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。     

QFN封装分层失效与湿-热仿真分析

采用湿度敏感度评价试验及湿-热仿真方法,分析了温湿度对于QFN封装分层失效的影响.通过C-SAM和SEM等观察发现,QFN存在多种分层形式,分层大多发生在封装内部材料的界面上,包括封装塑封材料和芯片之间的界面、塑封材料和框架之间的界面等.此外,在封装断面研磨的SEM图像上发现芯片粘结剂内部有空洞出现.利用有限元数值模拟的方法,对QFN封装的内部湿气扩散、回流过程中的热应力分布等进行了模拟,分析QFN分层失效的形成原因.结果表明,由于塑封器件材料、芯片、框架间CTE失配,器件在高温状态湿气扩散形成高气压条件下易产生分层.最后提出了改善QFN分层失效的措施.     

粘片胶固化对塑封集成电路可靠性的影响

当具有诸如分层等可靠性缺陷的微电子器件焊接在线路板上,通过回流焊时会产生塑封体裂缝、塑封体鼓胀等重要缺陷。粘片胶未充分固化、水汽未完全排除、环境湿气较大易吸湿等原因导致水汽沿着塑封体与引线引脚向内部扩散。表现为各结合面的分层,粘片胶与芯片之间、塑封体与引线之间、塑封体与芯片之间,各种分层在快速加热产生的热应力下水汽快速膨胀,从而引起器件可靠性隐患。     

热膨胀系数不匹配导致的塑封器件失效

塑封器件使用过程中由于塑封材料和芯片之间热膨胀系数的不匹配,导致在外界温度变化时的应力释放对芯片造成损伤。文中通过VLSI失效分析,对这种应力造成的芯片损伤进行了研究,并提出利用环境应力试验和可靠性分析的方法暴露热膨胀系数不匹配导致芯片损伤的技术。     

苛刻环境下多芯片FC-PBGA器件板级温循可靠性研究

研究了宇航用多芯片扁出型倒装芯片球栅（FC-PBGA）器件在温度循环下的板级可靠性。通过宏观形貌观察、微观结构演变等手段对温循后的器件进行了分析,并利用有限元仿真对器件的疲劳寿命进行了评估。研究结果表明,经温度循环后FC-PBGA器件表面三防漆出现起皮现象,但封装组件未出现失效故障。器件中倒装芯片的布局对温度循环后危险焊点的位置具有显著的影响。危险焊点在温度循环后产生了明显的塑性变形,在靠近芯片端的界面处形成了显微裂纹。通过有限元仿真结合修正的Coffin-Manson方程计算得到了组件的温度循环疲劳寿命约为1 002次,具有较好的可靠性,为塑封器件的高可靠性应用提供了参考。     

大功率LED封装的温度场和热应力分布的分析

针对典型的大功率发光二极管(LED)塑封器件,建立了3D模型,对器件内部从初始到正常工作状态过程中的温度分布和热应力分布进行了有限元分析模拟,并测试了实际器件表面特征点的温度变化.瞬态热分析的结果与实际的器件表面温度实测数据吻合较好.考虑到不同散热条件的影响,给出了该大功率LED塑封器件中热应力集中区域和该类器件适当的工作范围.     

表面安装塑封体吸湿性引起的开裂问题及其对策

随着SMT 封装技术的发展,表面安装型塑封体由于吸湿性引起的开裂问题越来越突出。本文主要对表面安装型塑封体(如SOP、PLCC、PQFP、PBGA 等)在回流焊时吸湿开裂机理以及相应对策等进行讨论分析。     

无铅倒装焊点的热应力与湿热应力分析

板上倒装芯片（FCOB）作为一种微电子封装结构形式得到了广泛的应用。微电子塑封器件中常用的聚合物因易于吸收周围环境中的湿气而对封装本身的可靠性带来很大影响。文章采用有限元软件分析了潮湿环境下板上倒装芯片下填充料在湿敏感元件实验标准MSL-1条件下（85℃／85％RH、168h）的潮湿扩散分布,进而分别模拟计算出无铅焊点的热应力与湿热应力,并加以分析比较。论文的研究成果不仅对于塑封电子元器件在潮湿环境中的使用具有一定的指导意义,而且对于FCOB器件在实际应用中的焊点可靠性问题具有一定的参考价值。     

