adhesion and reliability of anisotropic conductive films ... · adhesion and reliability of...

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

Hyoung-Joon Kim and Kyung-Wook Paik

Nano Packaging and Interconnects Lab. (NPIL)

Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST)

373-1, Guseong-dong, Yuseong-gu, Daejeon, 305-701, Republic of Korea Tel: +82-42-869-3375, Fax: +82-42-869-3310

* e-mail: [email protected]

Abstract The effect of final metal finishes of Cu electrodes on the

adhesion and reliability of anisotropic conductive film (ACF) joints was investigated. Two different metal surface finishes, electroless Ni/immersion Au (ENIG) and organic solderability preservatives (OSPs) coated on Cu, were selected in this study. These are commonly used as final metal finish materials in the printed circuit board (PCB) industry. However, the effect of OSPs coated on Cu on the adhesion and reliability of ACF joints has not been studied yet. Therefore, the adhesion and reliability of ACF/OSP joints were investigated. The results of ACF/OSP joints were compared with those of ACF/ENIG joints to evaluate the feasibility of ACF/OSP joints in real packaging applications.

For electrical continuity and reliability tests, patterned flexible substrates and FR4 rigid substrates were prepared. A flexible substrate was bonded to two types of organic FR4 substrates with different metal surface finishes, ENIG and OSP, using ACFs. According to the result of contact resistance measurement, the initial contact resistances of the ACF/OSP joints, approximately 15 mΩ, were almost the same as those of the ACF/ENIG joints. For the reliability test, a pressure cooker test (PCT) was performed in order to verify the stability of the ACF/OSP joints in high temperature and humid environments. According to the results of PCT, ACF/OSP joints showed better reliability compared to ACF/ENIG joints.

The adhesion strength of ACF/OSP joints was higher than that of ACF/bare Cu and ACF/ENIG joints. The fracture site of the ACF/bare Cu and ACF/ENIG joints was the ACF/metal interface, while that of ACF/OSP joints was inside the ACF. TEM and FT-IR analyses showed that the OSP coating layer on the Cu electrodes remained after ACF bonding, and OSP layer acted as an adhesion promoter to ACFs.

1. Introduction An electroless Ni/immersion Au (ENIG) layer has been

used as a common metal surface finish of printed circuit boards (PCBs) for solder bonding as well as ACF bonding. However, ENIG has a higher processing cost, and it has reliability problems. One such problem is known as the “black pad”, which causes a brittle failure at ENIG/solder joints. Therefore, various alternative metal surface finish technologies such as an organic solderability preservative (OSP), direct immersion Au (DIG), electroless Ni/electroless

Pd/immersion Au (ENEPIG), and others have been proposed to replace the ENIG system. Essentially, OSP is a thin organic layer coated on the surface of Cu electrodes. It can protect the surface of Cu electrodes from oxidation and tarnishing. Although the protective function of OSP disintegrates at elevated temperatures due to instability of OSPs at high temperatures, the use of OSP has increased recently. The simple processes, environmental considerations, and its lower cost are the driving forces in growing of the use of OSPs. The OSP process provides more than a 50% of cost reduction versus the ENIG process [1]. Moreover, the need for coplanar SMT surfaces and the advent of chip scale packages and BGA expand the need of OSP due to its good co-planarity [2].

Benzotriazole was one of the earliest OSP formulations. However, the copper-benzotriazole complex coating layer showed very poor oxidation resistance under thermal cycles, indicating that the heat resistance of OSP needed to be enhanced. Therefore, benzimidazole has been used as an OSP material to improve the thermal stability of OSPs, resulting in the OSP coating layer to be survived after several thermal cycles [3]. In fact, benzimidazole has been used extensively in industries as a good corrosion inhibitor for transition metals and their alloy surfaces, as it forms a protective layer, particularly in copper [4-7]. The protective layer is formed initially through a complexing reaction with copper to form an organo-metallic bond, followed by a build-up of the benzimidazole-copper complexes (see figure 1). Higher thermal stability of the benzimidazole coated layer allows OSP to be Pb-free compatible. Many studies on the interfacial phenomena and reliability issues of various Pb-free solders and OSP have been reported.

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Figure 1. A schematic of an OSP layer on a copper surface as the result of a benzimidazole-copper complexing reaction.

