- Email: [email protected]
Fundamental understanding of ACF conduction establishment with emphasis on the thermal and mechanical analysis
ARTICLE IN PRESS
International Journal of Adhesion & Adhesives 24 (2004) 135–142
Fundamental understanding of ACF conduction establishment with emphasis on the thermal and mechanical analysis Woon-Seong Kwon*, Kyung-Wook Paik Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejon 305-701, South Korea Accepted 15 July 2003
Abstract This paper presents the thermal and mechanical contribution of anisotropic conductive films (ACFs) to the electrical conduction establishment of ACF joint. The conduction mechanism of ACF joint strongly depends on the thermal and mechanical properties of ACF. Therefore, it is important to understand the relationship of thermal and mechanical properties of ACF in a bonding process with the electrical conduction establishment. In this study, ACF flip chip process was fully designed based on the material characterization and in situ process monitoring. Moreover, the effect of degree of cure on the ACF conduction establishment was investigated in a bonding process window. The important mechanical mechanism of ACF conduction for good bonding quality is the joint clamping force due to curing and cooling-down processes of ACFs. The build-up behavior of z-axis shrinkage stress in ACF joint during curing and cooling-down processes of ACF materials was experimentally investigated with thermo-mechanical measurement of ACF. These results reveal that shrinkage stress in ACF joint developed during bonding process is the important parameter to establish the electrical conduction of interconnects using ACF material. r 2003 Elsevier Ltd. All rights reserved. Keywords: C. Thermal analysis; C. Stress analysis; Anisotropic conductive films (ACFs); Thermo-mechanical analysis
1. Introduction Flip chip technology has been of much attention because they not only make products thinner, smaller and lighter but also offer good electrical performance due to short interconnection length. Several flip chip assembly technologies such as solder flip chip, stud bump bonding using isotropic conductive adhesives (ICAs) and alloy bonding have been demonstrated. Recently, flip chip interconnection using anisotropic conductive films (ACFs) has been introduced as a promising flip chip alternative. There are some ACF flip chip advantages such as lower bonding temperature, lower cost assembly due to simple processing steps and environmentally friendly process (no lead, no flux and no solvents) [1]. Because of these advantages, ACF bonding technology has been extensively used in areas such as displays, smart card and flip chip applications. *Corresponding author. Tel.: +82-42-869-3375; fax: +82-42-8693310. E-mail address: [email protected] (W.-S. Kwon). 0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.07.003
Effects of deformation characteristics of conductive filler on the ACA electrical conduction development and the relationship between contact resistance change and conductor particle deformation have been reported [2– 4]. While ACF joint is electrically interconnected through conductive fillers and bump, it is mechanically maintained by z-axis joint clamping due to compressive force of ACF after being fully cured and cooled. Therefore, ACF flip chip reliability depends not only on the contact characteristics of conductive particles but also on the ACF joint clamping force established by ACF shrinkage after bonding process. However, fundamental understanding about this compressive force along the z-axis developed by thermal reaction of ACF is still lack. The critical sources of the compressive force (joint clamping force) that establish the ACF electrical conduction are originated from thermo-mechanical properties such as thermal shrinkage and build-up stiffness during or after thermal reaction of ACFs. This shrinkage stress, influenced by thermal shrinkage and build-up stiffness, is responsible for contact integrity in an ACF joint. Shrinkage stress level in an ACF joint is
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closely related with the degree of ACFs cure that has an effect on the ultimate thermo-mechanical properties of ACFs. In order to design a reliable ACF flip chip joint, the development of z-axis shrinkage stress during heating and cooling should be quantitatively understood with the consideration of shrinkage behavior and thermo-mechanical properties. In this study, qualitative relationship between ACF electrical conduction and degree of ACFs cure was investigated. The temperature responses of both zdirectional ACFs shrinkage and material properties of ACFs were experimentally measured. z-axis shrinkage stresses developed during ACF curing and cooling-down processes were then calculated by taking into account not only the thermal shrinkage but also thermomechanical properties.
