Abstract— Flip-chip assembly has been widely adaptedto various electronic devices due to advantages, such asminiaturization of electronic devices and high density integration.The chip-on-flex (COF) package used in this paper is a flip-chippackage with an anisotropic conductive adhesive flim (ACF)interconnection and shows flexible features and reducedthickness compared with chip-on-board (COB) packages.All electronic packages experience temperature variation duringservice conditions and under environmental changes. Undertemperature variation, stresses emerge due to the differences inthe coefficient of thermal expansion among components. In orderto evaluate the thermomechanical reliability of a COF package,a thermal cycling (TC) test was conducted. A moiré experimentusing Twyman/Green interferometry was performed to observethe warpage behavior of the package under a TC condition.Through the experiment, the rate of change of chip warpage withrespect to temperature (dw/dT) as a parameter of the thermaldamage model was obtained. A finite element analysis (FEA)was also performed to calculate the maximum shear stress atthe ACF layer as another parameter of the model. From theexperiment and FEA results, the thermal damage model canaccurately represent the TC life of the COF package. However,based on observations of different warpage behavior of theCOF package compared with a COB package from the moiréexperiment, a modified thermal damage model that can predictthe TC life of both packages more accurately was proposed.
Manuscript received November 5, 2013; revised March 9, 2014 andApril 14, 2014; accepted April 23, 2014. Date of publication June 6,2014; date of current version June 30, 2014. This work was supportedin part by the Industrial Core Technology Development Program, KoreanMinistry of Knowledge Economy, under Grant 10033309, and in part by theDevelopment Program of Manufacturing Technology for Flexible Electronicswith High Performance under Grant SC0970 through the Korea Institute ofMachinery and Materials. Recommended for publication by Associate EditorS. Mahalingam upon evaluation of reviewers’ comments.
J.-W. Jang was with the Department of Mechanical Engineering,Korea Advanced Institute of Science and Technology (KAIST),Daejeon 305-701, Korea. He is now with the MechanicalEngineering Research Institute, KAIST, Daejeon 305-701, Korea(e-mail: [email protected]).
K.-L. Suk was with the Department of Materials Science andEngineering, Korea Advanced Institute of Science and Technology,Daejeon 305-701, Korea. She is now with the Semiconductor Researchand Development Center, Samsung Electronics, Suwon 443-742, Korea(e-mail: [email protected]).
J.-H. Park is with the DMC Research and Development Center, SamsungElectronics, Suwon 443-742, Korea (e-mail: [email protected]).
K.-W. Paik is with the Department of Materials Science and Engineering,Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea(e-mail: [email protected]).
S.-B. Lee is with the Department of Mechanical Engineering, KoreaAdvanced Institute of Science and Technology, Daejeon 305-701, Korea(e-mail: [email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TCPMT.2014.2325975
Index Terms— Chip-on-flex (COF) package, finite elementanalysis (FEA), life prediction, modified thermal damage model,moiré experiment.
I. INTRODUCTION
PROGRESSIVE enhancement of science and technologiesoffers numerous benefits to our daily lives. The develop-
ment of mobile devices allows us to freely access Internet,communicate with others abroad, and watch news anywhereand anytime. To increase the mobility of such devices, theyhave been gradually getting lighter and thinner. At the sametime, the devices have become multifunctionalized with highperformance. In other words, components of devices, such aselectronic packages, have become thinner and lighter with highdensity integration. To meet the current trend of electronicpackages, a flip-chip assembly using an adhesive for chip tosubstrate interconnection is widely used due to advantages,such as small package size, fine pitch capability, and environ-mental friendliness [1], [2]. In particular, a chip-on-flex (COF)package, a kind of flip-chip package has been widely used fordisplay driver integrated circuit [3], [4] and has a potentialusage for flexible electronic devices.
Despite the small size of electronic packages, it is difficultto avoid thermomechanical reliability problems caused byCTE mismatches among component materials and temperaturevariation during operation conditions or environmental change.In general, thermomechanical reliability can be evaluatedthrough an accelerated life test, such as a thermal cycling (TC)test which more severe loads than the service condition areapplied to reduce time cost. From the test results, it is alsopossible to approximately predict the life of a package underthe service condition using life prediction models [5]–[7].Proper application of accelerated life tests and life predictionmodels is important under the trend that the developmentperiod of electronic devices is becoming shorter.
