基于扇出型封装塑封材料性能的表征研究

扇出型封装在塑封过程中会出现芯片偏移及翘曲等缺陷,详细了解环氧塑封材料(EMC)的特性能够准确预测封装材料、结构、塑封工艺对塑封效果的影响。针对用于扇出型封装的EMC材料采用动态机械分析仪、差示扫描量热仪、流变仪测试其动态力学性能、固化动力学性能、流变学性能和热容,并建立可用于有限元分析的材料特性数学模型。结果表明,EMC在150℃等温固化60 min后具有最少残余固化;100℃环境下黏度随温度增加速率最快;时温等效原理可预测实验频率以外的力学行为。模型曲线与实验数据的拟合优度均大于0.982,材料表征模型满足准确性与适用性的要求。     

对表面安装封装体的粘弹性翘曲问题的分析

为了确保表面安装封装体优良的焊点连接,减少LSI封装的翘曲是一关键性的问题。把有限元和各种计算方法进行不同地结合,是研究预测薄型小外形封装(TSOP)翘曲问题的最佳方法。研究结果表明预测翘曲问题最合适的方法是使用多层壳体元缩减切变模量和体积模量,计算出粘弹性GK。所有的计算结果证明化合物厚度比为1.2,会使大芯片TSOP的翘曲问题最小化。对小芯片TSOP而言,化合物厚度比为2.0～2.9,减轻了封装翘曲问题。小芯片TSOP的翘曲显示出严重的鞍形状况。封装的翘曲率及翘曲幅度依赖于模塑料的特性。     

Warpage evolution of overmolded ball grid array package during post-mold curing thermal process

Two of the main driving forces for warpage deformation and residual stress in electronic packages are the thermal expansion mismatch between dissimilar package constituents and the crosslinking reaction of polymers during packaging thermal processes. For the purpose of quantitatively characterizing these two driving forces and assessing the process effect on warpage deformation, experimental and numerical analyses were applied to study the warpage evolution of overmolded ball grid array (BGA) package under post-mold curing (PMC) thermal histories. From in situ shadow Moiré warpage analyses on bimaterial and package specimens, it was observed that, during the isothermal curing condition, a significant increase in specimen warpage occurred as a result of molding compound shrinkage. A numerical modeling procedure that incorporates the models for the thermochemical cure kinetics, the curing- and chemical aging-induced shrinkage strains, and the cure-dependent viscoelastic relaxation modulus for the molding compound was then applied to simulate and compare to the experimentally obtained warpage evolutions. It can be seen from the analysis results that the evolution of package warpage over multiple thermal histories can be superpositioned by the thermal expansion mismatch-driven warpage change during non-isothermal stages and the chemical shrinkage-induced warpage evolution during isothermal aging at temperatures above the material glass transition point.     

Predicting the process induced warpage of electronic packages using the P–V–T–C equation and the Taguchi method

A critical issue in the manufacturing of electronic packages is the warpage induced during the molding process as a result of differences in the shrinkage of the constituent materials. Package warpage causes serious problems such as the quality degradation of devices and yield loss in manufacturing processes. Loss of lead coplanarity happens due to package warpage and causes difficulty in device testing and surface mount assembly. Internal stresses associated with package warpage can also cause device failures such as die cracking, broken circuits and package cracking.Warpage in IC package has drawn intensive attention in the past. Although the effects of thermal shrinkage were extensively investigated in the literatures, the influence of the cure shrinkage on package warpage had received less attention. Accordingly, this study develops a numerical approach for generating more accurate predictions of the package warpage by taking the effects of both thermal shrinkage and cure shrinkage into account. A three-dimensional finite element model of the small outline package (TSOP) DBS-27P is constructed and the proposed numerical approach, which is based on the P–V–T–C (pressure–volume–temperature–conversion) equation and the CTEs (coefficients of thermal expansion) of the package materials, is employed to predict the warpage at each of its corners under various packaging processing conditions. Using the Taguchi method, the relative influences of the transfer pressure, the packing pressure, the mold temperature and the curing time on the degree of package warpage are identified and the optimal processing conditions are established. A series of experimental packaging trials are performed using the optimal processing conditions. It is found that the warpage of the actual package is in good agreement with that predicted numerically. Therefore, the accuracy of the proposed numerical approach is confirmed. Moreover, the results also demonstrate the capability of the Taguchi method to identify the optimal packaging processing parameters on the basis of a limited number of simulation runs.     