2. Experimental 2.1. Test chip and PCB materials The dimension of test chips with daisy-chain structure, which is a pattern with interconnected joint of flip chip assembly, was 14.7 mm 8.5 mm 0.7 mm. Gold bumps were formed on aluminum pads of test chips. Bump size was 80–90 mm in diameter and the bump pitch was 800 mm. The 1.2 mm thick FR4 substrate had a thick Ni/Au finish. The 50 mm thick epoxy-based ACFs containing 5 mm diameter Au/Ni-plated polymer ball as a conductive particle and 0.8 mm diameter SiO2 filler as a nonconductive particle were used. 2.2. Degree of ACF cure The degree of cure of ACF materials was obtained by using the Perkin-Elmer DSC7. The amount of exothermic heat from an as-received ACFs was defined as reference. The amounts of the exothermic heat from heat-treated ACFs at either isothermal condition or real bonding condition were defined as the heats from cured ACFs. The heating profile of DSC experiment was in all cases programmed to run from 30 C to 200 C at a rate of 10 C/min. The degree of cure is defined as degree of cureðaÞ DHðuncured ACFsÞ Dðheat treated ACFsÞ ¼ 100: DHðuncured ACFsÞ 2.3. Thickness contraction measurements Dimensional changes of ACFs during heating and cooling were monitored by thermal analyzer with thermal analysis software over a temperature range
Sensing Probe Silicon cover Uncured ACF Silicon cover Sample platform
Fig. 1. Schematics of thermal shrinkage measurement setup of ACFs during curing.
from 30 C to 180 C with heating rate of 5 C/min in N2 gas environment (Fig. 1). ACFs with a size of 5 mm 5 mm 0.05 mm were held between two pieces of silicon cover. Static force applied to sensing probe for the monitoring of z-directional changes was set to minimum value to prevent the squeeze-out of ACF. The z-directional change of two silicon covers was also measured and subtracted from the total z-directional dimension changes to isolate only the z-directional shrinkage of ACFs. Because the upper and lower surface of the ACF sample is held to parallel silicon cover plates, where the shrinkage in the x2y plane is constrained, uni-axial strain condition can be assumed for the free axial shrinkage condition. zdirectional shrinkage of ACFs during the curing and cooling down could be obtained from the dimensional change of TMA measurement. Many repetitions of thickness change measurement confirmed that measurement of thickness contraction by TMA was experimentally reproducible with a variation of less than 10%. 2.4. Modulus measurement The shear modulus of uncured ACF films was measured by using a controlled stress rotational rheometer. The temperature control unit used in this study was an electrically heated parallel plates system. Temperature control unit was used with disposable lower plates. ACF films were placed on the lower plate and the temperature ramped at 5 C/min from 30 C to 200 C. The sample was tested in oscillation mode at a frequency of 1 Hz and a strain amplitude of 2%. Elastic moduli of cured ACF were experimentally determined by a thermal analyzer (Seiko Instruments TMA/SS 6100) equipped with thermal analysis software over a temperature range from 30 C to 200 C with a heating rate of 5 C/min.
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3. ACF thermal reaction In order to optimize the bonding parameters of ACFs, the curing characteristics of the ACF such as curing temperature and isothermal curing time should be fully understood. Degree of cure was investigated for the analysis of bonding parameter, because degree of cure was one of the critical parameters affecting the bonding quality of assembly using ACF. Fig. 2 shows the DSC analysis results of ACFs. Fig. 2(a) shows degree of cure for the ACF with increasing temperature. As shown in Fig. 2(a), the ACF curing reaction started at around 90 C. The inflection point in transition region is called the curing point. In Fig. 2(a), the curing point of ACF was at 115–120 C. Even though ACF curing started at 90 C, it will be fastest at this curing point. Generally, the bonding temperature is isothermally set above curing peak temperature to increase the throughput. Fig. 2(b) shows the degree of cure as a
110 100
Degree of cure (%)
90 80 70 60 50 40 30 20 10 0 -10 20
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Temperature (°C)
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Degree of cure (%)
80 o
150 C o 180 C
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function of curing time at 150 C and 180 C isothermal condition. As shown in Fig. 2(b), while fully cured state (>90%) of ACF at 150 C was attained at about 60 s, ACF curing at 180 C was finished within 25 s. Accordingly, in order to obtain the good bonding quality at the bonding temperature of 180 C, isothermal curing time should be about 25 s.
