Four-point bending cycling: The alternative for thermal cycling solder fatigue testing of electronic components

This paper deals with an alternative testing approach for quantifying the life time of board level solder joint reliability of components. This approach consists of applying a relative shear displacement between component and Printed Circuit Board (PCB) through cyclic board bending. During the cycling, the temperature is kept constant, preferably at elevated temperature in order to accelerate the creep deformation of the solder joint. This is done in a four-point bending setup which allows to apply an equal loading on all components lying between the inner bars. The scope of the paper is, firstly, to evaluate if the four point bending testing generates the same fatigue fracture as in thermal cycling; secondly, that the measured life times can be also predicted through finite element simulations; and thirdly if the technique can finally accelerate the cycling frequency to reduce the testing time.     

Evaluation of analytical models for thermal analysis and design of electronic packages

The objective of this study is to evaluate the use of several analytical compact heat transfer models for thermal design, optimization, and performance evaluation in electronic packaging. A model for heat spreading in orthotropic materials is developed. The developed model is used in conjunction with the other available heat transfer models in a resistance network for calculation of heat transfer rate and junction temperatures in a multi-chip module (MCM). Refrigeration cooled MCM of an IBM server is used to illustrate the methodology. Results of the analytical model and resistance network analysis are compared with a numerical solution. Capability of the analytical model in predicting the thermal field is discussed and effectiveness of using the analytical models in thermal design and optimization of electronic packages is demonstrated.     

Effect of glue on reliability of flip chip BGA packages under thermal cycling

Glue is widely used to improve the reliability of ball grid array (BGA) under mechanical shock and vibration. Although it has been demonstrated to have a positive effect on the reliability of BGA under mechanical impact, it can have adverse effects on BGA under thermal cycling. This paper investigates the effect of glue on the reliability of BGA under thermal cycling using both experimental and numerical methods. The digital image correlation (DIC) technique was used to obtain the thermal mechanical behavior of the package. The experimental results explain in detail how the glue negatively affects the reliability of the BGA. Furthermore, a finite element analysis was performed and its results were verified with experimental results. A numerical parametric study was carried out on various mechanical properties, configurations of the glue, and introduction of a stiffener using the validated FEM model. The results show that the reliability of BGA strongly depends on geometries and material properties of the glue. Based on the results, a guideline of glue selection for BGA reliability under thermal cycling is formulated.     

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.     

Advancement in simulating moisture diffusion in electronic packages under dynamic thermal loading conditions

The normalization approach has been accepted as a routine to overcome the concentration discontinuity at multi-material interfaces in the moisture diffusion simulation by the finite element method. However, applying the normalization approach directly in the commercial finite element software under a dynamic thermal loading condition, such as reflow process, may lead to erroneous results. Special techniques are needed to obtain correct results. In this study, different approaches that can deal with the moisture diffusion under dynamic thermal loading conditions were reviewed and compared with case studies. Advantages and disadvantages of each approach were analyzed and discussed. Theoretical derivation was developed to show the direct concentration approach (DCA) violates the law of mass conservation. The effect of Fourier number on the accuracy of the DCA was also examined. The internal source approach was shown to be a universal method in modeling the moisture diffusion in a multi-material system under dynamic thermal loading conditions. The newest version of ANSYS also showed good performance in solving the diffusion problem under transient thermal loadings if a proper time step size was used.     

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.     

改进的多目标粒子群算法优化设计及应用

针对粒子群算法存在易陷入局部最优点的缺点,提出了一种改进的带变异算子的多目标粒子群优化算法。采用非支配排序策略和动态加权法选择最优粒子,引导种群飞行,提高帕累托(Pareto)最优解的多样性。与其他优化算法相比,该算法易于实现并且计算速度更快。通过计算Pareto前沿最优解设计最佳多层电磁吸收体,在吸收体的厚度与反射系数之间取得最佳折衷。通过对反射系数函数与吸收体厚度函数测试验证,该算法能够在保持优化解多样性的同时具有较好的收敛性。     

Mechanical and electrical characterisation of Au wire interconnects in electronic packages under the combined vibration and thermal testing conditions

