Solder Material Properties Research Articles

An insight into dynamic properties of SAC305 lead-free solder under high strain rates and high temperatures

The development of high density and high integration of electronic chips puts forward stricter requirements on electronic packaging structures. Under the premise of lead-free rules, Sn-3.0Ag-0.5Cu (SAC305) has developed into a competitive solder material. However, the reliability of its mechanical properties has been attracting much attentions especially under extreme conditions. In this paper, the high-strain rate experiments are performed by a split-Hopkinson pressure bar (SHPB) to realize different plastic strain rate, while the high temperature environment is achieved by incorporating a heating furnace. By combining these two devices, the dynamic responses of solder specimens can be measured in the strain rate range from 833 s−1 to 1961 s−1 and the temperature conditions of 70 °C and 120 °C. It is worth noting that as a typical strain-rate hardening process, the maximum stress undergoes an initial ascending stage as the applied strain rate increases. Nevertheless, the hardening process is followed by a descending stage to indicate a softening process of maximum stress due to a further temperature increase during the rapid material deformation at higher strain rates. To gain a further insight into this maximum stress transition, the microstructure characterizations of the impacted solder specimens are carried out by a scanning electron microscope (SEM). It is found that the β-Sn phases and eutectic phases appear to experience refinement and then coarsen as the plastic strain rate continues to increase. A finite element (FE) model is created in the commercial nonlinear FE software ABAQUS/Explicit by considering all the experimental details. Since none of the existing material models in the material library is available for such a maximum stress transition with different ranges of strain rate, the mechanical properties of solder material are emphasized by proposing an improved Johnson-Cook model which can be readily utilized in the numerical evaluations of packaging structures under extremely high strain-rate conditions.

Unveiling the damage evolution of SAC305 during fatigue by entropy generation

Low-cycle thermal-mechanical fatigue loadings induce progressive and permanent degradation of mechanical properties of lead-free solder materials, and thus reduce the fatigue life of electronic devices. In this study, damage evolution and accumulation of Sn-3.0Ag-0.5Cu (SAC305), the most successfully commercialized lead-free solder material, was investigated by performing strain-controlled fatigue tests at different temperatures (288–373 K) and strain rates (0.001–0.004 s−1). Unlike existing empirical models, a fatigue damage model was proposed based on entropy generation related to the thermodynamic nature of fatigue damage. To be intrinsic to entropy generation, the proposed model was calibrated with the peak stress degradation at different temperatures and strain rates. Our findings showed that the damage parameter is closely related to temperature and strain rate and monotonically increases from 0 to 1 during the low-cycle fatigue loading, which unveiled the fact regarding the irreversibility of the internal entropy generation. For the first time, the damage evolution is found to be more associated with the applied strain rate than the temperature. By observations using an optical microscopy, the physical damage mechanism is elucidated for SAC305 solder by correlating microstructures and damage evolutions. The evolving dendritic β-Sn phase and the surrounding Sn-Ag-Cu ternary eutectic network also explained the effects of temperature and strain rate based on the energy dissipation. Our proposed damage model reconciled the damage accumulation of SAC305 solder subjected to the low-cycle fatigue loading, which is readily adopted to predict the fatigue life of the electronic packaging structures.

The effect of Bi on the microstructure, electrical, wettability and mechanical properties of Sn-0.7Cu-0.05Ni alloys for high strength soldering

This paper elucidated the effects of the Bi element (0 wt%, 0.5 wt% and 1.5 wt%) on the microstructure, electrical, wettability and mechanical properties of the Sn-0.7Cu-0.05Ni as a high strength solder. Besides using the conventional cross-sectioned microstructure image, the real-time synchrotron radiation imaging and synchrotron micro-X-ray fluorescence (XRF) technique was also used to investigate the microstructure, focusing on the in-situ growth behaviour of the primary (Cu,Ni)6Sn5 intermetallic and elemental distribution that had occurred in the Sn-0.7Cu-0.05Ni-1.5Bi. Other essential properties of solder material, such as wettability, electrical resistance, and shear strength, were also determined. The results showed that the addition of 1.5 wt% Bi refined the primary (Cu,Ni)6Sn5 intermetallics formation in the solder joint, where it grew earlier and faster relative to that in the Sn-0.7Cu-0.05Ni/Cu joint. Additionally, the addition of 1.5 wt% Bi resulted with a 3% reduction of its electrical resistance while increasing the wettability of the solder alloy. 1.5 wt% addition of the Bi element also found to have contributed to a significant increment of shear strength relative to that of the Sn-0.7Cu-0.05Ni. The results confirmed that the developed material is applicable as a potential high strength solder material in the context of advanced interconnecting applications.

