Effect of microwave preheating on the bonding performance of flip chip on flex joint

Microelectronics Reliability 44 (2004) 815 821 www.elsevier.com/locate/microrel Effect of microwave preheating on the bonding performance of flip chip on flex joint R.A. Islam, Y.C. Chan * Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon tong, Kowloon, Hong Kong Received 7 July 2003; received in revised form 17 September 2003 Abstract A microwave (MW) preheating mechanism of anisotropic conductive adhesive film (ACF) has been introduced in order to reduce the bonding temperature for flip chip technology. Thermal curing of epoxy shows a very sluggish and non-uniform curing kinetics at the beginning of the curing reaction, but the rate increases with time and hence requires higher temperature. On the other hand MW radiation has the advantage of uniform heating rate during the cycle. In view of this, MW preheating (for 2/3 s) of ACF prior to final bonding has been applied to examine the electrical and mechanical performance of the bond. Low MW power has been used (80 and 240 W) to study the effect of the MW preheating. It has been found that 170 C can be used for flip chip bonding instead of 180 C (standard temperature for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (0.015 0.025 X) is low in these samples where the standard resistance is 0.017 X (bonded at 180 C without prior MW preheating). The shear forces at breakage were satisfactory (152 176 N) for the samples bonded at 170 C with MW preheating, which is very close and even higher than the standard sample (173.3 N). For MW preheating time of 2 s, final bonding at 160 C can also be used because of its low contact resistance (0.022 0.032 X), but the bond strength (137.3 145 N) is somewhat inferior to the standard one. Ó 2003 Elsevier Ltd. All rights reserved. 1. Introduction * Corresponding author. Tel.: +852-2788-7130; fax: +852-2788-7579. E-mail addresses: rashed@ee.cityu.edu.hk (R.A. Islam), eeycchan@cityu.edu.hk (Y.C. Chan). Anisotropic conductive film (ACF) consisting of conducting particles and adhesives provides both attachment and electrical interconnections between electrodes. They are widely used for high-density interconnections between liquid crystal display (LCD) panels and tape automated bonding (TAB) as a replacement of traditional soldering or rubber connectors. ACF has the advantage of low temperature assembly, high-density interconnections, fluxless bonding and low fabrication cost [1 4]. Therefore, their use is also expected to increase as new interconnect materials in semiconductor packaging which requires smaller and thinner dimensions. The application on flexible substrate such as smart card, disc drives and driver chips for LCDs have attracted much interests and widespread use [5]. ACFs are adhesive films with anisotropic properties induced by dispersing 0.5 5 vol% of conducting particles into polymer matrices. By controlling the volume fractions of conducting particles, anisotropic conductivity can be imparted to the adhesive film. In a thermomechanical system the adhesive resin cured with the application of heat and pressure and conducting particles dispersed in the matrix is trapped between the conducting surfaces. The mechanical contacts between conducting particles and bonding electrodes are retained by strong adhesion strength of the polymer matrix. Therefore, the electrical and mechanical properties of the polymer matrix have a significant influence on the interconnection reliability of ACFs [6,7]. Polymerization, as the key processing step for generating polymer, is directly related to the polymers performance and is therefore of great importance. 0026-2714/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2003.08.014

816 R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815 821 Fig. 1. Epoxy molecule re-orientation and conductive particle distribution by curing process: (a) uncured state, (b) after preheated by MW radiation and (c) cured state. Conventionally, thermal heating is used to initiate and carry out the polymerization. But in thermal curing, a non-uniform heating at the early stage of the process produce localized curing and needs higher curing temperature or higher curing time to complete the polymerization. Furthermore thermal curing (during die bonding) is costly and therefore it is always desirable to find an alternative curing process in the ACF technology. Other than thermal, MW radiation can be a possible candidate for performing the curing operation [8]. Microwave (MW) as a heating source for curing polymer and polymer-based composites has been of interest for many years. The proposed curing mechanism is that, the alternating electric field of MW causes re-orientation of the long-chain molecules of polymers, which results in the friction among molecules; this friction converts the MW energy into the thermal heat [9,10]. For example, a high heating rate and a more uniform temperature distribution can be easily obtained because all molecules are heated simultaneously without the requirement of thermal conduction. Most literature reported the same kinetics for curing reaction between MW and thermal cure. Discrepancy was merely centered in the issue of curing extent. Some authors reported the partial completion of cross-linking by MW, which is believed that MW radiation accelerated the curing rate at the early stages of reaction and the induced rapid cross-linking created a molecular network [11,12]. However, partial polymer