In this paper, we proposed a modified thermal damagemodel that can predict the TC life not only of a thick chip-on-board (COB) package, but also a thin and flexible COFpackage with extremely low thickness using anisotropic con-ductive film (ACF) interconnection. TC tests were conductedto determine the TC life of a COF package. Some damageparameters used for the thermal damage model were obtainedfrom a moiré experiment and finite element analysis (FEA).From the moiré experiment, the value of warpage variationwith respect to temperature was obtained. Warpage behavior of
JANG et al.: WARPAGE BEHAVIOR AND LIFE PREDICTION OF A COF PACKAGE 1145
the COF package under a TC condition was also observed andthe reasons for its different behavior compared with that of theCOB package have been discussed. In addition, an FEA wasperformed to obtain the shear stress on the ACF layer. Valida-tion of the thermal damage model was then carried out usingthe obtained parameters and TC life of the COF package.Based on the investigation of the warpage behavior of theCOF package compared with that of the COB package, somemodifications were reflected on the thermal damage model toestablish a modified thermal damage model that is applicableto both packages.
II. LIFE PREDICTION MODELS FOR ADHESIVE-TYPE
FLIP-CHIP PACKAGE
Some studies related to thermal life prediction of flip-chip packages with an adhesive-type interconnection havebeen reported. For enhancement of the TC reliability ofCOB packages, reduction of the shear strain of the ACFby reducing the CTE mismatch between the chip and thesubstrate, otherwise known as global CTE mismatch, is highlyrecommended [8]. The shear strain in the ACF layer was alsoemphasized as a dominant parameter of thermally inducedfailure and the use of ACF with a low CTE high modu-lus [9], and high Tg [10] is recommended. Liu [6] proposedan equation for predicting the TC life of an anisotropicconductive adhesive joint under thermal cycling conditions andcompared the predicted lives with the experimental results.The chip warpage model introduced in [5] showed that theremaining life (or cumulative damage) of a COB packagecan be assessed by observing dw/dT. This model is basedon the concept that thermally induced warpage decreases asdelamination length at the interface between the adhesive layerand the substrate increased. Park et al. [7] emphasized thata single parameter, such as dw/dT or shear strain cannoteffectively represent the current status of damage of COBpackages, and proposed a thermal damage model representedas follows:
Damage index = τmax
(dw
dT
)(1)
where τmax is the maximum shear stress of the ACF layer.They conducted TC tests using various geometric specifica-tions of COB packages. Notably, the proposed model canpredict the TC life of COB packages with some variationsof geometric specifications, i.e., the thickness of each compo-nent. This is useful information for packaging engineers anddesigners to design their product. However, the specificationrange used for verification of the thermal damage modeldid not comprehensively cover all of the recently designedand manufacturing electronic packages. This means that thevalidity of the predicted TC life of a thin package, suchas a COF package, calculated by using the model cannotbe guaranteed. The ACF was used for the chip to substrateinterconnection in both COB and COF packages. The geo-metric difference of the COF package compared with thepreviously used COB package is that each component in theCOF package is thinner than the corresponding component inthe COB package. As a result, the flexibility of the substrate
Fig. 1. Top view of (a) COB and (b) COF package, and (c) cross-sectional schematic diagram of both packages (exaggerated; adapted from [10]and [12]).
of the COF package is highly enhanced relative to that ofthe COB package. The flexibility of the COF package canresult in different warpage behavior different from that ofthe COB package [11], which can affect the TC life of thepackage. Therefore, investigation of this topic is important forlife prediction and design optimization of a COF package.
III. SPECIMENS AND METHODOLOGIES
A. Specimens
Both COB and COF packages used in this paper areflip-chip packages using adhesives for the chip to substrateinterconnection. In particular, ACF, a kind of adhesive thatis widely employed for realizing high density interconnectswas used for the interconnection. The packages consist of asilicon ship, ACF and a substrate, and top views are shownin Fig. 1(a) and (b). As shown in Fig. 1(c), Au bump andCu electrode were formed on the chip and the substrate side,respectively, and electrical interconnection is achieved by thecaptured conductive particles between them. The specificationsof the packages are listed in Table I. Park et al. [7] usedeight kinds of COB packages for verification of their thermaldamage model. Compared with the COB packages, all compo-nents of the COF package are thinner. Furthermore, the totalthickness of the COF package is more than three times lessthan that of S1 the thinnest COB package considered in theirpaper.
