Thermal fatigue life estimation and delamination mechanics studies of multilayered MEMS structures

This paper is concerned with the application of a Physics of Failure (PoF) methodology to assessing the reliability of Micro-Electro-Mechanical-System (MEMS) switches. Numerical simulations, based on the finite element method (FEM) using a sub-domain approach, were performed to examine the damage onset (e.g. yielding) due to temperature variations and to simulated the crack propagation arising after thermal fatigue. In this work remeshing techniques were employed in order to develop a damage tolerance approach based on the assumption that initial flaws exist in the multi-layered structure due, for instance, to manufacturing processes and/or are originates after thermal fatigue, as preliminary experimental tests has shown.     

A Probabilistic Approach to Predict Thermal Fatigue Life for Ball Grid Array Solder Joints

Numerous studies of the reliability of solder joints have been performed. Most life prediction models are limited to a deterministic approach. However, manufacturing induces uncertainty in the geometry parameters of solder joints, and the environmental temperature varies widely due to end-user diversity, creating uncertainties in the reliability of solder joints. In this study, a methodology for accounting for variation in the lifetime prediction for lead-free solder joints of ball grid array packages (PBGA) is demonstrated. The key aspects of the solder joint parameters and the cyclic temperature range related to reliability are involved. Probabilistic solutions of the inelastic strain range and thermal fatigue life based on the Engelmaier model are developed to determine the probability of solder joint failure. The results indicate that the standard deviation increases significantly when more random variations are involved. Using the probabilistic method, the influence of each variable on the thermal fatigue life is quantified. This information can be used to optimize product design and process validation acceptance criteria. The probabilistic approach creates the opportunity to identify the root causes of failed samples from product fatigue tests and field returns. The method can be applied to better understand how variation affects parameters of interest in an electronic package design with area array interconnections.     

Fast reliability qualification of SiP products

For the purpose of rapidly identifying the functional weak points of SiP products and defining appropriate design rules, a new methodology is proposed to achieve fast reliability qualification. This new methodology is based on the moisture absorption behavior along the critical interface of a SiP carrier and on the most sensitive zone to delamination of the SiP carrier, determined by simulation and experimentally checked. In this paper, a new accelerated preconditioning is proposed and a new non destructive thermal method to monitor the delamination is presented. The effectiveness of this new stress test to accelerate the failure mechanism of the SiP carrier and the ability to detect delamination are evaluated by performing a DOE.     

Dependence of Flip Chip Solder Reliability on Filler Settling

Thermomechanical reliability of solder joints in flip-chip packages is usually analyzed by assuming a homogeneous underfill ignoring the settling of filler particles. However, filler settling does impact flip chip reliability. This paper reports a numerical study of the influence of filler settling on the fatigue estimation of flip-chip solder joints. In total, nine underfill materials ( 35 vol% silica filler in three epoxies with three filler settling profiles for each epoxy) are individually introduced in a 2-D finite element (FE) model to compare the thermal response of flip chip solder joints that are surrounded by the underfill. The results show that the fatigue indicators for the solder joints (inelastic shear strain increments and inelastic shear strain energy density) corresponding to a gradual, nonuniform filler profile studied in this paper can be smaller than those associated with the uniform filler profile, suggesting that certain gradual filler settling profiles in conjunction with certain resin grades may favor a longer solder fatigue lifetime. The origin of this intriguing observation is in the fact that the solder fatigue indicators are a function of the thermal mismatch among the die, substrate, solder, and underfill materials. The thermal mechanics interplayed among these materials along with a gradual filler profile may allow for minimizing thermal mismatch; and thus lead to lower fatigue indicators.      

Signal Integrity Flow for System-in-Package and Package-on-Package Devices

As data rates required for systems in package (SiPs) increase and their complexity increases, signal integrity issues become increasingly difficult to address. The design flow of the SiP should therefore take into account these issues from the beginning. A design flow aimed at designing the SiP tracks is presented; its suitability for the design of packages comprising multiple stacked memories is verified through a design example. The proposed flow for signal integrity can be integrated easily within the complete design of the SiP.      

