Improving the reliability of ball grid arrays under random vibration by optimization of module design

Abstract

In this work, three different arrangements of circuit board in an electronic device were designed and the effects of random vibration frequencies on the reliability of ball grid arrays (BGA) in these arrangements were evaluated. The failure criterion in solder balls was the root mean square of peeling stress exerted during the dynamical loadings. According to the finite element method (FEM) results, the uttermost stress concentration was generated at the interface of printed circuit board (PCB) and the solder balls. It was also revealed that the increase of input power spectral density (PSD) decreased the fatigue life of solder joint in all the arrangements. Considering the arrangements of circuit boards, it was found that the maximum domain of peeling stress had a minimum effect on the solder balls when the heat sinks were away from the packages. The microstructural characterization of critical zone in solder balls indicated that with the increase of maximum peeling stress, the crack initiated and propagated along the interface.     

Microstructural damage analysis of SnAgCu solder joints and an assessment on indentation procedures

In solder joint reliability, solder/pad adjoining interface is crucial, the quality of which is determined by the metallization. In this paper, microstructural analyses of SnAgCu alloy and soldered joints are conducted in direct connection with the metallization. Solder balls, solder paste and cast SnAgCu are reflowed on Cu, Ni/Au and Cu/Ni(V)/Au. Substrate influence on bulk and interfacial metallurgy is examined and a correlation between interfacial microstructure and the corresponding damage paths is established. Damage localizes at the bonding interfaces with a strong influence of intermetallic layers and primary crystals. Crack propagation is studied with Cu and Ni/Au substrates and the cracking mechanism in principal directions is scrutinized. In BGA production, different reflow parameters are investigated, and an optimum bumping procedure is established. Nano-indentation is used for the mechanical characterization of the solder alloy. An assessment on indentation parameters for soft solders is conducted and the influence of Ag content on material properties of SnAgCu is presented.     

A cyclic viscoplastic and creep damage model for lead free solder alloys

The reliability of microelectronic components under cyclic thermomechanical loading is an important problem especially for new leadfree solder alloys. To investigate the low cycle fatigue strength of solder joints, material models are required, that can describe the constitutive inelastic deformation and damage behavior of solder materials. Such models form the basis for advanced numerical analyses by the finite element method. In the present contribution an appropriate material model that combines the viscoplastic constitutive model of Chaboche-type with the damage law of A.C.F. Cocks for porous creep will be introduced. The algorithm is reported for an implementation as a user defined material subroutine into the FEM-code ABAQUS®. The necessary parameters of the material model are identified using results of miniaturized double lap-shear experiments and tensile tests for a Sn96Ag3Cu1 solder alloy at various temperatures. The comparison of experimental and numerical results shows a good agreement with respect to strain rate sensitivity, relaxation and damage behavior of the investigated solder material. Finally, some numerical applications to surface mounted microelectronic devices are presented.     

板级跌落冲击载荷下无铅焊点形状对BGA封装可靠性的影响

焊点高度和焊盘尺寸相同情况下,分析焊点形状（桶形、柱形、沙漏形）对BGA封装在板级跌落冲击载荷下可靠性的影响。根据不同焊点形状建立3种3D有限元模型,采用Input-G方法将加速度曲线作为数值模型的载荷输入,对BGA封装件在板级跌落冲击载荷下的可靠性进行分析。结果表明：在跌落冲击过程中,在0.1ms左右PCB板出现最大弯曲变形;焊点形状对BGA封装件在跌落冲击过程中的可靠性有较大的影响;以最大剥离应力作为失效准则对三种焊点进行寿命预测, 沙漏形焊点的平均碰撞寿命值最大,其次是柱形焊点,桶形焊点最小,表明沙漏形焊点在跌落测试中表现出较好的抗跌落碰撞性能。     

Computational modeling of material aging effects

Summary Progress is being made in our efforts to develop computational models for predicting material property changes in weapon components due to aging. The first version of a two-dimensional lattice code for modeling thermomechanical fatigue, such as has been observed in solder joints on electronic components removed from the stockpile, has been written and tested. The code does a good qualitative job of presenting intergranular and/or transgranular cracking in a polycrystalline material when undergoing thermomechanical deformation. Algorithms to couple codes at different length-scales were also exercised. The current progress is an encouraging start for our long-term effort to develop multi-level simulation capabilities for predicting age-related effects on the reliability of weapons.     