reaction was only observed at the early stages of cross-linking, which implies that further curing by thermal energy would enable the reaction to complete uniformly [13,14]. Fig. 1 illustrates the molecular state of epoxy before curing, after MW preheating and after the final curing. Continued heating with MW can be the cause of conductive filler metal arcing. So an identical or even much higher curing degree can be obtained with MW preheating followed by thermo-mechanical bonding even at lower temperature than that used in conventional thermal bonding. The objective of this study is to see the effect of MW preheating of ACF on the bonding performance of flip chip on flex circuit. Here the contact resistance and the shear force at breakage of the bond will characterize the electrical and the mechanical performance of the bonding. 2. Experimental The flex substrates are 50 lm thick and the height of gold/electroless nickel coated copper (Au/Ni/Cu) electrodes is about 14 lm (the pitch between two Au/Ni/Cu electrode is 35 lm in average). Sony ACF that has the capability for the fine pitch on flex is used in this study. The thickness of ACFs is about 35 lm. Conductive particles with diameter of 3.5 lm (resin + Ni/Au plating + insulating coating) are distributed in the adhesive matrix, with a concentration of 3.5 million/mm 3. A schematic view is shown in Fig. 2. The test chips have a size of 11 3mm 2 with 300 Au/Ni bump having a height of 4 lm and a size of 70 50 lm 2. The MW preheating was done in a electronically controlled Sharp R652B microwave oven used for domestic purpose (having a fixed frequency of 2.45 GHz and a maximum power of 800 W) with the power of 80 and 240 W and the exposure time of 2 or 3 s. The built in controller of the oven, having a very high resolution, was set to desired time and output power and the oven was turned on for preheating. Then the prebonding process is carried out using the Karl Suss FCM manual flip chip bonder at a temperature of 90 C with 0.3 MPa pressure for 5 s. Final bonding was done by using the Toray FC 2000 semi-automatic flip chip bonder at 150, 160 and 170 C and 60 N load for 10 s. For every condition, five samples were prepared using the same experimental setup. The control/standard sample was prepared by using no MW preheating and the final bonding temperature of 180 C with other variables remaining the same [15,16]. Again the five samples were prepared to compare with the newly bonded samples. The contact resistance was measured by the 4-point probe method. Then the chip package is mounted on the FR4 board as shown in Fig. 2. The die shear test was carried out by using INSTRON Mini 44 Tester with a cross-head speed of 10 mm/min. The experimental setup is shown in Fig. 3. The shear force was measured at breakage when chip is detached from the substrate. The tests were repeated five times for each condition using the same

R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815 821 817 Polymer Thin Au Layer Flex Substrate ACF Chip FR4 Board Thin Ni Layer Insulating layer Fig. 2. Schematic view of a conductive particle and the side view of a chip package prepared for die shear test. Crosshead Chip Package Specimen Holder BASE Fig. 3. Experimental setup for die shear test. at 80 W and bonded at 170 C. The samples preheated at 240 W show some higher contact resistance for both the case. Here some factors have to be considered. They are the initial cross-linking of molecules by MW heating, final cross-linking by thermal heating, distribution of conductive particles etc. Thermal curing, during the flip chip bonding, is a localized heating process. Here the cross-linking starts at some molecules getting thermally excited. Then the number of excited molecules increases with increasing temperature and finally the curing process completes. experimental setup and the average results with data bar have been reported (Figs. 8 and 9). The samples were then cross-sectioned and mounted on epoxy resin. The Philips XL40 FEG scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDX) was used to inspect and analyze the micro-joints of the FCOF packages, especially the chip/conductive particle metallization interface. Optical microscopy was used for examining the presence of air bubbles in the fracture surface. 3. Results and discussion Contact Resistance (ohm) 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0.017 ohm 0 Bonding temperature in degree celsius 80 watt 240 watt Fig. 4. Variation of contact resistance with bonding temperature (MW preheating time: 2 s). 3.1. Contact resistance Figs. 4 and 5 represent the contact resistance vs. bonding temperature at 2 and 3 s MW preheating respectively. It is quite clearly observed that with increasing MW power and MW exposure time the contact resistance increases except at 170 C. With the same MW power and the MW preheating time, the higher the bonding temperature, lower the contact resistance. In both the figures the black line represents the contact resistance of the standard sample using no MW preheating and bonded at 180 C. It is clearly observed that the samples preheated in MW oven with 80 W and 2/3 s can reach the contact resistance very close to the standard one and even smaller, especially for 3 s preheating Contact Resistance (ohm) 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0.017 ohm 0 Bonding temperature in degree celsius 80 watt 240 watt Fig. 5. Variation of contact resistance with bonding temperature (MW preheating time: 3 s).

