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1 Available online at Microelectronics Reliability 48 (28) Effects of bondin temperature on the properties and reliabilities of anisotropic conductive films (ACFs) for flip chip on oranic substrate application J.S. Hwan a, *, M.J. Yim b, K.W. Paik a a Department of Materials Science and Enineerin, KAIST, Taejeon 35-17, South Korea b Center for Electronic Packain Materials (CEPM), KAIST, Taejeon 35-17, South Korea Received 29 March 26; received in revised form 28 July 26 Available online 5 July 27 Abstract The effects of bondin temperatures on the composite properties and reliability performances of anisotropic conductive films (ACFs) for flip chip on oranic substrates assemblies were studied. As the bondin temperature decreased, the composite properties of ACF, such as water absorption, lass transition temperature (T ), elastic modulus (E ) and coefficient of thermal expansion (a), were improved. These results were due to the difference in network structures of cured ACFs which were fully cured at different temperatures. From small anle X-ray scatterin (SAXS) test result, ACFs cured at lower temperature, had denser network structures. The reliability performances of flip chip on oranic substrate assemblies usin ACFs were also investiated as a function of bondin temperatures. The results in thermal cyclin test ( 55 C/+15 C, 1 cycles) and PCT (121 C, 1% RH, 96 h) showed that the lower bondin temperature resulted in better reliability of the flip chip interconnects usin ACFs. Therefore, the composite properties of cured ACF and reliability of flip chip on oranic substrate assemblies usin ACFs were stronly affected by the bondin temperature. Ó 27 Elsevier Ltd. All rihts reserved. 1. Introduction Due to the recent innovation of IC packain technoloy, hih density and hih performance interconnection method has become essentially important in flip chip interconnection technoloy. Moreover, recent concerns about environmental problems, such as toxicity from lead and chlorofluorocarbon-based flux cleaners, make the use of anisotropic conductive film (ACFs) more useful as an interconnection material for flip chip technoloy [1 3]. ACFs are adhesive films consist of mainly epoxy resins includin curin aents, conductive particles and other additives like couplin aents. Epoxy resins are bein used as the most common adhesive by virtue of its hih cohesive * Correspondin author. Tel.: ; fax: address: hjs@kaist.ac.kr (J.S. Hwan). and adhesive strenth, low shrinkae, and versatility in formulatin and processin. It is well known that epoxy resins need to be fully cured for desirable physical and mechanical properties and those properties of cured epoxy resins stronly depend on the cure conditions, such as cure temperature and time [4]. Furthermore, durin the curin process, residual stress and warpae are enerated in the packae due to the cure shrinkae and mismatch of coefficient of thermal expansion (CTE) between the ACFs and substrates. Assembly warpae has lon been reconized as an important factor for determinin the packae reliability. Warpae of flip chip assembly could cause not only IC failure due to delamination and die crack, but also assembly yield problems in the subsequent process due to dimensional instability and noncoplanarity, etc. [5]. There were several studies about the effects of bondin conditions on the properties and reliability of ACFs but /$ - see front matter Ó 27 Elsevier Ltd. All rihts reserved. doi:1.116/j.microrel

2 294 J.S. Hwan et al. / Microelectronics Reliability 48 (28) these were focused on deree of cure effects on the properties and reliability of ACFs. In other words, the previous studies were only concentrated on the relationship between the curin deree of ACFs, the properties and reliability of ACFs, which were bonded at different temperature and/or time. And the previous researches about warpaes were disreardin the deterioration of physical and mechanical properties oriinated from different cure temperatures. This paper investiates the effect of bondin temperatures on the physical and mechanical properties of fully cured ACFs, and the reliability of flip chip on oranic substrate usin ACFs considerin the relationships amon warpae, moisture absorption and curin temperature. 