Improving the reliability of the plastic IC package in reflow soldering process by optimization

In order to reduce stress in the plastic small outline J-lead (SOJ) package and prevent the interface delamination under the infrared (IR) soldering process, parameterization, parametric study and optimization are used. The design variables of dimensions and material properties are determined among all the possible variables from the parametric study. Their optimized values are determined by applying the constraint optimization to the parameterized integrated circuit (IC) package.     

塑封器件回流焊与分层的研究

由于无铅焊料的应用,回流焊的温度提高影响了塑封器件的质量和可靠性。针对实际的LQFP器件,利用有限元软件建立三维模型,分析了塑封器件在潮湿环境中的湿气扩散及回流焊中的形变和热应力分布,并讨论了塑封料参数及细小裂纹对分层的影响。结果表明,在湿热的加载下,塑封器件的顶角易发生翘曲现象;芯片与塑封料界面处易分层,导致器件失效。     

Optimization modeling of the cooling stage of reflow soldering process for ball grid array package using the gray-based Taguchi method

The current paper introduces an effective numerical technique for optimization of the cooling stage of reflow soldering process. The study aims to optimize the thermal stress of a solder joint and cooling rate of a ball grid array package using the gray-based Taguchi method. The current methodology comprised numerical techniques for computational fluid modeling of the internal flow in the reflow oven coupled with the structural heat modeling at the board and package levels. The Multi-physics Code Coupling Interface (MpCCI) was used as the coupling software. Various parameters, such as printed circuit board density, cooling temperature, inlet velocity, and conveyor speed, were considered. Nine simulated experiment runs based on Taguchi L₉ orthogonal arrays were performed. The optimal parameter settings for multiple performance characteristics were determined and compared with the single performance characteristic. From the analysis of variance (ANOVA) result, the most influential factor for the multiple performance characteristics was found to be the inlet velocity. Simulated experiment results showed that the performance characteristics during the cooling stage in the reflow oven obviously improved. The new approach greatly helps in reducing soldering defects and enhances solutions to lead-free reliability issues.     

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material suppliers. The moisture absorption during preconditioning at JEDEC Level 1, and moisture desorption at various high temperatures are characterized. The moisture diffusivity is a few orders higher at reflow temperature than moisture preconditioning temperature. Due to coefficient of moisture expansion mismatch among various materials, hygro-mechanical stress is induced. The concept is analogous to coefficient of thermal expansion mismatch which results in thermo-mechanical stress. Thermal diffusivity is much faster than the moisture diffusivity. During reflow, the internal package reaches uniform temperature within a few seconds. The vapor pressure can be calculated based on the local moisture concentration after preconditioning. Results show that the vapor pressure saturates much faster than the moisture diffusion, and a near uniform vapor pressure is reached in the package. The vapor pressure introduces additional strain of the same order as the thermal strain and hygrostrain to the package. Subsequently, the interfacial fracture mechanics model is applied to study the effect of crack length on die/mold compound and die/die attach delamination.     

回焊炉内氧气体积分数对焊接可靠性的影响

SMT(表面贴装技术)回焊炉工作时,炉中氧气体积分数的差异很大,从100×10–6至10000×10–6不等,在已验证氧气体积分数[(O2)]超过4000×10–6会造成产品不良的前提下,针对500×10–6,1000×10–6,3000×10–6以及4000×10–6这四种不同的氧气体积分数,分别对回焊炉焊接的铜片上的锡焊点进行润湿角、EDS分析,对组装印制电路板(PCBA)上的锡焊点进行X射线、推力、通孔填充量等测试。结果显示,(O2)在4000×10–6以下,元件的焊接可靠性并无明显差异。选择(O2)=4000×10–6可降低回焊炉的氮气用量,节约成本。     