818 R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815 821 Fig. 6. SEM images showing uneven distribution of conductive particle bonded at 150 C (bond line thickness: 0.592 lm). The reason behind this is the initial reaction rate is much slower because of the poor thermal conduction. On the other hand, MW radiation is a volumetric heat source, which accelerates the curing process at the early stage of the curing operation. All the molecules get excited simultaneously from the beginning. As the nucleation or polymerization starts everywhere due to the replacement of thermal heating by MW radiation at the early period of the curing, set a stage for continued growth and crosslinking of epoxy molecules by the thermal heat. If the MW preheating time is much longer then the internal temperature achieved could be higher then it will be difficult for thermal heating at 170 C to allow the reaction to proceed and additional shrinkage to occur, which is a typical irreversible behavior of thermosetting polymers. But if the preheating time in MW oven is just enough to initiate polymerization volumetrically then additional thermal heat is required to reach the desired cure point. This can be observed in Fig. 4. With MW preheating time of 2 s at 80 W, even the final bonding at 160 C produces a very low contact resistance (0.022 X), which is very close to the standard value (0.017 X). The final bonding at 170 C at this condition produces even lower contact resistance (0.018 X), which can be considered as almost the same with the standard one. Thermosetting polymers are irreversible polymers. Once set, they cannot be reshaped. They are formed by a large amount of cross-linking of linear prepolymers. Once the cross-linking has been established and it is cooled down, it cannot be altered and it will be very difficult to enhance the curing [17]. In Fig. 5, same feature can be seen that is the bonding at 150 and 160 C produces very high contact resistance even in 80 W. This can be explained as the preheating time of 3 s reaching a level of the curing process where the final bonding at 150/160 C is not very effective to overcome the already cured state. But a higher temperature of 170 C could complete the curing process within the bonding time. Thus it produces a very low contact resistance (0.015 X), even lower than the standard value. Another factor that affects the contact resistance is the distribution of conductive particles. During curing, the epoxy molecules interconnect by cross-linking and the conductive particles distribute evenly during the adhesive flow. In the heating and polymerization (curing) process, there will be a good adhesive flow at an appropriate temperature, which results in an even distribution of the conductive particles. When this situation prevails, there is a very good chance that between every contacting electrode, there will be a considerable amount of conductive particles. The outcome will be a very low contact resistance. This feature has been found in the case of samples bonded at 170 C with MW preheating and even some of the samples of 160 (MW preheating time of 2 s). However, for higher MW power exposures the viscous flow stage is bypassed and there is potentially an uneven distribution of conducting particles. According to this situation there is a very little chance of trapping a good amount of well-distributed conductive particles between every contacting electrodes. This can be clearly shown in Fig. 6. Here uneven distribution of particles is shown. In one joint there is only one particle within the bonding electrode and in other joint there are several number of conducting particles found very close to each other in the samples bonded at 150 C. In Fig. 7 an even distribution of particles are shown in the samples bonded at 170 C. Fig. 7. SEM image showing the even distribution of conductive particle between the joints bonded at 170 C (bond line thickness: 0.296 lm).