2. Experiments 2.1. Materials preparation An adhesive resin used for the ACFs was epoxy based formulation due to its ood adhesion to various substrates, hih lass transition temperature, and favorable melt viscosity required for the thermo-compression bondin process. Imidazole derivative was used for curin aent, and Au coated polymer particles in 5 lm size were used as conductive particles. The epoxy based resin and fine conductive particles were mixed uniformly, and then fabricated as B-stae films in 45 lm. thickness on the carrier PET film. The test chip and substrate used for flip chip assembly was 67 lm thick silicon chip and 1 mm thick FR-4 oranic board. The silicon chip was desined with four point Kelvin structure to measure connection resistance of ACF flip chip joint between chip and oranic board. The specifications of test chip and oranic board were summarized in Table 1. ACF flip chip assembly process consists of three steps. First, ACF was pre-bonded to the oranic substrate with the conditions of 8 C and 1 kf/cm 2 for 3 s. Then, the I/ pads on chip and board were alined each other. Finally, bondin pressure of 1 f/bump and bondin temperature was applied for the thermo-compression bondin of flip chip on oranic substrate usin ACF. Table 1 Specifications of test chip and oranic substrate Specification ranic substrate Material FR-4 Size (mm mm) 3 3 Final metallization Au Test IC Size (mm mm) 9 9 umber of I/s 8 Pitch (lm) 25 Pad size (lm lm) Bump heiht (lm) 17 ± 3 Bumps Au plated bumps 2.2. Characterization Differential scannin calorimetry (DSC) measurements A Perkin Elmer DSC-7, which was calibrated with hih purity indium and zinc standards, was used for measurement of the cure kinetics of ACFs as a function of time. The isothermal scan curves were achieved usin about 5 1 m of sample with a ramp-up rate of 25 C min 1 under a nitroen flow about 4 ml min 1. Samples were sealed in aluminum pans, heated to taret temperature with a ramp-up rate and then hold with a taret temperature for 5 min Fourier-transform infrared spectroscopy (FT-IR) To investiate the cure derees of ACF materials, infrared spectroscopic measurements were performed on FT-IR spectrometers (MB154-BMEM). Samples, which were cured at different temperatures, were analyzed in the form of films. Twenty-five scans were collected at a 4 cm 1 in the mid-ir rane from 4 to 4 cm 1. The extent of cure was monitored by the characteristic epoxide absorption peak at 916 cm Small anle X-ray scatterin (SAXS) measurements X-ray scatterin measurements were carried out to investiate the relationship between bondin temperature and resulted microstructure of ACF material. The X-ray source was a rotatin copper anode operated at 47 kv and 18 ma with a wave lenth of 1.54 Å selected with a raphite mono-chromator. Data in the q =.25.5 Å 1, where q =(4p/k)sinH, rane were obtained usin onedimensional (1D) detector. All data were corrected by dark currents and empty cell scatterin. Sample thickness and transmission corrections were also implemented for each scatterin result Thermo- and dynamic mechanical analysis (TMA/DMA) Coefficient of thermal expansion (CTE) and elastic modulus (E ) were measured usin Seiko Instruments thermomechanical analyzer (TMA/SS 61). The dimension of specimen was 2 mm width, 15 mm lenth and 45 lm thickness. Sample was subjected to a uniaxial tension mode from 3 C to 18 C with a heatin rate of 5 C min 1. The CTE values were determined from the linear section of thermal expansion versus temperature in TMA curves. To determine the elastic modulus of ACFs, Seiko Instruments thermo-mechanical analyzer (TMA/SS 61) was used. The dimensions of specimens were same as CTE measurement samples. To characterize the modulus, cured ACFs were fastened vertically between TMA rips and then a sinusoidal stress was applied to the specimen. An initial tension of 2 m was applied to the film and then the tension force oscillated ±1 m at a frequency of.2 Hz. The dimensional chanes in sample lenth were recorded with the cyclin force as the ACF specimen was

3 heated with a heatin rate of 5 C min 1. The data was then processed to determine the viscoelastic properties usin the DMA software. J.S. Hwan et al. / Microelectronics Reliability 48 (28) Water absorption of cured ACFs Water absorption of ACF samples, cured at different temperatures, was measured. To calculate the amount of water absorption, ACF samples were placed at pressure cooker test chamber and aed for 96 h at 121 C, 1% RH conditions. Then the amount of water absorption was calculated by measurin the difference of sample weiht before and after ain Reliability test To investiate the reliability of ACF flip chip assembly, connection resistance of flip chip joints usin ACF was monitored durin the environmental test. The initial connection resistance was measured usin 4-point probe method and after each time interval, connection resistance was measured up to the completion of environmental test. For the test items, pressure cooker test (PCT; 121 C, 1% RH, 96 h) and thermal cyclin test (T/C; 55 C to15 C, 1 cycles) were employed. 