Crack growth calculations in solder joints based on microstructural phenomena with X-FEM

Determining the lifetime of solder joints subjected to thermomechanical loads is crucial to guarantee the quality of electronic devices. The fatigue process is heavily dependent on the microstructure of the joints. We present a new methodology to determine the lifetime of the joints based on microstructural phenomena. Random microstructures are generated to capture the statistical variety of possible microstructures and crack growth calculations are performed. The extended finite element method is used to solve the structural problem numerically which allows a complete automation of the process. Numerical examples are given and compared to experimental data.     

Reliability behavior of lead-free solder joints in electronic components

With more consumer products moving towards environmentally friendly packaging, making solder Pb-free has become an urgent task for electronics assemblies. Solder joints are responsible for both electrical and mechanical connections. Solder joint does not have adequate ductility to ensure the repeated relative displacements due to the mismatch between expansion coefficients of the chip carrier and the circuit board. Materials behavior of solder joints involves a creep–fatigue interaction, making it a poor material for mechanical connections. The reliability of solder joints of electronics components has been found playing a more important role in service for microelectronics components and micro-electro-mechanical systems. So many researchers in the world investigated reliability of solder joints based on finite element simulation and experiments about the electronics devices, such as CR, QFP, QFN, PLCC, BGA, CSP, FCBGA and CCGA, which were reviewed systematically and extensively. Synchronously the investigation on reliability of solder joints was improved further with the high-speed development of lead-free electronic packaging, especially the constitutive equations and the fatigue life prediction equations. In this paper, the application and research status of constitutive equations and fatigue life prediction equations were reviewed, which provide theoretic guide for the reliability of lead-free solder joints.     

Fatigue life estimations of solid-state drives with dummy solder balls under vibration

An investigation of the effect of dummy solder balls on the fatigue life of a solid-state drive (SSD) under vibration was studied. Finite element analysis and forced vibration experiments of an SSD were conducted to evaluate the fatigue life of an SSD. Global–local analysis techniques were adopted in the finite element analysis to calculate the stress of the solder balls in the SSD. In the vibration experiments, the SSD with dummy solder balls was excited via a sinusoidal sweep vibration around the first resonant frequency until the SSD failed. Using the stress results from global–local analysis and the cycle to failure results from the experiment, an S–N curve was derived for the SSD. This study showed that the solder joints at the corners of the controller package were the most vulnerable components, and that applying the dummy solder balls to those areas decreased the stress of the solder balls and increased the fatigue life of the SSD.     

Probabilistic modeling of fatigue crack growth in Ti–6Al–4V

This paper proposes an approximate approach to efficient estimation of some variabilities caused by the material microstructural inhomogeneities. The approach is based on the results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti–6Al–4V. A simplified experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests. The experimental studies suggest that the variabilities in long fatigue crack growth rate data and the Paris coefficient are well described by the log-normal distributions. The variabilities in the Paris exponent are also shown to be well characterized by a normal distribution. The measured statistical distributions are incorporated into a probabilistic fracture mechanics framework for the estimation of material reliability. The implications of the results are discussed for the probabilistic analysis of fatigue crack growth.     

The ‘IMC rings’ growth mechanism in Sn–Ag–Cu solder ball and NiAu-plated BGA ball pad

Due to environmental pollution concerns, the law says the lead (Pb) inside electronics devices must be eliminated. Lots of lead-free materials have been introduced and been used for electronic products and Sn–Ag–Cu (SAC) is one of most popular lead-free representatives and has been used in high-volume production. The most popular IC packages, BGA packages which have higher I/O counts, and better thermal and electrical performance than lead-frame type packages, use solder balls of SAC for lead-free applications to connect with printed circuit boards. A particular phenomenon, so-called ‘IMC rings’, is only observed on BGA solder ball pad surfaces after the SAC solder balls are mounted on BGA ball pads which are plated with NiAu. It has not been found in either eutectic solder or Sn–Ag solder welding on plated NiAu pads. No significant evidence exists to show that ‘IMC rings’ degrade the strength of solder joints or cause earlier failures in mechanical tests. ‘IMC rings’ appear to be an inevitable outcome after the SAC is soldered onto a plated NiAu ball pad. This study is to find the growth mechanism of ‘IMC rings’ on the ball pad which is created between SAC ball and plated NiAu pad during solder ball temperature reflow. The design of the experiment and data have been discussed.     

