adhesion and reliability of anisotropic conductive films (acfs

Adhesion and Reliability of Anisotropic ConductiveFilms (ACFs) Joints on Organic Solderability Preservatives(OSPs) Metal Surface Finish

HYOUNG-JOON KIM1,2 and KYUNG-WOOK PAIK1

1.—Nano Packaging and Interconnects Laboratory (NPIL), Department of Materials Science andEngineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong,Yuseong-gu, Daejeon 305-701, Republic of Korea. 2.—e-mail: [email protected]

The effect of final metal finishes of Cu electrodes on the adhesion and reli-ability of anisotropic conductive film (ACF) joints was investigated. Two dif-ferent metal surface finishes, electroless Ni/immersion Au (ENIG) and organicsolderability preservatives (OSPs) coated on Cu, were selected in this study forACF bonding. The adhesion strength of ACF/OSP joints was higher than thatof ACF/bare Cu and ACF/ENIG joints. The fracture sites of the ACF/bare Cuand ACF/ENIG joints were ACF/metal interfaces, while those of ACF/OSPjoints were inside the ACF. Transmission electron microscope (TEM) andFourier-transform infrared (FT-IR) analyses showed that the OSP coatinglayer on the Cu electrodes reacted with the epoxy resin of the ACFs but stillremained at the bonding interface. According to the in-depth X-ray photo-electron spectroscopy (XPS) analysis, additional C-N bonds formed after theOSP-epoxy reaction and the outermost nitrogen of the OSP layer participatedin curing of the epoxy resin of the ACF. Therefore, the OSP layer acted as anadhesion promoter to ACFs. Furthermore, this role of the OSP layer enhancedthe reliability of the ACF/OSP joints under high temperature and humidenvironments, as compared to the ACF/ENIG joints.

Key words: Anisotropic conductive film (ACF), organic solderabilitypreservative (OSP), adhesion, surface finish, flexible substrate,flex-on-board (FOB)

INTRODUCTION

An electroless Ni/immersion Au (ENIG) layer hasbeen used as a common metal surface finish ofprinted circuit boards (PCBs) for solder bonding aswell as anisotropic conductive film (ACF) bonding.However, ENIG has a higher processing cost, and ithas reliability problems. One such problem is knownas the black pad, which causes a brittle failure atENIG/solder joints. Therefore, various alternativemetal surface finish technologies such as an organicsolderability preservative (OSP), direct immersionAu (DIG), electroless Ni/electroless Pd/immersionAu (ENEPIG), and others have been proposed to

replace the ENIG system. Essentially, OSP is a thinorganic layer coated on the surface of Cu electrodes.It can protect the surface of Cu electrodes from oxi-dation and tarnishing. Although the protectivefunction of OSP disintegrates at elevated tempera-tures due to the instability of OSPs at high temper-atures, the use of OSP has increased recently. Thesimple processes, environmental considerations, andits lower cost are the driving forces in the growinguse of OSPs. The OSP process provides more than a50% cost reduction versus the ENIG process.1

Moreover, the need for coplanar surface mount(SMT) surfaces and the advent of chip scale pack-ages and ball grid arrays (BGAs) expand the need forOSPs due to its good co-planarity.2

Benzotriazole was one of the earliest OSPformulations. However, the copper–benzotriazole

(Received February 27, 2007; accepted January 18, 2008;published online March 29, 2008)

Journal of ELECTRONIC MATERIALS, Vol. 37, No. 7, 2008 Regular Issue Paper

DOI: 10.1007/s11664-008-0397-4 2008 TMS

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complex coating layer showed very poor oxidationresistance under thermal cycles, indicating that theheat resistance of OSP needed to be enhanced.Therefore, benzimidazole has been used as a basematerial of OSP to improve the thermal stability ofOSP, resulting in an OSP layer that can surviveseveral thermal cycles.3 In fact, benzimidazole hasbeen used extensively in industry as a good corro-sion inhibitor for transition metals and their alloysurfaces, as it forms a protective layer, particularlyin copper.4–7 The protective layer is formed initiallythrough a complexing reaction with copper to forman organometallic bond, followed by a build-up ofthe benzimidazole–copper complexes. The greaterthermal stability of the benzimidazole–copper com-plex layer allows OSP to be Pb-free compatible.Many studies on the interfacial phenomena andreliability issues of various Pb-free solders on anOSP-finished copper surface have been reported.8–11

Recently, modular approach has been the trend ofhandheld products manufactured for their higherfunctionality and smaller size. Each functionalmodule, on flexible substrates, is connected to themain organic rigid substrates. Organic rigid sub-strate–flexible substrate (RS–FS) bonding usingACFs is one of the most promising module assemblymethods. As a result, ACF bonding of a flexiblesubstrate on an OSP-finished rigid substrate hasbecome more important. However, the effect of anOSP coating on the adhesion and reliability of theACF/OSP joints has not been investigated. Fur-thermore, the feasibility of ACF bonding on theOSP-coated surface should be evaluated.

