' Department of Electronic Engineering

Degradation Mechanisms of Anisotropic Conductive Adhesive Joints for Flip Chip on Flex Applications Y.C. Chad, K.C. Hung', C.W. Tang', and C.M.L. Wu* ' Department of Electronic Engineering * Department of Physics & Materials Science City University ofhong Kong Tat Chee Avenue, Kowloon, Hong Kong eevcchaniij)cit\;u.edu. hk Abstract Flip chip on flex (FCOF) using anisotropic conductive film (ACF) has been demonstrated. Two types of conductive particle in ACF are uscd in this paper to investigate the effect of pinholes of the electroless nickel bumps on electrical connection of ACF joints of FCOF samples. The conduction mechanisms of both types of ACF joint due to the effect of pinholes have been discussed. After high temperature and high humidity storage, Ni filled and Aumi coated polymer filled ACF joints using non-aged or aged bump chips show slightly and dramatic increases in connection resistance respectively. Detail degradation mechanisms for these ACF joints have been proposed. Introduction As the trend in requirements of electronic packaging is toward higher I/O, greater performance, higher density, and lighter weight, flip chip is becoming an increasingly attractive technology. However, conventional lead-tin soldering is incompatible with extremely fine pitch interconnection and undesirable from an environmental point of view. Anisotropic Conductive Adhesives (ACA) or Films (ACF) are key technology to deal with mentioned issues, having the advantages of extreme fine-pitch capability, being lead free, and of flexible and simple processing at low temperatures. These flip chip applications on flexible substrates such as smart cards, disk drives and driver chips for LCDs have attracted much interests and widespread uses [ 11. However, one critical drawback of the current conductive adhesive technology is it's unstable contact resistance which usually shows substantial increases over time particularly under high temperature and high humidity conditions (8S0C/85%RH). Several possible mechanisms including metal oxidation, water absorption, and electrochemical corrosion have been suggested for the unstable contact resistance. Normally, direct metal oxidation will occur when metal exposes to an oxygen environment either in the presence or absence of moisture, particularly at high temperature [2]. For dry condition, it has been found that the connection resistance of the ACF joints for flip chip on flex (FCOF) shows gradual increase over time under high-temperature storage at - 120 "C [l]. This means that the effect of direct metal oxidation under dry condition on the ACF joints is not significant since the cured adhesives have the ability to avoid the exposure of the joints to the oxygen environment. Electrochemical corrosion will occur when two metals are in contact under wet condition. The less noble metal (electrochemical potential less than 0.4 V) will act as an anode while the metal with high electrochemical potential will act as a cathode [3]. The following reaction will occur: At cathode 2H20+4e- +02-4OH- ; (1) At anode M -ne- + M"+. (2) According to the above equations, metal hydroxide will be formed and it finally becomes a more stable form of metal oxide [3]. Therefore, it indicates that electrochemical corrosion will only occur under wet condition between metals with different electrochemical potential. Results of the reliability test for FCOF using gold bumps and ACF filled with Ni particles have shown that the connection resistance increases over time under high temperature and high humidity storage [1,4]. In this case, Au bump may act as a cathode while Ni particle may act as an anode and nickel oxidc will eventually form on the surface ofni particle. In addition, it is believed that swelling of cured adhesives will occur under 85"C/85%RH wet condition since the water absorption rate of most adhesives is about 2 to 3 wt% under this temperature and humidity storage [SI. This swelling effect may reduce the contact surface area between the conductive particles and electrode and thus causing the increase in connection resistance of the ACF joints. Moreover, water not only diffuses into the adhesive layer but it also penetrates into the interface between adhesive and substrate/chip causing the reduction of adhesion strength. Fig. 1 is a schematic diagram showing typical degradation mechanisms that may occur at the ACFJoints. Metal osidati on Corrosion ---;-- Chip 1 Electrode Flex - L- U] Water Figure I. Typical degradation mechanisms in ACF joints... 0-7803-6460-0/00/$10.00 02000 IEEE 141

