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

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1 Adhesion and eliability of Anisotropic onductive Films (AFs) Joints on rganic Solderability Preservatives (SPs) Metal Surface Finish Hyoung-Joon Kim and Kyung-Wook Paik ano Packaging and Interconnects Lab. (PIL) Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 373-1, Guseong-dong, Yuseong-gu, Daejeon, , epublic of Korea Tel: , Fax: * kwpaik@kaist.ac.kr Abstract The effect of final metal finishes of u s on the adhesion and reliability of anisotropic conductive film (AF) joints was investigated. Two different metal surface finishes, electroless i/immersion Au (EIG) and organic solderability preservatives (SPs) coated on u, were selected in this study. These are commonly used as final metal finish materials in the printed circuit board (PB) industry. However, the effect of SPs coated on u on the adhesion and reliability of AF joints has not been studied yet. Therefore, the adhesion and reliability of AF/SP joints were investigated. The results of AF/SP joints were compared with those of AF/EIG joints to evaluate the feasibility of AF/SP joints in real packaging applications. For electrical continuity and reliability tests, patterned flexible substrates and F4 rigid substrates were prepared. A flexible substrate was bonded to two types of organic F4 substrates with different metal surface finishes, EIG and SP, using AFs. According to the result of contact resistance measurement, the initial contact resistances of the AF/SP joints, approximately 15 mω, were almost the same as those of the AF/EIG joints. For the reliability test, a pressure cooker test (PT) was performed in order to verify the stability of the AF/SP joints in high temperature and humid environments. According to the results of PT, AF/SP joints showed better reliability compared to AF/EIG joints. The adhesion strength of AF/SP joints was higher than that of AF/bare u and AF/EIG joints. The fracture site of the AF/bare u and AF/EIG joints was the AF/metal interface, while that of AF/SP joints was inside the AF. TEM and FT-I analyses showed that the SP coating layer on the u s remained after AF bonding, and SP layer acted as an adhesion promoter to AFs. 1. Introduction An electroless i/immersion Au (EIG) layer has been used as a common metal surface finish of printed circuit boards (PBs) for solder bonding as well as AF bonding. However, EIG has a higher processing cost, and it has reliability problems. ne such problem is known as the black pad, which causes a brittle failure at EIG/solder joints. Therefore, various alternative metal surface finish technologies such as an organic solderability preservative (SP), direct immersion Au (DIG), electroless i/electroless Pd/immersion Au (EEPIG), and others have been proposed to replace the EIG system. Essentially, SP is a thin organic layer coated on the surface of u s. It can protect the surface of u s from oxidation and tarnishing. Although the protective function of SP disintegrates at elevated temperatures due to instability of SPs at high temperatures, the use of SP has increased recently. The simple processes, environmental considerations, and its lower cost are the driving forces in growing of the use of SPs. The SP process provides more than a 50% of cost reduction versus the EIG process [1]. Moreover, the need for coplanar SMT surfaces and the advent of chip scale packages and BGA expand the need of SP due to its good co-planarity [2]. Benzotriazole was one of the earliest SP formulations. However, the copper-benzotriazole complex coating layer showed very poor oxidation resistance under thermal cycles, indicating that the heat resistance of SP needed to be enhanced. Therefore, benzimidazole has been used as an SP material to improve the thermal stability of SPs, resulting in the SP 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 SP to be Pb-free compatible. Many studies on the interfacial phenomena and reliability issues of various Pb-free solders and SP have been reported. u u u u u u u u u Figure 1. A schematic of an SP layer on a copper surface as the result of a benzimidazole-copper complexing reaction. n /07/$ IEEE Electronic omponents and Technology onference