Properties of molding compounds to improve package reliability ofSMDs

Package cracking during reflow soldering process is the great problem in the reliability of plastic packages. The technique of lowering the glass transition temperature (Tg) of a molding compound is very effective for improvement of the package cracking resistance because of the properties of low moisture absorption and high adhesion strength for a molding compound. But the package reliability except for the package cracking resistance is also important. In this study, the effects of the Tg for molding compounds on the package reliability was discussed. It was confirmed that decreasing the crosslinking density was an important factor to improve the package cracking resistance. There was no problem in thermal resistance, even if the molding compound has low Tg. However, decreasing the crosslinking density by the lowering of Tg may not satisfy humidity resistance in some cases. It was found to be important to decrease the crosslinking density of molding compounds without lowering Tg in order to improve both the package cracking resistance and the humidity resistance. It was also confirmed that the introduction of rigid structural segments into the matrix resin molecules of the molding compound was a useful technique for achieving excellent package reliability     

Effect of the plasma cleaning process on plastic ball grid array package assembly reliability

Interface delamination is recognized as one of the major failures of microelectronics packaging. It can result from various factors, including stresses from mismatch of adherent materials, hygrothermal stress from the release of vapor pressure of moisture during soldering reflow process, and interface material adhesion strength. The failure mechanisms are associated with cyclic loads, temperature and moisture condition as well as interface adhesion strength degradation. This paper focuses on the evaluation of plasma cleaning on PBGA assembly, including resistance to interface delamination. Two different plasma systems, powered by radio frequency (RF) and microwave (MW) energy, are studied. The optimized plasma cleaning process parameters are obtained by surface contact angle measurements. The plasma cleaning results are also verified by scanning electron microscopy (SEM) as well as physical pull and shear tests. The test vehicles are 27/spl times/27 mm 292-ball PBGAs. The results from encapsulation peel tests, die and encapsulant pull tests, bonding wire pull tests and C-Mode SAM (C-SAM) examination are presented. It is clear that an optimal plasma cleaning process can be achieved with different plasma systems. The experimental results also demonstrate that plasma cleaning has little effect on wire bonding process and die attach pull strength for given substrates and assembly materials. In all the cases, optimal plasma cleaning steps improve PBGA resistance against interface delaminations for cases where plasma cleaning is carried out before encapsulation process. Moreover, different plasma cleaning techniques would affect the assembly productivity, investment and yield. This paper demonstrates that the optimized plasma cleaning process would enhance PBGA package qualification level and improve the process yields and productivity.     

How to improve intrinsic and extrinsic reliability of vias by process optimization

A systematic study of various processes and their impact on intrinsic reliability has been performed on Cu dual damascene interconnects. The most significant improvement for intrinsic reliability is the ‘break-through’ liner. A strong impact on stressmigration (SM) was revealed using a HDP based SiN deposition on top of Cu lines. Early failures in electromigration (EM) studies are present with insufficient cleaning processes. No reliability impact was detected with different plating and slurry chemistries and liner thickness increase. Extrinsic via reliability is assessed with a special test chip comprising 3E9 via/wafer. High ohmic vias are identified before and after thermal stress. As an example, the failure rates in Cu dual damascene levels with relaxed pitch before and after cleaning optimization are discussed.     

PBGA封装热可靠性分析

对PBGA封装体建立了有限元数值模拟分析模型。模型采用无铅焊点,完全焊点阵列形式。研究了封装体在经历IPC9701标准下的五种不同温度循环加载后,受到的热应力、应变,以及可能的失效形式。结果表明,焊点是封装体结构失效的关键环节,焊点所受应力大小与焊点位置有关。比较了不同温度循环下封装体的疲劳寿命。其结果为提高封装体的可靠性和优化设计提供了理论依据。     

Applications of fracture mechanics to quantitative accelerated life testing of plastic encapsulated microelectronics