凸点材料的选择对器件疲劳特性的影响

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

Effect of randomness of Cu-Sn intermetallic compound layerthickness on reliability of surface mount solder joints

A statistical reliability analysis on thermal fatigue lifetime of surface mount solder joints, considering randomness of Cu-Sn intermetallic compound (IMC) layer thickness, is presented. Based on published thermal fatigue life test data, the two-parameter Weibull distribution of the thermal fatigue lifetime for a fixed IMC layer thickness is found, and a K-S goodness-of-fit test is conducted to examine the goodness of fit of the assumed Weibull distribution. Then, the Weibull parameters as functions of IMC layer thickness are obtained. Considering the randomness of IMC layer thickness, the MTTF and reliability of surface mount solder joints on thermal cycles are analyzed. For surface mount solder joints formed under the same conditions and loaded during the same thermal cycling as stated in the publication, numerical results of the MTTF and reliability are presented. The results show that when the mean value of MC layer thickness is low (e.g., smaller than 1.5 μm), the effect of randomness of IMC layer thickness is significant; i.e., the MTTF has strong dependence on IMC layer thickness distribution; and the reliability is significantly different at high thermal cycles. When the mean value of IMC layer thickness is high (e.g., greater than 2.0 μm), the effect of randomness of IMC layer thickness is negligible. Therefore, the presented results are important to the reliability study of surface mount solder joints. Even though the validity of the presented results based on the test data remains to be verified from other sources of data, the proposed statistical method is generally applicable for thermal fatigue reliability analysis of surface mount solder joints. By combining the proposed method with the forming mechanism of IMC layer under varying manufacturing and loading conditions, a comprehensive reliability analysis on thermal fatigue lifetime of surface mount solder joints can be expected     

A Birnbaum-Saunders accelerated life model

The 2-parameter family of probability distributions introduced by Birnbaum and Saunders characterizes the fatigue failure of materials subjected to cyclic stresses and strains. It is shown that the methods of accelerated life testing are applicable to the Birnbaum-Saunders distribution for analyzing accelerated lifetime data, and the (inverse) power law model is used due to its justification for describing accelerated fatigue failure in metals. This paper develops the (inverse) power law accelerated form of the Birnbaum-Saunders distribution, and explores the corresponding inference procedures-including parameter estimation techniques and the derivation of the s-expected Fisher information matrix. The model approach in this paper is different from an earlier work, which considered a log-linear form of a model with applications to accelerated life testing. Here, using an example data set, the fitted model is effectively used to estimate lower distribution percentiles and mean failure times for particular values of the acceleration variable. The benefits of having an operable closed form of the Fisher information matrix, which is unique to this article for this model, include interval estimation of model parameters and LCB on percentiles using relatively simple computational procedures     

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.     

Thermo-mechanical reliability analysis of a RF SiP module based on LTCC substrate

The RF SiP module based on LTCC substrate has attracted considerable attention in wireless communications for the last two decades. However, the thermo-mechanical reliability of this 3D LTCC architecture has not been well-studied as common as its traditional ceramic package structure. A practical RF SiP module based on LTCC substrate was presented and its thermo-mechanical reliability was analyzed in this paper, with emphasis on the reliability of heat reflow process, the operating state and fatigue of second-level solder joints. The configuration and assembly process of the SiP module were briefly introduced at first, and qualitative analysis was made according to the reliability problem that may occur in the manufacturing process and the operating state. Through FEM simulation, this paper studied the warpage and stress variation of the RF SiP module, as well as parametric studies of some key package dimensions. Solder joint reliability under temperature cycling condition was also analyzed in particular in this paper. The results show that for the heat reflow process and operating state, the maximum warpage is both on the top LTCC substrate, but the maximum stresses are on the outermost solder ball and the kovar column at the corner, respectively. There is a large residual stress on the critical solder ball at the end of the reflow process and the key package dimensions has little effect on it. The thickness of top LTCC substrate has a significant impact on the thermal deformation and thermal stress, followed by the height of kovar columns. The reason for the considerable thermal stress on the kovar column is the non-uniform of temperature distribution. The key to reducing thermal deformation and stress in the operating state is the employment of effective cooling measures. It is found by comparison that the reliability of critical solder joints can be greatly improved by adding suitable underfill.     

