IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 7, NO. 10, OCTOBER 2017 1583
The Effect of Anisotropic Conductive FilmsAdhesion on the Bending Reliability of Chip-in-Flex
Packages for Wearable Electronics ApplicationsJi-Hye Kim, Tae-Ik Lee, Taek-Soo Kim, and Kyung-Wook Paik
Abstract— In this paper, the effects of adhesion propertiesof anisotropic conductive films (ACFs) interconnection on thechip-in-flex (CIF) bending reliability were investigated. Oxy-gen plasma treatment was conducted to increase the adhesionstrength between ACFs and Si chip or ACFs and flexible printedcircuit (FPC) substrates. In order to characterize the enhancedadhesion properties of the CIF packages, surface energy, sur-face roughness, elemental composition, and peel strength weremeasured. A digital image correlation method was used withcross-sectional scanning electron microscopy images to visualizethe stress development at the ACFs interconnection. It was foundthat the interface of ACFs resin and FPC substrate showed theweakest adhesion, where the delamination was initiated. As aresult of the improved adhesion at the ACFs resin and FPCsubstrate, the location of stress concentration was changed to theinterface of Si chip and ACFs resin, leading to better dynamicbending reliability. When the oxygen plasma was treated bothon the Si chip and FPC substrate, the stress concentration wasobserved not at the ACFs interfaces, but inside of the ACFsresin, resulting in further improved dynamic bending reliability.With the optimized plasma treatment condition and the ACFsmaterials, the dynamic bending reliability of the CIF packageswas successfully demonstrated up to 160 000 bending cycles at a7.5-mm bending radius without any electrical failures.
Index Terms— Adhesion, anisotropic conductive films (ACFs),chip-in-flex (CIF) package, cyclic bending, flexible package,plasma treatment.
I. INTRODUCTION
WEARABLE electronics have attracted great researchinterest in the recent years according to its potential
applications in the next-generation smart devices. Therefore,flexible interconnection technology becomes very importantto achieve fully flexible electronics under various operatingconditions [1], [2]. With these trends, advanced flexible chippackaging technology has been needed for wearable electronic
Manuscript received July 24, 2016; revised March 8, 2017; acceptedMay 25, 2017. Date of publication July 21, 2017; date of current versionOctober 5, 2017. This work was supported by the Wearable Platform MaterialsTechnology Center funded by the National Research Foundation of KoreaGrant of the Korean Government (MSIP) under Grant 2016R1A5A1009926.Recommended for publication by Associate Editor M. Bakir upon evaluationof reviewers’ comments. (Ji-Hye Kim and Tae-Ik Lee contributed equally tothis work.) (Corresponding author: Kyung-Wook Paik.)
J.-H. Kim and K.-W. Paik are with the Department of Materials Science andEngineering, Korea Advanced Institute of Science and Technology, Daejeon305-701, South Korea (e-mail: [email protected]).
T.-I. Lee and T.-S. Kim are with the Department of Mechanical Engineering,Korea Advanced Institute of Science and Technology, Daejeon 305-701, SouthKorea.
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.2017.2718186
devices with higher performance and miniaturization. For theflexible chip packages, chip-on-flex (COF) interconnectiontechnology consisting of ultrathin chips and flexible printedcircuit (FPC) substrates can provide excellent flexibility andbending performance [3], [4]. Moreover, as an interconnectionmaterial, anisotropic conductive films (ACFs) have widelybeen used for the COF interconnection technology [5], [6].ACFs are composed of a polymer resin for mechanical adhe-sion and conductive particles of typical metal-coated polymerballs for the electrical interconnection between the twoelectrodes [7].
Recently, chip-in-flex (CIF) packages have also been intro-duced to provide higher flexibility and bending reliability bymodifying the COF package [8], [9]. However, when the CIFpackages are subjected to dynamic bending tests, two failuremodes were observed: the cracking of a silicon chip and thedelamination of ACFs epoxy layer [10], [11]. In our previousstudies, interfacial delamination observed at the low-modulusACFs-assembled CIF packages resulted in lower dynamicbending reliability [8], [9]. Especially, the ACFs interfacialdelamination is related to poor adhesion strength of the ACFsadhesive layer [12].