MATERIALS AND EXPERIMENTS

Test Vehicle Preparation

An ACF material for PCB bonding applicationwas used in this study. The details of the ACFmaterial are listed in Table I. In general, the ACFsfor PCB bonding applications use Ni metal balls asconductive particles instead of metal-coated poly-mer balls, due to the need for high current-carryingcapability and a high surface roughness of the metalelectrode on rigid substrates.

The FR-4-based rigid substrate had a 12 lm-thickCu metal electrode. The final metal surface finishconditions were ENIG with a 5-lm-thick electrolessNi and 0.3-lm-thick immersion Au layer, andOSP on copper surface. The pitch of the Cu metalelectrodes was 400 lm, consisting of Cu metal

electrodes with a 220 lm width and a 180 lm gapbetween adjacent Cu metal electrodes. A polyimide-based casting-type flexible substrate (EspaNexTM)with 25-lm-thick polyimide (PI) and 12-lm-thick Cumetal electrodes was used. The final metal finish offlexible substrates was 0.5 lm Ni/0.1 lm Au. Topviews of the rigid and flexible substrates are shownin Fig. 1. The contact structures of ACF/OSP andACF/ENIG joints were observed using a focused ionbeam (FIB). For the contact resistance measure-ment, Kelvin-patterned flexible and rigid substrateswere bonded using the ACF. To assemble the testvehicles, conventional thermocompression ACFbonding was performed by applying heat and pres-sure simultaneously for 20 s. The in situ tempera-ture at the ACF layer was measured by using anadhesive-type thermocouple, and the measuredtemperature profile is shown in Fig. 2. Bondingpressure was 7.2 MPa.

The Characteristics of the ACF/OSP Reaction

The morphologies of an OSP coating layer beforeand after ACF bonding were observed by FIB and atransmission electron microscope (TEM). Theseobservations were crucial for understanding the roleof the OSP coating layer during ACF bonding.

In order to investigate the reactivity between theepoxy resin of ACF and the OSP, an ACF withoutconductive particles and a latent curing agent wasprepared. This ACF was applied onto each of thethree type of Cu foils (bare Cu, ENIG finish, andOSP finish), and these three foils were heated to190C on a hot plate. Then, they were cooled to roomtemperature. After heating and cooling, the ACFson each Cu foil were removed with acetone. Becausethese ACFs did not contain a curing agent, no cur-ing reaction occurs even at a high temperature of190C. However, if there were any interfacial reac-tions between the epoxy resin of ACF and the sur-face finish materials, the residues or products ofchemical reactions would remain at the Cu foilsurface even after acetone cleaning. For this reason,the differences in the Cu foil surfaces were initiallychecked optically. And then, the surfaces wereanalyzed using a Fourier-transform infrared(FT-IR) spectroscopy and an X-ray photoelectronspectroscopy (XPS). XPS was performed in anultrahigh-vacuum (UHV) system at a base pressureof 10-10 Torr. Photoelectrons were excited by non-monochromatized Mg Ka (1253.6 eV) radiation. Thebinding energies were calibrated by setting theinstrument work function to give an Ag3d5/2 lineposition at 368.3 eV. A wide scan was taken in orderto survey all spectra, and high-resolution spectrawere also analyzed.

Adhesion Test

Three types of Cu foils (bare Cu, ENIG finish, andOSP finish) and two copper-clad laminates (CCLs)with an ENIG finish and an OSP finish were pre-pared in order to investigate the effect of the OSP

Table I. Details of the ACF Material

Base Resin Type Bisphenol A Type Epoxy

Tg (C) 108.5Thickness (lm) 40Width (mm) 2.5Conductive particle 6-lm-diameter Au-coated Ni ball

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finish on the adhesion of ACF joints. ACFs wereapplied on CCLs, and prebonding was performed at70C. After removing the releasing paper of ACFs,each Cu foil was bonded on CCLs using ACFs, asshown in Fig. 3. Following this step, a 90 deg peeltest was performed with a 10 mm s-1 test speed.After the peel test, the fractured sites of the CCLsand Cu foils were observed by using a scanningelectron microscope (SEM).

