ieee transactions on components, packaging and manufacturing technology, vol. 9, no. 3...

IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 9, NO. 3, MARCH 2019 405

Effects of the Types of Anisotropic ConductiveFilms on the Bending Reliability

of Chip-in-Plastic PackagesJi-Hyun Kim, Ji-Hye Kim, and Kyung-Wook Paik

Abstract— Chip-in-plastic (CIP) packages using three types ofACFs have been investigated as a flexible display package solutionfor high-definition (HD) display applications. By optimizing theCIP packages, higher flexibility could be achieved comparedwith the chip-on-plastic packages. Three types of ACFs wereprepared: the conventional ACFs, nanofiber ACFs, and anchoringpolymer layer (APL) ACFs. The electrical properties and bendingreliability of CIP packages using these ACFs were evaluated.For electrical characteristics, unlike the conventional ACFs, thenanofiber and APL ACFs showed a 100% insulation property ata 20-µm pitch. The rate of increase in contact resistance aftera dynamic bending test of 100 000 cycles at a 15-mm bendingradius was 0.6%, 0.9%, and 1.1% for the APL, nanofiber, andconventional ACFs, respectively. The APL ACFs showed themost stable resistance changes during a dynamic bending test.Specifically, they showed the highest modulus of 1.8 GPa and thelowest accumulated plastic strain. Therefore, it was confirmedthat the CIP packages using APL ACFs are the most suitablepackage structure for flexible display driver IC packages andHD display applications.

Index Terms— Anchoring polymer layer (APL), anisotropicconductive films (ACFs), bending reliability, flexible display.

I. INTRODUCTION

THERE is a strong demand for display technologywith higher resolution, from high definition (HD) to

4K HD and 8K HD. Recently, together with higher resolution,flexible displays have also been required for various appli-cations. A representative example is virtual reality (VR), astate-of-the-art technology that enables people to experience avirtual experience created by a computer. For a more realisticexperience in the VR, curved display technology with ultra-high resolution with 8K HD using an organic light-emittingdiode (OLED) should be implemented. With these trends,flexible and ultrafine pitch display driver integrated circuitassembly and interconnection methods for OLED panels arenecessary for flexible display packaging.

Manuscript received July 20, 2018; revised November 16, 2018 andDecember 26, 2018; accepted January 4, 2019. Date of publication Jan-uary 21, 2019; date of current version March 13, 2019. Recommendedfor publication by Associate Editor D. Lu upon evaluation of reviewers’comments. (Corresponding author: Kyung-Wook Paik.)

The authors are with the Department of Materials Science andEngineering, Korea Advanced Institute of Science and Technology,Daejeon 34141, South Korea (e-mail: [email protected]; [email protected];[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.2019.2893979

An available candidate for a flexible driver IC packagestructure is a chip-on-plastic (COP) package in which athinned chip is assembled on a flexible plastic panel [1].However, the Si chip in the COP assembly is exposed, andit can easily break during bending [2], [3]. To address theseproblems, chip-in-plastic (CIP) packages wherein the chip iscovered by a plastic film are introduced in this paper.

As an interconnection method for flexible packaging, ACFs,which consist of a thermosetting resin and conductive parti-cles, are a promising material providing anisotropic electricalconduction and adhesion between a chip and substrate pan-els [4], However, conventional ACFs have a problem of shortcircuit due to agglomeration of conductive particles duringthe resin flow at an ultrafine pitch interconnection. As asolution, nanofiber ACFs and anchoring polymer layer (APL)ACFs were previously introduced by our research group. Thenanofiber and APL ACFs suppress the movement of conduc-tive particles and can achieve a 100% insulation property byincorporating conductive particles into high tensile strengthnanofiber or film type polymer materials [5].

In this paper, conventional, nanofiber, and APL ACFs wereapplied to a CIP package and the electrical properties andthe bending reliability according to the ACF types in the CIPpackages were investigated. The CIP package was optimizedby changing the cover and adhesive film thickness. The effectof the ACFs type on the bending reliability was evaluated bya dynamic bending test.

II. EXPERIMENT

A. Test Vehicles and Optimization of CIP Packages

A silicon chip with the dimension of 31.3 mm × 1.3 mmand the thickness of 80 μm was used. Au bumps witha size of 12 μm × 75 μm × 8 μm and 24-μm pitchwere electroplated on a chip. A plastic panel with a sizeof 65 mm × 22 mm and 128-μm thickness was used, andelectrodes were composed of 40 nm Ti/500 nm Al/70 nm Tiwith a size of 10 μm × 110 μm × 610 nm and 24-μm pitch.