Study of warpage due to P-V-T-C relation of EMC in IC packaging

There were many studies about the prediction of warpage due to thermal mismatch , . However, cure induced warpage is usually ignored and the results can be inaccurate. To minimize this problem, a thorough understanding of epoxy molding compound (EMC) with pressure-volume-temperature-cure (P-V-T-C) relation is necessary. This paper used the P-V-T-C relation of an encapsulation material to formulate the stress-strain relationship. With the help of finite element method (FEM) and mold flow analysis, warpage predictions combined with P-V-T-C relation were performed and the results show that this approach is practical. For a given P-V-T-C relation, the shrinkage direction is pointing toward the gate and maximum warpage usually occurs at the boundary of an integrated circuit (IC) package. Variation of specific volume difference along the flow direction is larger than that perpendicular to the flow direction. When temperature difference is small in thickness direction, specific volume difference in thickness direction varies only slightly.     

Time-domain viscoelastic constitutive model based on concurrent fitting of frequency-domain characteristics

A numerical procedure for constructing the multiaxial viscoelastic model for polymeric packaging material over a wide range of temperature is presented. By using the proposed best-fitting procedure, experimentally measured frequency-domain Young's and shear storage moduli are used to calculate the time-domain bulk and shear relaxation moduli which describe the three-dimensional constitutive behavior of a viscoelastic solid. The numerical procedure incorporates restrictions that ensure that the derived time-domain material function is physics compatible. The proposed procedure was applied to construct the viscoelastic constitutive models of epoxy molding compounds (EMCs), and compared to results obtained by using approximate-formula based direct conversion procedure. It was shown that, without using the proposed procedure, the directly calculated time-dependent Poisson's ratio oscillates significantly in the rubbery regime and is physically inadmissible. To validate the constitutive model constructed by using the proposed procedure, a numerical finite element model that incorporates the viscoelastic constitutive model of the EMC was applied to simulate warpage of an overmolded package under the solder reflow process and compared to experimental shadow Moiré measurements.     

Design of a novel chip on glass package solution for CMOS image sensor device

This paper presents a chip-on-glass (COG) package solution for CMOS image sensors based on highly precise and reliable bumping and flip-chip bonding techniques. The package is fabricated using three core techniques, namely the damage-free image sensor bumping technique, the wafer form glass substrate patterned technique, and the damage-free flip-chip bonding technique. Since the proposed package concept is new, the effects of the package geometry and material properties on the package reliability are uncertain in the initial design stage. A three-dimensional nonlinear finite element model of the proposed CMOS image sensor package is created. In the simulations, the applied thermal load is cooled from 200 °C to ambient temperature (25 °C) to model the thermal deformation and warpage of the package during the practical ACF assembly cooling process. The design parameters influencing the reliability of the package, i.e. the material properties of the ACF, the thickness of the image chip and the thickness of the optical glass are investigated. Two control levels are specified for the chip, glass, and ACF factors and a 2³ factorial design is created to determine the appropriate combination of material properties and geometric size. It is found that the glass thickness and the ACF properties significantly affect the thermal deformation of the package, while the chip and glass factors, and the interaction between them, significantly affect the warpage. Regression models are developed to perform a series of surface response simulations. Using the developed statistical tests and regression models, suitable material selection criteria and geometric sizes can be specified to satisfy various reliability considerations in the initial design stage.     

Controlling warpage of molded package for inkjet manufacturing

A system consisting of four silicon dies and several discrete components was encapsulated with epoxy mold material and the package was used for the evaluation of inkjet manufacturing in electronics. Experimental samples showed that the molding process induces thermo-mechanical stresses that warp the package after the mold resin is cured at elevated temperature. The molding process was modeled using the finite element method (FEM), and different package structures were simulated to see how those affect the final warpage of the package. Material properties of the mold material were measured and used in the FE model. The viscoelastic behavior of mold material was modeled with a sum of Prony series terms and a time-temperature shift factor was used to include the temperature effect. To verify the modeling assumptions, the surface profile of an experimental package was measured with an optical profilometer and the measurements were compared with the simulated profile. A good correlation was found between the measured and simulated profile of the package. The simulations with different package structures showed that e.g. an additional film placed on top of the mold resin reduces significantly the warpage of the package.     

The impact of moisture in mold compound preforms on the warpage of PBGA packages

The moisture in the mold compound preforms influenced the resultant mechanical properties as well as the warpage of IC packages after the molding and post mold curing (PMC) process. Moisture will diffuse into the mold compound preforms (compound pallet) after thawing from cold room and exposing to clean room condition before molding process. The moisture will cause the package to swell, and combine with thermal stress, and finally result in the warpage of molded package after molding process. The main objective of this paper is to address the impact of moisture in the compound preforms on the warpage of the PBGA packages and explain the change in mechanical properties under different moisture conditions, e.g., the variations of the flexural modulus, T_g, and CTE with respect to moisture level. The compound preforms exposure to a clean room condition were simulated by a series of experiments. The warpage of PBGA packages were measured by the moiré test. The moisture control during the IC manufacturing process was highlighted in terms of the mechanical properties variation and warpage measurement due to the moisture effects on the mold compound preforms.     