4. ACF flip chip bonding processes and electrical monitoring 4.1. Process sequences Fig. 3 shows schematic ACF flip chip bonding processes. An FR4 substrate was held on the bonding stage of a flip chip bonder heated at 80 C. ACFs were then pre-bonded on an FR4 substrate. Bumps of a test chip on the heating tool heated at 80 C were aligned to pads on an FR4 substrate. After chip to substrate alignment, bonding force was applied. ACFs placed between a test chip and an FR4 substrate flow and fill the gap between a test chip and an FR4 substrate, because ACF is still the uncured and low viscosity state. At this process stage, electrical contact is established and maintained by the external bonding force with 3N/ bump. Right after bonding force was applied, the temperature of the heating tool ramped up from 80 C to 200 C and bonding stage from 80 C to 190 C, resulting in ACF temperature of 180 C. At this stage, ACF resin starts curing reaction, which increases viscosity and modulus due to the development of cross-linking structure. Both bonding force and temperature were continuously applied for ACFs to be fully cured. After ACF resin was fully cured, bonding force was removed, and assembly was cooled to room temperature. During the cooling process, shrinkage stress can be developed due to thermal shrinkage and build-up stiffness of ACFs. Although the external bonding force was removed after bonding process, the electrical conduction of ACF joint could be mechanically maintained by compressive force due to thermal shrinkage of ACF. 4.2. In situ ACF temperature and connection resistance monitoring
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Isothermal curing time (seconds)
Fig. 2. DSC study of the ACF: (a) dynamic scan at 10 C/min ramp rate and (b) isothermal scan at different temperatures.
Connection resistances between a test chip and a substrate and ACF temperature were monitored during bonding process window at various main bonding times (30, 40, 60 and 90 s). The changes of ACF temperature, measured by a specially designed thermocouple embedded in ACF resin, was shown in Fig. 4. ACFs temperature was initially about 70 C because of a substrate temperature
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80oC
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Substrate Conductive filler
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Heating tool Die
ACF
Thermo-compression bonding
Substrate 80oC
190oC Die
ACF
Substrate
Assembly cooling
Fig. 3. Schematics of ACF flip chip bonding process.
80oC
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Transient temperature response
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ACF Temperature (°C)
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Fig. 5. In situ monitoring of connection resistance at various bonding process condition.
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Time (sec) Fig. 4. In situ monitoring of actual transient temperature for ACFs at various bonding process condition. (Temperature monitoring point is at the center of ACFs.)
of 80 C. During ACF bonding, the ACF temperature increased linearly up to 180 C. In situ monitoring result of interconnect resistance during bonding process was shown in Fig. 5. The measured connection resistance is sum of electrical
resistance of PCB Cu traces, Al metallization lines and Au bumps. The electrical connection between FR4 substrate and silicon chip is established through ACF contacts between Au bumps and substrate pads. The connection resistance rapidly decreased and stayed at about 0.22 O, when electrical conduction was formed due to ACF contact by external bonding force of heating tool. As a heating tool was heated up to 200 C, connection resistance increased due to temperaturedependent resistivity of metal. After the external bonding force and heat were removed, connection resistance decreased during the cooling of assembly. However, for the bonding time of 30 s, the connection
ARTICLE IN PRESS W.-S. Kwon, K.-W. Paik / International Journal of Adhesion & Adhesives 24 (2004) 135–142
resistance rapidly became back to high-resistance values, after the bonding force and heat are removed. This is because the degree of ACF cure is not sufficient enough to maintain good ACF contacts between bumps and substrate pads. 4.3. Relationship between in situ connection resistance and degree of ACF cure
Degree of cure (%)
The in situ measurement of both degrees of ACFs cure and connection resistance changes during bonding process windows is very important to optimize real bonding condition and to obtain reliable ACF joint. The degree of ACF cure at various bonding times was shown in Fig. 6. In order to calculate the degree of ACFs cure after specific assembly bonding time, exothermic heats of ACFs that undergo the same bonding profile as the bonding process window were measured with DSC. As shown in Fig. 6, cross-linking reaction of epoxy chains rapidly occurs during 25–40 s bonding time. And then after 40 s, curing reaction was almost ended and degree of ACFs cure reached almost more than 90%. In situ measurement results of interconnect resistance and degree of cure during ACF bonding process showed that degree of ACFs cure significantly affected conduction establishment of ACF joints. When the degree of cure was below 60%, the connection resistance bounce up phenomena was observed during the cooling of assembly. However, as the degree of cure increased more than 90%, stable ACF interconnect was established. It is presumably because high degree of ACFs cure may result in sufficient shrinkage stress for the stable electrical conduction between chip and substrate. And the degree of ACFs cure is also closely related with the evolution of shrinkage strain and mechanical stiffness of ACF. Therefore, the degree of ACFs cure should be
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sufficiently high in order to obtain both ultimate ACF material properties and compressive force for a stable ACF electrical conduction.