2 ecently, 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. rganic rigid substrate-flexible substrate (S-FS) bonding using AFs is one of the most promising modulation assembly methods. As a result, AF bonding of a flexible substrate on an SP finished rigid substrate has become more important. However, the effect of an SP coating on the adhesion and reliability of AF/SP joints has not been investigated. Furthermore, the feasibility of the AF bonding on the SP coated surface should be evaluated. 2. Materials & Experiments 2.1. AF material An AF material for out lead bonding (LB) application was used in this study. The details of the AF used in this study are listed in the Table 1. In general, AFs for PB bonding applications use i 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 Interface observation & electrical continuity test The contact structures of AF/SP and AF/EIG 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 AF. The rigid substrate had a 12 um-thick u metal trace on a 1 mm-thick F-4. The final metal surface finish conditions were EIG with a 5 um-thick electroless i and 0.3 um-thick immersion Au layer and SP on copper surface. The pitch of the u metal s was 400 um, consisting of u metal s with a 220 um width and a 180 um gap between adjacent u metal s. A polyimide based casting type flexible substrate (Espaex TM ) with 25 um-thick polyimide (PI) and 12 um-thick u metal traces was used. The final metal finish of flexible substrates was 0.5 um i/0.1 um Au. The top-views of the rigid and flexible substrates are shown in figure The characteristic of AF/SP reaction The morphologies of an SP coating layer before and after AF bonding were observed by FIB and a FE-TEM. These observations were crucial for understanding the role of the SP coating layer during AF bonding. In order to investigate the reactivity between the epoxy resin of AF and SP, AF without conductive particles and a latent curing agent was prepared. This AF was applied onto each of the three type of u foils (bare u, EIG finish, and SP finish), and these three foils were heated to 190 o on a hot plate. Then, they were cooled to room temperature. After heating and cooling, the AFs on each u foil were removed with acetone. Because these AFs did not contain a curing agent, thus no curing reaction occurs even at a high temperature of 190 o. However, if there were any interfacial reactions between the epoxy resin of AF and the surface finish materials, the residues or products of chemical reactions would remain at the u foil surface after acetone cleaning. For Table 1. Details of the AF material. Base resin type Bisphenol A type epoxy Tg ( o ) Thickness (um) 40 Width (mm) 2.5 onductive particle 6 um diameter Au-coated i ball (b) Top-view of a (a) Top-view of a rigid substrate flexible substrate Figure 2. Top-views of the rigid and flexible substrates. this reason, the differences in the u foil surfaces were initially checked optically. Second, the surfaces were analyzed using FT-I 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 ( ev) radiation. The binding energies were calibrated by setting the instrument work function to give an Ag 3 d 5/2 line position at ev. A wide scan was taken in order to survey all spectra, and high-resolution spectra were also analyzed Adhesion test Three types of u foils (bare u, EIG finish, and SP finish) and two copper clad laminates (Ls) with an EIG finish and an SP finish were prepared in order to investigate the effect of the SP finish on the adhesion of AF joints. AFs were applied on Ls, and pre-bonding was performed at 70 o. After removing the release paper of AFs, each u foil was bonded on Ls using AFs, as shown in figure 3. Following this step, a 90 o peel test was performed with a 10 mm/sec test speed. After the peel test, the fractured sites of the Ls and u foils were observed via SEM. (a) EIG u foil bonded on (b) SP u foil bonded on EIG L SP L Figure 3. Samples for the adhesion measurement eliability test After patterned S-FS bonding using AFs, a pressure cooker test was performed as a reliability assessment. The Electronic omponents and Technology onference