声学扫描显微镜检查在塑封器件检测中的应用

随着塑封器件在武器系统中的使用越来越广泛,塑封器件在使用中也暴露出了一些问题,如塑封器件易打磨、翻新,内部易进入水汽产生爆米花效应或内部界面分层等。作者总结近几年塑封器件DPA试验中出现的各种失效,重点对塑封器件内部界面分层以及分层产生的原因、危害进行了论述。同时,论述了声学扫描显微镜检查对内部界面分层的辨别、原理及其相关试验标准等,提出了塑封器件在型号产品中的使用建议。     

芯片涂胶的塑封器件超声检测工艺研究

基于超声检测原理,提出一套针对芯片涂胶的塑封器件超声检测工艺,防止将涂覆胶误判为分层缺陷,并通过理论分析与试验结果进行检测工艺适用性的验证,为利用超声检测手段检测和评价芯片涂胶的塑封器件内部界面分层提供判定准则。     

Comprehensive treatment of moisture induced failure-recent advances

This paper describes a comprehensive treatment of moisture induced failure in integrated circuit (IC) packaging with emphasis on recent advances. This includes advanced technique for modeling moisture diffusion under dynamic boundary conditions such as experienced by packages during solder reflow, autoclave, and temperature-humidity cycling; advanced characterization technique for moisture sorption and diffusion properties of packaging materials including effect of edge diffusion on transverse diffusivity, anisotropic diffusivity in organic laminates, impact of non-Fickian sorption; advanced techniques for modeling vapor pressure during solder reflow; advanced techniques for modeling dynamic delamination propagation during solder reflow; interfacial fracture strength as a function of temperature and moisture; as well as plastic analysis of popcorn cracking.     

PBGA器件潮湿扩散和湿热应力的有限元分析

塑料封装器件暴露在一定的潮湿环境下将会吸收潮湿的现象已经得到广泛的认同。针对实际的PBGA器件,采用通用有限元软件分析和计算了器件在潮湿环境下的潮湿扩散。并且计算了由于吸潮使器件在高温下产生的湿热应力。有限元模拟计算表明,不同潮湿环境下器件的潮湿扩散状态是不一样的,这导致在其后的高温焊接过程中器件内部产生的湿热应力的不同。     

热–机械应力和湿气引起QFN器件层间开裂分析

采用通用有限元软件分析和计算器件在潮湿环境下的潮湿扩散,并计算由于吸潮使器件在无铅回流的高温下产生蒸汽压力,并对层间开裂现象进行分析。结果表明,气压是层间开裂的主要原因,在气压和热机械应力的作用下,层合面上的微孔洞扩张并结合起来,导致器件最后失效。     

Advances in Vapor Pressure Modeling for Electronic Packaging

Two advanced techniques have been developed for modeling vapor pressure within the plastic IC packages during solder reflow. The first involves the extension of the "wetness" technique to delamination along multimaterial interface and during dynamic solder reflow. Despite its simplicity, this technique is capable of offering reliable and accurate prediction for packages with high flexural rigidity. For packages with low flexural rigidity, the new "decoupling" technique that integrates thermodynamics, moisture diffusion, and structural analysis into a unified procedure has been shown to be more useful. The rigorous technique has been validated on both leadframe-based as well as laminate-based packages. With high accuracy and computational efficiency, these dynamic modeling tools will be valuable for optimization of package construction, materials, and solder reflow profile against popcorn cracking for both SnPb and Pb-free solders     

Interface delamination analysis of TQFP package during solder reflow

Interface delamination during solder reflow is a critical reliability problem for the plastic IC packages. The main objective of this study is to apply modified virtual crack closure method (MVCCM) for the analysis of interface delamination between the leadframe pad and the encapsulant during a lead-free solder reflow after the level 1 moisture preconditioning. In this study, the moisture diffusion parameters and the coefficient of moisture expansion (CME) of two different epoxy molding compounds (EMC) are characterized for moisture diffusion analysis and the deformation analysis due to hygroscopic swelling. At the same time, the entire thermal and moisture history of Thin Quad Flat Pack (TQFP) package is simulated from the start of level 1 moisture preconditioning (85 °C/85%RH for 168 h) to subsequent exposure to a lead-free solder reflow process. Finally, the transient development of the stress intensity factors due to thermal stress only K_t, hygrostress only K_h, vapor pressure only K_p and combined energy release rate G_tot are computed and studied by using MVCCM. Based on the calculated stress intensity factors and energy release rates, it seems that for the EMC, the Young’s modulus, moisture diffusion coefficient, CME and adhesion strength with leadframe at high temperature appear to be the most significant variables for the MSL performance of TQFP package and this matches well with the experimental finding.     