In fact, different materials of bump, conductive particle in ACF, substrate and electrode chosen may lead to different dominant degradation mechanisms. Different degradation mechanisms actually require different approaches for the stabilization of contact resistance. For example, if the use of Au bumps and Ni conductive particles causes the increase in connection resistance of ACF joints which due to different electrochemical potential, one can change either Ni particles to gold coated polymer particles or Au bumps to electroless Ni bumps so as to minimize the difference of electrochemical potential between Au and Ni. Recently, electroless Ni bumping process has been introduced and intensively used for low cost purpose of ACF applications since no expensive facilities for sputtering or masking are necessary [6]. Normally, there is a gold flash on top of the electroless Ni bump for protection purpose. Too thick an Au layer will increase the plating and manufacturing costs, and more importantly, it may increase the risk of corrosion like using Au bumps. However, too thin an Au layer will result in pinholes on top of the bump. Au plates with less than 0.5 pm in thickness have traditionally been considered too porous for providing effective protection [7-81. It is because pinholes may cause oxidation of the bumps. Pinholes may either promote oxidation of the exposed area or provide a diffusionless path for the delivery of oxygen to the gold-nickel interface. Yet, no detail study of the effect of pinholes on the conduction and degradation mechanisms of the ACF joints has been investigated. The aims of this paper are to study and understand the fundamental conduction and dcgradation mechanisms of ACF joints for FCOF application due to the effect of pinholes. Only a clearer fundamental understanding of these conduction and degradation mechanisms can allow manufacturers to develop high reliable, low cost and better performance electronics products using ACF FCOF applications. Experimentation The test chips have a size of 3 mm square, 54 electroless Ni bumps, bump pitch of IOOpm, bump diameter of 90 pm and bump height of 20 pm. The electroless Ni bumping process steps are A1 cleaning, AI activation, electroless Ni deposition, and immersion Au coating. In our previous work [9], Au thickness less than 800 A will have pinholes on top of the Ni plating. A SEM image of the electroless Ni bump is shown in Fig. 2. It is found that both small and big pinholes on top of the electroless Ni bump are present. Fig. 3 is a magnified SEM image, which shows a clearer surface morphology of the pinhole area that indicates the exposure of pinholes to the environment. In order to accelerate the aging effect on the pinholes of the electroless Ni bumps, some bumped chips are put into a furnace to anneal at temperature of 200 "C for I to 2 hours. The flex substrates used in this study are of 50 pm thickness and the electrode is gold/electroless nickel coated copper (Au/Ni/Cu). Two types of ACF that have the capability for fine pitch are used in this study. The detail specifications as well as the bonding parameters given by the manufacturers are summarized in Table 1. Figure 2. SEM image ofthe electroless Ni bzrnip. Figure 3. Surface niorphologv qf the area around pinhole 0r.1 the electroless Ni bump. Table 1. SpeciJication, bonding parameters and properties for hvo tvwes ofacfs. - ~~ Description Film thickness (Fin) Conductive particle Insulation coated Type A 30 Nickel NO Type B 30 Au/Ni coated resin Yes Bonding temperature ("C) Bonding time (s) Bonding pressure (MPa) 180 I90 20 10 IO0 60 145 125 55 63 The FCOF assemblies are carried out by using the Karl Suss FCM manual flip chip bonder and the bonding procedure is as follow: 1. substrate setting, 2. lamination of ACF, 3 peeling off cover film, 4. chip alignment, and 5. final bonding The reliability tests for these bonded FCOF samples will then be performed under 85OC/85%RH humidity storage. The samples before and after the humidity test have been carried out a series of measurements including optical measurement electrical measurement and scanning acoustic microscopic (SAM) measurement. Finally, for interfacial joining, examination, FCOF samples are mounted in epoxy and then sectioned using a slow speed diamond saw. Sectioned samples 142

were then grounded and polished. The chemical and microstructural analyses of the cross-sectioned samples were obtained by using the Philips XL 40 FEG Scanning Electron Microscope (SEM) equipped with Energy Dispersive X-ray Analysis (EDX). Results & Discussion The connection resistance of ACF joints of FCOF is measured by using four-point probe method which is shown in Fig. 4. In this measurement, we apply a 1mA constant current to the circuit and we then obtain the connection resistance of ACF joint by a simple calculation using R = V/I. The connection resistance of ACF joints for different types of ACF film area summarized in Table 2. It can be observed that the initial connection resistance of Type B (Au/Ni coated polymer spheres filled) ACF joints is larger than that of Type A (Ni particles filled) ACF joints. In order to have a clearer explanation, a schematic diagram illustrating the bonding of ACF joints using different types of ACF film are shown in Fig. 5. In fact, the effective contact surface area of rigid Ni particles is larger than that of the compliant Au/Ni coated spheres after ACF FCOF bonding. This increase in effective contact surface area will lead to a decrease in contact resistance between conducting particles and bumps/electrodes. Figure 4. Connection resistance measurement for ACF joints using four-point probe method. When electroless Ni bumped chips with pinholes are aged for 2 hours, it is