5. Experimental analysis for the development of shrinkage stress As described above, shrinkage stress in a joint resulting from full-cure of ACF is important because it establishes reliable ACF flip chip joint. The development of shrinkage stress is generally governed by the evolution of thickness contraction and mechanical stiffness during curing and cooling-down processes. Typical ACF bonding processes involve two steps in viewpoint of thermal and mechanical properties: (1) curing at an elevated bonding temperature where ACFs chemically shrink and build up stiffness; (2) cooling from elevated bonding temperature to room temperature where ACFs physically shrink and become stiff. Shrinkage stress developed along the thickness direction during these bonding steps is the source of joint clamping force which builds a stable ACF electrical conduction. The shrinkage stress along the z-axis can be expressed as an integral of the thickness contraction and elastic modulus as shown below: Z sz ¼ EdEz ; ð1Þ T
where E is the temperature-dependent modulus. Ez denotes the thickness change of ACFs. The shrinkage stress calculated with Eq. (1) has an analogy to the compressive force of ACF layer along the thickness direction, which supports the compressed state of conductive particles at ACF flip chip joint. In other words, both thickness shrinkage and build-up stiffness of the ACF layer cause the ACF flip chip joint to be compressed state but the ACF matrix be stretched state. In this study, the compressive force acted on the interconnect due to thickness shrinkage of ACF is called ‘‘shrinkage stress’’. Experimental analysis of shrinkage stress estimated from Eq. (1) will be helpful to understand the ACF electrical conduction established by the z-axis mechanical clamping during curing and cooling down. 5.1. Evolution of shrinkage and mechanical stiffness
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Assembly bonding time (sec)
Fig. 6. Degree of ACF cure as a function of assembly bonding times in bonding process window. Assembly bonding time is not equivalent to isothermal ACF cure time.
The shrinkage stress along the z-axis was obtained in terms of the thickness contraction and modulus. Using thermo-mechanical analysis, these polymeric properties were monitored during curing and cooling-down processes of ACF bonding. The z-directional dimension changes of ACFs as a function of temperature were shown in Fig. 7. As shown
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Dimensional change (%)
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Fig. 7. z-directional dimension changes of ACFs during curing and cooling down.
5.2. Shrinkage stress build-up Elastic modulus change and thickness contraction during the temperature history can be simultaneously plotted with changing temperature. Shear modulus in
Storage modulus (GPa)
in Fig. 7, with increasing temperature, ACF started to expand because of polymer chain movement. This early expansion of ACF has no significant contribution to the evolution of stress in ACFs. When ACFs acquired enough thermal energy for curing reaction to start, ACFs started cross-linking reaction to form a polymer network structure. When curing reaction started at about 100 C, cure shrinkage became detectable as shown in Fig. 7. Finally as the cure reaction finished, cure shrinkage due to cross-linking reaction diminished. And cured ACFs started to expand again at about 130 C. During cooling down to room temperature, dimensional changes exhibited two different slopes differentiated by glass transition temperature at about 110 C. It is important to recognize that during ACFs bonding process, ACFs experience both cure shrinkage due to epoxy cross-linking and thermal shrinkage due to cooling down from bonding temperature to room temperature. Fig. 8(a) shows how G 0 (elastic shear modulus) and G 00 (viscous shear modulus) of ACFs change as a function of temperature during ACFs curing. As temperature increased, ACFs started softening and moduli decreased up to 80 C. After ACFs acquired enough thermal energy for cross-linking reaction to take place, modulus started to rise rapidly in the B-stage region. Fig. 8(b) shows the elastic modulus changes of fully cured ACF during subsequent cooling down. Upon cooling below Tg ; ACFs changed from rubbery materials to glassy solid and shrinkage stress may be primarily developed during cooling down below 120 C.
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Fig. 8. ACF modulus as a function of temperature: (a) modulus changes of uncured ACFs during ACF curing and (b) modulus changes of cured ACFs during cooling down to room temperature.