1-4244-0985-3/07/$25.00 ©2007 IEEE 1707 2007 Electronic Components and Technology Conference

Recently, modulation has been the trend of hand-held products manufactured for their higher functionality and smaller size. Each functional module in flexible substrates is connected to the main organic rigid substrates. Organic rigid substrate-flexible substrate (RS-FS) bonding using ACFs is one of the most promising modulation assembly methods. As a result, ACF bonding of a flexible substrate on an OSP finished rigid substrate has become more important. However, the effect of an OSP coating on the adhesion and reliability of ACF/OSP joints has not been investigated. Furthermore, the feasibility of the ACF bonding on the OSP coated surface should be evaluated.

2. Materials & Experiments

2.1. ACF material An ACF material for out lead bonding (OLB) application

was used in this study. The details of the ACF used in this study are listed in the Table 1. In general, ACFs for PCB bonding applications use Ni metal balls as a conductive particle instead of metal-coated polymer balls, due to the need of high current-carrying capability and a high surface roughness of the metal trace on rigid substrates.

2.2. Interface observation & electrical continuity test The contact structures of ACF/OSP and ACF/ENIG joints

were observed using a FIB (Focused Ion Beam). For the electrical continuity test, kelvin-patterned flexible and rigid substrates, designed for contact resistance measurement, were bonded using ACF. The rigid substrate had a 12 um-thick Cu metal trace on a 1 mm-thick FR-4. The final metal surface finish conditions were ENIG with a 5 um-thick electroless Ni and 0.3 um-thick immersion Au layer and OSP on copper surface. The pitch of the Cu metal electrodes was 400 um, consisting of Cu metal electrodes with a 220 um width and a 180 um gap between adjacent Cu metal electrodes. A polyimide based casting type flexible substrate (EspaNexTM) with 25 um-thick polyimide (PI) and 12 um-thick Cu metal traces was used. The final metal finish of flexible substrates was 0.5 um Ni/0.1 um Au. The top-views of the rigid and flexible substrates are shown in figure 2.

2.3. The characteristic of ACF/OSP reaction The morphologies of an OSP coating layer before and

after ACF bonding were observed by FIB and a FE-TEM. These observations were crucial for understanding the role of the OSP coating layer during ACF bonding.

In order to investigate the reactivity between the epoxy resin of ACF and OSP, ACF without conductive particles and a latent curing agent was prepared. This ACF was applied onto each of the three type of Cu foils (bare Cu, ENIG finish, and OSP finish), and these three foils were heated to 190oC on a hot plate. Then, they were cooled to room temperature. After heating and cooling, the ACFs on each Cu foil were removed with acetone. Because these ACFs did not contain a curing agent, thus no curing reaction occurs even at a high temperature of 190oC. However, if there were any interfacial reactions between the epoxy resin of ACF and the surface finish materials, the residues or products of chemical reactions would remain at the Cu foil surface after acetone cleaning. For

Table 1. Details of the ACF material.

Base resin type Bisphenol A type epoxy Tg (oC) 108.5

Thickness (um) 40 Width (mm) 2.5

Conductive particle 6 um diameter Au-coated Ni ball

(a) Top-view of a rigid substrate (b) Top-view of a flexible substrate

Figure 2. Top-views of the rigid and flexible substrates.

this reason, the differences in the Cu foil surfaces were initially checked optically. Second, the surfaces were analyzed using FT-IR and X-ray photoelectron spectroscopy (XPS). XPS was performed in a UHV (Ultra High Vacuum) system at a base pressure of ~10-10 torr. Photoelectrons were excited by non-monochromatized Mg Kα (1253.6 eV) radiation. The binding energies were calibrated by setting the instrument work function to give an Ag3d5/2 line position at 368.3 eV. A wide scan was taken in order to survey all spectra, and high-resolution spectra were also analyzed.

2.4. Adhesion test Three types of Cu foils (bare Cu, ENIG finish, and OSP

finish) and two copper clad laminates (CCLs) with an ENIG finish and an OSP finish were prepared in order to investigate the effect of the OSP finish on the adhesion of ACF joints. ACFs were applied on CCLs, and pre-bonding was performed at 70oC. After removing the release paper of ACFs, each Cu foil was bonded on CCLs using ACFs, as shown in figure 3. Following this step, a 90o peel test was performed with a 10 mm/sec test speed. After the peel test, the fractured sites of the CCLs and Cu foils were observed via SEM.

(a) ENIG Cu foil bonded on

ENIG CCL (b) OSP Cu foil bonded on

OSP CCL Figure 3. Samples for the adhesion measurement.