Physics-of-failure models of erosion wear in electrohydraulic servovalve,and erosion wear life prediction method

A series of Physics-of-Failure (PoF) models for particle erosion wear of electrohydraulic servovalves (EHSV), and the PoF based erosion wear service life prediction models, are established. Because there are only few correlative quantitative researches on the effect of erosion wear to EHSV, establishing the PoF models is of great significance to solve the problem. These models can also help to design a high-reliability and long-life EHSV, to establish relevant specifications, or to carry out accelerate life tests of EHSV. Before modeling, this paper analyzed the failure mechanism; and deduced mathematical models of servovalve’s critical performance parameters (CPP) that involved in the Slide Valve’s pressure gain, null leakage flow, and the Flapper nozzle Valve’s null bias, which may be affected by particle erosion wear. The factors that influence servovalves lifetime are the hardness, shape and size, velocity, concentration of particle, the design and assembling parameters, and the application method. With erosion wear equations, we connect the factors to mathematical models of CPP to establish the PoF models. The methods to ascertain parameters in the models are proposed. Contaminant erosion wear experiments are carried out at fluid cleanliness level 7. The results show the PoF models can be utilized to precisely describe the degradation process and accurately estimate the erosion wear life of various servovalves.     

Physics-of-Failure (PoF) methodology for qualification and lifetime assessment of supercapacitors for industrial applications

Supercapacitors are used nowadays in an extensive range of battery-powered devices such as GPS/GPRS transceivers, active RFID tags, industrial PDAs, electronic locks, micro medical pumps, digital cameras, mobile phones and others. They are also used in vehicle's and machine's sub-systems requiring short but robust powerful current pulses. This covers back-up, delivery and leveling of high peak power as well as storing harvested energy.They are designed to meet high power requirements that cannot be fulfilled by standard batteries. Furthermore, their size and cost are very attractive for industries.However, selecting the right supercapacitor for a specific application remains a real challenge to industry. The specifications of these components only provide limited information about their lifetime for specific stress values. This information is not enough for industries to design a robust product and avoid high field returns.In this paper, we apply the Physics-of-Failure (PoF) methodology for qualification and lifetime assessment of electronic systems, to derive PoF models for supercapacitors at different stresses relevant for some industrial applications. It is expected from these models to better understand the performance of supercapacitors at different stresses and to predict accurate lifetime of supercapacitors allowing industry to robustly design their products and avoid high field returns.     

Ultra-thin high-density LSI packaging substrate for advanced CSPs and SiPs

An ultra-thin high-density LSI packaging substrate, called multi-layer thin substrate (MLTS), is described. It meets the demand for chip scale packages (CSPs) and systems in a package (SiPs) for use in recently developed small portable applications with multiple functions. A high-density build-up structure is fabricated on a Cu plate, which is then removed, leaving only an ultra-thin, high-density multi-layer substrate. MLTS has (1) excellent registration accuracy, which enables higher density and finer pitch patterning due to the use of a rigid, excellent-flatness Cu base plate; (2) a thinner multi-layer structure due to the use of a core-less multi-layer structure; (3) excellent reliability, supported by the use of an aramid-reinforced epoxy resin dielectric layer; and (4) a cost-effective design due to the use of fewer layers fabricated using a conventional build-up process. A prototype high-density CSP (0.4-mm pitch/288 pins/4 rows/10 mm²) was fabricated using a 90-μm-thick MLTS (with a solder resist layer). Testing demonstrated that it had excellent long-term reliability. A prototype ultra-thin, high-density SiP (0.5-mm pitch/225 pins/11 mm²/0.93 mm thick) was also fabricated based on MLTS. MLTS consists of only two conductor layers (total thickness: 90 μm) while an identical-function build-up printed wiring board needs four conductor layers (total thickness: 300 μm). With its thinner core-less multi-layer structure, MLTS enables the fabrication of ultra-thin, high-density SiPs.     

