. As a result, the wetness equation is not directly analogous to the standard diffusion equation. This study presents the peridynamic wetness modeling for time dependent saturated concentration for computation of moisture concentration in electronic packages. It is computationally efficient as well as easy to implement without any iterations in each time step. Numerical results concerning the one-dimensional analysis illustrate the accuracy of this approach. Moisture concentration calculation in a three-dimensional electronic package configuration with many different material layers demonstrates its robustness.     

Tubulation techniques for locating fine-leak sites in hermetic electronic packages

Bubble testing is often regarded, at best, as a poor method for locating gross leaks. However, it is demonstrated that, under proper conditions, bubble testing can be used to detect both gross and fine leaks, as well as their precise locations. Although not applicable in all situations, these leak-tests offer rapid and effective methods for identifying leak locations in components that are not adversely affected by the increased differential pressure. This method offers additional advantages such as not being fooled by virtual leaks, and indicating exact leak-location of single or multiple leaks in both the gross and fine leak regimes.     

S-parameter based technique for simultaneous switching noiseanalysis in electronic packages

A method based on S-parameters is developed for the analysis of simultaneous switching noise (SSN) in electronic packages. A two-port Z matrix of the package pin/trace, and the coupling between the pins/traces are modeled by analytical equations. SSN is analyzed as a function of the number of switching drivers and switching time. Frequency domain measurements are performed to demonstrate the accuracy of the model. The modeling methodology is applied to both leaded and area array packages     

A multi-scale approach for investigation of interfacial delamination in electronic packages

Interfacial delamination, due to the presence of dissimilar material systems, is one of the primary concerns in electronic package design. Continuum models such as J-integral have been developed to evaluate potential delamination by defining a pre-defined crack at selected interface in the package. The interface is modeled as a discontinuity across the mating material layer. However, at molecular level, the mating materials are held together by a force field which is prescribed by their chemical structures. Molecular dynamics (MD) simulation is an effective tool used to model such force fields. The MD model can be used to simulate the reaction of the interfacial force field under external loading and the result can be utilized to characterize the material properties at the interface. The material properties at the interface are not necessarily consistent with those of the bulk materials forming the interface.The present study is focused on incorporating material behavior at the interface derived from MD simulations into the continuum model. The MD simulations were conducted to construct the constitutive relation of interface under the external loading. The constitutive relations from MD simulations in the form of a traction-displacement plot were introduced into the cohesive zone model to study the constitutive response of interface under the external loading, which is traversed across the length scale from nanoscale to macroscale.     

Failure rate estimation of known failure mechanisms of electronic packages

Product reliability is one of the key factors for a successful product launch. However, electronic components can still fail in various stages of applications due to certain failure mechanisms. A constant failure rate typically describes a majority of non-solder joint related package failures in the accelerated testing or the field application. Historically, the failure rate for a constant failure phenomenon is estimated by using the Chi-square value or the expected number of failures.This paper will discuss the statistical characteristics of the number of failures observed in tests or applications and their confidence bounds. Several methods used to estimate the confidence bounds will be described, and a new approach will be proposed and validated through case studies. The estimation of the acceleration factor (AF) used in the failure rate modeling is also discussed. The conclusion will help engineers to understand the statistical meaning of the failures observed in stress tests or in the field applications, additionally, obtain a meaningful failure rate based on the expected failure data.     

Reliability enhancement of electronic packages by design of optimal parameters

Reliability growth testing involves the selection of optimal design parameters to enhance a product's reliability. This paper proposes a split plot experimental design that accommodates the restriction on randomization on the order of experimental runs caused by the experimental nature of accelerated reliability testing. The proposed experimental design provides statistically relevant solutions about the choice of design parameters, in terms of their reliability impact, in a much shorter time. A degradation model that aids in predicting the failure time for the given problem further supplements this discussion.     

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

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

Evaluation of board-level reliability of electronic packages under consecutive drops

In general, the drop reliability of a board-level electronic package is characterized by the number of drops to failure according to a certain failure criterion. This implies that damage of solder joints evolves during each drop and eventually leads to failure. Development of a numerical method capable of obtaining accumulated stresses and strains under consecutive drop conditions is therefore in need because without these damage factors, accurate predictions for the board-level drop reliability of electronic packages are unattainable. We implement in this paper the support excitation scheme incorporated with the implicit time integration scheme to study transient structural responses of a board-level chip-scale package subjected to consecutive drops. Accumulated stresses, plastic strains, and plastic strain energy densities on the solder joints under repetitive drop impacts are investigated.