Thermal fatigue life of Sn–3.0Ag–0.5Cu solder joint under temperature cycling coupled with electric current

In this paper, the physical and mechanical properties of Sn–3.0Ag–0.5Cu (SAC305) solder and the thermal fatigue properties of solder joints under different electric current densities and heat sink temperatures are investigated. The thermal and electrical conductivities of the SAC305 specimens are measured for the electric current density between 6.37 × 102 A/cm2 and 2.55 × 103 A/cm2. Through comparisons against the experimental results, it is found that the prediction results of finite element simulations can reasonably characterize the mechanical properties of SAC305 solder materials. In order to evaluate the effect on mechanical behaviour of solder alloys, a typical solder joint of mechanical properties for the upper and lower bounds under thermo-electric coupling conditions are analyzed by finite element simulations. The original Coffin–Manson model is improved to predict the fatigue life of SAC305 solder joints under temperature cycling coupled with electric current. Comparisons show that the different mechanical behaviour of SAC305 solder materials results in different performances at varying electric current densities and heat sink temperatures. For low current densities and heat sink temperatures, the solder material with lower yielding and ultimate strengths has a higher fatigue life. On the contrary, at high current densities and heat sink temperatures, the solder material with higher yielding and ultimate strengths will exhibit advantages in terms of fatigue life. From the numerical simulation point of view, this study reveals the performance advantages of lead-free solders with higher yielding and ultimate strengths in terms of thermal fatigue and current density for high temperature and power applications.

Atomistic analysis of the thermomechanical properties of Sn–Ag–Cu solder materials at the nanoscale with the MEAM potential

Sn-Ag-Cu, commonly known as SAC, is considered to be among the most promising of all lead-free solder alloys. Research aimed at making electronic components smaller has pointed to the possible use of nanosized solder joints in the future. In this study, for the first time, molecular dynamics simulations were used to analyze the thermomechanical properties of SAC solder materials at the nanoscale. The modified embedded-atom method (MEAM) potential was utilized in the simulations of the SAC solder materials. The dimensions of the structures considered were 55 × 55 × 59Å. Four different SAC solders were studied, with Ag percentages ranging from 1% to 4% (SAC105, SAC205, SAC305, and SAC405). Thus, the effects of the Ag percentage on the thermomechanical properties of the solder at the nanoscale were identified. The impacts of the temperature on the uniaxial tensile properties and coefficient of thermal expansion (CTE) values of the SAC solder materials were investigated by performing simulations of the materials at different temperatures. Results suggest that as the Ag percentage increases, the uniaxial tensile strength and CTE of the solder increase whereas the failure strain and thermal conductivity decrease. The results presented should prove useful in the electronic packaging industry.

Elevated Standoff Heights of Solder Joint Interconnections Can Result in Appreciable Stress and Warpage Relief

Abstract The “head-in-pillow” (HnP) defects in lead-free solder joint interconnections of Integrated Circuit (IC) packages with conventional (small) standoff heights of the solder joints, and particularly in packages with fine pitches, are attributed by many electronic material scientists to the three major causes: attributes of the manufacturing process, solder material properties, and design-related issues. The latter are thought to be caused primarily by elevated stresses in the solder material, as well as by the excessive warpage of the Printed Circuit Board (PCB)-package assembly and particularly by the differences in the thermally induced curvatures of the PCB and the package. In this analysis, the stress and warpage issue is addressed using an analytical predictive stress model. The model is a modification and an extension of the model developed back in 1980s by the first author. It is assumed that it is the difference in the postfabrication deflections of the PCB-package assembly that is the root cause of the solder material failures and particularly and perhaps the HnP defects. The calculated data based on the developed stress model suggest that the replacement of the conventional ball grid array (BGA) designs with designs with elevated standoff heights of the solder joints could result in significant stress and warpage relief and, hopefully, in a lower propensity of the IC package to HnP defects as well. The general concepts are illustrated by a numerical example, in which the responses to the change in temperature of a conventional design, referred to as BGA, and a design with solder joints with elevated standoff heights, referred to as column grid array (CGA), are compared. The computed data indicated that the effective stress in the solder material was relieved by about 40% and the difference between the maximum deflections of the PCB and the package was reduced by about 60%, when the BGA design was replaced by a CGA system. Although no definite proof that the use of solder joints with elevated standoff heights will lessen the package propensity to the HnP defects is provided, the authors nonetheless think that there is a reason to believe that the application of solder joints with elevated standoff heights could result in a substantial improvement in the general IC package performance, including, perhaps, its propensity to HnP defects.