R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815 821 819 This feature is another explanation of why there is a high contact resistance in the samples bonded at 150 C (all samples) and 160 C (for 3 s preheating time). 3.2. Die shear test The important characteristics governing bonding outcome is the viscosity of the adhesive and hence its flow property. If the viscosity of the adhesive is low only for a short time, wetting will be far from complete and air bubbles will easily be entrapped in the joint, which will act as a stress raiser [18]. In the figures (Figs. 8 and 9) for shear force vs. bonding temperature, the curves show a similar trend. For 2/3 s MW preheating time, a smaller die shear force is required for the specimens bonded at 150 C. The force increases with the bonding temperature, indicates the increase in the formation of molecular network. If there is air bubble present, proper bonding cannot take place due to reduction of cohesion force between the adhesive and substrate and also between the adhesive and die. The viscosity of an adhesive can be lowered by heat energy (MW radiation and temperature) leading to an increase in the degree of wetting. Again bonding at temperature 150 and 160 C produces a thick bond line, which generally is a weakening feature as the mechanical strength of the unsupported resin film is likely to be less than that of the substrates. However, this type of bond line can, in general, be more flexible than the substrates and is also more susceptible to failure when a pressure is applied [18]. In Fig. 6, a thick bond line of 0.592 lm is observed for the samples bonded at 150 C, whereas much thinner bond thickness (0.296 lm) is observed for Shear force (at breakage) in N 180 170 160 150 140 130 120 80 Watt 110 240 Watt 100 Bonding Temperature in degree celsius 173. 3 N Fig. 8. Variation of force required to shear the die with bonding temperature (MW preheating time: 2 s). Shear force (at breakage) in N 180 170 173. 3 N 160 150 140 130 120 80 Watt 110 240 Watt 100 Bonding Temperature in degree celsius Fig. 9. Variation of force required to shear the die with bonding temperature (MW preheating time: 3 s). the sample bonded at 170 C which is due to the complete curing at the curing temperature. So this is another point why the cohesive force is much lesser in the samples bonded at 150/160 C. Fig. 10. Optical images showing various amount of air bubble affecting the bond strength.

820 R.A. Islam, Y.C. Chan / Microelectronics Reliability 44 (2004) 815 821 MW radiation is a volumetric heat source, which starts curing (or polymerization) the epoxy adhesive at all nuclei of the material. So it takes very short time for the curing process. That is why for high power and increased time, there is a chance of reaching a level of polymerization beyond which further heat at identical temperature cause little effect to the cured adhesive. In this case the adhesive would be less viscous for a very short time and results in improper flow of adhesive. High MW power can heat very fast and generate voids or trap air bubbles in the adhesive, which is very detrimental for strong adhesive joint [18]. These features are observed with 240 W samples the shear force at breakage is less than that of the samples preheated at 80 W. MW preheating time of 3 s at 240 W can easily reach such a situation and there is a significant reduction of cohesion energy, therefore, a smaller force is required to shear the die. Fig. 10 shows the air bubble trapped between the ACF and the substrate. At 150 C a huge amount of air bubbles are trapped. The amount decreases with increasing bonding temperature, decreasing preheating power and time. 4. Conclusions MW activated bonding process has been studied in order to reduce the final bonding temperature. It has been successful to reduce the maximum curing temperature by 10 170 C for curing if MW preheating of ACF is used for 2 3 s prior to final bonding. When high bond strength is not required, 160 C curing can also be used as final bonding temperature after the MW preheating of ACF for 2 s at very low power (80 W). The benefits of reducing the final bonding temperature up to 10 20 C are the reduction in the operation cost and processing time. Above 100 C, it is quite expensive to affect a small increment of temperature and also time consuming to reach at high temperature. So by introducing the MW preheating, the processing time and operation cost can be minimized. Here the contact resistance for samples bonded at 170 C and a MW preheating of ACF is used prior to that has shown a low contact resistance of 0.015 X (MW: 3 s at 80 W), which is even lower than the samples bonded at 180 C without any MW preheating. The shear force measured at breakage in the die shear test was found 176.8 N maximum, which is higher than the standard one (173.3 N). Optical microscopy of this joint shows a low concentration of air bubble which is the basic criteria for the high bonding strength. The samples bonded at 160 C for 2 s preheating time also show low contact resistance (0.022 0.032 X), which is very close to the standard value (0.017 X) but the bond strength is not up to the mark due to considerable amount of air bubble entrapment. So it can be concluded that, 80 W is the ideal power for MW preheating and the time depends upon the bonding temperature. For 170 C, 2 3 s would be ideal but for 160 C, not more than 2 s preheating time must be provided. Acknowledgements The authors would like to acknowledge the financial support from Strategic Research Grant of City University of Hong Kong Comparative Study of Thermal and Microwave Cured Anisotropic Conductive Joints for Liquid Crystal Display Application (project no: 7001419). Special thanks must be given to Mr. Ahmed Sharif and Ms. Samia Islam for their endless support. References [1] Fu Y, Willander M, Liu J. Statistic of electric conductance through anisotropically conductive adhesive. IEEE Trans Compon Pack Technol 2000;24(2):250 5. [2] Chang DD, Fulton JA, Lyons AM, Nis JR. Design considerations for the implementation of anisotropically conductive adhesive interconnections. Proc Nepcon W. 1992. p. 1381. [3] Yim M, Paik K. 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