This paper concerns the reliability of thermosonically bonded 25 μm Au wires in the combined high temperature with vibration conditions, under which the tests have been carried out on wire-bonded 48-pin Dual-in-Line (DIL) High Temperature Co-fired Ceramic (HTCC) electronic packages. Mechanical, optical and electrical analysis has been undertaken in order to identify the failure mechanisms of bonded wires due to the combined testing. The results indicated a decrease in the electrical resistance after a few hours of testing as a result of the annealing process of the Au wire during testing. In general, ball shear and wire pull strength levels remained high after testing, showing no significant deterioration due to the tests under the combined high temperature and vibration conditions. However, a trend of the variation in the strength values is identified with respect to the combined conditions for all wire-bonded packages, which may be summarised as: (i) increase of the testing temperature has led to a decrease of both the shear and pull strength of the wire bonds; (ii) the mechanical behaviour of the wires is affected due to crystallisation that leads to material softening and consequently the deformation of wire.     

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.     

A modular approach for simulation-based optimization of MEMS

The importance of MEMS optimization concerning performance, power consumption, and reliability is increasing. A variety of specialized tools is available in the MEMS design flow. FEM tools (ANSYS, CFD-ACE+) are widely used on component level. System level simulations are carried out using simplified models and simulators like Saber or ELDO. There is a lack of simulator-independent optimization support. Only a few simulators offer optimization capabilities.Our approach aims at a flexible combination of simulators and optimization algorithms by partitioning the optimization cycle into simulation, error calculation, optimization and model generation. This new method is translated into a modular optimization system named Moscito implemented in Java. Several optimization algorithms are available: methods without derivatives, methods using derivatives and stochastic approaches. Interfaces to simulators like ANSYS, Saber, MATLAB are implemented. Thus the optimization problem can be handled on different levels of model abstraction (FEM, ordinary differential equations, generalized networks, block diagrams, etc.). A graphical user interface supports control and visualization of the optimization. The modules of the optimization system are able to communicate via the Internet to allow a distributed, web-based optimization.     

Structural design optimization for board-level drop reliability of wafer-level chip-scale packages

In this paper, the Taguchi optimization method is applied to obtain the optimal design in enhancing board-level drop reliability of a wafer-level chip-scale package (WLCSP) under JEDEC drop test condition B, which features a half-sine impact acceleration pulse with a peak acceleration of 1500 G and a pulse duration of 0.5 ms. An L₉ (3⁴) orthogonal array is arranged for the optimization of four control factors that involve compositions of solder alloys and thickness of die and polyimide passivation layers. The submodeling technique capable of dealing with path-dependent features, including elastoplastic responses of solder joints and structural nonlinearity under drop impacts, is applied so that delicate structures of passivation, under bump metallurgy (UBM), and redistribution line (RDL) in a WLCSP package can be taken into account. Effects of these control factors on the drop reliability of WLCSP are compared and ranked.     

Measurement matrix design for CS-MIMO radar using multi-objective optimization

In this paper, we design a measurement matrix for a compressive sensing-multiple-input multiple-output radar in the presence of clutter and interference. To optimize the measurement matrix, three main criteria are considered simultaneously to improve detection and sparse recovery performance while suppressing clutter and interference. To this end, we consider three well-known criteria including Bhattacharyya distance, mutual coherency of sensing matrix, and signal-to-clutter-plus-interference ratio. Due to the use of simultaneous multi-objective functions, a multi-objective optimization (MOO) framework is exploited. Some numerical examples are provided to illustrate the achieved improvement of our proposed method in target detection and sparse recovery performance. Simulation results show that the proposed MOO technique for measurement matrix design can achieve superior performance in target detection compared with Gaussian random measurement matrix technique.     

Cyclic bending reliability of wafer-level chip-scale packages

In this paper, both experimental and numerical studies are conducted to investigate board-level reliability of wafer-level chip-scale packages under four-point cyclic bending conditions that combine different deflection amplitudes and excitation frequencies. In addition to the fatigue lives of the test vehicle, locations and modes of fractured solder joints are observed. In the numerical modeling, inertia forces along with rate-dependent material properties of the solder joints are considered in order to capture frequency-dependent characteristics of this particular test methodology. Through the dynamic finite element analysis, plastic strain energy densities accumulated per bending cycle within the critical solder joint are calculated and together with the experimental results, parameters for the Morrow fatigue model are calibrated.     