3 condition of PT 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 PT were observed using a SEM. Table 2. Pressure cooker test conditions Temperature Humidity Pressure Test time 121 o 100 %H 2 atm 144 hours 3. Materials & Experiments 3.1. omparison of AF joint structure & contact resistance of EIG and SP finished joints Figure 4 shows the cross-sections of the AF joints and the deformation of the conductive particles in each surface finished sample. As shown in figure 4, the i conductive particles were well squeezed between both the u s of the rigid and the flexible substrates. Essentially, contacts between the u s and conductive particles were mechanically established by the contraction of AF resin. Therefore, in both cases (EIG finish and SP 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. thickness of the SP coating layer was about 70 ~ 100 nm, and a small amount of thickness deviation was caused by the roughness of the u. To observe the change in the SP layer after AF bonding, an SP/AF joint was crosssectioned using the FIB. As shown in figure 6 (a), it was difficult to distinguish the SP layer from the AF layer by SEM observation, therefore, a TEM analysis was performed. As described in figure 6 (b), the initial SP coating layer remained after the AF bonding, and a well-defined AF/SP interface was observed. This result indicates that the SP coating layer is not decomposed or disappears during AF bonding. Moreover, the total thickness of the SP layer decreased from as-coated 100 nm to 20 nm after AF bonding. Therefore, based on these thickness change observations, it is obvious that the SP layer reacts with the AF resin during the AF bonding process. (a) Top-view of SP finished (b) ross-section of SP u finished u Figure 5. bservation of the SP coating layer on u surface before AF bonding using a FIB. SP u of F4 EIG u of flex SP u of F4 AF (a) EIG finished S-FS (b) SP finished S-FS bonding bonding Figure 4. The tilted view of the AF joint structures and the deformation of conductive i particles in each surface finished sample by FIB cross-sectional analysis. Table 3. Measured initial contact resistances. (40 measurement for each sample) Sample type ontact resistance (mω) EIG finished S-FS ± 0.59 SP finished S-FS ± SP layer observation before and after AF bonding Figure 5 (a) shows an SP finished u surface before AF bonding. In this figure, a rectangular hole was formed on the SP finished u surface, resulting in sputtering of Ar ions in the FIB chamber. Figure 5 (b) shows the existence of the SP coating layer on a u surface. The AF (a) ross-section of AF/SP interface AF u of F4 Pt layer AF SP layer u of F4 (b) ross-sectional TEM of the AF/SP interface Figure 6. bservation of the SP/AF interface after AF bonding using FIB and TEM Electronic omponents and Technology onference

4 3.3. haracterization of the SP/AF reactivity To investigate the reactivity between SP and AF during AF bonding, an AF without a curing agent was prepared. This AF was then applied on three types of u foils (bare u, EIG finish, and SP finish) and these AFapplied u foils were heated up to the AF curing temperature and then cooled. Figure 7 shows the differences of each u foil surface after the thermal treatment. It was found that no reaction took place at the AF/bare u and AF/EIG interfaces. Therefore, the AFs on both bare and EIG finished u foils were completely removed by acetone cleaning. However, as shown in figure 7 (b), a stained area where the AF was previously applied was observed on the SP finished u foil surface, even after an acetone cleaning. This result implies that a certain chemical reaction between the epoxy resin of AF and the benzimidazole of SP takes place at the AF curing temperature, causing the reacted residue to remain even after acetone cleaning. For an in-depth surface analysis, FT-I and XPS were performed on the stained area. Figure 8 (a) shows the FT-I analysis result on an asreceived SP finished u foil surface. This graph is in good agreement with the reported result of SP [8]. In addition, figure 8 (b) shows the FT-I result of the stained region of the u 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 -H antisymmetric stretching mode [9]. As shown in figure 8 (a), no sharp peak was observed at 2700 ~ 3000 cm -1 from the asreceived SP finished u surface, because the benzimidazole, the base material of SP, 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 AF. That is, it is clean that the epoxy resin of AF can react with the benzimidazole of SP at the AF curing temperatures. Bare u SP EIG AF w/o curing agent (a) Before heating Bare u SP EIG Stained area (b) after heating and cooling Figure 7. bservation of the reactivity between AF and three surface metal finishes. Intensity (a.u.) Intensity (a.u) As-received SP finished SP u u surface surface Wavenumber (cm -1 ) (a) SP finished u foil surface (as-received) After SP F u surface cleaning on after SP SP-AF