To resolve the ACFs adhesion problem, we used the oxygenplasma treatment to enhance the adhesion strength at theinterface of ACFs resin and electrode surfaces. Plasma treat-ment is a method to modify the surface properties, and low-pressure plasma allows better surface uniformity at the mod-ified surface [13]. In this study, the oxygen plasma treatmentwas conducted on the Si chip and FPC surfaces. To analyzethe enhanced adhesion properties of CIF packages, contactangle, surface energy, and surface roughness were measured.In addition, the higher ACFs resin modulus was used for theCIF assembly because it is known that the adhesion strengthimproves as the modulus of the adhesive layer increases [12].Cyclic bending test was conducted to evaluate the bendingreliability of the CIF packages. Electrical resistance of thepackage specimens was monitored in real time to quantify thepackage performance under the dynamic bending condition.A microscopic deformation analysis was conducted using adigital image correlation (DIC) method on cross-sectionalscanning electron microscopy (SEM) images to visualize theeffect of adhesion enhancement at the ACF interconnection.
II. EXPERIMENT
A. Materials and Test VehiclesA 40-μm-thick 10 × 10-mm2-size silicon chip was designed
with a peripheral and an area array Au bumps at the center.
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TABLE I
SPECIFICATIONS OF ACFs USED IN THIS EXPERIMENT
Fig. 1. Test vehicles. (a) Chip design. (b) Au bumps on a chip.(c) FPC substrate design.
300-μm-pitch Au bumps with a size of 150 × 150 × 12 μm3
were electroplated on I/O pads. Flexible FPCs were fabricatedusing a 60-μm-thick polyimide with a size of 40 × 20 mm2,the electrodes were designed with dimensions of150 × 150 × 12 μm3, and a 300-μm-pitch Cu withelectroless nickel immersion gold (ENIG) substrate padfinishes was used. Moreover, the FPC substrate has a daisychain resistance test pattern for measuring the electricalcontinuity. Fig. 1 shows the design layout of the siliconchip, Au bumps, and the FPC substrate. In addition, twoACFs with different modulus were used, and the ACFsconsist of epoxy-based resin and 20-μm-diameter Ni- andAu-coated polymer balls as a conductive particle. Table Ishows the specifications of ACFs including mechanical andthermodynamic properties.
B. Fabrication of CIF Package Structures
Fig. 2 shows cross-sectional schematic images of the COFand CIF packages. Before the fabrication of the CIF pack-ages, the COF bonding was conducted using the flip-chip
Fig. 2. Schematic of (a) COF and (b) optimized CIF packages.
bonder (Fineplacer, FineTech). The Si chip with a 40-μmthickness was bonded onto the ACFs laminated FPCs at200 °C for 10 s at a 2-MPa pressure. After fabrication ofthe COF packages, a 30-μm-thick cover adhesive film waslaminated on a 60-μm-thick polyimide film at 60 °C by aroll laminator at a speed of 1 cm/s to remove voids betweenthe cover adhesive film and the polyimide film. As shownin Fig. 2(b), the CIF package was constructed with nearlysymmetric structure in the thickness direction so that theneutral axis is located at the center of the silicon chip [8].For the cover adhesive film, epoxy-based nonconductive filmswere used. After attaching the cover adhesive film and thepolyimide film on the COF package, vacuum lamination wasperformed at 200 °C for 1 min with a 0.5-MPa pressure.
C. Oxygen Plasma Treatment
Plasma treatment was performed by using the oxygenreactive-ion etcher (RIE). Chip and FPC substrates were putin the RIE chamber and treated at a 100-W power for 3 min.The working pressure of the oxygen plasma was 20 mTorrwith a constant oxygen gas flow of 100 sccm. In order toinvestigate the effects of adhesion properties on the dynamic
KIM et al.: EFFECT OF ACFs ADHESION 1585
Fig. 3. Schematic of three types of CIF packages. (a) No plasma treatment.(b) FPC-side plasma treatment only. (c) Both chip and FPC sides plasmatreatment.
bending reliability, three types of specimens were preparedas shown in Fig. 3: (a) no treatment, (b) plasma-treated FPCsubstrate only (“one side treatment”), and (c) both plasma-treated chip and FPC substrate (“both sides treatment”). In thecase of one side treatment, oxygen plasma was induced onFPC substrates to enhance the adhesion with the ACFs. Forboth sides treatment, oxygen plasma was induced on both chipand the FPC substrate.
