集成湿热及蒸汽压力对PBGA器件可靠性的影响

塑封器件在高温焊接之前的存储过程中，会吸收环境中的潮湿水分，这些潮湿水分在随后的焊接高温下会汽化因而产生蒸汽压力。本文将首先对PBGA器件内部潮湿水分高温下产生的蒸汽压力模型进行讨论，并结合湿／热一机械应力得到集成应力，最后分析蒸汽压力、集成应力对PBGA封装可靠性的影响。分析结果表明：在蒸汽压力、集成应力计算中，芯片、DA材料和EMC材料三相交界处发生了应力集中，如果在此界面存在初始裂纹，那么在这些局部集中的力的作用下，极易使裂纹扩展导致层间开裂。     

基于CZM法QFN器件热冲击载荷下的界面脱层研究

引进内聚力模型（CZM）法,利于有限元软件MSC．Marc对热冲击栽荷下QFN器件各材料界面之间的脱层开裂情况进行了研究。并分析了不同Diepad厚度对器件脱层失效的影响。结果表明：脱层开裂均发生在各个界面的两端,并逐渐沿着界面向里扩展,Diepad与芯片粘结剂之间的界面最容易发生脱层开裂;Diepad厚度对器件的脱层开裂影响较大,增加Diepad厚度能较大幅度的提高QFN器件的抗脱层开裂能力。     

QFN器件在无铅焊工艺中湿热导致的界面开裂失效研究

界面开裂是塑封IC器件的主要失效模式之一。电子封装用高聚物具有的多孔特性致使封装材料易于吸潮。在无铅回流焊工艺中，整个器件处于相对较高的温度下，致使高聚物吸收的潮湿会膨胀并在材料内部空洞产生很高的蒸汽压力。界面开裂在热机械、湿机械和蒸汽压力的耦合作用下极易发生。本文的主要目的就是研究无铅回流焊工艺中，温度、潮湿和蒸汽压力耦合作用对QFN器件开裂失效的影响。文章对塑料封装QFN器件从168小时的JEDECLevell标准（85℃／85％RH）下预置吸潮到后面的的无铅回流焊的整个过程进行了有限元仿真，并且对温度、湿度和蒸汽压力耦合作用下裂纹的裂尖能量释放率也通过J积分进行了计算。论文的研究结果表明QFN器件吸潮后封装体界面的潮湿成为界面开裂扩展的主要潜在因素，EMC材料、芯片和粘合剂的交点处应力最大，在该处预置裂纹后分析表明回流峰值温度时刻裂纹最易扩展且随裂纹长度增加扩展的可能性在提高。     

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。     

湿热环境下EMC尺寸对PBGA器件可靠性的影响

塑料封装器件由于湿热导致层间开裂是影响器件可靠性的关键问题之一。采用有限元分析软件模拟和计算了不同的模塑封材料(EMC)尺寸的PBGA(塑封焊球陈列)器件在358.15K、RH60%条件下吸潮168h和398.15K、RH0环境下干燥50h后器件内部的应力对界面可靠性的影响。结果表明,在吸潮情况下,EMC厚度为0.85mm的器件的界面可靠性最低,最大湿应力为0.528MPa;在干燥阶段,EMC厚度为1.25mm的器件的界面可靠性最高,最大湿应力仅为0.124MPa。     

湿热环境下吸潮对QFN器件可靠性的影响研究

由吸潮引起的微电子塑封器件失效已经越来越多地引起人们的关注.选用QFN器件作为研究对象,首先进行QFN器件在高温高湿环境下吸潮17 h、50 h、96 h试验;然后利用有限元软件分析和模拟潮湿在QFN器件中的扩散行为,并建立湿气预处理阶段应力计算模型;最后,通过试验与仿真相结合,分析潮湿对封装可靠性的影响.研究表明:微电子塑封器件的潮湿扩散速度与位置有着重要的关系;在高温高湿环境下,微电子器件吸潮产生的湿热应力在模塑封装材料(EMC)、硅芯片(DIE)和芯下材料(DA)的交界处最大;QFN器件在高温高湿环境下吸潮产生的裂纹主要出现在硅芯片与DA材料交界面的边界.     

PBGA封装的耐湿热可靠性试验研究

塑封电子器件作为一种微电子封装结构形式得到了广泛的应用,因此湿热环境下塑封电子器件的界面可靠性也越来越受到人们的关注.为了研究塑封器件及其所用材料在高湿和炎热(典型的热带环境)条件下的可靠性,采用耐湿温度循环的试验方法,以塑封球栅平面阵列封装(PBGA)器件为例进行测试.试验结果表明,芯片是最容易产生裂纹的地方,试验后期器件产生的裂纹主要出现在芯片和DA材料界面处及芯片、DA材料和EMC材料三种材料的交界处.空洞缺陷是使界面层间开裂的主要原因,在高温产生的蒸汽压力和热机械应力的作用下,界面上的微孔洞扩张并结合起来,导致器件最后失效.     