3. Results and discussion 3.1. Material characterization Determination of process conditions The purpose of this study is to investiate the effect of bondin temperatures on the composite properties and reliabilities of flip chip joints by fully cured ACF materials. Therefore, it is necessary to determine the bondin time at each test temperatures to obtain fully cured ACF materials. From isothermal scan results of DSC measurements, the relationships between the bondin time and the deree of cure at various bondin temperatures can be calculated. Fi. 1 represents the deree of cure as a function of bondin time at 15 C, 18 C and 21 C. To obtain more Absorbance Before bondin Bonded at 15 ºC for 25 sec. Bonded at 18 ºC for 16 sec. Bonded at 21 ºC for 15 sec Wavenumber (cm -1 ) Fi. 2. Fourier-transform infrared spectrum of the ACF materials before and after bondin at various bondin conditions. than 9% of cure deree, it needs 22 s at 15 C, 13 s at 18 C, and 12 seconds at 21 C. From the isothermal scan results, the bondin time was determined 25 s at 15 C, 16 s at 18 C and 15 s at 21 C. To confirm the cure deree at each process condition, infrared spectroscopic measurements were performed on FT-IR spectrometers (MB154-BMEM). Samples, which were bonded at different temperatures, were analyzed in the form of bonded state. Twenty-five scans were collected at a 4 cm 1 in the mid-ir rane from 4 to 4 cm 1. The extent of cure was monitored by the characteristic epoxide absorption peak at 916 cm 1 [6]. The FT-IR spectrums of the ACF materials, which were bonded at 15 C, 18 C and 21 C, were shown in Fi. 2. There were epoxy band at 916 cm 1 in uncured ACF materials. But after bondin process, epoxy band at 916 cm 1 was disappeared in all bondin conditions. From these results, the determined processin conditions from DSC measurement were reasonable to study the effect of bondin temperatures on the ACF properties and its reliability in flip chip assembly. Deree of cure (%) Bonded at 15 ºC Bonded at 18 ºC Bonded at 21 ºC Cure time (sec) Fi. 1. The plot for deree of cure versus cure time Effect of curin temperatures on the microstructures of cured ACF Fi. 3 shows schematics of cure mechanism involvin epoxy and curin aent. Generally, the reaction mechanism of ACF cure can be described two distinct steps [7]. The first step is the initiation reaction via the formation of an intermediate (III) from the curin aent (II) and epoxy monomer (I). And then, propaation reaction is arisin from the consecutive etherification reactions between the intermediate (III) and epoxy monomer (I) or between the reactive intermediates (III). When ACF was cured at relatively low temperatures, the reactions between the intermediate (III) and epoxy monomer (I) are more dominant. This is because that the intermediates (III), which have larer molecular weiht than epoxy monomers, have lesser mobility, and as a result, it is difficult to react with each other. The molecular weiht distribution, resulted from monomer

4 296 J.S. Hwan et al. / Microelectronics Reliability 48 (28) H 2 C 2 Epoxy monomer (I) Curin Aent (II) (I) to intermediate (III) reaction, shows narrow and it leads dense networks of cured ACF materials. But at the hiher curin temperatures, the reaction between larer intermediate molecules (III) may also occur and this leads to a wide distribution of molecular weihts of epoxy networks. Conclusively, lower curin temperature leads denser and more homoeneous networks in cured ACF materials, and as a result, cured ACF materials have better physical and mechanical properties. n the contrary, hiher curin temperature leads loose and heteroeneous networks, and as a result, cured ACF shows contrary effect on the composite properties. To confirm the structural chanes occurrin in epoxy networks resultin from the different curin temperatures, small anle X-ray scatterin (SAXS) measurement was employed [8 1]. Brominated DGEBA epoxy monomer, obtained from Aldrich Co., was added into the ACF composition to provide adequate X-ray contrast. The scatterin intensity versus scatterin vector (Q =(4p/k) sin H) curves for cured ACF networks, which were cured at 15 C, 18 C and 21 C, are shown in Fi. 4. Scatterin 2 2 Intermediate (III) 1. Epoxy Monomer H 2 C 2. ther 2 2 Intermediate Intermediate (III) 2 Epoxy etworks Fi. 3. Cure mechanism involvin epoxy and curin aent (imidazole derivative). peaks are located at approximately.36 for all of the ACF networks cured at 21 C, 18 C and 15 C. These similar locations of peaks mean that the intra-network correlation amon the epoxy sements separated by curin aent linkaes was not affected by curin temperature. But scatterin intensity of ACF networks was affected sinificantly by the curin temperature. The scatterin intensity depends on the heteroeneity of the