D. Surface and Adhesion Characterization
Adhesion strength analysis of chips and FPC substratestreated by the oxygen plasma was performed. As shownin Fig. 1, FPC substrate consists of Cu with an ENIG substratepad metal finish and a polyimide surface, while the siliconchip consists of Au bump and SiN passivation surface layer.It means that the chip and FPC surface materials in contactwith ACFs are Au, polyimide, and SiN passivation layer[Fig. 3(a)]. In order to precisely analyze the oxygen plasmatreated surface, 100-μm-thick Au plates, polyimide films, andSiN deposited silicon chips were prepared. First, a contactangle of these three materials was measured by using acontact-angle analyzer with deionized water and glycerol. The
Fig. 4. Dynamic bending test apparatus with a loaded CIF specimen.
contact angles were used to calculate the surface energiesof Au, polyimide, and SiN surfaces after the oxygen plasmatreatment using the geometric-mean method [14].
The surface morphologies and surface roughness of theoxygen plasma treated surfaces were investigated by an atomicforce microscope (AFM) (Nanoman, VEECO). In each mea-surement, an area of 10 × 10 μm2 was scanned using a tappingmode. The root-mean-square surface roughness (Ra) wascalculated from the roughness profile measured by the AFM.
In order to investigate chemical structure changes onthe plasma treated surfaces, an X-ray photoelectron spec-troscopy (XPS) (Sigma Probe, Thermo VG Scientific) analysiswas performed to study the elemental composition analysis.
Last, a 90° peel test was performed to measure the adhesionstrength of CIF packages at a peeling velocity of 10 mm/min.In order to measure the adhesion strength of interface ACFsand test vehicles, ACFs-bonded COF assembly was fixed ontothe jig of the peel test machine, and then FPC substrate wasdetached at 90° from the COF assembly [15]. The minimum offive specimens was tested to obtain the average peel strengthsof the CIF packages specimens.
E. Dynamic Bending Test of the CIF Packages
To evaluate the bending fatigue reliability, a dynamicbending test was conducted with a lab-designed flexible testmachine using a linear actuator, as shown in Fig. 4. Test sam-ples for the dynamic bending test were prepared by attachingthe CIF package on a 188-μm-thick polyethylene terephtha-late (PET) supporting substrate. The PET substrate size wasthe same with the PI FPC substrate (20 × 40 mm2). The two-side edges were attached by two pieces of Scotch tape. Thespecimen was placed on a pair of aluminum blocks. One blockis fixed, and the other moves back and forth, pushing the edgeof the sample without any gripping. This method ensures theformation of uniform bending curvature during the dynamicbending test [11]. The cyclic bending test was conducted byrepeating the bending radius from 30 (0.033 mm−1), whichis almost flat, to 7.5 mm (0.133 mm−1) with a frequencyof 1 Hz. In situ daisy chain resistance was recorded duringthe test using soldered copper wires. The bending test was
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Fig. 5. 3-D AFM images of Au, polyimide, and SiN surfaces with noplasma treatment and Au, polyimide, and SiN surfaces after the oxygen plasmatreatment for 3 min.
Fig. 6. Changes of surface energies. Test surfaces with (a) no oxygen plasmatreatment and (b) after the oxygen plasma treatment for 3 min.
performed in the convex bending direction in order to providea cyclic tensile stress to the ACFs joint.