Fig. 8(a) was converted to Young’s modulus in order to calculate the shrinkage stress as expressed in Eq. (1). The uncured ACF exhibits the liquid-like behavior. Moreover, the fully cured ACF exhibits the elastomericlike behavior above its glass transition temperature. Therefore, the polymeric ACF which exhibits the liquidlike and elastomeric behavior can be regarded as the elastic incompressibility material, and consequently Poisson’s ratio approaches a value of 0.5. In the result, the shear modulus in Fig. 8(a) was converted to Young’s modulus as shown below: G¼
E E ¼ ; therefore E ¼ 3G: 2ð1 þ nÞ 3
ð2Þ
Young’s modulus versus thickness contraction curves during curing and cooling-down processes were presented in Fig. 9. Each starting points in these curves (Fig. 9(a) and (b)) individually correspond to the shrinkage onset temperature (100 C) and bonding temperature (180 C). The progress of shrinkage stresses with increasing temperature is proportional to the area
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Shrinkage stress (MPa)
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Fig. 9. Modulus (E) versus thickness contraction (de) during (a) curing and (b) cooling down. The area under the curve corresponds to the shrinkage stress.
of each curve for the curing and cooling-down processes. Fig. 10 shows shrinkage stresses developed during ACF curing and cooling-down processes, estimated by numerical integration according to Eq. (1), as a function of temperatures. As shown in Fig. 10(a), in early heating stage up to 110 C, total stress could be negligible because the modulus of uncured ACFs was virtually zero. As curing reaction was proceeded above 100 C, ACFs started to chemically shrink and build up stiffness, resulting in cure shrinkage stress build-up as shown in Fig. 10(a). Upon cooling ACF from bonding temperature to Tg ; the shrinkage stress was almost negligible because rubbery ACF had an extremely low modulus. At lower than Tg of ACFs, shrinkage stress was considerably developed as a result of ACF shrinkage and significant modulus increase. It is evident that shrinkage stress during ACF bonding process is mainly due to the cooling-down process below Tg : Therefore, glass transi-
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180
Temperature (°C)
(b)
Fig. 10. Calculated shrinkage stresses along the z-axis during (a) ACF curing and (b) cooling down. Shrinkage stresses were calculated by numerical integration according to Eq. (1).
Table 1 Comparison of shrinkage stresses during the curing and cooling-down processes of ACF Process step
Young’s modulusa (MPa)
Thickness contractiona (%)
Shrinkage stressb (MPa)
Cure Cooling below Tg
70 2600
1.46 2.2
0.2 35.1
Total a b
35.3
Measured. Numerical integration according to Eq. (1).
tion temperature is the important factor to determine the shrinkage stress level in ACF joint. Shrinkage stresses from the different parts of curing and cooling-down processes were summarized in Table 1. The cure shrinkage stress, 0.2 MPa, was very
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small when compared with shrinkage stress during cooling down, 35.1 MPa.
6. Conclusions Conduction establishment of flip chip joint using ACFs with emphasis on thermal and mechanical behaviors was experimentally investigated. Measurement of degree of cure for ACF materials during bonding process is important for choosing proper bonding condition and ensuring optimal performance. Moreover, the degree of cure of ACF has an effect on the evolution of shrinkage strain and mechanical stiffness associated with the shrinkage stress. The relationship of connect resistance with degree of cure during ACF bonding revealed that conduction establishment in ACF joint was greatly affected by the degree of cure of ACF resulting in the development of shrinkage stress. Based on the thermal shrinkage and thermomechanical properties of ACF, the build-up of z-axis shrinkage stresses during cure and cool-down process of ACF was experimentally investigated. The considerable development of shrinkage stress was due to the cooldown process below Tg : Therefore, it is considered that
glass transition temperature is the important onset point for the development of shrinkage stress resulting in reliable ACF joint. Consequently, fundamental understanding about ACF flip chip joint in viewpoint of thermal and mechanical behaviors is important for the conduction establishment and optimal ACF joint.
References [1] Liu J. An overview of advances of conductive adhesive joining technology in electronics applications. Mater Technol 1995;10: 247–52. [2] Yim MJ, Paik KW, Kim YK, Hwang HN. A study on the electrical conduction mechanism of anisotropically conductive film for LCD packaging application. In: Advances in electronic packaging, International Electronic Packaging Technical Conference and Exhibition (Inter PACK), vol. 19-1. New York: EEP; 1997. p. 65–72. [3] Fu Y, Wang Y, Wang X, Liu J, Lai Z. Experimental characterization and theoretical characterization of electrical contact in anisotropically conductive adhesives. IEEE Trans CPMT B: Adv Packaging 2000;23(1):15–21. [4] Lai Z, Liu J. Anisotropically conductive adhesive flip-chip bonding on rigid and flexible printed circuit substrates. IEEE Trans CPMT B: Adv Packaging 1996;19(3):644–60.