electronic density and the size of the heteroeneous structure. For a twophase system, the intensity of the scattered radiation is described as follows: I / Dqu a u b where Dq is the density difference between the phases, and u a, u b are the volume fractions of the phase. As mentioned above, when cured at hiher temperature, ACF has looser networks portion (u a ), which is resulted in the reaction between larer intermediate molecules, and denser networks portion (u b ) which is resulted in the reaction between larer intermediate molecule and epoxy monomer, at the same time. Therefore, as the cure temperature increased, the resulted ACF networks have more u a portion than when cured at lower temperature. Therefore, hiher temperature cured ACF shows stroner scatterin peak intensity than the lower temperature cured one Thermo-mechanical properties Storae modulus (E ) of ACFs, which were cured at different temperatures, was shown in Table 2. As curin temperature decreased, the storae modulus and lass transition temperature (T DMA ) of the cured ACFs were increased. The test results of TMA experiments of ACFs, which were cured at different temperatures, were listed at Table 3. The inflection points of TMA curves were defined as lass transition temperature (T TMA ) and the coefficient of thermal expansion (CTE) values were calculated at the linear section of thermal expansion versus temperature. CTE of cured ACF below the T TMA (a1) and above T TMA ða2þ are Intensity Cured at 15 ºC Cured at 18 ºC Cured at 21 ºC Table 2 Elastic modulus (E ) and lass transition temperature (T DMA ) of ACFs as a function of cure temperature Cure temperature ( C) Elastic modulus (GPa) Glass transition temperature T DMA ( C) Q ( -1 ) Fi. 4. X-ray scatterin intensity versus scatterin vector from ACF networks which were cured at different temperatures. Table 3 T TMA and CTE values below and above of T TMA cure temperature of ACFs as a function of T TMA ( C) a1 (ppm) a2 (ppm) Cured at 15 C Cured at 18 C Cured at 21 C

5 J.S. Hwan et al. / Microelectronics Reliability 48 (28) important parameters in determinin the reliability of ACF flip chip interconnection. Almost all failures of ACF flip chip on oranic board applications were caused by the warpae problems which were caused by the CTE mismatch between the chip and the substrates. Therefore, lower CTE values are desirable for the reliability improvement of ACF flip chip assembly. From Table 3, the T TMA values increased and the CTE values of cured ACF below the T TMA ða1þ and above T TMA ða2þ decreased as the curin temperature decreased. As discussed above, the networks of the ACF cured at lower temperature showed hiher crosslink density than the one cured at hiher temperature. Therefore, the ACF materials cured at lower temperature have hiher crosslink density, leadin to hiher modulus, lower CTE and hiher T values than those cured at hiher temperature Water absorption It is well known that the absorption of water by epoxy resins adversely affect the mechanical properties of cured ACFs. To calculate the amount of water absorption, ACF samples were placed in pressure cooker test chamber and aed for 96 h at 121 C, 1% RH conditions. Then the amount of water absorption was calculated by measurin the difference of sample weiht before and after ain. Water absorption of ACF materials, which were cured at 15 C, 18 C and 21 C, was 3.27%, 4.38%, and 4.71% respectively. This tendency was also oriinated from the differences of the network structures as proved at SAXS measurements. From the results of the SAXS measurements, the network structures of cured ACF materials were more homoeneous and denser as the curin temperature decreased. This means that ACF cured at lower temperature has lesser free volume. The free volume of cured ACF offers the space for the water molecules to be trapped in. Therefore, the decrease of free volume of cured ACF can lead improvement in water absorption as well as other properties such as T, modulus and CTE Thermal cyclin reliabilities of ACF flip chip assembly ne of the most important issues in flip chip technoloy usin ACF is thermal cyclin reliability (T/C, 55 C to 15 C, 1 cycles). The thermal cyclin fatiue life of ACF flip chip assembly is limited by the CTE mismatch between the chip and the oranic substrate. The thermal cyclin reliability is affected by two major factors, bondin temperature and ACF composite properties. Adhesive flip chip assemblies are typically manufactured at an elevated (curin) temperature and subsequently cooled down to a low temperature (Fi. 5). Because thermo-mechanical properties of bonded materials are different, thermally induced stresses and strains, which are caused by the CTE mismatch of these