Reliability Test

After the Kelvin-patterned RS–FS bonding usingACFs, a pressure-cooker test (PCT) was performedas a reliability assessment. Test conditions for thePCT were 121C, 100% relative humidity (RH), and2 atm. During the test, the changes in the contactresistances were measured every 24 h for a totaltest time of 144 h. After the test, delaminations orcracks were observed using a cross-sectional SEM.

RESULTS AND DISCUSSION

Comparison of ACF Joint Structure andContact Resistance

Figure 4 shows cross-sections of the ACF jointsand the deformations of the conductive particles ineach surface-finished sample. As shown in Fig. 4,

the Ni conductive particles were well squeezedbetween both the Cu electrodes of the rigid andthe flexible substrates. Essentially, the contactsbetween the Cu electrodes and the conductive par-ticles were mechanically established by the con-traction of the ACF resin. Therefore, the electricalconduction was established through particle con-tacts, and initial contact resistances were nearlyidentical for both ENIG and OSP surface-finishedsamples, as shown in Table II.

OSP Layer Observation before and after ACFBonding

Figure 5a shows an OSP-finished Cu electrodesurface before ACF bonding. In this figure, a rect-angular hole was formed on the OSP-finished Cuelectrode, resulting in sputtering of Ar ions in theFIB chamber. Figure 5b shows the existence of theOSP coating layer on a Cu electrode. The thicknessof the OSP coating layer was about 70 nm to100 nm, and a small amount of thickness deviationwas caused by the roughness of the Cu electrode. Toobserve the change in the OSP layer after ACFbonding, an ACF/OSP joint was cross-sectionedusing the FIB. As shown in Fig. 6a, it was difficultto distinguish the OSP layer from the ACF layer bySEM observation, therefore a TEM analysis wasperformed. As described in Fig. 6b, the initial OSPcoating layer remained after the ACF bonding, anda well-defined ACF/OSP interface was observed.

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Fig. 1. Top views of (a) a rigid substrate and (b) a flexible substrate.

Fig. 3. (a) ENIG Cu foil-ENIG CCL and (b) OSP Cu foil-OSP CCLcombinations, prepared for the adhesion test.

Adhesion and Reliability of Anisotropic Conductive Films (ACFs) Joints onOrganic Solderability Preservatives (OSPs) Metal Surface Finish

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This result indicates that the OSP coating layer isnot totally decomposed or removed during ACFbonding. Moreover, the total thickness of the OSPlayer decreased from the as-coated 100 nm to 20 nmafter ACF bonding. Therefore, based on thesethickness changes, it is obvious that the OSP layerreacts with the epoxy resin of the ACF during theACF bonding process.

Characterization of the OSP/ACF Reactivity

To investigate the reactivity between the OSPand the ACF during ACF bonding, an ACF withouta curing agent was prepared. This ACF was then

applied to three types of Cu foils (bare Cu, ENIGfinish, and OSP finish) and these ACF-applied Cufoils were heated to the ACF curing temperatureand then cooled to room temperature.

Figure 7 shows the differences between each Cufoil surface after the thermal treatment. It wasfound that no reaction took place at the ACF/bareCu and ACF/ENIG interfaces. Therefore, the ACFson both bare and ENIG-finished Cu foils werecompletely removed by acetone cleaning. However,as shown in Fig. 7b, a stained area where the ACFwas applied was observed on the OSP-finished Cufoil surface, even after an acetone clean. This resultimplies that a certain chemical reaction between theepoxy resin of ACF and the benzimidazole of theOSP takes place at the ACF curing temperature,causing the reacted residue to remain even afteracetone cleaning. For an in-depth surface analysis,FT-IR and XPS were performed on the stained area.

Figure 8a shows the FT-IR analysis result on anas-received OSP-finished Cu foil surface. This graphis in good agreement with the reported result for

Table II. Measured Initial Contact Resistances(40 Measurements for Each Sample)

Sample Type Contact Resistance (mX)

ENIG-finished RS–FS 14.74 ± 0.59OSP-finished RS–FS 15.45 ± 0.59

Fig. 4. The deformation of conductive Ni particles in ACFs at (a) the ENIG-finished RS–FS bonding and (b) the OSP-finished RS–FS bondingACF joints by FIB cross-sectional analysis.