found that oxidation of nickel at the exposed area and Ami interface of bumps will occur due to the direct exposure of electroless Ni bumps through pinholes. As a result, a very thin layer of oxide is formed in the Ami interface of the bumps and thus, the connection resistance of ACF joints using aged bump chips should be larger. From Table 2, the connection resistance of Type B ACF joints (68.43 ma) using aged bump chips is larger than that (49.38 mq) using non-aged bump. However, the connection resistance of Type A ACF joints (29.63 d) using aged bump chips is just slightly larger than that (23.33 d) using non- aged bump chips. It is because Ni particles can break and penetrate the oxide layer at the Au/Ni interface of the bumps which result in a better electrical connection between bump and electrode. Bump Electrode Bump Electrode Electroless Ni-P => Au I I +!>AU Electroless Ni-P Figure 5. A schematic diagram showing the bonding of ACF joints using (a) Type A and (b) Type B ACFfilms. After short period of 8S C/85%RH humidity storage, both Type A ACF joints using aged and non-aged bump chips show an increase in connection resistance which is nearly twice as large as the initial values. In fact, for our samples having very thin gold layer on top of the bumps and electrodes, the effect of electrochemical corrosion for short period is not very significant. Instead, swelling due to water absorption of the adhesive may be the dominant degradation mechanism governing the increase in Connection resistance of ACF joints. In addition, Fig. 6 shows the SAM images for Type A FCOF samples before and after humidity storage. It can be seen that there are small delaminations occurred at both edges of the bump side after humidity storage. In order to investigate where the delaminations occur, Fig. 7 shows the SAM images using TAM1 scan which layer by layer signals can be obtained. The first image is the SAM image of the die bottom layer while the second one is the SAM image of the substrate top layer. It is indicated that small delaminations mainly occur just under the bottom of the chip is at the chip/adhesive interface and only a few occur at the adhesive/substrate interface. These small delaminations may be induced due to the water penetration at the chipladhesive and adhesivekubstrate interfaces since no delamination is observed from the SAM images before humidity storage. Moreover, delaminations only occur at both edges of bump sides and no delamination is found at the upper and lower edges of non bump sides as seen in Fig. 7. It is because the peel stresses arise at the adhesive/substrate and chip/adhesive interfaces around the bumps after cooling will support delamination [IO]. Therefore, swelling as well as water penetration causing small delaminations at the chip/adhesive interface could be the degradation mechanisms leading to the increase in connection resistance of Type A ACF joints. 143

Table 2. Averape connection resistance of ACF joints. Connection resistance of ACF,joints(d) ACF No Aging Aging 85"C/85%RH (hrs) 85"C/85%WI (hrs) 0 10 0 IO Type A Type 3 23.33 f 2.74 38.63 f 9.01 29.63 f 11.38 75.00 f 38.04 49.38 f 8.73 291.75 f 103.82 68.43 f 9.47 1798.86 f 1031.67 I::I Pam 1 01 2 Before humidity storage Pnak Amnl itrido chip/adhesive interface will lead to an increase in connection resistance like Type A ACF joints as discussed above However, if delamination occurs at thc adhesive/substratc interface, the problem will bccome more serious, especially for flex substrate. From Fig. 10, once the delamination at thc adhesive/flex interface due to water penetration is appeared, water in flex substrate may diffuse into the delamination which causes a further increase in volume and propagation ol' the delamination. As a result, these big swelling and propagation of delamination may lead to a serious increase in the connection resistance of Type B ACF joints. In addition. from Table 2, the connection resistance of ACF joints using, aged bump chips is much more larger than that using nonaged bump chips after humidity storage. The reason may be due to the continuous increase in thickness of the oxide layer at Au/Ni interface of the aged bumps. Therefore, swelling 01' delamination due to water penetration at the adhesive/flex interface could be the main degradation mechanism that leads to a serious incrcase in connection resistance of Type B ACF joints. Pmk AMDI itride After humicllty storage Figure 6. SAM images for Type A FCOF sample bejbre and after humidity storage. For Type B ACF joints after humidity storage, it is shown from Table 2 that the connection resistance of these joints is approximately 6 and 26 times the initial value for non-aged and aged bump chips respectively. Fig. 8 shows the TAMI scan SAM images for Type B FCOF sample using non-aged bump chips. It is shown that the delaminations mainly occur not only at the chip/adhesive interface but also at the adhesive/substrate interface. Fig. 9 shows the TAMI scan SAM images for sample using aged bump chip also indicates that there are serious delaminations both at the chip/adhesive and adhesive/substrate interfaces. In order to explain such dramatic increase in connection resistance of Type B ACF joints, Fig. 10 shows a schematic diagram of our ACF joints configuration. When a FCOF sample is put into a humidity environment, the adhesives may absorb water which cause swelling of adhesives. However this swelling effect on the connection resistance should not be so serious since the water absorption of cured adhesives is only 3 to 5 wt% [5]. Furthermore, water penetration causing delamination at the.. u.u t U 2.w J.U nm F1 ragc 4 01 31 WP 5 478 E Die bottom I: I Peal: Aaulltode 13 3 0 0 1.0 2.a 3.0 m Fag- 10 UT 31 liil UP: 5.178 F Substrate top Figure 7. TAM scan SAM images for Type A FCOF sample after humidity storage. 144