Thermomechanical properties of the materials constitutingCOF package, such as elastic modulus (E), Poisson’s ratio (ν),glass transition temperature (Tg), and CTE, are listed inTable II.
B. Methodologies
To examine the validation of the thermal damage model,some experiments and analysis are needed. First, the TClife of the COF package is determined through a TC test.Proposed damage parameters of the thermal damage model
1146 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 4, NO. 7, JULY 2014
TABLE I
SPECIFICATIONS OF PACKAGES [7], [12]
TABLE II
THERMOMECHANICAL PROPERTIES OF MATERIALS [12]
such as dw/dT and maximum shear stress at ACF should beobtained using a moiré experiment and FEA, respectively.More detailed descriptions of the methodologies are intro-duced in this section.
1) Thermal Cycling Test: Thermal cycling tests were con-ducted to evaluate the thermal fatigue life of specimens.The test condition, resistance measurement method for thechip to the substrate interconnection, and failure criteria ofthe specimen are the same as those used in [7] to ensurea consistent evaluation process. The temperature range ofthe test is from −40 °C to 150 °C. The holding time ateach maximum and minimum temperature (Tmax and Tmin)condition is 10 min and TC rate is 2 cycle per hour. Thefailure criterion of the specimen was determined as the bumpcontact resistance reaches 220 m�.
2) Moiré Experiment: In this paper, Twyman/Green inter-ferometry was used to obtain the out-of-plane deformation,i.e., warpage, of the specimens during the TC condition. Theexperimental setup is shown in Fig. 2(a).
As shown in Fig. 2(b), which is a schematic diagram ofTwyman/Green interferometry, the separated laser beams atthe beam splitter are incident to the reference mirror and thespecimen, respectively. The reflected beams from the mirror
and the specimen merge near the beam splitter. As a result,the different optical path length between the separated laserbeam reflected from the reference mirror and the specimencreates interference fringes [13]. The relationship betweenout-of-plane displacement (w) and fringe order at each pointin the fringe pattern (N) is
w (x, y) = λ
2N (x, y) (2)
where λ is the wavelength of the laser beam. In this paper, aHe-Ne laser beam with 633-nm wavelength was used.
3) Finite Element Analysis: According to the thermal dam-age model [7], the maximum shear stress at the ACF layeris needed as a parameter for evaluating the damage of thespecimen. Due to the low thickness of the COF package, anFEA was alternatively employed to obtain the shear stressinstead of using an analytic solution or an experimentalmethod.
IV. EXPERIMENTAL AND FEA RESULTS
A. TC Test
Table III shows the TC test results of the COF package.Due to the large deviation of specimen 7 compared with the
JANG et al.: WARPAGE BEHAVIOR AND LIFE PREDICTION OF A COF PACKAGE 1147
Fig. 2. (a) Experimental setup and (b) schematic diagram of Twyman/Greeninterferometry (adapted from [13]).
TABLE III
TC LIFE OF COF PACKAGE
others, the median value was considered as the representativevalue of TC life. The results are coplotted with those of theCOB package in the next section.
Fig. 3. Moiré fringe variation of COF package under temperature variation.
B. Moiré Experimental Results
Fig. 3 shows the warpage change of the COF packageunder different temperatures. Under the temperature variation,global CTE mismatch between the chip and the substrate isthe major reason for the fringe variation. Some fringes atcertain temperatures show anisotropic fringe patterns. This canbe explained by the anisotropic material properties of thecomponents as well as the flexible characteristic and initialwarpage of the substrate of the COF package [12]. From thefringe images, the warpages along the x-axis at the centerline were obtained by using (1) and they are plotted in Fig. 4.Warpage behavior of COB packages [7] is also plotted in Fig. 4for comparison of the results with those of the COF package.
The concave and convex shapes of the packages are assignedas positive and negative values, respectively. Remarkable dif-ferences between the warpage behavior of the COB and COFpackages observed from Fig. 4 are summarized in Table IV.