International Journal of Adhesion & Adhesives 24 (2004) 135–142
Fundamental understanding of ACF conduction establishment with emphasis on the thermal and mechanical analysis Woon-Seong Kwon*, Kyung-Wook Paik Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejon 305-701, South Korea Accepted 15 July 2003
Abstract This paper presents the thermal and mechanical contribution of anisotropic conductive films (ACFs) to the electrical conduction establishment of ACF joint. The conduction mechanism of ACF joint strongly depends on the thermal and mechanical properties of ACF. Therefore, it is important to understand the relationship of thermal and mechanical properties of ACF in a bonding process with the electrical conduction establishment. In this study, ACF flip chip process was fully designed based on the material characterization and in situ process monitoring. Moreover, the effect of degree of cure on the ACF conduction establishment was investigated in a bonding process window. The important mechanical mechanism of ACF conduction for good bonding quality is the joint clamping force due to curing and cooling-down processes of ACFs. The build-up behavior of z-axis shrinkage stress in ACF joint during curing and cooling-down processes of ACF materials was experimentally investigated with thermo-mechanical measurement of ACF. These results reveal that shrinkage stress in ACF joint developed during bonding process is the important parameter to establish the electrical conduction of interconnects using ACF material. r 2003 Elsevier Ltd. All rights reserved. Keywords: C. Thermal analysis; C. Stress analysis; Anisotropic conductive films (ACFs); Thermo-mechanical analysis
1. Introduction Flip chip technology has been of much attention because they not only make products thinner, smaller and lighter but also offer good electrical performance due to short interconnection length. Several flip chip assembly technologies such as solder flip chip, stud bump bonding using isotropic conductive adhesives (ICAs) and alloy bonding have been demonstrated. Recently, flip chip interconnection using anisotropic conductive films (ACFs) has been introduced as a promising flip chip alternative. There are some ACF flip chip advantages such as lower bonding temperature, lower cost assembly due to simple processing steps and environmentally friendly process (no lead, no flux and no solvents) [1]. Because of these advantages, ACF bonding technology has been extensively used in areas such as displays, smart card and flip chip applications. *Corresponding author. Tel.: +82-42-869-3375; fax: +82-42-8693310. E-mail address: [email protected] (W.-S. Kwon). 0143-7496/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2003.07.003
Effects of deformation characteristics of conductive filler on the ACA electrical conduction development and the relationship between contact resistance change and conductor particle deformation have been reported [2– 4]. While ACF joint is electrically interconnected through conductive fillers and bump, it is mechanically maintained by z-axis joint clamping due to compressive force of ACF after being fully cured and cooled. Therefore, ACF flip chip reliability depends not only on the contact characteristics of conductive particles but also on the ACF joint clamping force established by ACF shrinkage after bonding process. However, fundamental understanding about this compressive force along the z-axis developed by thermal reaction of ACF is still lack. The critical sources of the compressive force (joint clamping force) that establish the ACF electrical conduction are originated from thermo-mechanical properties such as thermal shrinkage and build-up stiffness during or after thermal reaction of ACFs. This shrinkage stress, influenced by thermal shrinkage and build-up stiffness, is responsible for contact integrity in an ACF joint. Shrinkage stress level in an ACF joint is
ARTICLE IN PRESS 136
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closely related with the degree of ACFs cure that has an effect on the ultimate thermo-mechanical properties of ACFs. In order to design a reliable ACF flip chip joint, the development of z-axis shrinkage stress during heating and cooling should be quantitatively understood with the consideration of shrinkage behavior and thermo-mechanical properties. In this study, qualitative relationship between ACF electrical conduction and degree of ACFs cure was investigated. The temperature responses of both zdirectional ACFs shrinkage and material properties of ACFs were experimentally measured. z-axis shrinkage stresses developed during ACF curing and cooling-down processes were then calculated by taking into account not only the thermal shrinkage but also thermomechanical properties.