Effect of substrate flexibility on solder joint reliability. Part II: finite element modeling

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue life significantly. The mechanism of substrate flexibility on improving solder joint thermal fatigue was investigated by thermal mechanical analysis (TMA) technique and finite element modeling. The reliability of solder joints in real flip chip assembly with both rigid and compliant substrates was evaluated by accelerated temperature cycling test. Finite element simulations were conducted to study the reliability of solder joints in flip chip on flex assembly (FCOF) and flip chip on rigid board assembly (FCOB) applying Anand model. Based on the finite element analysis results, the fatigue lives of solder joints were obtained by Darveaux’s crack initiation and growth model. The thermal strain/stress in solder joints of flip chip assemblies with different substrates were compared. The results of finite element analysis showed a good agreement with the experimental results. It was found that the thermal fatigue lifetime of FCOF solder joints was much longer than that of FCOB solder joints. The thermal strain/stress in solder joints could be reduced by flex buckling or bending and flex substrates could dissipate energy that otherwise would be absorbed by solder joints. It was concluded that substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.     

Sintered silver finite element modelling and reliability based design optimisation in power electronic module

This paper discusses the design for reliability of a sintered silver structure in a power electronic module based on the computational approach that composed of high fidelity analysis, reduced order modelling, numerical risk analysis, and optimisation. The methodology was demonstrated on sintered silver interconnect sandwiched between silicon carbide chip and copper substrate in a power electronic module. In particular, sintered silver reliability due to thermal fatigue material degradation is one of the main concerns. Thermo-mechanical behaviour of the power module sintered silver joint structure is simulated by finite element analysis for cyclic temperature loading profile in order to capture the strain distribution. The discussion was on methods for approximate reduced order modelling based on interpolation techniques using Kriging and radial basis functions. The reduced order modelling approach uses prediction data for the thermo-mechanical behaviour. The fatigue lifetime of the sintered silver interconnect and the warpage of the interconnect layer was particular interest in this study. The reduced order models were used for the analysis of the effect of design uncertainties on the reliability of the sintered silver layer. To assess the effect of uncertain design data, a method for estimating the variation of reliability related metrics namely Latin Hypercube sampling was utilised. The product capability indices are evaluated from the distributions fitted to the histogram resulting from Latin Hypercube sampling technique. A reliability based design optimisation was demonstrated using Particle Swarm Optimisation algorithm for constraint optimisation task consists of optimising two different characteristic performance metrics such as the thermo-mechanical plastic strain accumulation per cycle on the sintered layer and the thermally induced warpage.     

倒装芯片焊点可靠性的有限元模拟法综述

随着集成电路封装技术的发展,倒装芯片技术得到广泛的应用。由于材料的热膨胀失配,使倒装焊点成为芯片封装中失效率最高的部位,而利用快捷又极具参考价值的有限元模拟法是研究焊点可靠性的重要手段之一。介绍了集成电路芯片焊点可靠性分析的有限元模拟法,概括了利用该方法对芯片焊点进行可靠性评价常见的材料性质和疲劳寿命预测模型。     

Reliability evaluations of under bump metallurgy in two soldersystems

Under bump metallurgy (UBM) reliability is one of the critical issues in the total reliability of a flip-chip bumping technology. Since the UBM materials and structures vary for different bumping technologies, the UBM strength and reliability need to be determined for each design and process. In addition, the stress that a UBM experiences during thermal cycles depends on the solder alloy used in the interconnect. Different solder alloys require different UBM structures and strengths to achieve good reliability in thermal cycling. In this study, a simplified stress model is developed to determine the UBM stress during thermal cycling. A simplified stress model for the UBM strength is also developed. These models are used to predict the stress and strength of the UBM under the die pull test and the thermal cycle conditions for both eutectic and high lead solder systems. A methodology for using the pull test results to evaluate UBM reliability is also discussed. This methodology can be extended to the studies of UBM's with other solder systems such as lead free solder systems     