In both cases, their warpage behavior can be dividedinto three regions: 1) linear region I (LR-I); 2) transitionregion (TR); and 3) linear region II (LR-II). When the packagewas heated from room temperature (Tr ), the warpage of thepackages increased linearly with increasing temperature, andthis is denoted by LR-I. It is well known that the globalCTE mismatch is the major cause of the warpage behavior.From −40 °C to Tr , which includes the temperature range
1148 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 4, NO. 7, JULY 2014
Fig. 4. Warpage behavior of COB and COF packages under the TC condition.
TABLE IV
DIFFERENCE OF WARPAGE BEHAVIOR BETWEEN
COB AND COF PACKAGES
of the TC condition in this paper is also considered as LR-I.Because the material properties of components do not changedramatically below Tr , in general, the warpage is still linearwith respect to temperature at this temperature region [5].Comparing dw/dT in LR-I [(dw/dT)I] of the COB and COFpackages, that of COF is larger than that of the COB packagedue to the lower flexural modulus induced by the low thicknessof the COF package compared with the COB package.
The maximum temperature of LR-I (Tmax,I ) is ∼20 °C–30 °C lower than Tg of ACF for both packages. In this region,dw/dT decreases gradually and is close to zero. The reasonfor this phenomenon is that mechanical coupling between thechip and the substrate cannot be gradually provided by ACFaccording to increasing temperature. Finally, the state of ACFis altered from glass to rubbery at Tg of ACF. The regionfrom Tmax,I to Tg of ACF is designated as TR. However, TRdisappeared after the first thermal cycle and (dw/dT)I duringand after the first heating have the same value similar to formerstudies [8], [10], [14]. The mechanism of the warpage behaviorof the package is altered through TR (or Tg of ACF) accordingto the following description.
In general, it is known that a flip-chip package has astressfree state at temperature over Tg of ACF [14]. Due to
the loss of binding capability in ACF warpage induced byglobal CTE mismatch does not emerge. Therefore, the chipand the substrate expand freely proportional to their CTEsand the temperature increment. This temperature region isdenoted by LR-II. In the case of the COB package, the chipwarpage maintains a constant value regardless of temperatureincrement. On the other hand, in the case of the COF package,the warpage shows linear behavior with respect to temperature;that is, dw/dT in LR-II [(dw/dT)II] has a nonzero value slightlylower than (dw/dT)I. In this region, the CTE mismatch betweenthe Cu electrode and the substrate instead of that between thechip and the substrate causes the substrate to warp. Due tothe larger CTE of the substrate than that of the Cu electrode(Table II), the substrate assumes a concave shape. The chipbecomes warped along the curvature of the substrate due toits low flexural modulus, which results from its low thickness.Even though the COB package has the same structure as theCOF package, the substrate of the COB package becomesalmost flat in spite of temperature increment because of thehigh thickness of the substrate compared with that of theCu electrode [8], [10], [14]. Therefore, it shows a constant(dw/dT)II at LR-II.
C. FEA Result
Fig. 5 shows the FE model of the COF package and theanalysis result. By applying isothermal loading from Tg ofACF, 111 °C to the minimum temperature of the TC condition,−40 °C, the maximum shear stress at the corner of the ACFlayer was calculated as 13.71 MPa.
V. MODIFIED THERMAL DAMAGE MODEL
From the parameters obtained through the moiré experimentand FEA in Section III, the damage index of the COF packagewas calculated by using the thermal damage model introducedin Section I. Fig. 6 shows the relationship between the TC lifeand the damage calculated using the thermal damage modelof the COF package. The results of the COB package [7] arealso coplotted in Fig. 6. The coefficient of determination (R2)for COB and COF packages (0.883) is slightly lower thanthat for only COB package (0.905). It appears that the thermaldamage model can accurately represent the TC life of not onlythe COB package but also the COF package. Even though thedamage index of the COF package is slightly overestimated,the thermal damage model can closely represent the flip-chip-type packages regardless of their design specifications underthe TC condition. However, based on the observation that theCOF package shows different warpage behavior at temperatureover Tg of ACF, we proposed a modified thermal damagemodel that accounts for its characteristics.