2.5. Reliability test After patterned RS-FS bonding using ACFs, a pressure

cooker test was performed as a reliability assessment. The

1708 2007 Electronic Components and Technology Conference

condition of PCT is listed in the Table 2. During the test, changes in the contact resistances were measured in every 24 hours. After the test, delaminations or cracks during PCT were observed using a SEM.

Table 2. Pressure cooker test conditions

Temperature 121oC Humidity 100 %RH Pressure 2 atm Test time 144 hours

3. Materials & Experiments

3.1. Comparison of ACF joint structure & contact resistance of ENIG and OSP finished joints

Figure 4 shows the cross-sections of the ACF joints and the deformation of the conductive particles in each surface finished sample. As shown in figure 4, the Ni conductive particles were well squeezed between both the Cu electrodes of the rigid and the flexible substrates. Essentially, contacts between the Cu electrodes and conductive particles were mechanically established by the contraction of ACF resin. Therefore, in both cases (ENIG finish and OSP finish) electrical conduction was established through particle contacts. Initial contact resistances were nearly identical for both surface finished samples as shown in the Table 3.

(a) ENIG finished RS-FS

bonding (b) OSP finished RS-FS

bonding Figure 4. The tilted view of the ACF joint structures and the deformation of conductive Ni particles in each surface finished sample by FIB cross-sectional analysis. Table 3. Measured initial contact resistances. (40 measurement for each sample)

Sample type Contact resistance (mΩ) ENIG finished RS-FS 14.74 ± 0.59 OSP finished RS-FS 15.45 ± 0.59

3.2. OSP layer observation before and after ACF bonding Figure 5 (a) shows an OSP finished Cu electrode surface

before ACF bonding. In this figure, a rectangular hole was formed on the OSP finished Cu surface, resulting in sputtering of Ar ions in the FIB chamber. Figure 5 (b) shows the existence of the OSP coating layer on a Cu surface. The

thickness of the OSP coating layer was about 70 ~ 100 nm, and a small amount of thickness deviation was caused by the roughness of the Cu electrode. To observe the change in the OSP layer after ACF bonding, an OSP/ACF joint was cross-sectioned using the FIB. As shown in figure 6 (a), it was difficult to distinguish the OSP layer from the ACF layer by SEM observation, therefore, a TEM analysis was performed. As described in figure 6 (b), the initial OSP coating layer remained after the ACF bonding, and a well-defined ACF/OSP interface was observed. This result indicates that the OSP coating layer is not decomposed or disappears during ACF bonding. Moreover, the total thickness of the OSP layer decreased from as-coated 100 nm to 20 nm after ACF bonding. Therefore, based on these thickness change observations, it is obvious that the OSP layer reacts with the ACF resin during the ACF bonding process.

(a) Top-view of OSP finished

Cu electrode (b) Cross-section of OSP

finished Cu electrode Figure 5. Observation of the OSP coating layer on Cu surface before ACF bonding using a FIB.

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Figure 6. Observation of the OSP/ACF interface after ACF bonding using FIB and TEM.


3.3. Characterization of the OSP/ACF reactivity To investigate the reactivity between OSP and ACF

during ACF bonding, an ACF without a curing agent was prepared. This ACF was then applied on three types of Cu foils (bare Cu, ENIG finish, and OSP finish) and these ACF-applied Cu foils were heated up to the ACF curing temperature and then cooled.

Figure 7 shows the differences of each Cu foil surface after the thermal treatment. It was found that no reaction took place at the ACF/bare Cu and ACF/ENIG interfaces. Therefore, the ACFs on both bare and ENIG finished Cu foils were completely removed by acetone cleaning. However, as shown in figure 7 (b), a stained area where the ACF was previously applied was observed on the OSP finished Cu foil surface, even after an acetone cleaning. This result implies that a certain chemical reaction between the epoxy resin of ACF and the benzimidazole of OSP takes place at the ACF curing temperature, causing the reacted residue to remain even after acetone cleaning. For an in-depth surface analysis, FT-IR and XPS were performed on the stained area.

Figure 8 (a) shows the FT-IR analysis result on an as-received OSP finished Cu foil surface. This graph is in good agreement with the reported result of OSP [8]. In addition, figure 8 (b) shows the FT-IR result of the stained region of the Cu foil surface as described in figure 7 (b). The largest peak difference between two graphs was observed in the 2700 ~ 3000 cm-1 range. This peak comes from the C-H anti-symmetric stretching mode [9]. As shown in figure 8 (a), no sharp peak was observed at 2700 ~ 3000 cm-1 from the as-received OSP finished Cu surface, because the benzimidazole, the base material of OSP, consisted of benzene rings and an imidazole structure. Therefore, the origin of this peak at 2700 ~ 3000 cm-1 comes from the epoxy resin of ACF. That is, it is clean that the epoxy resin of ACF can react with the benzimidazole of OSP at the ACF curing temperatures.