高功率连续激光辐照CFRP层合板热力破坏效应多尺度分析模型

建立了能够反映高功率连续激光辐照碳纤维增强复合材料(CFRP)层合板时材料发生烧蚀、热解与层间开裂等热力损伤效应的多尺度分析模型。从细观尺度分别建立了纤维和基体的热解动力学方程,通过热重分析获得热解动力学参数,进而得到CFRP层合板宏观的热物与力学性能参数。通过内聚力模型建立了激光辐照引起层间开裂的分析模型,提出并建立了热解和层间开裂效应阻碍能量传递的热阻模型。将多尺度模型获得的热-力学性能参数与热力耦合数值模型相结合,模拟了高功率连续激光引起的烧蚀、热解及层间开裂行为,模拟结果与实验结果吻合较好。     

CCD多晶硅层间绝缘介质对器件可靠性的影响

采用扫描电子显微镜和电学分析技术研究了电荷耦合器件(CCD)多晶硅层间绝缘介质对器件可靠性的影响.研究结果表明,常规热氧化工艺制作的多晶硅介质层,在台阶侧壁存在薄弱区,多晶硅层间击穿电压仅20 V,器件在可靠性试验后容易因多晶硅层间击穿而失效.采用LPCVD淀积二氧化硅技术消除了多晶硅台阶侧壁氧化层薄弱区,其层间击穿电压大于129 V,明显改善了器件可靠性.     

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。     

Interface delamination in plastic IC packages induced by thermal loading and vapor pressure - a micromechanics model

A micromechanics model and an associated computational scheme are proposed to study interface delamination in plastic integrated circuit (IC) packages induced by thermal loading and vapor pressure. The die and die-pad are taken as elastic materials, while the die-attach and molding compound are taken as elasto-visco-plastic materials. The interface between molding compound and the die-pad is characterized by a cohesive law. The key parameters of this law are the interface strength and interface energy. The vapor-induced pressure along the interface is incorporated by way of a micromechanics model. Parametric studies are conducted to understand interface properties and vapor pressure effects on interface delamination. Under purely thermal loading, both weak and strong interfaces are highly resistant to interface failure. However, the combined effects of thermal loading and vapor pressure arising from moisture trapped within the interface can cause total delamination at the interface. Once delamination has initiated at a weak interface, no significant increase in thermal loading and vapor pressure is required for the delaminated zone to grow to a macro-crack and subsequently to catastrophic failure referred to as popcorn cracking. The critical factors controlling the occurrence of popcorn cracking are the interface adhesion strength and interface vapor pressure.     

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.     

Evaluation of the Delamination in a Flip Chip Using Anisotropic Conductive Adhesive Films Under Moisture/Reflow Sensitivity Test

Anisotropic conductive adhesive films (ACFs) have been used for electronic assemblies such as the connection between a liquid crystal display panel and a flexible printed circuit board. ACF interconnection is expected to be a key technology for flip chip packaging, system-in-packaging, and chip size packaging. This paper presents a methodology for quantitative evaluation of the delamination in a flip chip interconnected by an ACF under moisture/reflow sensitivity tests. Moisture concentration after moisture absorption was obtained by the finite element method. Then, the vapor pressure in the flip chip during solder reflow process was estimated. Finally the delamination was predicted by comparing the stress intensity factor of an interface crack due to vapor pressure with the delamination toughness. It is found that the delamination is well predicted by the present methodology.     

Investigation of moisture-induced failures of stacked-die package

The present work studies the moisture induced failures of a stacked-die package. The study includes various experimental tests to find the characteristics of constituent packaging materials such as interfacial strength, hygro-swelling property and vapor pressure, and a comprehensive finite element analysis integrating the effects of the hygro-swelling stress, the thermo-mechanical stress and the vapor pressure. Specifically, the vapor pressures with respect to the moisture concentration and the temperature in epoxy molding compound were characterized experimentally by combined TMA/TGA technique. In the numerical stress analysis, the interfacial delamination was newly simulated by applying a cohesive element modeling technique, in which a quadratic traction damage initiation law and an exponential displacement separation law were used.     

Investigation of delamination control in plastic package

Interfacial delamination control is important for the mechanical reliability of plastic package, especially facing the challenge of lead-free requirement. Efforts of delamination performance improvement including structure, material and process for plastic package were made. It was demonstrated that the improvement of packaging material mechanical properties should depend on the package and purpose and should be the comprehensive consideration of advantage and disadvantage. The fracture mechanics was introduced to explain the mechanism of bumpy interface as the delamination retardant. The moisture absorption experiments of three kinds of molding compound were carried out to confirm that large difference of delamination performance among different molding compounds for the same device in our project was induced by moisture absorption difference, and the relation between the moisture weight absorbed and weight ratio of resin was obtained, given compounds of the same resin. It was substantiated that the moisture absorption of molding compound observes the Fick’s law at (Moisture Sensitivity Level 3) MSL3. Quantitative analyses of the moisture influence at the risk location were conducted to evaluate the risk of delamination with the consideration of vapor pressure during solder reflow, moisture and thermal expansion.     