Effect of substrate flexibility on solder joint reliability

Solder joint fatigue failure is a serious reliability concern in area array technologies, such as flip chip and ball grid array packages of integrated-circuit chips. The selection of different substrate materials could affect solder joint thermal fatigue lifetime significantly. The reliability of solder joint in flip chip assembly for both rigid and compliant substrates was evaluated by accelerated temperature cycling test. Experimental results strongly showed that the thermal fatigue lifetime of solder joints in flip chip on flex assembly was much improved over that in flip chip on rigid substrate assembly. Debonding area of solder joints in flip chip on rigid board and flip chip on flex assemblies were investigated, and it was found that flex substrate could slow down solder joint crack propagation rate. The mechanism of substrate flexibility on improving solder joint thermal fatigue was investigated by thermal mechanical analysis (TMA) technique. TMA results showed that flex substrate buckles or bends during temperature cycling and this phenomenon was discussed from the point of view of mechanics of the flip chip assembly during temperature cycling process. It was indicated that the thermal strain and stress in solder joints could be reduced by flex buckling or bending and flex substrates could dissipate energy that otherwise would be absorbed by solder joints. It was concluded that substrate flexibility has a great effect on solder joint reliability and the reliability improvement was attributed to flex buckling or bending during temperature cycling.     

Lifetime estimation of IGBT modules for MMC-HVDC application

Modular multilevel converters (MMCs) usually work in harsh operating environments due to their compact layouts and adverse mission profiles, which accelerate the thermomechanical fatigue process in insulated-gate bipolar transistor modules (IGBTs). Accurate lifetime estimation is desired to conduct reliability prediction and develop maintenance policies. This paper presents an analytical approach to estimating the lifetimes of IGBTs for MMC-HVDC application based on the thermal cycles, which are influenced by the transmission power profile and ambient temperature profile. The structure and operating principle of MMCs are studied to develop an analytical model for computing the IGBT power loss. A thermal equivalent network in the form of a Foster model is adopted to link the power losses and junction temperature. Next, an RC equivalent circuit analytical method for characterizing the fundamental-frequency thermal cycles, developed using electrothermal analogy theory, is proposed. The rainflow counting algorithm is applied to extract the low-frequency thermal cycles from the annual junction temperature data computed at every minute. The Bayerer model is employed to predict the IGBTs lifetime. Finally, the lifetime distribution, mission profiles and comparison of different IGBTs are analyzed via case studies.     

Fatigue life evaluation of wire bonds in LED packages using numerical analysis

Reliability of LED packages is evaluated using several tests. When a thermal shock test, which is one of the reliability tests, is conducted, the most common failure mode is wire neck breakage. In order to evaluate the wire bonding reliability of LED packages, performing the thermal shock test is time-consuming. In this paper the wire bonding reliability for LED packages is evaluated by using numerical analysis. A wire bonding lifetime model for the thermal shock test was developed, which is based on Coffin-Manson fatigue law. The model was calibrated from fatigue data of thermal shock tests and volume averaging accumulated plastic strains. The accumulated plastic strains were calculated by using finite element analysis corresponding to the test conditions. The test conditions were changed by silicones, package sizes, wire bonding diameters, heights, and lengths. The calibrated model was used to estimate the number cycle to failure so that the wire bonding reliability for the thermal shock test was evaluated by performing the numerical analysis. Furthermore, we used a response surface methodology to study the relationship between the wire loop and the accumulated plastic strain to determine the optimal wire loop. The plastic strain was a function of diameter, height and length. At the optimal point, the number of cycle to failure for the thermal shock test was suggested using the wire bonding lifetime model.