materials, arise after bondin process. So these thermal stresses and strains are one of the most serious problems for the reliability of electronic packae and can lead to mechanical and functional failure in IC Chip ACF Electrode ranic substrate Expandin by heatin durin the bondin process IC Chip ACF ranic substrate Au bump Contractin by coolin after bondin process Fi. 5. Thermal and mechanical deformation of ACF flip chip packae durin and after bondin process. adhesive flip chip assemblies durin the thermal cyclin environments. Furthermore, the composite properties of ACFs also depend on the curin temperatures as discussed above, and the thermal cyclin reliability of ACF flip chip assembly is also affected by the differences of composite properties. Fi. 6 shows the method (a) and results (b) of warpae measurements of ACF flip chip assemblies, which were bonded at different temperatures. Warpae of ACF flip chip assemblies was measured alon the diaonal line of the chip and the measurement results was 4.5 lm, 7.9 lm and 8.6 lm when bonded at 15 C, 18 C and 21 C, respectively. The test results of contact resistances of ACF flip chip assembly joint durin thermal cyclin test were shown in Table 4. The resistance values are averaes for each test lot. Furthermore, the joint is not included in the averae values and it is considered as failed joint if the resistance value is increased over 5 mx or if joint has opened durin the test. The resistance values of ACF flip chip assembly joints were increased durin the ain cycle. But the ACF joint bonded at lower temperature resulted in better resistance values than the ACF joint bonded at hiher temperature. And the rate of failed joints (numbers of failed joint/numbers of total tested joint 1) was lower when bonded at 15 C (11.6%) than the others at the end of thermal cyclin test as shown in Table 4. The reason for the improved reliability in thermal cyclin test was also oriinated from the better composite properties and less warpaes with decreasin bondin temperatures as mentioned above Pressure cooker test (PCT) reliabilities of ACF flip chip assembly The cumulative distribution of contact resistances of the ACF flip chip joints after 96 h of PCT were shown in Fi. 7. As discussed above, the amount of water absorption increased as the bondin temperature increased due to the loose network structures which were oriinated from different cure mechanisms. From Fi. 7, the 71% of ACF assembly joints, bonded at 15 C, maintained below 5 mx. But as the bondin temperature increased, the ACF assembly joints, which were below 5 mx, decreased to 58% (bonded at 18 ) and 45% (bonded at 21 C). These results were

6 298 J.S. Hwan et al. / Microelectronics Reliability 48 (28) Chip center (mm) Displacement ( ) Chip ede (6mm) Chip ranic board -2 Displacement ( ) Distance from chip center (mm) Fi. 6. Method (a) and result (b) of warpae measurement of ACF assemblies bonded at different temperatures. Table 4 The effect of ACF bondin temperatures on the connection resistance of flip chip assembly durin thermal cyclin test ( 55 C to 15 C for 1 cycles) Bondin temperature ( C) well explained by the water absorption and other composite properties behaviors. Therefore, the PCT reliability of ACF flip chip joint was also sinificantly affected by the bondin temperature. 4. Conclusion umber of cycles Percentae failed (%) In this paper, we investiated the effects of bondin temperatures on the ACF material properties and reliabilities of ACF flip chip assemblies. When the ACF was bonded at lower temperature, the network structures of cured ACF showed denser and more homoeneous than the ACF materials which were cured at hiher temperatures. Dense and homoeneous network structure of cured ACF materials lead to hiher lass transition temperature (T ), hiher modulus (E ), lower coefficient of thermal expansion (CTE), and lesser water absorption properties. Therefore, the warpae of ACF assembly could be reduced and, as a result, the thermal cyclin (T/C) reliability was Cumulative distribution (%) Contact Resistance ( mω ) enhanced when bonded at lower temperature. Furthermore, the water absorption was also reduced when bonded at lower temperature and it leads to enhance the PCT reliability. Conclusively, it is important to control the bondin temperature to obtain reliable flip chip assemblies usin ACFs. Acknowledement Cured at 15 ºC Cured at 18 ºC Cured at 21 ºC Fi. 7. Connection resistance of ACF assemblies, bonded at different temperatures. (After PCT at 121 C, 1% RH for 96 h.) Financial support from Center for Electronic Packain Materials (CEPM) of Korea Science and Enineerin Foundation is ratefully acknowleded.

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