The effect of hygro-mechanical and thermo-mechanical stress on delamination of gold bump

The reliability of the FC–CSP (flip chip–chip scaled package) package with gold bump at the MRT (moisture resistance test) reflow temperature, was evaluated by using the finite element method. The moisture properties of EMC (epoxy molding compound) obtained from the test described in JEDEC standard, were used to characterize the local moisture concentration analysis by transient moisture diffusion, the hygro-mechanical analysis by CME, the vapor pressure analysis and the thermo-mechanical analysis by CTE mismatch. Also, after precondition, the package reliability under the reflow process was predicted, by comparing and integrating each factors, package swelling and stress due to by vapor pressure, as well as thermo-mechanical stress. Consequently, the result showed that the effects on hygro-mechanical stress and vapor pressure in a package could not be negligible, when it is compared with that of the thermo-mechanical stress by CTE mismatch, which is recognized as the main effect on the package crack under reflow temperature. The stress was concentrated at interface between gold bump and die, where most of delamination occurred.     

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.     

A study of moisture diffusion in plastic packaging

The absorption and desorption processes of moisture in plastic material were studied with experimental measurement and finite element (FE) simulation. The diffusion coefficient and the saturate concentration were determined by experiment and simulation results with Fick’s law. The water molecules inside the plastic material were chemically bonded with polymers by hydrogen bonds in the microholes formed by the polymer molecule chains. On the saturate concentration, the moisture density in the effective volumes was 100 times larger than the vapor density in standard state. However, it is only 8% of liquid water. The water inside the plastic material was in a liquid situation. The delamination and the delamination recovery of flip-chip packaging inspected by C-mode scanning acoustic microscropy (C-SAM) during a high-temperature and high-humidity accelerating test could be explained with the state change of the water in plastic material space. The delamination recovery resulted from the increase in content of liquid water. The bonding of water molecules and polymers reduced the adhesive strength at the interface between epoxy material and die, and the delamination on the interface was initiated. A comparison of three cases with noncoated film, top-side SiC_x coated, and both-sides SiC_x coated indicated that the delamination would occur when the moisture concentration was between 50% and 95% of the saturate concentration. During the reflow process, the low interface-adhesive strength and the high vapor pressure of the wet sample might cause popcorning of the plastic packaging. Popcorning could be predicted by simulation of the moisture concentration at the interface.     

Reliability assessment and hygroswelling modeling of FCBGA with no-flow underfill

In the flip-chip ball grid array (FCBGA) assembly process, no-flow underfill has the advantage over traditional capillary-flow underfill on shorter cycle time. Reliability tests are performed on both unmolded and molded FCBGA with three different types of no-flow underfill materials. The JEDEC Level-3 (JL3) moisture preconditioning, followed by reflow and pressure cooker test (PCT) is found to be a critical test for failures of underbump metallization (UBM) opening and underfill/die delamination. In this paper, various types of modeling techniques are applied to analyze the FCBGA-8×8 mm on moisture distribution, hygroswelling behavior, and thermomechanical stress. For moisture diffusion modeling, thermal-moisture analogy is used to calculate the degree of moisture saturation in the multi-material system of FCBGA. The local moisture concentration along the critical interface, e.g. die/underfill, is critical for delamination, because the moisture weakens the interfacial adhesion strength, generates internal vapor pressure during reflow, and induces tensile hygroswelling stress on UBM during PCT. The results of moisture distribution can be used as loading input for the subsequent hygroswelling modeling. The magnitude of hygroswelling stress acting on UBM is found to be greater than the thermal stress induced during reflow, both in tensile mode which may cause the UBM-opening failure. Underfill with lower saturated moisture concentration (C_sat) and coefficient of moisture expansion (CME) are found to induce lower UBM stress and has better reliability results. Molded package generally has higher stress level than unmolded package. Parametric studies are performed to study the effects of no-flow underfill materials, package type (molded vs. unmolded), die thickness, and substrate size on the stresses of UBM during reflow and PCT.     

Thermomechanical Analysis of Plastic Ball Grid Arrays With Vapor Pressure Effects

This study adopts a mechanism-based computational approach to gain insights into the delamination and cracking of plastic ball grid array (PBGA) packages under moisture sensitivity test (MST) conditions. The possible crack paths in the molding compound are first examined by modeling the fully porous overmold with void-containing cell elements. These computational cells are governed by a Gurson constitutive relation, extended to account for vapor pressure effects. We show that the corner of the die/die-attach interface presents a likely site for crack initiation under MST conditions. Failure along this interface of interest is then examined by deploying a single row of computational cells along the die/die-attach interface. Under combined thermal and vapor pressure loading, delamination concurrently occurs at both the die corner and the die center; these competing damage sites lead to the rapid and complete delamination of the die/die-attach interfaces.