Numerical modeling of warpage induced in QFN array molding process

Warpage is a critical issue for QFN array molding process. In this paper, a cure-dependent viscoelastic constitutive model is established to model the cure-induced warpage in array molding process. For the relaxation modulus functions of the packaging polymer, the equilibrium moduli are modeled with a model based on scaling analysis and the relaxation behavior of the transient part is described by the cure-dependent relaxation amplitude and reduced relaxation times which are based on the time-conversion superposition principle. The cure-dependent parameters are characterized by using an integrated approach of dynamical mechanical analysis (DMA) and differential scanning calorimetry (DSC) measurements. Finite element modeling is carried out for three configurations of a carrier package map mould and the warpage induced during the curing process and cooling down is predicted. The results show that warpage induced during the curing process has significant contribution on the total warpage of the map.     

Characterization of chip scale packaging materials

The mechanical stability of Chip Scale Packages (CSP) used in surface mount technology is of primary concern. The dominant issues are package warpage and solder fatigue in solder joints under cyclic loads. To address these issues, molding compound and die attach film were characterized with finite element method which employed a viscoelastic and viscoplastic constitutive model. The model was verified with experiments on package warpage, PCB warpage and solder joint reliability. After the correlation was observed, the effect of molding compound and die attach film on package warpage and solder joint reliability was investigated. It was found that package warpage tremendously affected solder joint reliability. Furthermore, a die attach film was developed based on results of the modeling. CSP with the developed die attach film are robust and capable of withstanding the thermal stresses, humidity and high temperatures encountered in typical package assembly and die attach processes. Also, a lead free solder is discussed based on the results of creep testing. This paper presents the viscoelastic and viscoplastic constitutive model and its verification, the optimum material properties, the experimental and simulated reliability and performance results of the u*BGA packages, and the lead free solder creep.     

Warpage simulation for the reconstituted wafer used in fan-out wafer level packaging

Fan-out packaging technology involves processing redistribution interconnects on reconstituted wafer, which takes the form of an array of silicon dies embedded in epoxy molding compound (EMC). Yields of the redistribution interconnect processes are significantly affected by the warpage of the reconstituted wafer. The warpage can be attributed to the crosslinking reaction and viscoelastic relaxation of the EMC, and to the thermal expansion mismatch between dissimilar materials during the reconstitution thermal processes. In this study, the coupled chemical-thermomechanical deformation mechanism of a commercial EMC was characterized and incorporated in a finite element model for considering the warpage evolution during the reconstitution thermal processes. Results of the analyses indicate that the warpage is strongly influenced by the volume percentage of Si in the reconstituted wafer and the viscoelastic relaxation of the EMC. On the other hand, contribution from the chemical shrinkage of the commercial EMC on warpage is insignificant. As such, evaluations based on the comprehensive chemical-thermomechanical model considering the full process history can be approximated by the estimations from a simplified viscoelastic warpage model considering only the thermal excursion.     

环氧模塑料在半导体封装中的应用

环氧模塑料是一种重要的微电子封装材料,是决定最终封装性能的主要材料之一,具有低成本和高生产效率等优点,目前已经成为半导体封装不可或缺的重要材料。本文简单介绍了环氧模塑料在半导体封装中的重要作用和地位;分析了环氧模塑料性能对半导体封装的影响,并对不同半导体封装对环氧模塑料的不同要求进行了分析;最后展望半导体封装和环氧模塑料的未来发展趋势,以及汉高华威公司在新产品开发中的方向。     

扇出型晶圆级封装中圆片翘曲研究

在扇出型晶圆级封装工艺中,由于芯片材料与塑封料之间的热膨胀系数差异,晶圆塑封过程中必然会形成一定的翘曲。如何准确预测晶圆的翘曲并对翘曲进行控制是扇出型晶圆级封装技术面临的挑战之一。在讨论圆片翘曲问题时引入双层圆形板弯曲理论与复合材料等效方法,提出一套扇出型晶圆级封装圆片翘曲理论模型,并通过有限元仿真与试验测试验证了该翘曲理论模型的计算精度。同时给出该理论模型在实际工程中的应用,对扇出型晶圆级封装产品设计与翘曲预测具有指导意义。     

Material property effects on solder failure analyses

The effects of material properties modeling on the solder failure analyses by numerical simulations are studied. The packaging structure of plastic ball grid array on printed circuit board was modeled. Two different types of molding compounds and two different types of substrates were employed and combined for the plastic ball grid array package modeling. The material properties were assumed as temperature dependent elastic and viscoelastic, and finite element method was used to calculate and analyze the strain energy densities of the solder balls. The chip warpage was also studied, and related with the solder ball reliability analyses by discussing the viscoelastic characteristics of the materials and their influences on the deformations. The results showed that the warpage developments of the packaging structure showed very different behaviors, and the mechanism of the strain energy density accumulations in the solder balls were also different depending on the material properties modeling and their combinations. This study demonstrates that appropriate modeling of the material properties is critical for the interpretation and understanding the microelectronics reliability mechanisms.     