Elevated Stand-Off Heights in Solder Joint Interconnections of Surface Mounted IC Packages Result in Appreciable Stress and Warpage Relief

Abstract The “head-in-pillow” (HnP) defects in lead-free solder joint interconnections of IC packages with conventional (small) stand-off heights of the solder joints, and particularly in packages with fine pitches, are attributed by many electronic material scientists to the three major causes: 1) attributes of the manufacturing process, 2) solder material properties and 3)design-related issues. The latter are thought to be caused primarily by elevated stresses in the solder material, as well as by the excessive warpage of the PCB-package assembly and particularly to the differences in the thermally induced curvatures of the PCB and the package. In this analysis the stress-and-warpage issue is addressed using an analytical predictive stress model. This model is a modification and an extension of the model developed back in 1980-s by the first author. It is assumed that it is the difference in the post-fabrication deflections of the PCB-package assembly that is the root cause of the solder materials failures and particularly and perhaps the HnP defects. The calculated data based on the developed analytical thermal stress model suggest that the replacement of the conventional ball-grid-array (BGA) designs with designs characterized by elevated stand-off heights of the solder joints could result in significant stress and warpage relief and, hopefully, in a lower propensity of the IC package to HnP defects as well. The general concepts are illustrated by a numerical example, in which the responses to the change in temperature of a conventional design referred to as ball-grid-array (BGA) and a design with solder joints with elevated stand-off heights referred to as column-grid-array (CGA) are compared. The computed data indicated that the effective stress in the solder material is relieved by about 40% and the difference between the maximum deflections of the PCB and the package is reduced by about 60%, when the BGA design is replaced by a CGA system. Although no proof that the use of solder joints with elevated stand-off heights will lessen the package propensity to the HnP defects is provided, the authors think that there is a reason to believe that the application of solder joints with elevated stand-off heights could result in a substantial improvement in the general IC package performance, including, perhaps, its propensity to HnP defects.

Constitutive behaviour and life evaluation of solder joint under the multi-field loadings

The mechanical behaviour and service life of the 63Sn-37Pb solder joint under coupled thermal-electrical loadings have been investigated. The mechanical properties of solder material are calibrated by uniaxial tensile experiments at different strain rates and working temperatures. In order to describe the mechanical responses of solder alloy, a constitutive model is proposed based on a thermo-visco-plastic flow rule. Furthermore, the fatigue life of solder joints is predicted by establishing a 3D finite element model under the coupled electrical-thermal-mechanical loadings. Temperature distribution, current density and thermal stress are numerically obtained in the solder joint. Based on the distribution of stress and plastic strain, the vulnerable location in the solder joint is determined, which is critical for predicting the service life based on the Coffin-Manson model. By varying the loading conditions, a parametric study is performed to develop an empirical formula of fatigue life, and the dominant factor under complicated working conditions is assessed. It is found that the current density and heat sink temperature pronouncedly influence the fatigue life of solder joint. The empirical predictions show reasonable accuracy compared with the Coffin-Manson model and finite element simulations.

Nonlinear and Temperature-Dependent Material Properties of AU/SN Alloy for Power Module

AbstractIn recent years, the material Au-20Sn eutectic solder, which is resistant to high temperatures, is used for electric interconnections in high-power modules, the material properties such as temperature and strain rate dependent stress-strain curve are critically needed for reliability assessment of Au-20Sn solder joint. Thus, this study was performed to determine the material properties of Au-20Sn eutectic solder under various strain rates and temperature loads. Many researches using shear test to determine the shear resistance of solder joint, however, the mechanical strength as measured by the shear test is the maximum shear strength of the package joint, but this measurement does not represent the stress-strain behavior of Au-20Sn material. To identify the material properties of Au-20Sn eutectic solder, the tensile test was performed to measure its mechanical strength and nonlinear material properties. The strain rate effect was examined in terms of the influence of the mechanical strength on the Au-20Sn eutectic solder at different tensile rates. The temperature-dependent material properties of Au-20Sn solder were also measured under various thermal loadings, and material properties of Au-20Sn obtained in this research can be applied to the simulation model, the thermomechanical behavior and reliability of the power module can be further analyzed and evaluated.