F. Strain Measurement for the ACFs JointUsing a DIC Technique
In order to investigate the deformation of ACFs resin and thebending strain at the interface of ACFs resin and Au surfacein the CIF packages under a dynamic bending test, a DICtechnique was used. The DIC software (ARAMIS professional,GOM, Germany) was adopted to directly quantify and visual-ize the degree of deformation by comparing the cross-sectionalSEM images of the ACFs interconnection for the flat and bentstates [9]. To measure the degree of deformation, first, specklepatterns should be formed randomly on the region of interest.These speckle patterns offer the means of reference pointsfor the position tracking, so that the deformation induced bymechanical loadings can be analyzed [16].
In order to obtain the SEM images, test specimens wereprepared by grinding the CIF package starting from the very
Fig. 7. Curve fitting results of the C1s spectrum of as deposited Au,polyimide, and SiN surfaces and the plasma-treated Au, polyimide, and SiNsurfaces.
Fig. 8. Peel strength results of CIF packages. (a) Without plasma treatment.(b) With oxygen plasma treatment on FPC surface only. (c) With oxygenplasma treatment on both chip and FPC surface.
edge without any external molding. The grinding process wascontinued up to the first line of interconnection region. Afterthe grinding process, 0.25-μm silica particles were dispersedby spraying on the specimen surface, and the specimen wasdried in oven at 125 °C for 5 min. The SEM images ofspecimen were obtained at flat and bent states of bendingradius of 7.5 mm. A full-field analysis was conducted toobtain the strain map on the ACFs interconnection. The vonMises strain applied on the joint region was quantified, andthe average values at the interfaces were compared.
III. RESULTS AND DISCUSSION
A. Adhesion Properties of CIF Packages WithOxygen Plasma Treatments
Fig. 5 shows the surface morphologies and surface rough-ness after 3-min plasma treatment on each surface measuredby AFM. In the case of Au surface, there were no big
KIM et al.: EFFECT OF ACFs ADHESION 1587
Fig. 9. Fracture path of the CIF package after the 90° peel test. (a) Without oxygen plasma treatment. (b) With oxygen plasma treatment on FPC.
Fig. 10. In situ daisy chain electrical resistances of CIF packages ofthree types of plasma treatment up to 17 500 cycles of the dynamic bendingtest at a 7.5-mm bending radius.
changes of the surface roughness (Ra), while that of thepolyimide surface increased from 8.7 to 19.1 nm. The Ra ofthe SiN surface also increased after the plasma treatment from2.8 to 5.2 nm. Consequently, from the increase of roughnesson polyimide surface and SiN surface, the adhesion betweenthe ACFs resin and polyimide surface or SiN surface can beenhanced by mechanical interlocking.
Fig. 6 shows the surface energy of Au, polyimide, and SiNsurfaces before and after the oxygen plasma treatment for3 min, calculated from the contact-angle measurements. Afterthe oxygen plasma treatment, contact angles of Au, polyimide,and SiN were reduced to ≈0° (a complete wetting). Thedecrease of contact angle corresponds to the higher surfaceenergy with increased wettability and adhesion. The calculatedsurface energies of Au, polyimide, and SiN were initially 40,32.5, and 56.27 N/m and increased by the plasma treatmentto 72.5, 74.6, and 73.84 N/m, respectively. This is attributedthat the plasma treatment with oxygen enhances the formationof polar groups on the surfaces and results in the strongeradhesion property [13], [18], [19]. That is, the improvedadhesion with the ACF resin was confirmed by the increasedsurface energy of the inorganic components.
Fig. 7 shows the XPS results of the surfaces of Au,polyimide, and SiN before and after the oxygen plasmatreatment. Carbon and oxygen from C1s and O1s scanningspectra were mainly detected on the surfaces, and the atomicsurface compositions of the test surfaces were evaluated.
Fig. 11. Cross-sectional SEM images of the ACFs joints of the CIF specimenswith (a) no plasma treatment, (b) one side plasma treatment, and (c) both sidesplasma treatment after the dynamic bending test of 160K cycles.