Warpage mechanism analyses of strip panel type PBGA chip packaging

The objective of this study is to analyze a warpage development mechanism by simulating a strip type packaging for plastic ball grid array. Molding compound and substrate materials were thermo-mechanically tested to obtain the mechanical properties by several test methods. Samples were fabricated using the same materials, and warpage developments were measured at room temperature after molding compound cure. Based on the tested materials property, the warpage developments were simulated by numerical calculations during cooldown process. The results were compared with the measurement data of the samples, and the warpage mechanism was investigated based on the elastic and viscoelastic simulation results. It was found that the relaxation behaviors of the molding compound and the substrate materials had significant effect on the warpage development. It was also found that the warpage development was dependent on the packaging geometry. The development mechanism was analyzed through the simulation calculations by combining different material properties modeling and geometries, and the results showed comprehensive consideration of the materials and the packaging design are essential to control the warpage.     

Prediction of cure induced warpage of micro-electronic products

Prediction of residual stresses in micro-electronic devises is an important issue. Virtual prototyping is used to minimize residual stresses in order to prevent failure or malfunction of electronic products.Already during encapsulation stresses build up due to polymerization induced shrinkage of the molding compound. Differences in coefficient of thermal expansion of the involved materials cause additional stresses during cooling down from molding to ambient temperature. Since industry is availed by reliable prediction methods, detailed material models are required. In electronic packaging, mechanical properties of most of the involved materials have constant mechanical properties. However, the viscoelastic properties of the encapsulation material depends highly on temperature and degree of cure. Reliable predictions of residual stresses require simulation models which take into account the effect of temperature and conversion level.In this paper, properties of molding compound are discussed which are relevant for the prediction of warpage of micro-electronics products. The models for the individual properties are combined to one single model suitable for finite element simulations. The numerical implementation in finite element code is not standard and is done by using user-subroutines.Validation experiments are performed in order to verify the developed material model which is done by measuring and predicting the warpage of a mold map. A Topography and Deformation Measurement (TDM) device is used to measure the deformations at elevated temperatures in a non-intrusive way such that the developed material model could be validated in a broad range of temperature.Finally, simulations are carried out with simplified material models of molding compound. The results of these simulations are compared with results obtained with the cure dependent viscoelastic model and real warpage data. From these comparisons it is concluded that for reliable prediction of warpage, the cure dependent viscoelastic model is has to be used.     

Virtual qualification of moisture induced failures of advanced packages

This paper presents a combined numerical and experimental methodology for predicting and preventing moisture induced failures in encapsulated packages. Prevention of such failures will enable efficient and optimal pre-selection of materials, their interfaces and geometric design with respect to the desired resistance to moisture. This virtual qualification methodology is illustrated for a specific BGA package which showed 50% failures (broken stitch-bonds) during HAST testing due to excessive warpage and/or delamination of different interfaces. For three different material combinations the moisture diffusion during the HAST test is predicted and subsequently thermo-mechanical-moisture simulations are performed where the effects of hygro-swelling, vapor pressure, thermal expansion and delamination on the failure mechanisms are predicted. The comparison of the simulation results of the different molding compounds with the observations of HAST testing indicates that the developed methods and models can predict the observed trends. Application of the presented methodology will result in shorter time-to-market and significant cost reduction due to reduced trial-and-error design cycles and effective material usage.     

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。     

Drude-Lorentz色散模型研究光学Tamm态的性质

废旧塑封芯片的界面分层会严重影响其回收重用价值, 一直是废弃电器电子产品(WEEE)资源化研究中关注的热点问题。基于有限元仿真方法, 分析了废旧QFP塑封芯片在线路板拆解温度下的翘曲变形和各材料界面应力分布。研究表明, 拆解温度过高时, 芯片翘曲过大容易导致材料膨胀力失配, 造成界面分层; 在粘结层、衬底和模塑料的结合区域应力水平非常高, 较易发生分层; 模塑料在拆解温度下为玻璃态, 与管芯及衬底的结合强度均降低, 其界面角点均为较易发生分层的位置。