Fig. 5. Observation of (a) a top view and (b) a cross-section of the OSP layer on Cu electrode surface before ACF bonding using a FIB.

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OSP.12 In addition, Fig. 8b shows the FT-IR resultof the stained region of the Cu foil surface asdescribed in Fig. 7b. The largest peak differencebetween two graphs was observed in the 2,700 cm-1

to 3,000 cm-1 range. This peak comes from the C-Hantisymmetric stretching mode.13 As shown inFig. 8a, no peak was observed at 2,700 cm-1 to3,000 cm-1 from the as-received OSP-finished Cufoil surface, because the benzimidazole, the basematerial of OSP, consisted of benzene rings and animidazole structure. Therefore, the origin of thispeak at 2,700 cm-1 to 3,000 cm-1 comes from theepoxy resin of ACF. That is, it is clear that the epoxyresin of ACF can react with the benzimidazole ofOSP at the ACF curing temperatures.

The wide scan of the XPS analysis is shown inFig. 9. The intensity of the Cu2p3/2 peaks increasedat the stained area. This indicates that the OSPlayer was consumed by the interfacial reaction withACF at the ACF curing temperature. Therefore, theintensity of the Cu background peaks increasedafter the OSP-ACF reaction. This result agrees wellwith the cross-sectional TEM result. For in-depthanalysis of the chemical bonding changes, C1speaks were deconvoluted (see Fig. 10). The largestdifference before and after the OSP-ACF reactionwas the change in the C with N bond portion. Asshown in Fig. 10b, the portion of the C-N bond peakincreased after the OSP-ACF reaction. This resultimplies that additional C-N bonds formed as a result

Fig. 6. Observation of (a) the OSP/ACF interface after ACF bonding using the FIB-SEM. (b) The remaining OSP layer can be observed evenafter ACF bonding by cross-sectional TEM.


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of the interfacial reaction between the OSP and theACF. In other words, due to the similarity of thechemical structure between the benzimidazole of

the OSP and the imidazole curing agent of ACF, theoutermost nitrogen atom of OSP can open theepoxide ring, and initiate the epoxy curing reactionduring ACF bonding. Therefore, strong chemicalbonding as well as mechanical adhesive bonding canbe formed at the ACF/OSP interface, resulting in anenhancement of the adhesion strength and reli-ability. The possible reaction site during ACFbonding on the OSP layer is illustrated in Fig. 11.

Effect of OSP on Adhesion of ACF Joints

The results of a 90 deg peel test are listed inTable III. The average peel strength of the ACF-bonded OSP CCL-OSP Cu foil combination showedthe strongest adhesion strength among three ACF-bonded CCL-Cu foil combinations. In particular, forthe ENIG CCL-ENIG Cu foil combination, thedelamination of the ENIG layer on the Cu surfacesignificantly reduced the adhesion strength. Afterthe peel test, the fractured surfaces of the CCLs andCu foils were observed by SEM. According to thefailure analysis, the adhesion between the bare Cusurface and ACF was lower than that of the OSPlayer and ACF. Therefore, as shown in Fig. 12,the fracture path moved from the bare Cu/ACF

Bare Cu OSP ENIG

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Bare Cu OSP ENIGBare Cu OSP ENIG(a)

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Bare Cu OSP ENIGBare Cu OSP ENIG(b)

Fig. 7. Observation of the reactivity between the ACF and threesurface metal finishes (a) before and (b) after heat treatment.

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Fig. 8. Results of the FT-IR spectrum of (a) the as-received OSP-finished Cu surfaces and (b) the stained area of OSP-finished Cu surfacereacted with ACF after an acetone cleaning.

Kim and Paik1008

interface to inside the ACF layer. The fracture pathsof the samples are illustrated in Fig. 13. The changein the fracture path implies that the OSP coatinglayer has good adhesion with the epoxy-basedadhesive, and that the OSP finish can enhance thebondability of ACF joints. This result correspondswith a previous study which reported that thepolymeric adhesion promoter, polybenzimidazole,which is similar as the OSP material, showed betteradhesion strength as it prevented the surface oxi-dation of copper.14 In addition, it is also reported

that the benzimidazole can improve the adhesionbetween polymeric materials such as epoxy orpolyimide, and copper.15,16 Generally, imidazole-type curing agents are widely used for epoxy curing.The chemical structure of the OSP material (benz-imidazole) is also based on the imidazole system.Therefore, as described in Fig. 11, the chemicalreaction between benzimidazole in OSP and ACFcan form strong bonding at the ACF/OSP interface,resulting in an enhanced adhesion.