The coupling effects of thermal cycling and high current density on Sn58Bi solder joints

Currently, one of the serious challenges in microelectronic devices is the miniaturization trend of packaging. As the decrease of joint dimension, electromigration (EM) and thermomechanical fatigue become critical issues for fine pitch packaging. The independent mechanisms of EM and thermomechanical fatigue are widely investigated and understood. However, the coupling effect of both conditions needs further exploration. The current study established the correlation between resistance and microstructure evolution of solder joint under the combination effect of thermal cycling and high current density and illustrated the different contributions of these two factors to the reliability of the joint through the comparison monopoly tests. The results revealed that cracks had more impact on resistance increase than phase segregation. The resistance evolution could be divided into three stages. First, the resistance mitigated due to the phase coarsening. Second, Joule heating effect made the resistance increase slowly. Third, EM led to the resistance increase rapidly. The high current density can help to improve the reliability of the solder joint under the coupling effect of thermal cycling and EM at the initial stage, but harmful to the consequence process.     

Degradation of Sn37Pb and Sn3.5Ag0.5Cu solder joints between Au/Ni (P)/Cu pads stressed with moderate current density

Sn37Pb (SP) and Sn3.5Ag0.5Cu (SAC) ball grid array (BGA) solder joints between Au/Ni (P)/Cu pads were stressed with a moderate current density of 6.0 × 10² A/cm² at an ambient temperature of 125°C up to 600 h. The solder joint reliability was evaluated in terms of temperature measurement, microstructural analysis and mechanical strength test. It was confirmed that no obvious electromigration occurred with this moderate current density. However, the local temperature of solder joints rose considerably due to massive Joule heating, which degraded the solder joint reliability seriously. Phase coarsening was observed for both solders and it was particularly apparent in the SP solder joints. Compared to the SP, the SAC was found to be more reactive and hence a thicker intermetallic compound (IMC) was developed during the current stressing. Nevertheless, the IMC thickening was not as remarkable as expected with current stressing at high temperature. It exhibited a sub-parabolic growth manner that was mainly controlled by grain boundary diffusion. However, a sufficiently thick IMC layer initially formed during reflow soldering and the low diffusivity of the Ni atoms retarded the growth. The shear strength of the solder joints was found to decrease severely with the current stressing time. This degradation was attributed to the large stresses arising from localized thermal mismatch, phase coarsening, volume shrinkage of IMC evolution, Ni–P layer crystallization and the pad cracking during current stressing.     

Issues related to the implementation of Pb-free electronic solders in consumer electronics

Consumer electronic applications are the primary target of the Pb-free initiative and package assembly and performance is affected by the move from eutectic Sn–Pb to Pb-free solder alloys. This paper outlines the key issues and mitigation possibilities for package assembly using Pb-free solders: High temperature reflow, Interfacial reactions, and Reliability. At the high temperatures required to reflow Pb-free alloys, moisture absorbed into the package can result in delamination and failure. The reaction of the Pb-free solder with Ni and Cu metallizations results in interfacial intermetallics that are not significantly thicker than with Sn–Pb but provide a path for fracture under mechanical loading due to the increased strength of the Pb-free alloys. The reliability issues discussed include thermomechanical fatigue, mechanical shock, electromigration and whiskering. The Pb-free alloys tend to improve thermomechanical fatigue and electromigration performance but are detrimental to mechanical shock and whiskering. Design trade-offs must be made to successfully implement Pb-free alloys into consumer applications.     

底充胶及其材料模型对倒装焊热循环可靠性的影响

对倒装焊电子封装可靠性进行了热循环实验和有限元模拟,结果表明,有底充胶（underfill)时,SnPb焊点的热循环寿命可提高约16倍,并确定了Coffin-Manson半经验方程的参数,采用3种底充胶材料模型,亦即定常弹性模型,温度相关弹性模型和粘弹性材料模型,描述了底充胶U8347－3的力学性能。模拟结果表明,材料模型影响计算得到的SnPb焊点的塑性应范围,封装形变以及底充胶／芯片界面应力,采用弹性材料模型可能过高估计了SnPb焊点的热循环寿命和界面应力。     

硅通孔转接板封装结构多尺度问题的有限元模型

三维硅通孔转接板封装结构中,存在大量的微凸点与微焊球,尺寸相差3个数量级,这种结构多尺度给有限元分析模型的建立带来困难。以板级封装焊锡接点热疲劳寿命的有限元计算为目标,采用均匀化方法将芯片与转接板间的微凸点/下填料层以及转接板与基板间的微焊点/下填料层等效为均匀介质,以解决结构多尺度带来的网格划分困难。在对比分析了几种均匀化方案的基础上,建议在计算三维硅通孔转接板板级封装焊锡接点的热疲劳寿命时,芯片与转接板间的微凸点/下填料层以及转接板与基板间的微焊点/下填料层可采用各自的下填料层替代建模。     