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Figure 7. Observation of the reactivity between ACF and three surface metal finishes.

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Figure 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 surface reacted with ACF after an acetone cleaning.

The wide scan of the XPS analysis is shown in figure 9.

The intensity of the Cu2p3/2 peaks increased in the stained area. This indicates that the OSP layer was consumed by the interfacial reaction with ACF at the ACF curing temperature. Therefore, the intensity of the Cu background peaks increased after the OSP-ACF reaction. This result is well-agreed with the cross-sectional TEM result. For in-depth analysis of the chemical bonding changes, C1s peaks were deconvoluted (see figure 10). The largest difference before and after the OSP-ACF reaction was the change in the C with N bond portion. As shown in figure 10 (b), the portion of the C-N bond peak increased after the OSP-ACF reaction. This result implies that additional C-N bonds formed as a result of the interfacial reaction between OSP and ACF. In other words, due to the similarity of the chemical structure between the benzimidazole of OSP and the imidazole curing agent, the outermost nitrogen atom of OSP opens the epoxide ring, and initiates the epoxy curing reaction during ACF bonding. Therefore, strong chemical bonding as well as mechanical adhesive bonding can be formed at the ACF/OSP interface, resulting in an enhancement of the adhesion strength and reliability. The possible reaction site during ACF bonding on the OSP layer is illustrated in figure 11.


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Figure 9. Results of the XPS analysis of bare (B-Cu) and OSP finished (OSP-Cu) and the stained area of OSP finished Cu surfaces reacted with ACF after and acetone cleaning (OSP-ACF-Cu).

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ACF after and acetone cleaning Figure 10. XPS results of the C1s peak deconvolution.

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

3.4. Effect of OSP on adhesion of ACF joints The results of a 90o peel test are illustrated in figure 12.

The average peel strength of the ACF bonded OSP CCL-OSP Cu foil combination showed the strongest adhesion strength among three ACF bonded CCL-Cu foil combinations. In particular, for the ENIG CCL-ENIG Cu foil combination, the delamination of the ENIG layer on the Cu surface significantly reduced the adhesion strength. After the peel test, fractured surfaces of CCLs and Cu foils were observed by SEM. According to the failure analysis, the adhesion between the bare Cu surface and ACF was lower than that of the OSP layer and ACF. Therefore, as shown in figure 13, the fracture path moved from the bare Cu/ACF interface to the inside of the ACF layer. The fracture paths of the samples are illustrated in figure 14. The change in the fracture path implies that the OSP coating layer has good adhesion with the epoxy-based adhesive, and that the OSP finish can enhance the bondability of ACF joints. This result corresponds with a previous study which reported that the polymeric adhesion promoter, polybenzimidazole, which is similar as the OSP material, showed better adhesion strength as it prevented the surface oxidation of copper [10]. In addition, it is also reported that the benzimidazole can improve the adhesion between polymeric materials such as epoxy or polyimide, and copper [11-12]. Generally, imidazole-type curing agents are widely used for epoxy curing. The chemical structure of the

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(a) Cu foil side: OSP CCL-Bare Cu foil

(b) Cu foil side: OSP CCL-OSP Cu foil

(c) CCL side: OSP CCL-

Bare Cu foil (d) CCL side: OSP CCL-

OSP Cu foil Figure 13. Observation of the fracture sites of CCLs and Cu foils.

(a) OSP CCL-bare Cu foil (b) OSP CCL-OSP Cu foil

Figure 14. Schematics of the fracture paths in each sample.

OSP material (benzimidazole) is also based on the imidazole system. Therefore, as described in figure 11, the chemical reaction between benzimidazole in OSP and ACF can form strong bonding at the ACF/OSP interface, resulting in an enhanced adhesion.