Table II summarizes the elemental compositions of modifiedsurface with the oxygen plasma treatment. For all the threesurfaces, carbon decreased and oxygen increased after exposedin the oxygen plasma presumably due to the surface cleaningeffect. In addition, it also means that functional polar groupsincluding oxygen such as single bond (C-O) and carboxylbond (O − C = O) increased, resulting in the less C-C singlebond at the binding energy of 284 eV. The presence of thesepolar groups greatly increases the hydrophilic nature of thesurfaces, resulting in improved adhesion [13]. To sum up, allthe surfaces were modified to be more reactive with highersurface energies and surface roughness by the oxygen plasmatreatment.
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TABLE II
ELEMENTAL COMPOSITION OF SURFACE BY XPS (ATOMIC %) BEFORE AND AFTER THE OXYGEN PLASMA TREATMENT
Fig. 12. DIC bending strain contours of the CIF packages with three typesof treatments. (a) No plasma treatment. (b) One side plasma treatment.(c) Both sides plasma treatment.
Fig. 8 shows the enhancement of peel strength after theoxygen plasma treatment on the FPC substrate. The peelstrength of the CIF packages without oxygen plasma treatmentwas 600 gf/cm, while that of the CIF packages with the oxygenplasma treatment was increased to 1900 gf/cm. Peel strengths
Fig. 13. In situ maximum daisy chain resistances of the CIF packagesassembled by using two different modulus ACFs after the both sides plasmatreatment up to 17 500 cycles of the dynamic bending test at a 7.5-mm bendingradius.
of one-side-treated and both-sides-treated CIF packages werethe same because the cohesive fracture of polyimide film wasobserved in both cases. Fig. 9 illustrates the fracture pathsof the CIF packages with and without the oxygen plasmatreatment. For the without plasma treatment, the fracture wasobserved at the interface between ACFs and FPC substratebecause the peel strength between ACFs and FPC substrate islower than the cohesive strength of the polyimide substrate.However, the cohesive fracture of the polyimide substrate wasinduced for both the one-side and both-sides treatment condi-tions because the adhesion strength between ACFs and FPCsubstrate was significantly increased by the oxygen plasmatreatment.
B. Effect of Enhanced ACFs Adhesion on theDynamic Bending Reliability
In order to investigate the adhesion effects on the dynamicbending reliability, ACFs with 0.9 GPa of Young’s moduluswas prepared and the three types of oxygen plasma treatmentswere conducted: no treatment, one side treatment, and bothsides treatment. Fig. 10 shows the in situ maximum daisychain electrical resistances versus the dynamic bendingcycles for ACFs-assembled CIF packages at a bending radiusof 7.5 mm. The failure criterion was defined as the daisy chainelectrical resistance of 100 �. The three CIF packages failedat 1500, 2500, and 6000 cycles for the no treatment, one sidetreatment, and both sides treatment, respectively. Unlike theboth sides treatment with the progressive increase of daisy
KIM et al.: EFFECT OF ACFs ADHESION 1589
Fig. 14. Cross-sectional SEM image of the ACFs joints of the CIF specimens assembled by using (a) low-modulus ACFs and (b) high-modulus ACFs afterthe both sides plasma treatment up to 160K cycles of the dynamic bending test.
Fig. 15. Bending strain contour plots of (a) low-modulus and (b) high-modulus ACFs-assembled CIF packages with the both sides plasma treatment.
chain electrical resistance during the dynamic bending test,both no treatment and one side treatment types showed rapidincrease of the daisy chain electrical resistance due to pooradhesion between the ACFs resin and Au surface.
