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 analysis of the warpage problem in TSOP

The reliability and the solderability of thin small outline package (TSOP) are significantly affected by the warpage that is generated after epoxy molding compound (EMC) molding process. This warpage problem mainly results from the mismatch of material properties such as Young's modulus, Poisson's ratio and coefficient of thermal expansion (CTE) and the geometric structure of each component for TSOP. The optimization of both material properties and geometric structures using the numerical analysis is necessary to reduce the warpage of TSOP. However, there are still some limitations for the numerical analysis to obtain proper results consistent with the practical warpage values. In this paper, the numerical analysis is performed under the assumption of elastic behavior for EMC. Furthermore, to solve the limitations, the material properties at the molding temperature and the degree of reaction rate at the end of the molding process of EMC are considered together for the analysis. This numerical analysis gives the higher warpage values than the measured ones, and is applicable to the practical design of the reliable electronic package.     

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.     

Study of package warp behavior for high-performance flip-chip BGA

To meet the future needs of high pin count and high performance, the LSI die and package size of flip-chip BGA (FC-BGA) devices have become larger. As a result, package warpage due to mismatch of the coefficients of thermal expansion among the construction materials has become a more serious problem for package reliability. In this paper, package warpage is successfully measured by a 3-D surface profile method in the temperature range from −55 to 230 °C. Furthermore, the package warpage of FC-BGA was investigated to clarify the effect of the thermomechanical properties of the underfill resin. Based on the results, we constructed a model of the mechanism of package warpage. This paper proposes an optimized underfill resin that can achieve low package warpage and a long fatigue life of the solder bump. The future trends in underfill resin will be to have properties of extremely low elastic modulus and non-linear properties such as creep.     

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.     

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.     

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.     

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.     

塑封IC翘曲问题

为分析封装材料及模塑过程参数对塑封IC翘曲的影响,本文先介绍塑料四边引线扁平封装(PQFP)使用的环氧模塑化合物(EMC)的热特性。并对不同模塑样品的聚合程度(DOC)、热膨胀系数(CTE)、玻璃转变温度Tg、剪切模量G等,运用各种热分析技术进行测量。结合不同封装过程参数,运用三维有限元分析法进行封装翘曲预测。最后对塑料四边引线扁平封装(PQFP)IC翘曲的测量值与预测值进行比较。     

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.     

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。     

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.     

Finite element simulation of package stress in transfer molded MEMS pressure sensors

The goal of this paper is to investigate the effect of residual packaging stress resulting from transfer molding of a micro electro mechanical system pressure sensor. A model of a silicon diaphragm micro electro mechanical system pressure sensor geometry was used to simulate the stresses developed during the molding process. The analyses were carried out with an assumption that the epoxy molding compound was a temperature dependent elastic material. Finite element analysis was used to calculate the residual packaging stress. The stress values were used to obtain the electrical output signal and sensitivity of the packaged sensor. In this way, a direct link was established between package stress and device performance.The calculated output signal and sensitivity were compared with experimental data to verify the simulated stress and hence determine the effect of the packaging process on the pressure sensor. Four different service temperatures were considered to examine the temperature effects on the output signal and the sensitivity for the packaged sensor.     

High reliability, high density, low cost packaging systems formatrix systems for matrix BGA and CSP by vacuum printing encapsulationsystems (VPES)

Ball grid array (BGA) and chip scale package (CSP) packaging markets are increasing. In general, transfer molding systems are used for these packaging processes. However, transfer molding systems are difficult to change the model for high expensive metal die. This paper describes a unique vacuum printing encapsulation system (VPES) we developed to solve such problems with lower cost than transfer molding. We used matrix type BGA and CSP for this test. Matrix type BGA and CSP make it easy to use printing technology for die-bonding, packaging, marking, and flux coating process. The total cost of this packaging is cheaper than the transfer molding process. We developed very low warpage and high reliability epoxy resin for matrix BGA and CSP. We succeeded in achieving high reliability and low cost packaging systems with this technology     

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。     

Using Taguchi method to obtain the optimal design of heat dissipation mechanism for electronic component packaging

Packaging technology developments in semiconductor chips are moving towards miniaturization, thinner products, lighter weights, and higher performance. However, in the process of packaging, warpage and residual stress have always been major problems, such as pin deviation, breakage, and weak signals. Further, the distinctive properties of the numerous materials that comprise a semiconductor chip demand different molding temperatures; thus, excessive internal thermal stresses are produced within the packaging structure which ultimately results in colloid warpage. This study used a 3D coordinate measuring machine to determine the levels of warpage produced in electronic packaging products and to verify the amount of warpage simulated by the finite element method. Then, Taguchi method was also utilized to analyze and discuss the four critical control factors namely: (1) shape of the heat sink; (2) thickness of molding; (3) molding temperature; and (4) thickness of soldering tin. Thus, the minimum thermal stress for electronic packaging components was obtained, which meant the optimal parameter combination for the packaging was a triangle-shaped heat sink, with a molding compound of 1.175 mm thick, a molding temperature of 170 °C, and a soldering tin that was 0.03 mm thick.     

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.     

Warpage simulation and experimental verification for 320 mm × 320 mm panel level fan-out packaging based on die-first process

Warpage for 320 mm × 320 mm panel level fan-out packaging based on die-first process was investigated by both simulation and experimental approaches. In the present paper, a simple and efficient FEA (Finite Element Analysis) method based on shell element was introduced. Finite element models were built by using the software of Ansys products to predict and analysis the warpage for feasibility of large panel fan-out packaging technology in aspect of material, package geometries, package size, process conditions and metal density. In order to verify the accuracy and the precision of the simulation method, test vehicle with dies was fabricated by using low cost ‘die first (face down)’ fan-out technology. Warpage of the test vehicle was measured by using Shadow Moiré method. The simulated warpage result and the experimental one exhibit good consistency.     

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