Reliability Test Results

According to the PCT results, OSP-finished sam-ples showed better PCT reliability compared withthe ENIG-finished samples. As shown in Fig. 14a,the degradation of the ACF joints in the ENIG-fin-ished samples started after 48 h of PCT. After 96 hof PCT, 50% of measurement points exceeded200 mX, and the degradation of the ACF joints wasaccelerated as the test time increased. However, the

Table III. Results of the 90 deg Peel Test

Sample Type Peel Adhesion Strength (gf)

ENIG CCL-ENIG Cu foil 144.7 ± 45.4OSP CCL-bare Cu foil 944.9 ± 90.6OSP CCl-OSP Cu foil 1167.4 ± 80.5

Fig. 11. A schematic of ACF bonding on the OSP-finished surface. A chemical reaction can occur between the outermost nitrogen of the OSPlayer and the epoxide ring of ACF.

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Original peakC combined HC combined N

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Fig. 10. XPS results of the C 1s peak deconvolution of (a) as-received OSP-finished Cu surface and (b) the stained area of OSP-finished Cusurface reacted with ACF.


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changes in the contact resistances of the ACF jointsin the OSP-finished samples were very stable up to120 h of PCT. Actually, the failure rate of the OSP-finished samples after 144 h of PCT was nearlyidentical to that of the ENIG-finished samplesafter 72 h of PCT. The main cause of the failure wasthe delamination of the ACF/PI of the flexible sub-strate interfaces in both surface-finished samples.

However, an additional failure site was observed inthe ENIG-finished samples. As shown in Fig. 15a,some additional delaminations and cracks occurredat the ACF/ENIG interface. This defect can act as apotential failure site during the reliability tests. Incontrast, no defects were observed in the OSP fin-ished samples. Consequently, it is considered thatthe OSP coating layer provides good adhesion withACFs, and that the improved initial adhesion influ-ences and enhances the reliability of ACF joints.

CONCLUSIONS

Benzimidazole, the base material of OSP,improved the adhesion strength of ACF joints.According to the results of TEM and FT-IR, theOSP-finished layer reacted with the epoxy resin ofthe ACF at the ACF curing temperature andenhanced the adhesion. That is, the OSP layer actedas an adhesion promoter. The improved adhesion atthe ACF/OSP interface affected the reliability ofassembled ACF joints. The ACF joint assembled

Fig. 12. Observation of the fractured (a) Cu foil and (b) CCL sides of OSP CCL-bare Cu foil bonded sample, and (c) Cu foil and (d) CCL sides ofOSP CCL-OSP Cu foil bonded sample.

Fig. 13. Schematics of the fracture paths of (a) OSP CCL-bare Cufoil, and (b) OSP CCL-OSP Cu foil.

Kim and Paik1010

with OSP-finished rigid substrates showed betterreliability than the ENIG-finished case. Besides, nodefects such as cracks and contact losses wereobserved at the ACF/OSP interface. These resultsimplied that the ACF/OSP interface was more sta-ble than the ACF/ENIG interface in high tempera-ture and humid environments. Therefore, OSP isapplicable to the final metal finish method formaking reliable ACF joints.

ACKNOWLEDGEMENT

This work was supported by the Center for Elec-tronic Packaging Materials (ERC) of MOST/KOSEF(Grant No. R11-2000-085-08005-0).

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Mater. 33, 157 (2004).12. M. Frederickson and B. Goers, Circ. World 24, 10 (1998).13. S. Yoshida and H. Ishida, J. Adhes. 16, 217 (1984).14. S.M. Song, K. Cho, C.E. Park, H.K. Yun, and S.Y. Oh,

J. Appl. Polym. Sci. 85, 2202 (2002).15. S. Siau, A. Vervaet, E. Schacht, S. Degrande, K. Callewaert,

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K.-W. Lee, J. Poly. Sci. Part B 37, 2806 (1999).

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Fig. 15. Cross-sections of (a) ENIG-finished and (b) OSP-finished samples after the pressure-cooker test.


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adhesion and reliability of anisotropic conductive films (acfs

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