finished u reaction surface Wavenumber (cm -1 ) (b) Stained region of the u foil surface Figure 8. esults of the FT-I spectrum of (a) the asreceived SP finished u surfaces and (b) the stained area of SP finished u surface reacted with AF after an acetone cleaning. The wide scan of the XPS analysis is shown in figure 9. The intensity of the u2p 3/2 peaks increased in the stained area. This indicates that the SP layer was consumed by the interfacial reaction with AF at the AF curing temperature. Therefore, the intensity of the u background peaks increased after the SP-AF reaction. This result is well-agreed with the cross-sectional TEM result. For in-depth analysis of the chemical bonding changes, 1s peaks were deconvoluted (see figure 10). The largest difference before and after the SP- AF reaction was the change in the with bond portion. As shown in figure 10 (b), the portion of the - bond peak increased after the SP-AF reaction. This result implies that additional - bonds formed as a result of the interfacial reaction between SP and AF. In other words, due to the similarity of the chemical structure between the benzimidazole of SP and the imidazole curing agent, the outermost nitrogen atom of SP opens the epoxide ring, and initiates the epoxy curing reaction during AF bonding. Therefore, strong chemical bonding as well as mechanical adhesive bonding can be formed at the AF/SP interface, resulting in an enhancement of the adhesion strength and reliability. The possible reaction site during AF bonding on the SP layer is illustrated in figure Electronic omponents and Technology onference

5 1s H 2 H 2 H 2 H 2 Intensity (arbitrary unit) 1s 1s B-u SP-u SP-AF-u u2p 3/2 H H H H H 2 H 2 u 2+ u 2+ u2+ 2+ u2+ 2+ H H H 2 H H H 2 AF layer SP layer ` Intensity (arbitrary unit) (a) Wide scan u2p 3/2 B-u SP-u SP-AF-u (b) Magnified u2p 3/2 peak Figure 9. esults of the XPS analysis of bare (B-u) and SP finished (SP-u) and the stained area of SP finished u surfaces reacted with AF after and acetone cleaning (SP-AF-u). Intensity (a.u.) riginal peak combined H combined doble bond (a) As-received SP finished u surface riginal peak combined H combined u Figure 11. A schematic of AF bonding on the SP finished surface. A chemical reaction can occur between the outermost nitrogen of the SP layer and the epoxide ring of AF Effect of SP on adhesion of AF joints The results of a 90 o peel test are illustrated in figure 12. The average peel strength of the AF bonded SP L-SP u foil combination showed the strongest adhesion strength among three AF bonded L-u foil combinations. In particular, for the EIG L-EIG u foil combination, the delamination of the EIG layer on the u surface significantly reduced the adhesion strength. After the peel test, fractured surfaces of Ls and u foils were observed by SEM. According to the failure analysis, the adhesion between the bare u surface and AF was lower than that of the SP layer and AF. Therefore, as shown in figure 13, the fracture path moved from the bare u/af interface to the inside of the AF layer. The fracture paths of the samples are illustrated in figure 14. The change in the fracture path implies that the SP coating layer has good adhesion with the epoxy-based adhesive, and that the SP finish can enhance the bondability of AF joints. This result corresponds with a previous study which reported that the polymeric adhesion promoter, polybenzimidazole, which is similar as the SP 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 Intensity (a.u.) Peel strength (gf/cm) (b) stained area of SP finished u surfaces reacted with AF after and acetone cleaning Figure 10. XPS results of the 1s peak deconvolution. 0 EIG-EIG u foil SP-Bare u foil SP-SP u foil Figure o peel test results in three metal finishes combination Electronic omponents and Technology onference