As mentioned in Section III, warpage of the COF packageincreased with increasing temperature over Tg of ACF; in otherwords, (dw/dT)II does not equal zero in contrast with the caseof the COB package. This nonzero (dw/dT)II can contribute toreduction of the danger of electrical disconnection between thechip and the substrate. Fig. 7 shows a schematic diagram ofthe warpage behavior of COB and COF packages over Tg ofACF. The chip of the COB package freely expands along the
JANG et al.: WARPAGE BEHAVIOR AND LIFE PREDICTION OF A COF PACKAGE 1149
Fig. 5. FE model of COF package and the FEA result.
Fig. 6. Relationship between the TC life and the damage index obtainedusing the thermal damage model of COB and COF packages.
flat substrate [Fig. 7(a)]; however, the chip of the COF packageexpands along the warped substrate [Fig. 7(b)]. It is known thatthe large shear deformation at temperature over Tg of ACFleads to electric disconnection of a flip-chip package [14].Due to the warped shape of the chip and the substrate inthe case of the COF package, the amount of misalignmentbetween the Au bump of the chip and the Cu electrodeof the substrate is smaller than that of the COB package.Furthermore, the warped substrate can prevent misalignmentbetween the neutral axes of the chip and the substrate, whichcould aggravate the misalignment between the Au bump andthe Cu electrode. Therefore, (dw/dT)II, which can reduce theshear deformation between the Au bump and the Cu electrodein the case of the COF package results in enhancement of thethermal reliability of the package and should be considered inthe thermal damage model. As a result, the modified thermaldamage model is proposed as follows:
Modified damage index=τmax
[CI
(dw
dT
)I− CII
(dw
dT
)II
](3)
Fig. 7. Schematic diagram of warpage behavior of (a) COB and (b) COFpackages over Tg of ACF (exaggerated).
where CI = Tmax,I − Tmin/Tmax − Tmin, and CII =Tmax − Tg/Tmax − Tmin. According to the definition of CI,it is the proportion of the temperature range of LR-I tothe total temperature range of the TC condition. Due tothe relatively small proportion of the temperature range ofTR and the expanded LR-I after the first thermal cycle as
1150 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 4, NO. 7, JULY 2014
Fig. 8. Relationship between the TC life and the damage index obtainedusing the modified thermal damage model of COB and COF packages.
explained in Section III, it could be approximately describedthat CI ≈ Tg − Tmin/Tmax − Tmin and CII ≈ 1 − CI. Therelation between the TC life and the damage index obtainedusing the modified thermal damage model of the COB andCOF packages is shown in Fig. 8. Comparing Figs. 6 and 8,median TC life of COF package evaluated by the modifieddamage model was placed somewhat closer toward the fittingline than that evaluated by the damage model. As a result, R2
for COB and COF packages obtained by the modified thermaldamage model (0.902) is higher than that obtained by thethermal damage model (0.883). That is, the modified thermaldamage model can more accurately evaluate the TC life ofboth packages than the thermal damage model.
VI. CONCLUSION
A thermal damage model was adapted for evaluation of theTC life of a COF package. To obtain parameters for the model,warpage behavior of the package was observed and shearstress was calculated through a moiré experiment and an FEA,respectively. In the case of the COF package, (dw/dT)II showsa nonzero value in contrast with the case of the COB package.The warped shape of the chip and the substrate at LR-IIresults in a reduction of the amount of misalignment betweenthe Au bump and the Cu electrode. This different warpagebehavior of COF package from COB package could influenceon the TC life. Therefore, the modified thermal damage modelthat accounts for the warpage behavior at temperature overTg of ACF was proposed and it can accurately represent theTC life of both COB and COF packages.
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Jae-Won Jang received the B.S. and Ph.D. degreesin mechanical engineering from Pusan National Uni-versity, Busan, Korea, and the Korea Advanced Insti-tute of Science and Technology (KAIST), Daejeon,Korea, in 2008 and 2014, respectively.
He is currently a Post-Doctoral Researcher withthe Mechanical Engineering Research Institute atKAIST. His current research interests include thereliability evaluation of high-density interconnects,mechanical properties characterization of thin mate-rials, and hybrid experimental-numerical analysis.
Kyoung-Lim Suk received the B.S. degree in nan-otechnology and advanced materials engineeringfrom Sejong University, Seoul, Korea, in 2007, andthe M.S. and Ph.D. degrees in materials science andengineering from the Korea Advanced Institute ofScience and Technology, Daejeon, Korea, in 2009and 2013, respectively.