.1 Peak nmpiitude U81 ztw Chip 2.8 1.e Delaminations \ 0.e Die bottom I:: I Peak. Amplitude Figure IO. A schematic diagram showing the swelling of adhesive due to water absorption and swelling of delamination due to the water dij$ision from flex. Conclusions n U 1 ' ~ nn YWC 'J 01 Jl UF: s 641 Substrate top Figure 8. TAM scan SAM images for Type B FCOF sample using non-aged bump chips after humidi@ storage. Die bottom Substrate top It is found that the pinholes on top of electroless Ni bumps have normally no obvious effect on the initial connection resistance of the ACF joints. The difference in the initial connection resistance between Type A (Ni filled) and Type B (Au/Ni coated polymer filled) ACF joints is only due to the difference in effective contact surface area between rigid Ni particles and compliant polymer particles bonding to the bump and electrode. However, once there is an oxidation occurred at the Au/Ni interface through pinholes, the effect of pinholes on the initial connection resistance has become significant. It is found that the initial connection resistance is depends on the types of conductive particles used. For aged bump chips, Ni filled ACF joints show less changes in connection resistance than Au/Ni coated polymer filled joints. The main reason is due to the fact that rigid Ni particles can break and penetrate the oxide layer at the Au/Ni interface of the bumps and make good electrical connection between bump and electrode. After high temperature and high humidity storage, both Ni filled ACF joints using aged and non-aged bump chips show an increase in connection resistance. Swelling as well as water penetration causing small delamination have been proposed to be the degradation mechanisms leading to such increase in connection resistance of Ni filled ACF joints. For Ami coated polymer filled ACF joints after humidity storage, these ACF joints show a tremendous increase in connection resistance, especially for aged bump chips. Swelling of delamination due to water penetration at the adhesive/flex intcrface as well as swelling of adhesive due to water absorption should be the main degradation mechanisms that leads to the drastic increase in connection resistance of these ACF joints. Moreover, the hrther increase in connection resistance of ACF joints using aged bump chips may be due to the increase in thickness of the oxide layer at the Au/Ni interface under humidity environment. Figure 9. TAM scan SAM images for Type B FCOF sample using aged bump chips after humidity storage. 145

Acknowledgements The authors would like to acknowledge the financial support provided by the RGC CRC (Project no. 8720003), Strategic Research Grants (Project no. 7001 080, 7000955), and Small-scale Research Grants (Project no. 9030790) of the City University of Hong Kong. References R. Aschenbrenner, R. MiePner and 1-1. Reichl, Adhesive Flip Chip Bonding on Flexible Substrates, Journal of Electronics Manufacturing, Vol. 7, No. 4, pp.245 (1997). U.R. Evans, The Corrosion and Oxidation of Metals: Scientific Principles and Practical Applications, Edward Arnold Ltd., London (1 960). D.Q. Lu, C.P. Wong, and Q.K. Tong, Mechanisms Underlying the Unstable Contact Resistance of Conductive Adhesives, 1999 Electronic Components and Technology Conference, pp. 342 (1 999). Anisotropic Conductive Material for Flip Chip Interconnection, Hitachi Chemical Data Sheet. D.C.C. Lam, F. Yang, and P. Tong, Chemical Kinetic Model of lnterfacial Degradation of Adhesive Joints, Proceedings of 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, pp. 44 (1 998). R. Aschenbrenner, A. Ostmann, G. Motulla, E. Zakel, and H. Reichl, Flip Chip Attachment using Anisotropic Conductive Adhesives and Electroless Nickel Bumps, IEEE Transactions on CPMT - Part C, Vol. 20, No. 2, pp. 95 (1997). C.J. Thwaites, Int. Met. Revs., Vol. 17, pp. 149 (1972). R.B. Cinque and J.W. Morris, The Effect of Gold- Nickel Metallization Microstructure on Fluxless Soldering, Journal of Electronic Materials, Vol. 23, pp. 533-539 (1994). K.C. Hung, Y.C. Chan, H.C. Ong, P.L. Tu, and C.W. Tang, Effect of Pinhole Au/Ni/Cu Substrate on Selfalignment of Advanced Packages, accepted for publication in Materials Science and Engineering B. [IO] R. Dudek, A. Schubert, S. Meinel, B. Michel, L. Dorfmuller, P.M. Knoll, and J. Baumbach, Flow Characterization and Thermo-mechanical Response of Anisotropic Conductive Films, Proceedings of 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, pp. 68 (1 998). 146