Accelerated testing must address the failure mechanisms active within the devices undergoing tests in order to assess lifetimes in a meaningful way. The assumption of constant temperature, thermally activated lifetime, based upon the Arrhenius assumptions, does not always provide the necessary understanding to interpret accelerated tests in microelectronics. Plastic encapsulants, dielectric polymers, and underfill materials are subject to delamination and cracking with thermal cycling. Crack propagation during use environment exposure, drives the potential for failure of microelectronic devices and is therefore a necessary focal point in qualification and life testing. This paper reviews the available research in the application of fracture mechanics to this class of problems in microelectronics including relevant test data. In addition, useful acceleration factor models are derived for polymer crack propagation based on principles of linear elastic fracture mechanics. Further, a simple approach to estimating the minimum temperature cycling ranges, necessary to propagate a crack, is also presented. Finally, a methodology of applying acceleration factors to develop testing plans is shown, with an example in spaceflight for a cubesat in low Earth orbit. Overall, this is a paper that shows a useful and appropriate process for creating physics of failure based life testing for delamination and cracking failures in microelectronic polymers in a temperature cycling environment.     

In-situ observation and simulation of the solidification process in soldering a small outline package with the Sn-Ag-Cu lead-free alloy

The formation process of solidification defects in solder fillet of circuit boards with a small outline package (SOP) soldered with Sn-3wt.%Ag-0.5wt.%Cu has been examined primarily by using an in-situ observation system and solidification simulation software. The lead frames of the SOP are two kinds of materials: Cu and Fe-42wt.%Ni (42 alloy). Microstructural observations were made of the SOP solder joints using metallographic cross sections. The β-Sn dendrite structure in the solder fillet of a SOP with 42-alloy lead frames is larger than that of a Cu lead frame. This is attributed to the slower cooling speed of the 42-alloy lead-frame SOP than that of the Cu lead-frame SOP. The solidification of a SOP joint is not uniform and locally time dependent. The solder surface of the slowly cooled region of the 42-alloy lead-frame SOP exhibited shrinkage voids and a rough surface because of the coarsening of the dendrite structure. According to the simulation of the solidification process, the relationship between the solidification process and formation characteristics of solidification defects for a SOP joint can be clarified.     

无铅回流焊焊点形成过程的实时监测与分析

利用实时摄像、图像采集、转录装置等对电容、电阻、塑封有引线芯片载体(PLCC)、二极管等元器件的无铅回流焊过程进行了实时监测和同步拍摄,实时记录了回流焊过程中焊点形成的全过程。特别地,分析了回流焊过程中焊膏对元件所表现出的极强的自对中能力及一些焊接缺陷(如立碑现象)的形成过程。     

Computational parametric analyzes on the solder joint reliability of bottom leaded plastic (BLP) package

The bottom-leaded plastic (BLP) package is a lead-on-chip type of chip scale package (CSP) developed mainly for memory devices. Because the BLP package is one of the smallest plastic packages available, solder joint reliability becomes a critical issue. In this study, a 28-pin BLP package is modeled to investigate the effects of molding compound and leadframe material properties, the thickness of printed circuit board (PCB), the shape of solder joint and the solder pad size on the board level solder joint reliability. A viscoplastic constitutive relation is adopted for the modeling of solder in order to account for its time and temperature dependence on thermal cycling. A three-dimensional nonlinear finite element analysis based on the above constitutive relation is conducted to model the response of a BLP assembly subjected to thermal cycling. The fatigue life of the solder joint is estimated by the modified Coffin-Manson equation. The two coefficients in the modified Coffin-Manson equation are also determined. Parametric studies are performed to investigate the dependence of solder joint fatigue life on various design factors.     