In order to fabricate the CIP packages, cover films of25-, 50-, 75-, and 135-μm-thick polyimide films and epoxy-based nonconductive films (NCFs) of 40-, 80-, 120-, and160-μm thickness as an adhesive film were prepared. Thecover film and adhesive film were laminated by a vacuumlaminator on the COP package at 110 °C and 50-psi pressure

2156-3950 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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https://orcid.org/0000-0003-1743-0376

406 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 9, NO. 3, MARCH 2019

Fig. 1. Fabrication of a CIP package.

Fig. 2. Schematic of three types of ACFs.

Fig. 3. Tensile strength of three types of ACFs.

for 5 min. Fig. 1 shows a schematic of a CIP package. Theflexibility of the CIP package was measured by a four-pointbending test and the minimum bending radius without chipcracking was determined [6]. The optimized thickness of thecover film and the adhesive film was selected at the smallestminimum bending radius condition. The four-point bendingtest was performed at a velocity of 30 m/s using a 10-mmupper span and a 20-mm lower span.

B. Preparation of Three Types of ACFs

Three types of ACFs were prepared for the CIP package.The first is the conventional ACFs, which are composed oflow viscosity thermosetting resins and conductive particles.The second is nanofiber ACFs. PVDF nanofiber was fabricatedby an electrospinning method at 12.5-kV for 3 min using aPVDF solution mixed with conductive particles at a 1:0.9 ratio.Four-layer structured nanofiber ACFs were prepared by lami-nating 2-μm-thick high-viscosity NCFs above and below thenanofiber and additional 8-μm-thick low-viscosity NCFs onthe top layer. Finally, APL ACFs were prepared by laminating

Fig. 4. Schematic of (a) static DIC and (b) dynamic DIC analysis.

Fig. 5. Minimum bending radius and the location of NA depending on thecover film thickness.

NCFs and an APL layer. The APL layer was coated using aPVDF solution by a film coater. Fig. 2 shows a schematicof the three types of ACFs. The modulus of the ACFs was


KIM et al.: EFFECTS OF THE TYPES OF ACFs ON THE BENDING RELIABILITY OF CIP PACKAGES 407

Fig. 6. Cross-sectional images of chip edge at the adhesive film thicknessof 10, 20, 30, and 40 μm.

Fig. 7. Minimum bending radius and the location of NA depending on theadhesive film thickness.

Fig. 8. Cross-sectional SEM images of optimized CIP package.

measured by the dynamic mechanical analysis method. Themodulus of the conventional, nanofiber, and APL ACFs was1.3, 1.6, and 1.8 GPa, respectively. The tensile strength ofthe three types of ACFs was measured by a tensile test.As shown in Fig. 3, the APL ACFs showed the highesttensile strength, followed by the nanofiber ACFs and theconventional ACFs.

C. Electrical Properties of the CIP Packages

COP bonding was performed at 190 °C for 5 s using thethree prepared types of ACFs. The bonding pressures wereadjusted to have the same gap height of 1.1 μm in all ACFs

Fig. 9. (a) Maximum tensile stress applied on a chip. (b) Minimum bendingradius of COP and optimized CIP packages.

joints. The capture rates of conductive particles were measuredby counting the number of conductive particles per bumparea before and after bonding using an optical microscope.The short-circuit rates and contact resistances were alsomeasured.

D. Bending Reliability of the CIP Packages

A dynamic bending test was performed to investigate thebending reliability of three types of ACF-assembled CIPpackages. In situ contact resistances were measured duringa dynamic bending test of 100 000 cycles at a bendingradius of 15 mm and a bending speed of 1 cycle/s. Therate of increase in contact resistance was measured after100 000 cycles.

Furthermore, a scanning electron microscope (SEM) digitalimage correlation (DIC) analysis was performed to analyzethe joint strains of the ACFs. The SEM DIC technique canquantitatively analyze the amount of strain by comparing theinitial and deformed images [7]. Two different types of SEMDIC analysis were conducted. One is a static DIC wherethe SEM images were obtained at flat and bent state witha bending radius of 20 mm. The other is a dynamic DICwhere the deformed SEM images were obtained after dynamic



Fig. 10. Optical and SEM images of (a) conventional, (b) nanofiber, and(c) APL ACFs.

Fig. 11. (a) Average number of captured conductive particles per a bump.(b) Capture rates of conductive particles for the conventional, nanofiber, andAPL ACFs.

bending of 100 000 cycles at a bending radius of 20 mm.Fig. 4 shows the detailed schematic of the static and dynamicDIC analysis methods.