Assessment of Constitutive Properties of Solder Materials Used in Surface-Mount Devices for Harsh Environment Applications

To perform a thermomechanical simulation of surface-mount devices (SMD), relevant solder constitutive properties are needed. This paper reviews some promising Pb and Pb-free solders that are candidates to harsh environment applications. It presents the discrepancy in their common mechanical properties reported in the literature. The mechanical test systems, solder microstructure under test, and the test conditions (temperature and strain rate) are among the major causes for the discrepancy. The use of such variations in the solder thermomechanical properties as input data for simulation studies may cause reliability prediction less meaningful. A novel and reliable test methodology to extract relevant solder properties for surface mount applications is presented in this paper. To fulfill the methodology requirements, lap shear tests with accurate control of the reflow profile and dimensionally accurate solder joint thickness need to be conducted to reproduce the SMD solder joint microstructures for determining solder joint properties for elastic, plastic, and creep model simulations. Two case studies with high-Pb and Pb-free solders are presented. Time-independent yield strength and ultimate tensile strength data obtained from our studies were found to be higher than the reported literature data. This methodology can be extended for other solder material systems with any surface finish plating materials.

Assessment of Joint Reliability of Sn–2.5Ag–0.5Cu Solder/Cu as a Function of Reflow Time

Solder interconnect reliability is exceedingly influenced by the solder material properties, the intermetallics formed during soldering, and the environmentally imposed loads. The current study involves the study of the wetting behaviour, interfacial reactions and growth of intermetallic compounds (IMCs) in Sn–2.5Ag–0.5Cu lead free solder solidified on Cu substrate as function of reflow time. The reflow temperature was maintained at 270 °C and reflow time was varied from 10 to 10,000 s. The solder alloy showed better wettability on the substrate with the increase in reflow time. A growth in Cu6Sn5 IMC thickness was observed up to a reflow time of 500 s. IMC layer formed during a reflow time of 10 s was 3.09 μm and its thickness increased to 4.40, 8.21, 10.96 μm during 100, 300 and 500 s reflow time respectively. A decrease in the thickness of IMC was observed for samples reflowed for 1000 s. The IMC thickness increased with further increase in reflow time. The joint reliability of Sn–2.5Ag–0.5Cu solder solidified on copper substrate surfaces was assessed by performing bond shear test. The average shear strength decreased with an increase in reflow time after an optimum value.

Development of a microwave sintered TiO2 reinforced Sn–0.7wt%Cu–0.05wt%Ni alloy

Abstract The use of reinforcing nano-size ceramic particulates is a promising method to improve the mechanical and thermal properties of lead-free solder materials. In addition, advanced fabrication processes routes such as microwave sintering powder metallurgy (PM) enhance properties in the fabrication of composite solders. To elucidate the mechanisms underlying the improvements in mechanical and thermal properties, Sn–Cu–Ni with TiO 2 nano-composite additions, fabricated via a microwave sintering PM method, were investigated using state-of-the-art characterization techniques. Synchrotron micro-X-ray fluorescence (XRF) results detected trace Ti in the solder matrix. This was consistent with X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) results which indicated that nano crystals were within the Sn matrix. It is possible these nano crystal form due to the migration of Ti during the rapid high energy microwave heating. A hypothesis of improved thermal and mechanical properties of nano-composite solders is discussed based on the results and the microwave sintering PM route was discussed as a promising method for next generation lead-free solder processing.

Highly Reliable Solder ACFs FOB (Flex-on-Board) Interconnection Using Ultrasonic Bonding

In this study, in order to improve the reliability of ACF interconnections, solder ACF joints were investigated interms of solder joint morphology and solder wetting areas, and evaluated the electrical properties of Flex-on-Board (FOB) interconncections. Solder ACF joints with the ultrasonic bonding method showed excellent solder wetting by broken solder oxide layers on solder surfaces compared with solder joints with remaining solder oxide layer bonded by the conventional thermo-compression (TC) bonding method. When higher target temperature was used, Sn58Bi solder joints showed concave shape due to lower degree of cure of resin at solder MP by higher heating rate. ACFs with epoxy resins and SAC305 solders showed lower degree of resin cure at solder MP due to the slow curing rate resulting in concave shaped solder joints. In terms of solder wetting area, solder ACFs with <TEX>$25-32{\mu}m$</TEX> diameters and 30-40 wt% showed highest wetted solder areas. Solder ACF joints with the concave shape and the highest wetting area showed lower contact resistances and higher reliability in PCT results than conventional ACF joints. These results indicate that solder morphologies and wetting areas of solder ACF joints can be controlled by adjustment of bonding conditions and material properties of solder and polymer resin to improve reliability of ACF joints.