Reliability of plastic ball grid array package

The reliability of plastic ball grid array (PBGA) package is studied for different materials. The reliability of the PBGA packages using conventional Bismaleimide-Triazine type PWB and our original product PWB that is made of high Tg epoxy resin is evaluated. The PBGA package using our original PWB has a feature of lower warpage for the package, and has similar performance regarding the thermal cycling stability and the endurance during reflow soldering as compared with the PBGA using the conventional PWB. The endurance during reflow soldering for each PBGA is JEDEC STD Method A112 level 3. In order to improve the endurance during reflow soldering, not only PWB materials but also other factors are investigated. As a result, the molding compound with the property of low moisture absorption and the die attach material with the properties of high adhesion strength and fracture strength are effective to improve the endurance property during reflow soldering. The package crack mechanism during reflow soldering is briefly described as follows     

PBGA封装的可靠性研究综述

通过传统BT类型的PWB材料与独特的PWB材料来PBGA封装的可靠性。相关的研究结果表明,后者同样具有相同的热循环稳定性和回流焊期间的疲劳强度,并具有较低的封装翘曲特点;模塑料的低吸湿性及粘片材料的高粘附强度和高断裂强度特性,有利于提高回流焊期间的疲劳强度和防止剥离现象的扩散。     

Interfacial Delamination Mechanisms During Soldering Reflow With Moisture Preconditioning

This paper first examines the commonly-used thermal-moisture analogy approach in thermal-moisture analogy approach. We conclude that such an analogy using a normalized concentration approach does not exist in the case of soldering reflow, when the solubility of each diffusing material varies with temperature or the saturated moisture concentration is not a constant over an entire range of reflow temperatures. The whole field vapor pressure distribution of a flip chip BGA package at reflow is obtained based on a multiscale vapor pressure model. Results reveal that moisture diffusion and vapor pressure have different distributions and are not proportional. The vapor pressure in the package saturates much faster than the moisture diffusion during reflow. This implies that the vapor pressure reaches the saturated pressure level in an early stage of moisture absorption, even the package is far from moisture saturated. However, the interfacial adhesion degrades continuously with moisture absorption. Therefore, the package moisture sensitivity performance will largely reply on the adhesion strength at elevated temperature with moisture. A specially designed experiment with a selection of six different underfills for flip chip packages was conducted. Results confirm that there is no correlation between moisture absorption and the subsequent interface delamination at reflow. The adhesion at high temperature with moisture is the only key modulator that correlates well with test data. Such a parameter is a comprehensive indicator, which includes the effects of thermal mismatch, vapor pressure, temperature and moisture. In this paper, a micromechanics based mechanism analysis on interfacial delamination is also presented. With the implementation of interface properties into the model study, it shows that the critical stress, which results in the unstable void growth and delamination at interface, is significantly reduced when the effect of moisture on debonding is considered.     

The Influence of Substrate Enhancement on Moisture Sensitivity Level (MSL) Performance for Green PBGA Packages

As the electronics industry migrates to the lead- and halogen-free (green) packages, many of the materials used in plastic ball-grid array (PBGA) substrates, in particular the molding compounds and die attaches, will have to be improved. The moisture sensitivity level (MSL) performance of the large nongreen PBGA packages are typically reduced by at least one JEDEC/IPC level at the lead-free reflow temperature of 260$^circ$C. Common failure mechanisms of traditional large size PBGA packages include popcorning, as well as delamination and cracks between the solder mask/copper interface in the multiple layer substrates. In this paper, the interfacial adhesion of traditional and advanced substrate materials and processing technologies are presented based on reliability tests of various PBGA packages subject to moisture soaking followed by reflow soldering at 260$^circ$C. It was found that substrate failures with delamination at the solder mask/copper interface were dramatically improved by introducing advanced materials and processes for multiple-layer substrates. However, the partial or full delamination at the mold compound/solder mask interface could still be observed after lead-free reflow soldering. There is an urgent need to improve the adhesion between mold compound/solder mask in order to achieve high MSL performance of large size and green PBGA packages.     

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.