Fig. 12. (a) Short-circuit rates and (b) contact resistance for the conventional,nanofiber, and APL ACFs.

In order to analyze the failure mechanism of the threeACFs-assembled CIP packages during a dynamic bendingtest, a cyclic tensile test was carried out for 100 cycles,where 1 cycle means holding the maximum load (underultimate tensile strength) for 0.5 s and holding without aload for 0.5 s. Through the cyclic tensile test, the accumu-lated plastic stain was measured and compared in the threetypes of ACFs. Finally, an 85 °C/85 RH% reliability testwas performed by measuring the contact resistances after100, 250, and 500 h.

III. RESULTS AND DISCUSSION

A. Optimization of the Cover and Adhesive Film Thickness

As the COP package is transformed to a CIP pack-age by attaching the cover and adhesive film, the posi-tion of the neutral axis (NA) is changed according to the



Fig. 13. In situ contact resistance changes of (a) conventional, (b) nanofiberACFs, and (c) APL ACFs-assembled CIP packages during the dynamicbending test of 100 000 cycles (at 15 mm R).

following equation:yN =

∑i Ei yi Ai

∑i Ei Ai

(1)

where yN is the NA, Ei is the elastic modulus of element i ,yi is the distance from bottom to the center of element i , andA is the cross-sectional area of element i .

Fig. 14. Increase rate of contact resistance after the dynamic bending testof 100 000 cycles.

The change of the NA location is related to the bendingstress according to the following equation:

σb = Ei × y

ρ(2)

where σb is the bending stress, Ei is the elastic modulus ofelement i , y is the perpendicular distance to the NA, and ρ isthe bending radius.

Raising the NA to the center of the chip will decrease themaximum value of “y,” which will reduce the bending stressimposed on the chip, as per (2). Therefore, optimization ofthe CIP package is needed depending on the thickness of thecover and the adhesive film.

The optimum thickness of the cover film and the adhesivefilm was selected at the smallest bending radius conditionthrough a four-point bending test. First, the adhesive film wasfixed at 40-μm thickness and the thickness of the cover filmswas changed to 25, 50, 75, and 135 μm. When chip crackingoccurred, the minimum bending radius was determined andFig. 5 shows the four-point bending test results depending onthe cover film thickness.

As the cover film thicknesses increased, the minimumbending radius decreased and saturated at 10 mm at 75 μmcover film thickness. As the cover film thickness increased,the distance from the bottom to the center of the cover filmbecomes larger. Based on (1), when the value of “y” for thecover film was increased, the NA moved to the centerline ofthe chip, resulting in a reduction of the stress applied to thechip surface. This is the reason why the minimum bendingradius decreased as a thicker cover film was applied.

Next, in order to optimize the thickness of the adhesivefilm, the cover film thickness was fixed at 75 μm. As shownin Fig. 6, the thickness of the adhesive film was changed to10, 20, 30, and 40 μm. If the adhesive film was too thin,the edge side of the chips was not fully encapsulated andvoids were entrapped after lamination. When the thickness ofthe adhesive film was 40 μm, there were no void entrapmentsand the edge side of a chip was encapsulated well.



Fig. 15. DIC bending strain of CIP packages with three types of ACFs at static and dynamic bending.

Fig. 7 shows the minimum bending radius (min. R) depend-ing on the adhesive thickness. As the adhesive film thicknesswas increased, the minimum bending radius decreased becausethe neural axis moved to the centerline of a chip. The adhesivefilm thickness was optimized at 80 μm, when the NA waslocated at 177.3 μm and the minimum bending radius wassaturated at 9.2 mm. Fig. 8 presents the cross-sectional imagesof the optimized CIP package in terms of the cover andadhesive film thickness.

Fig. 9 shows the minimum bending radius of the COP andoptimized CIP packages. Based on (2), the calculated tensilestress applied at the backside of a chip at a bending radiusof 15 mm was 498 and 463 MPa at the COP and the optimizedCIP, respectively. By attaching the cover film and adhesive filmon the COP package, tensile stress on a chip was reduced andthe flexibility was improved in the CIP package.