Effect of geometric complexities and nonlinear material properties on interfacial crack behavior in electronic devices

Interfacial failures are often found in solder joints between electronic components and PWAs, under shock and drop loading. These interfacial fractures are often either between layers of dissimilar intermetallic compounds (IMCs), or between the solder and IMC layer. Studies have revealed that these interfaces are usually scalloped (wavy and non-planar) and that the waviness decreases with continued thermal aging, accompanied by a reduction of the apparent resistance to interfacial crack initiation. This study investigates the effects of the interfacial waviness, nonlinear solder material properties, local geometric complexities, and the initial crack length, on the resistance to crack initiation. Most of the studies in this field until this point have been either simple theoretical models not including all complexities or few limited experimental work. A computational framework is generated here to comprehensively include all important parameters that can then make efficient predictions cost-effectively and provide insights on the dependency of the interfacial fracture properties on all these parameters. Accurate and efficient assessment of such fracture properties will inherently help in designing reliable electronic assemblies and prevention of premature field failures.

Impact of Copper Densities of Substrate Layers on the Warpage of IC Packages

STATS ChipPAC Inc., 47400 Kato Road, Fremont, CA 94538 (Received September 13, 2013: Corrected September 30, 2013: Accepted December 2, 2013)Abstract: In this paper, the impact of the copper densities of substrate layers on IC package warpage is studiedexperimentally and numerically. The substrate strips used in this study contained two metal layers, with the metal densitiesand patterns of these two layers varied to determine their impacts. Eight legs of substrate strips were prepared. Leg 1 toleg 5 were prepared with a HD (high density) type of strip and leg 6 to leg 8 were prepared with UHD (ultra high density)type of strip. The top copper metal layer was designed to feature meshed patterns and the bottom copper layer was designedto feature circular patterns. In order to consider the process factors, the warpage of the substrate bottom was measuredstep by step with the following manufacturing process: (a) bare substrate, (b) die attach, (c) applying mold compound(d) and post reflow. Furthermore, after the post reflow step, the substrate strips were diced to obtain unit packages andthe warpage of the unit packages was measured to check the warpage trends and differences. The experimental resultsshowed that the warpage trend is related to the copper densities. In addition to the experiments, a Finite Element Modeling(FEM) was used to simulate the warpage. The nonlinear material properties of mold compound, die attach, solder mask,and substrate core were included in the simulation. Through experiment and simulation, some observations wereconcluded. Keywords: warpage, substrate layers

Investigation of the Mechanical Properties of Lead-Free Solder Materials

Reliability and life-time of electric/electronic products is still a highly actual and discussed topic. Every electric/electronic product is heterogeneous system which consists not only from components and constructional parts, but also from many interconnections. Basic segments of interconnection are solder joints, recently created by lead-free materials. The passing from well-proven lead solders to lead free compositions brought many new aspects in reliability and life-time of electronic systems. Some of the negative factors are their poor electro-mechanical properties, which are based on complicated microstructure of interlayer interface. This fact causes defects in solder joints and consequential failure of the function. This article is focused on an investigation of mechanical properties of lead-free solder joints. There are investigated various properties of common lead-free solder paste depending on various concentrations of O2 by reflow process and on various components size. Next, there are shown microstructures by micro sections for various concentrations of O2. Mechanical properties (mechanical strength) are tested by shear test equipment DAGE 2400.

The Wiedemann–Franz–Lorenz relation for lead-free solder and intermetallic materials

Lead-free solders are replacing lead-rich solders in the electronics industry. Due to the limitation of available experimental data for thermal conductivity of lead-free solder and intermetallic compound (IMC) materials, the Wiedemann–Franz–Lorenz (WFL) relation is presented in this paper as a possible solution to predict thermal conductivity with known electrical conductivity. The method is based upon the fact that heat and electrical transport both involve free electrons. The thermal and electrical conductivities of Cu, Ni, Sn and different Sn-rich lead-free solder and IMC materials are studied by employing the WFL relation. Generally, analysis of the experimental data shows that the WFL relation is obeyed in both solder alloy and IMC materials, especially matching close to the relation for Sn, with a positive deviation from the theoretical Lorenz number. Thus, with the available electrical conductivity data, the thermal conductivity of solder and IMC materials can be obtained based on the proper WFL relation, and vice versa. A coupled thermal–electrical three-dimensional finite element analysis is performed to study the behavior of lead-free solder/IMC interconnects. Solder and IMC material properties predicted using the WFL relation are adopted in the computational model. By applying the WFL relation, the number of experiments required to determine the material properties for different lead-free solder/IMC interconnects can be significantly reduced, which can lead to pronounced savings of time and cost.