Recasting modified nodal analysis to improve reliability in numerical circuit Simulation

Modified nodal analysis (MNA) can be considered as the most adopted method in circuit simulation programs. Once node equations were written and complemented with those from the current controlled branches, a nonlinear system of algebraic and/or differential equations is obtained. In general, by using linear multistep integration methods, differential equations are recast as nonlinear algebraic ones, which are solved through the Newton method at each integration time step. While computer precision is not an issue in most situations, some specific but frequent cases yield ill-conditioned or singular matrices even in originally well posed circuits. This drawback can occur in different situations namely in presence of strongly nonlinear elements and/or when very small integration time steps are used. In the second case, very large conductances in the companion model of capacitors can introduce roundoff errors. In this paper, a transformation of the MNA that overcomes these problems is proposed. It is based on a suitable recombination of equations and electrical variables of the conventional MNA and it can be easily implemented in a circuit simulator.     

SCSP中芯片黏结层在焊接时的可靠性

提出了一个细观力学模型,该模型同时考虑了热膨胀和蒸汽膨胀对叠层芯片尺寸封装(SCSP)中芯片黏结层变形的影响.当初始温度确定时,由该模型可求得给定温度下芯片黏结层内部的蒸汽压力和孔隙率,从而判断芯片黏结层在焊接回流时的可靠性.当温度从100℃升高到250℃时,芯片黏结层的饱和蒸汽压、等效弹性模量及孔隙率分别从0.10 ...     

Development of a solder material process to relieve the plastic constraint associated with thin joints

High aspect ratio (large diameter/thickness) solder joints which are plastically constrained develop large hydrostatic stresses (Friction Hill) greatly in excess of their yield strength. Because the local high triaxial stresses arising from the Friction Hill prevent homogeneous yielding and, in a strain controlled system, will localize plastic deformation within the regions near free surfaces, abrupt brittle fracture through an intermetallic or along an interface can occur. In such situations, the service life of the joint during fatigue situations such as thermal cycling will be greatly reduced. The prevention of triaxial stress build up within such a strain controlled environment which can occur in, for example, leadless chip carrier solder joints requires a distribution of internal free surfaces within the joint. The solder system developed in this study is a thin porous metal film with a regular distribution of pores. The solder material is formed from the usual components, tin and lead. Small lead or tin particles are coated with a thin film of the other component, mixed with flux paste, and the temperature is raised to just above the eutectic temperature. Solid state diffusion occurs across the lead-tin interface until its composition reaches the melting point. The particles then are interconnected by a thin near eutectic liquid film. Additional metal from the solid particle dissolves into the liquid increasing its position and, thus its melting point. Diffusion into the liquid continues until it solidifies isothermally. This forms an interconnecting network of solder “mini-elements” with a dense pore structure.     

塑封器件的贮存环境与使用可靠性

介绍了用潮热环境试验模拟塑封器件在长期贮存时对水汽的吸附以及用潮热试验和焊接热的综合影响来评价塑封器件耐潮湿性能的方法.     

孔洞对BGA封装焊点热疲劳可靠性影响的概率分析

采用有限元分析、蒙特卡罗模拟和概率分析相结合的方法,研究了BGA(Ball Grid Array)封装焊点内部孔洞对焊点的热疲劳可靠性的影响规律。先用X-Ray检测仪对BGA封装进行无损检测,获得焊点内部孔洞尺寸及其分布规律,然后通过有限元软件建立BGA封装模型进行计算分析,针对危险焊点进行参数化建模,建立了含孔洞尺寸及位置呈随机分布的焊点有限元分析子模型。通过后处理获取塑性应变能密度作为响应值,构造随机孔洞参数与塑性应变能密度的代理模型,并运用蒙特卡罗随机模拟方法,研究了孔洞对焊点热疲劳可靠性的影响规律。结果表明,除了位于焊点顶部区域的小孔洞以外,大部分孔洞的出现都会提高焊点的热疲劳可靠性。     

提高电气自动化控制设备可靠性的方法

本文简单阐述电气自动化控制设备可靠性的重要性,深入分析如何测试电气自动化控制设备可靠性以及对其产生影响的各方面因素,最终总结出提高电气自动化控制设备可靠性的方法.