基于GA的系统冗余可靠性设计多目标优化模型

系统冗余可靠性多目标设计是一个复合最优化问题。传统的用于解决此类问题的方法,如拉格朗日乘子法、动态规划法、直接寻查法等,在系统单元较多的情况下存在着计算量大、难以获取全局最优解等问题。针对此问题,本文建立了基于遗传算法（GA）的系统冗余可靠性设计的多目标优化模型,该模型可在系统可靠性约束条件下,使系统的成本费用、体积、重量等指标达到最优设计。     

Growth of tin whiskers for lead-free plated leadframe packages in high humid environments and during thermal cycling

Growth behavior of tin whiskers from pure tin and tin–bismuth plated leadframe (LF) packages for elevated temperature and high humidity storages and during thermal cycling was observed. In the storage at 60 °C/93% relative humidity (RH) and 85 °C/85%RH the galvanic corrosion occurred at the outer lead toes and shoulders where the base LF material is exposed, forming tin oxide layers of SnO₂. The corroded layers spread inside the film and formed whiskers around the corroded islands. Many whiskers were observed to grow from grain boundaries for the Fe–42Ni alloy (alloy42) LF packages. It was confirmed that the corrosion tends to occur on the side surfaces of outer leads adjacent to the mold flash. The contribution of ionic contaminants in epoxy mold compound (EMC) to the corrosion was not identified. During thermal cycling between −65 °C and +150 °C whiskers grew out of as-deposited grains for pure tin-plated alloy42 LF packages and they grew linearly with an increase of number of cycle. Growth mechanisms of the whiskers from grain boundaries and as-deposited grains were discussed from the deformation mechanism map for tin and mathematical calculation with a steady-state diffusion model.     

Growth of tin whiskers for lead-free plated leadframe packages in high humid environments and during thermal cycling

Growth behavior of tin whiskers from pure tin and tin-bismuth plated leadframe (LF) packages for elevated temperature and high humidity storages and during thermal cycling was observed. In the storage at 60 °C/93% relative humidity (RH) and 85 °C/85%RH the galvanic corrosion occurred at the outer lead toes and shoulders where the base LF material is exposed, forming tin oxide layers of SnO₂. The corroded layers spread inside the film and formed whiskers around the corroded islands. Many whiskers were observed to grow from grain boundaries for the Fe–42Ni alloy (alloy42) LF packages. It was confirmed that the corrosion tends to occur on the side surfaces of outer leads adjacent to the mold flash. The contribution of ionic contaminants in epoxy mold compound (EMC) to the corrosion was not identified. During thermal cycling between −65 °C and +150 °C whiskers grew out of as-deposited grains for pure tin plated alloy42 LF packages and they grew linearly with an increase of number of cycle. Growth mechanisms of the whiskers from grain boundaries and as-deposited grains were discussed from the deformation mechanism map for tin and mathematical calculation with a steady-state diffusion model.     

A review on thermal cycling and drop impact reliability of SAC solder joint in portable electronic products

Currently, the portable electronic products trend to high speed, light weight, miniaturization and multifunctionality. In that field, solder joint reliability in term of both drop impact and thermal cycling loading conditions is a great concern for portable electronic products. The transition to lead-free solder happened to coincide with a dramatic increase in portable electronic products. Sn–Ag–Cu (SAC) is now recognized as the standard lead free solder alloy for packaging interconnects in the electronics industry. The present study reviews the reliability of different Ag-content SAC solder joints in term of both thermal cycling and drop impact from the viewpoints of bulk alloy microstructure and tensile properties. The finding of the study indicates that the best SAC composition for drop impact performance is not necessarily the best composition for optimum thermal cycling reliability. The level of Ag-content in SAC solder alloy can be an advantage or a disadvantage depending on the application, package and reliability requirements. As a result, most component assemblers are using at least two (and in many cases even more) lead-free solder sphere alloys to meet various package requirements.