3.5. Reliability test results According to the PCT results, OSP finished samples

showed better PCT reliability compared with the ENIG finished samples. As shown in figure 15 (a), the degradation of the ACF joints in the ENIG finished samples started after 48 hours of PCT. After 96 hours of PCT, 50% of measurement points exceeded 200 mΩ, and the degradation of the ACF joints was accelerated as the test time increased. However, the changes in the contact resistances of the ACF joints in the OSP finished samples were very stable up to 120 hours of PCT. Actually, the failure rate of the OSP finished samples after 144 hours of PCT was nearly identical to that of

the ENIG finished samples after 72 hours of PCT. The main cause of the failure was the delamination of the ACF/PI of the flexible substrate interfaces in both surface finished samples. However, an additional failure site was observed in the ENIG finished samples. As shown in figure 16 (a), some additional delaminations and cracks occurred at the ACF/ENIG interface. This defect can act as a potential failure site during the reliability tests. In contrast, no defects were observed in the OSP finished samples. Consequently, it is considered that the OSP coating layer provides good adhesion with ACFs, and that the improved initial adhesion influences and enhances the reliability of ACF joints.

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Figure 15. Results of the changes of contact resistance during the pressure cooker test.

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(a) ENIG finished sample (b) OSP finished sample Figure 16. Cross-sections of ENIG finished and OSP finished samples after the pressure cooker test.

OSP Cu elect. of FR4

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OSP Cu elect. of FR4

Cu elect. of flex


4. Conclusions (1) Benzimidazole, the base material of OSP, improved the

adhesion strength of ACF joints. According to the results of TEM and FT-IR, the OSP finished layer reacted with epoxy resin of ACF at the ACF curing temperature, and it enhanced the adhesion. Therefore, OSP layer played as an adhesion promoter.

(2) ACF joints, assembled with OSP finished rigid substrates, showed better reliability than ENIG finished case. Besides, no defects were observed at ACF/OSP interface. These results implied that the ACF/OSP interface was more stable than the ENIG/ACF interface under high temperature and humid environments.

(3) Therefore, OSP is applicable to the final metal finish method for making reliable ACF joints.

Acknowledgments This work was supported by the Center for Electronic

Packaging Materials (ERC) of MOST/KOSEF (Grant No. R11-2000-085-08005-0).

References 1. E. Stafstron, “Untraveling the Final Finishing Mystery”,

Circuit Assembly, No. 11, (Nov 2000), pp. 56-62. 2. M. Carano, “OSP Evolution”, Printed Circuit Fabrication,

Vol. 20, No. 7, (1997), pp. 28-31. 3. J. D. Debiase, “Organic Solderability Preservatives:

Benzotriazoles and Substituted Benzimidazoles”, Surface Mount International, (1996), pp. 763-776

4. G. Lewis, “The Corrosion Inhibition of Copper by Benzimidazole”, Corrosion Science, Vol. 22, No. 6, (1982), pp. 579-584.

5. D. P. Drolet, et al., “FT-IR and XPS Study of Copper(II) Complexes of Imidazole and Benzimidazole”, Inorganica Chimica Acta, Vol. 146, (1988), pp. 173-180.

6. Yu. I. Kuznetsov, et al., “Chemical Structure Benzoimidazoles and Their Protective Effects on Zinc and Copper in Phosphate Electrolytes”, Protection of Metals, Vol. 40, No. 2, (2004), pp. 130-135.

7. G. Xue, et al., “The Formation of a Compact Polymer Film on a Copper Electrode from Benzimidazole Solution”, J. Electroanal. Chem., Vol. 270, (1989), pp. 163-173.

8. M. Frederickson, et al., “Real-time Control of Advanced Solderability Preservatives”, Circuit World, Vol. 24, No. 2, (1998), pp. 10-13.

9. S. Yoshida, et al., “A FT-IT Reflection-Absorption Spectroscopy Study of an Epoxy Coating on Imidazole-Treated Copper”, J. Adhesion, Vol. 16, (1984), pp. 217-232.

10. S. M. Song, et al., “Synthesis and Characterization of Water-Soluble Polymeric Adhesion Promoter for Epoxy Resin/Copper Joints”, J. Appl. Poly. Sci., Vol. 85, (2002), pp. 2202-2210.

11. S. Siau, et al., “Chemical Modification of Buildup Epoxy Surfaces for Altering the Adhesion of Electrochemically Deposited Copper”, J. the Electrochem. Socie., Vol. 152, No. 9, (2005), pp. D136-D150.

12. J. Yu, et al., “Miscibility of Polyimide with Polymeric Primer and Its Influence on Adhesion of Polyimide to the Primed Copper Metal: Effect of Precursor Origin”, J. Poly. Sci. Part B, Vol. 37, (1999), pp. 2806-2814.


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