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.

Shear Property of Pb-Free Solder Joint According to Temperature Condition by SP and FEM

The use of Pb-free solder alloys are just newly developed materials and their properties are not well established and documented yet. So, the research to substitute the critical problem of the Pb solder alloy are increasing widely. To evaluate the shear property of solder joints according to temperature condition was predicted through the Shear-Punch test and FEM. By using the database according to this, the reliability evaluation technology of the solder joints tries to be formulated. In this paper, the solder used the Sn37Pb, Sn-4Ag and the Sn4Ag0.5Cu. When solder melted in the copper interface, the solder joints according to reflow times, each modeling required for the Shear- Punch was made in consideration of the IMC layer thickness. And the micro shear punch test was performed to obtain material properties of solder according to temperature condition. The shear properties of specimen which was modeled was predicted through FEM. It compared with the test value in which it obtains from Shear-Punch. Also, a method which can obtain shear properties of Pb-free solder joints easily and quickly using FEM was proposed.

점탄성 물성치를 고려한 WB-PBGA 패키지의 열-기계적 변형 거동

WB-PBGA 패키지를 구성하는 솔더볼 재료나 수지 복합재의 열-기계적 물성치는 온도에 대단히 큰 영향을 받을 뿐 아니라, 온도가 유지되는 시간에도 큰 영향을 받는 것으로 알려져 있다. 본 논문에서는 무연 솔더 WB-PBGA 패키지의 변형 거동을 신뢰성 있게 해석하기 위하여 재료의 비선형성을 고려한 유한요소 해석을 수행하고 무아레 간섭계 실험결과와 비교하였다. 먼저 수지 복합재의 점탄성 거동을 파악하기 위해 수지 접합재와 패키지 기판으로 구성된 이종접합체를 대상으로 하여 수지 복합재의 온도와 시간에 종속적인 점탄성 거동에 대해 유한요소 해석을 수행하고 결과를 분석하였다. 무연 솔더가 실장된 WB-PBGA의 열-기계적 거동을 파악하기 위하여 솔더는 점소성 물성치를, 수지 복합재는 점탄성 물성치를 적용하여 온도 변화에 따르는 유한요소 변형해석을 수행하여 실험결과와 비교하였다. 결과적으로 패키지의 변형은 수지 복합재의 재료 모델에 따라 대단히 크게 달라지며, 수지 복합재는 온도와 시간에 영향을 받는 점탄성 물성으로 해석해야 함을 알 수 있었다. 본 논문에서와 같은 SAC 계열 무연 솔더 WB-PBGA 패키지의 경우 유리전이 온도가 <TEX>$135^{\circ}C$</TEX> 정도로 비교적 높은 B-type 수지 복합재의 점탄성 물성치를 적용했을 때 상대적으로 신뢰성 있는 해석 결과를 얻을 수 있는 것으로 밝혀졌다. It is known that thermo-mechanical properties of solder material and molding compound in WB-PBGA packages are considerably affected by not only temperature but elapsed time. In this paper, finite element analysis (FEA) taking material nonlinearity into account was performed for more reliable prediction on deformation behavior of a lead-free WB-PBGA package, and the results were compared with experimental results from moire interferometry. Prior to FEA on the WB-PBGA package, it was carried out for two material layers consisting of molding compound and substrate in terms of temperature and time-dependent viscoelastic effects of molding compound. Reliable deformation analysis for temperature change was then accomplished using viscoplastic properties for solder ball and viscoelastic properties for molding compound, and the analysis was also verified with experimental results. The result showed that the deformation of WB-PBGA packages was strongly dependent on material model of molding compound; thus, temperature and time-dependent viscoelastic behavior must be considered for the molding compound analysis. In addition, viscoelastic properties of B-type molding compound having comparatively high glass transition temperature of <TEX>$135^{\circ}C$</TEX> could be recommended for reliable prediction on deformation of SAC lead-free WB-PBGA packages.