Fig. 11 shows cross-sectional SEM images of the ACFsjoints after the dynamic bending test. In the case of theno treatment type, the interfacial delamination was observedbetween ACFs resin and the Au surface of FPC substrates.According to the bending strain theory, the neutral axis wasplaced on the center of the silicon chip due to the symmetricstructure of the CIF specimen [8], [17]. Because the dynamicbending test was performed in a convex bending direction,tensile stress was applied to the ACFs joint. Therefore, the dis-tance from the neutral axis to the ENIG metal surface of FPCsubstrate was longer than that to the Au bump surface of asilicon chip, resulting in the higher tensile stress at the ACFsresin and the ENIG surface of FPC substrate interface. As aresult, the interfacial delamination was usually observed at theACFs–FPC interface. In the case of one-side plasma treatment,oxygen plasma was applied on the ENIG metal surface ofthe FPC substrates. As shown in Fig. 11(b), the interfacialdelamination occurred at the interface of ACFs resin andAu bump surface of the silicon chip due to the enhancedadhesion between the ACFs and the ENIG metal surface ofFPC substrates. Last, for the-both sides plasma treatment caseshown in Fig. 11(c), no interfacial delamination was observedbecause of the improved adhesion for both the upper Au bumpand the lower ENIG metal surface interfaces. Some polymer
delamination was only observed inside the polymer balls andACFs resin cracking.
Because the dynamic bending properties and failure locationof CIF packages showed the strong dependence on the plasmatreatment type, a DIC analysis technique was used to quan-tify and visualize the bending strain applied at the interfaceand find the location of stress concentration. Fig. 12 showsthe Mises strain of the CIF specimens with (a) no plasmatreatment, (b) one side plasma treatment, and (c) both sidesplasma treatment. The strain contours were overlapped on thecross-sectional SEM images, as shown in Fig. 11. In the caseof no plasma treatment, the bending strain was concentratedat the interface of ACFs resin and the ENIG metal surfaceof FPC substrates. However, when the oxygen plasma wasapplied to FPC substrates, the location of stress concentrationwas changed to the interface of ACFs resin and the Au bumpof a silicon chip. For both-sides plasma treatment, stress wasconcentrated not at the interfaces of ACFs and chips and FPCsbut inside the ACFs resin itself. Also, average bending strainwas reduced from 5%, 5.2%, and 3.2% for the no, one-side,and both-sides plasma treatment specimens, respectively.
C. Effects of the Enhanced Adhesion Properties of HigherModulus ACFs on the Dynamic Bending Reliability
As mentioned above, as the modulus of the adhesive layerincreases, the bond adhesion usually improves [12]. In orderto examine the effect of resin modulus, high-modulus ACFs
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with a value of 1.8 GPa were prepared and both-sides plasmatreatment was applied. Fig. 13 shows the in situ maximumdaisy chain resistance of CIF packages assembled with thetwo different modulus ACFs under the dynamic bending testwith a bending radius of 7.5 mm. Dynamic bending reliabilitywas significantly improved by using the higher modulus ACFs.
As shown in Fig. 14, in the case of higher modulus ACFs,neither delamination nor resin crack was observed in ACFsjoints, unlike low-modulus ACFs with both sides plasmatreatment. It was verified that the higher modulus ACFs wereeffective in reducing plastic strain accumulation and enhancingthe dynamic bending reliability [8]. The SEM DIC analysiswas used to measure the bending strain applied at the highermodulus ACFs resin. Fig. 15 shows the bending strain mapsof low- and high-modulus ACFs-assembled CIF packages.For both the ACFs, bending strain was concentrated insidethe ACFs resin due to the enhanced adhesion between ACFsresin and Au surfaces by the both sides plasma treatment. Forthe high-modulus ACFs-assembled CIF packages as shownin Fig. 15(b), the lower bending strain was distributed insideACFs resin compared to that of the low-modulus ACFs resin.The average bending strain values of the low- and high-modulus ACFs were 3.2% and 2.1%, respectively. Whilethe both-sides plasma-treated CIF package with the low-modulus resin failed at 6000 cycles of dynamic bending, thehigh-modulus ACFs resin maintained the initial daisy chainresistance up to 160K cycles.