6 the EIG finished samples after 72 hours of PT. The main cause of the failure was the delamination of the AF/PI of the flexible substrate interfaces in both surface finished samples. However, an additional failure site was observed in the EIG finished samples. As shown in figure 16 (a), some additional delaminations and cracks occurred at the AF/EIG interface. This defect can act as a potential failure site during the reliability tests. In contrast, no defects were observed in the SP finished samples. onsequently, it is considered that the SP coating layer provides good adhesion with AFs, and that the improved initial adhesion influences and enhances the reliability of AF joints. (a) u foil side: SP L- Bare u foil (c) L side: SP L- Bare u foil (b) u foil side: SP L- SP u foil (d) L side: SP L- SP u foil Figure 13. bservation of the fracture sites of Ls and u foils. umulative percentage (%) ontact resistance (mω) (a) EIG finished sample EIG- PT 0 hrs 24 hrs 48 hrs 72 hrs 96 hrs 120 hrs 144 hrs umulative percentage (%) SP- PT 0 hrs 24 hrs 48 hrs 72 hrs 96 hrs 120 hrs 144 hrs (a) SP L-bare u foil (b) SP L-SP u foil Figure 14. Schematics of the fracture paths in each sample. SP material (benzimidazole) is also based on the imidazole system. Therefore, as described in figure 11, the chemical reaction between benzimidazole in SP and AF can form strong bonding at the AF/SP interface, resulting in an enhanced adhesion eliability test results According to the PT results, SP finished samples showed better PT reliability compared with the EIG finished samples. As shown in figure 15 (a), the degradation of the AF joints in the EIG finished samples started after 48 hours of PT. After 96 hours of PT, 50% of measurement points exceeded 200 mω, and the degradation of the AF joints was accelerated as the test time increased. However, the changes in the contact resistances of the AF joints in the SP finished samples were very stable up to 120 hours of PT. Actually, the failure rate of the SP finished samples after 144 hours of PT was nearly identical to that of ontact resistance (mω) (b) SP finished sample Figure 15. esults of the changes of contact resistance during the pressure cooker test. u elect. of flex EIG u elect. of F4 rack & delamination u elect. of flex SP u elect. of F4 (a) EIG finished sample (b) SP finished sample Figure 16. ross-sections of EIG finished and SP finished samples after the pressure cooker test Electronic omponents and Technology onference

7 4. onclusions (1) Benzimidazole, the base material of SP, improved the adhesion strength of AF joints. According to the results of TEM and FT-I, the SP finished layer reacted with epoxy resin of AF at the AF curing temperature, and it enhanced the adhesion. Therefore, SP layer played as an adhesion promoter. (2) AF joints, assembled with SP finished rigid substrates, showed better reliability than EIG finished case. Besides, no defects were observed at AF/SP interface. These results implied that the AF/SP interface was more stable than the EIG/AF interface under high temperature and humid environments. (3) Therefore, SP is applicable to the final metal finish method for making reliable AF joints. Acknowledgments This work was supported by the enter for Electronic Packaging Materials (E) of MST/KSEF (Grant o ). eferences 1. E. Stafstron, Untraveling the Final Finishing Mystery, ircuit Assembly, o. 11, (ov 2000), pp M. arano, SP Evolution, Printed ircuit Fabrication, Vol. 20, o. 7, (1997), pp J. D. Debiase, rganic Solderability Preservatives: Benzotriazoles and Substituted Benzimidazoles, Surface Mount International, (1996), pp G. Lewis, The orrosion Inhibition of opper by Benzimidazole, orrosion Science, Vol. 22, o. 6, (1982), pp D. P. Drolet, et al., FT-I and XPS Study of opper(ii) omplexes of Imidazole and Benzimidazole, Inorganica himica Acta, Vol. 146, (1988), pp Yu. I. Kuznetsov, et al., hemical Structure Benzoimidazoles and Their Protective Effects on Zinc and opper in Phosphate Electrolytes, Protection of Metals, Vol. 40, o. 2, (2004), pp G. Xue, et al., The Formation of a ompact Polymer Film on a opper Electrode from Benzimidazole Solution, J. Electroanal. hem., Vol. 270, (1989), pp M. Frederickson, et al., eal-time ontrol of Advanced Solderability Preservatives, ircuit World, Vol. 24, o. 2, (1998), pp S. Yoshida, et al., A FT-IT eflection-absorption Spectroscopy Study of an Epoxy oating on Imidazole- Treated opper, J. Adhesion, Vol. 16, (1984), pp S. M. Song, et al., Synthesis and haracterization of Water-Soluble Polymeric Adhesion Promoter for Epoxy esin/opper Joints, J. Appl. Poly. Sci., Vol. 85, (2002), pp S. Siau, et al., hemical Modification of Buildup Epoxy Surfaces for Altering the Adhesion of Electrochemically Deposited opper, J. the Electrochem. Socie., Vol. 152, o. 9, (2005), pp. D136-D J. Yu, et al., Miscibility of Polyimide with Polymeric Primer and Its Influence on Adhesion of Polyimide to the Primed opper Metal: Effect of Precursor rigin, J. Poly. Sci. Part B, Vol. 37, (1999), pp Electronic omponents and Technology onference