B. Electrical Properties of CIP Packages Using ThreeTypes of ACFs

The conventional, nanofiber, and APL ACFs were prepared,as shown in Fig. 10. For the COP/CIP assembly, in the casesof the conventional and the nanofiber ACFs, the bondingpressure was 60 MPa. However, the APL ACFs required70-MPa bonding pressure to obtain the same gap heightas the other two ACFs. In order to ensure the number ofcaptured conductive particles would be 13 in all ACFs afterbonding, the number of particles on a bump before bondingwas designed as 40, 19, and 18 particles for the conven-tional, nanofiber, and APL ACFs, resulting in capture ratesof 32%, 68%, and 71%, respectively, as shown in Fig. 11.It was already reported that the nanofiber and APL polymercan significantly suppress the movement of the conductiveparticles, resulting in a higher capture rate than in the case ofthe conventional ACFs [5]. As shown in Fig. 12, 20% shortcircuit occurred in the conventional ACFs due to the initialhigher particle content. However, the nanofiber and APL ACFs

showed 100% electrical insulation. The contact resistanceswere almost the same in all ACFs because the ACFs havethe same joint gap height and the same number of capturedparticles per bump.

C. Dynamic Bending Reliability of CIP Packages UsingThree Types of ACFs

Fig. 13 shows the change in the contact resistance measuredin situ during the dynamic bending test. As shown in Fig. 13,the resistance differences between the bent and the flat stateswere 7, 5, and 4 m� for the conventional, nanofiber, and APLACFs, respectively. In addition, the rates of increase of contactresistance after 100 000 cycles were 1.1%, 0.9%, and 0.6% forthe conventional, nanofiber, and APL ACFs, respectively. Asshown in Fig. 14, the conventional ACFs showed the highestthe rate of increase, and the APL ACFs showed the lowest therate of increase in contact resistance.

Fig. 15 presents the strains at the ACF joints measuredby the DIC analysis. In the static bending DIC analysis,the bending strain at the ACF joint was 3.2%, 2.2%, and1.7% for the conventional, nanofiber, and APL ACFs, respec-tively. After the dynamic bending test of 100 000 cycles,the bending strain was increased to 5.8%, 3.1%, and2.2%, respectively. The rate of increase in the bendingstrain between static and dynamic bending was 81%, 41%,and 29% for the conventional, nanofiber, and APL ACFs,respectively.

These behaviors can be explained by the effect of themodulus of the ACFs. The moduli of the three ACFs were1.3, 1.6, and 1.8 GPa for the conventional, nanofiber, and APLACFs, respectively. The APL ACFs with the highest modulusproduced the lowest plastic strain at the ACF joint. To analyzethe effect of the modulus of the ACFs on the accumulatedplastic strain during the dynamic bending test, a cyclic tensiletest was performed. When tensile stress of 40 MP (less thanthe ultimate tensile strength from tensile test) was repeatedly



Fig. 16. Cyclic tensile test results of (a) conventional, (b) nanofiber,and (c) APL ACFs.

applied, the conventional ACFs failed at 42 cycles, but thenanofiber and APL ACFs did not fail up to 100 cycles,as shown in Fig. 16. Furthermore, at 40 cycles, the accu-mulated plastic strain was 0.19%, 0.15%, and 0.08% for theconventional ACFs, nanofiber, APL ACFs, respectively. As aresult, the APL ACFs, which had the highest modulus, showedthe lowest plastic strain during the dynamic bending test,resulting in the highest bending reliability among the threetypes of ACFs [8].

IV. CONCLUSION

Optimized CIP packages using a 75-μm-thick cover filmand an 80-μm-thick adhesive film showed the smallest bendingradius, 9 mm, while the COP packages showed a 15-mmbending radius. This is ascribed to the reduction of the tensilestress on the chip surface of the CIP package as a result ofmoving the NA location.

The capture rate of the conductive particles was 32%, 68%,and 71% for the conventional, nanofiber, and APL ACFs,respectively. The insulation rate of the conventional ACFs was80%, but the nanofiber and APL ACFs showed a perfect insu-lation property due to suppressed movement of the conductiveparticles.

In the dynamic bending test, CIP packages using APL ACFsshowed the lowest change in contact resistance between theflat and bent state and the lowest rate of increase after100 000 cycles of dynamic bending test. From the results of thecyclic tensile test, it was verified that the accumulated plasticstrain decreased as the modulus of the ACFs increased, andthe APL ACFs, which had the highest modulus (1.8 GPa),showed the lowest accumulated plastic strain.

In conclusion, CIP packages using the high-modulusAPL ACFs are one of the most suitable ACFs materialsfor flexible and ultrafine pitch display driver IC packagingapplications.

REFERENCES

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Authors’ photographs and biographies not available at the time of publication.


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