She is currently a Senior Engineer with theSemiconductor Reseach and Development Center,Samsung Electronics Company, Ltd., Suwon, Korea.Her current research interests include flip-chip
assembly and adhesive materials for advanced packaging, nanomaterials, andits applications for highly reliable electronic packaging.
JANG et al.: WARPAGE BEHAVIOR AND LIFE PREDICTION OF A COF PACKAGE 1151
Jin-Hyoung Park received the M.S. and Ph.D.degrees in mechanical engineering from the KoreaAdvanced Institute of Science and Technology,Daejeon, Korea, in 2005 and 2009, respectively.
He has served as an Engineer with the DMCReseach and Development Center, Samsung Elec-tronics Company, Ltd., Suwon, Korea, since 2009.His current research interests include reliability eval-uations of flip-chip electronic packaging using opti-cal measurement techniques and the developmentof an in-plane micromoiré technique using a phase-
shifting method.
Kyung-Wook Paik (M’95) received the B.S. degreein metallurgical engineering from Seoul NationalUniversity, Seoul, Korea, in 1979, the M.S. degreefrom the Korea Advanced Institute of Science andTechnology (KAIST), Daejeon, Korea, in 1981, andthe Ph.D. degree in materials science and engineer-ing from Cornell University, Ithaca, NY, USA, in1989.
He was a Research Scientist at KAIST from 1982to 1985, and was responsible for the developmentof gold bonding wires. After the Ph.D. degree, he
was with the General Electric (GE) Corporate Research and DevelopmentCenter, Niskayuna, NY, USA, from 1989 to 1995, as a member of the SeniorTechnical Staff, Interconnect (HDI) multichip module technology, and powerIC Packaging. He rejoined KAIST in 1995 as a Professor with the Departmentof Materials Science and Engineering. He was a Visiting Professor at thePackaging Research Center, Georgia Institute of Technology, Atlanta, GA,USA, from 1999 to 2000, where he was involved in packaging educationand integrated passives research programs. He was visiting Portland StateUniversity, Portland, OR, USA, in 2005, where he was involved in the areas offlip-chip polymer materials evaluation. He has authored more than 80 technicalpapers, and currently holds 16 U.S. patents and four U.S. patent pending. Hiscurrent research interests include flip-chip bumping and assembly, adhesivesflip-chip, embedded capacitors, and display packaging technologies.
Dr. Paik has been the Chairman of the Korean IEEE Components, Pack-aging, and Manufacturing Technology Chapter since 1995, and is a memberof the International Microelectronics Assembly and Packaging Society, theSociety for Emergency Medicine India, and the Materials Research Societyof India.
Soon-Bok Lee (M’11) received the B.S. degree inmechanical engineering from Seoul National Uni-versity, Seoul, Korea, in 1974, the M.S. degreefrom the Korea Advanced Institute of Science andTechnology (KAIST), Daejeon, Korea, in 1976, andthe Ph.D. degree in mechanical engineering fromStanford University, Stanford, CA, USA, in 1980.
He joined KAIST in 1988 where he became aProfessor with the Department of Mechanical Engi-neering. His current research interests include relia-bility in electronics packaging, thin films, micro and
nanoscale measurement, characterization of materials at elevated temperatures,fatigue, fracture mechanics, and failure analysis of various structures inindustry. He has published more than 290 technical papers and authored theKorea’s Electronics Industry.
Dr. Lee was actively involved in the National Reliability EnhancementProgram as the Chairman and a Committee Member of the National ReliabilityCounsel for Parts and Materials in the Ministry of Knowledge Economyfrom 2001 to 2008. He has organized the 3rd International Symposium onElectronics Materials and Packaging in 2001, the 5th International Conferenceon Experimental Mechanics as the General Chairman in 2006, and the 10thInternational Conference on Mechanical Behavior of Materials as the GeneralCo-Chairman in 2007. He served as the Vice President of the Korea ReliabilitySociety and also as the President of the Reliability Division in the KoreanSociety of Mechanical Engineers (KSME). He is a member of KSME, theKorean Reliability Society, the American Society of Mechanical Engineers,and the Society for Experimental Mechanics.