Critical studies to improve service reliability of Sn–Ag–Cu solder joints by thermal treatments

Lead-free electronic packages intended for use in applications such as aerospace, military, and other highly demanding service conditions, necessitate exceptional mechanical reliability of lead-free electronic solder joints under realistic service conditions. Most current design strategies employed for improving the reliability of lead-free electronic solder joints are aimed at developing suitable alloying additions and reinforcements to the solder itself. At present there exists no suitable methodology to minimize the effects of service conditions while the solder joint is in service. Since thermomechanical fatigue reliability of electronic solder joints is closely related to the crack nucleation that occurs during very early stages of repeated thermal excursions, this study is based on subjecting solder joints to a limited number of thermal shock (TS) cycles in a chosen temperature regime to nucleate cracks, then evaluating their effectiveness in improving reliability when the solder joints are subjected to additional TS cycles in a different temperature regime. This study is a preliminary investigation, aimed at developing suitable methodology to minimize the effects of damage to lead-free solder joint specimens subjected to repeated thermal excursions during service, by imposing appropriate thermal treatments. These thermal treatments can be automatically implemented at programmed intervals during the service life of the electronic packages. Methods employed in these studies may also be useful to enhance long-term service reliability and to obtain a conservative estimate of long-term service reliability.     

微电子封装领域焊膏回流曲线的优化研究

对电子封装工业生产中无铅焊料的回流曲线工艺进行了研究,焊料的回流曲线影响界面处的金属间化合物的生成,通过引入加热因子开展研究,同时也进行了焊料的剪切强度实验用于评价封装的可靠性,研究结果表明:较厚的金属间化合物会降低焊球的剪切强度,而回流曲线的优化对焊料焊接的质量,可靠性有很强的实践意义.     

Packaging Parameters Analysis for the Fatigue Reliability of Stacked Chip Ball Grid Array by Using the Optimal Equivalent Solder Balls

A global/local method with the modified sub‐modeling approach of the two‐parametric optimal equivalent volume solder balls is introduced to predict the deformation and reliability of the package. The equivalent solder balls as exhibit in this method can obviously reduce the required elements/nodes quantities to enhance computing efficiency. A package model of wire‐bonded stacked chip ball grid array under cyclic thermal loading is used as a test vehicle to verify the influences of design factors by fatigue life indicator. Comparing the proposed method with the global fine mesh model, it is found that the difference in the accumulated strain energy density is merely 5.77%, but the optimal equivalent model has highly saved 90% finite element analysis required elements, which means the adopted method can effectively replace the global fine mesh model because both results are in accordance with each other. Using design of experiments to efficiently verify each factor influence with their cross‐coupling effects, this paper adopts two kinds of response surface methods that confirm the fatigue life of the proposed approach can be improved by as much as 123.7% for the dual response surface method and 126.3% for the mixed response surface method when comparing with the baseline model. In addition, the optimization of generic algorithm for both response surface methods is demonstrated in this study. From the reviews of factor coupling effects, it is concluded that the response surface method is eligible to achieve the optimum design for package reliability improving. Copyright © 2013 John Wiley & Sons, Ltd.     

Fatigue failure of pb-free electronic packages under random vibration loads

The electronic equipment are used in several fields like, automotive, aerospace, consumer goods where they are subjected to vibration loads leading to failure of solder joints used in these equipment. This paper presents a methodology to predict the fatigue life of Pb-free surface mounted BGA packages subjected to random vibrations. The dynamic characteristics of the PCB, such as the natural frequencies, mode shapes and damping ratios were determined. Spectrum analysis was used to determine the stress response of the critical solder joint and the cumulative fatigue damage accumulated by the solder joint for a specific duration was determined.     

Microstructure history effect during sequential thermomechanical processing

The key to modeling the material processing behavior is the linking of the microstructure evolution to its processing history. This paper quantifies various microstructural features of an aluminum automotive alloy that undergoes sequential thermomechanical processing which is comprised hot rolling of a 150-mm billet to a 75-mm billet, rolling to 3 mm, annealing, and then cold rolling to a 0.8-mm thickness sheet. The microstructural content was characterized by means of electron backscatter diffraction, scanning electron microscopy, and transmission electron microscopy. The results clearly demonstrate the evolution of precipitate morphologies, dislocation structures, and grain orientation distributions. These data can be used to improve material models that claim to capture the history effects of the processing materials.