IV. CONCLUSION
In this study, the oxygen plasma treatment was appliedto investigate the effects of adhesion properties of ACFs onthe dynamic bending reliability of the ACFs-assembled CIFpackages. Oxygen plasma treatments improved the adhesionof the ACFs interconnection effectively, which was verified byincrease of the surface roughness, surface energy, and oxygenamount at the surface. As the result, the adhesion strength ofthe plasma-treated ACFs bonding increased more than threetimes compared to the untreated one. With the plasma-treatedCIF specimens, the dynamic bending test was conducted ata bending radius of 7.5 mm up to 160K cycles. Both-sidesplasma treatment type showed the slowest increase of daisychain resistance, followed by the one-side plasma treatmentand no plasma treatment ones. Through a DIC technique withcross-sectional SEM images, the bending strains at the ACFsinterconnection were quantitatively evaluated and visualized.In the case of no plasma treatment type, ACFs interfacialdelamination was observed at the interface of ACFs resinand FPC substrates. After the oxygen plasma treatment, stressconcentration and delamination location were changed to theinterface of ACFs resin and a silicon chip. After both-sidesplasma treatment, the bending stress was concentrated not atthe interfaces but at the ACFs resin itself, resulting in smallincrease of the daisy chain resistance. In order to furtherincrease the adhesion properties, the CIF package was fabri-cated with the higher modulus ACFs resin as well as the both-sides plasma treatment. The high-modulus ACFs-assembledCIF packages showed the stable daisy chain resistance com-pared to the low-modulus ACFs-assembled CIF package. From
the SEM DIC analysis, it was verified that the high-modulusACFs shows the lower bending strain inside the ACFs resinthan the low-modulus ACFs. By combining the oxygen plasmatreatment and the high-modulus ACFs, the highly reliable CIFpackage with improved dynamic bending performance wasdemonstrated for wearable electronics applications.
ACKNOWLEDGMENT
The authors would like to thank the anonymous reviewersfor all the valuable comments and suggestions that helpedimprove the quality of this paper.
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Ji-Hye Kim received the B.S. degree in materialsscience and engineering from Sejong University,Seoul, South Korea, in 2013, and the M.S. degreefrom the Korea Advanced Institute of Science andTechnology, Daejeon, South Korea, in 2015, whereshe is currently pursuing the Ph.D. degree with theNano-Packaging and Interconnect Laboratory.
Her current research interests include flexiblepackaging technologies using adhesive interconnec-tions for wearable electronics application.
Tae-Ik Lee received the B.S. and M.S. degrees inmechanical engineering from the Korea AdvancedInstitute of Science and Technology, Daejeon, SouthKorea, in 2013 and 2015, respectively, where heis currently pursuing the Ph.D. degree with theAdvanced Packaging and Thin Film Laboratory.
His current research interests include the experi-mental mechanics for measuring mechanical prop-erties of thin or soft materials and the simulationanalysis for improving reliability of flexibleelectronics.
Taek-Soo Kim received the B.S. degree in mechani-cal engineering from Yonsei University, Seoul, SouthKorea, in 2001, and the M.S. and Ph.D. degreesin mechanical engineering from Stanford University,Stanford, CA, USA, in 2006 and 2010, respectively.
He is currently an Associate Professor of mechan-ical engineering with the Korea Advanced Instituteof Science and Technology, Daejeon, South Korea,and also the Director of the Advanced Packaging andThin Film Laboratory. His current research interestsinclude the mechanics-related subjects of advanced
packaging and thin films: adhesion, deformation, fracture, fatigue, reliability,and stress- and strain-induced multiphysics phenomena.
Kyung-Wook Paik received the B.S. degree inmetallurgical engineering from Seoul National Uni-versity, Seoul, South Korea, in 1979, the M.S. degreefrom the Korea Advanced Institute of Scienceand Technology (KAIST), Daejeon, South Korea,in 1981, and the Ph.D. degree in materials scienceand engineering from Cornell University, Ithaca, NY,USA, in 1989.
He was a Research Scientist with KAIST from1982 to 1985 and was involved in the developmentof gold bonding wires. He was a Senior Technical
Staff Member of the Interconnect Multichip Module Technology and Power ICPackaging, General Electric Corporate Research and Development, Brookline,MA, USA, from 1989 to 1995. He joined the Department of Materials Scienceand Engineering, KAIST, as a Professor, in 1995, where he is currently withthe Nano-Packaging and Interconnect Laboratory, and is involved in the flip-chip bumping and assembly, adhesive flip-chips, embedded capacitors, anddisplay packaging technologies.