2. Experimental 2.1. Test chip and PCB materials The dimension of test chips with daisy-chain structure, which is a pattern with interconnected joint of flip chip assembly, was 14.7 mm 8.5 mm 0.7 mm. Gold bumps were formed on aluminum pads of test chips. Bump size was 80–90 mm in diameter and the bump pitch was 800 mm. The 1.2 mm thick FR4 substrate had a thick Ni/Au finish. The 50 mm thick epoxy-based ACFs containing 5 mm diameter Au/Ni-plated polymer ball as a conductive particle and 0.8 mm diameter SiO2 filler as a nonconductive particle were used. 2.2. Degree of ACF cure The degree of cure of ACF materials was obtained by using the Perkin-Elmer DSC7. The amount of exothermic heat from an as-received ACFs was defined as reference. The amounts of the exothermic heat from heat-treated ACFs at either isothermal condition or real bonding condition were defined as the heats from cured ACFs. The heating profile of DSC experiment was in all cases programmed to run from 30 C to 200 C at a rate of 10 C/min. The degree of cure is defined as degree of cureðaÞ DHðuncured ACFsÞ Dðheat treated ACFsÞ ¼ 100: DHðuncured ACFsÞ 2.3. Thickness contraction measurements Dimensional changes of ACFs during heating and cooling were monitored by thermal analyzer with thermal analysis software over a temperature range
Sensing Probe Silicon cover Uncured ACF Silicon cover Sample platform
Fig. 1. Schematics of thermal shrinkage measurement setup of ACFs during curing.
from 30 C to 180 C with heating rate of 5 C/min in N2 gas environment (Fig. 1). ACFs with a size of 5 mm 5 mm 0.05 mm were held between two pieces of silicon cover. Static force applied to sensing probe for the monitoring of z-directional changes was set to minimum value to prevent the squeeze-out of ACF. The z-directional change of two silicon covers was also measured and subtracted from the total z-directional dimension changes to isolate only the z-directional shrinkage of ACFs. Because the upper and lower surface of the ACF sample is held to parallel silicon cover plates, where the shrinkage in the x2y plane is constrained, uni-axial strain condition can be assumed for the free axial shrinkage condition. zdirectional shrinkage of ACFs during the curing and cooling down could be obtained from the dimensional change of TMA measurement. Many repetitions of thickness change measurement confirmed that measurement of thickness contraction by TMA was experimentally reproducible with a variation of less than 10%. 2.4. Modulus measurement The shear modulus of uncured ACF films was measured by using a controlled stress rotational rheometer. The temperature control unit used in this study was an electrically heated parallel plates system. Temperature control unit was used with disposable lower plates. ACF films were placed on the lower plate and the temperature ramped at 5 C/min from 30 C to 200 C. The sample was tested in oscillation mode at a frequency of 1 Hz and a strain amplitude of 2%. Elastic moduli of cured ACF were experimentally determined by a thermal analyzer (Seiko Instruments TMA/SS 6100) equipped with thermal analysis software over a temperature range from 30 C to 200 C with a heating rate of 5 C/min.
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3. ACF thermal reaction In order to optimize the bonding parameters of ACFs, the curing characteristics of the ACF such as curing temperature and isothermal curing time should be fully understood. Degree of cure was investigated for the analysis of bonding parameter, because degree of cure was one of the critical parameters affecting the bonding quality of assembly using ACF. Fig. 2 shows the DSC analysis results of ACFs. Fig. 2(a) shows degree of cure for the ACF with increasing temperature. As shown in Fig. 2(a), the ACF curing reaction started at around 90 C. The inflection point in transition region is called the curing point. In Fig. 2(a), the curing point of ACF was at 115–120 C. Even though ACF curing started at 90 C, it will be fastest at this curing point. Generally, the bonding temperature is isothermally set above curing peak temperature to increase the throughput. Fig. 2(b) shows the degree of cure as a
110 100
Degree of cure (%)
90 80 70 60 50 40 30 20 10 0 -10 20
40
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Temperature (°C)
(a) 0
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100 90
Degree of cure (%)
80 o
150 C o 180 C
70 60 50
function of curing time at 150 C and 180 C isothermal condition. As shown in Fig. 2(b), while fully cured state (>90%) of ACF at 150 C was attained at about 60 s, ACF curing at 180 C was finished within 25 s. Accordingly, in order to obtain the good bonding quality at the bonding temperature of 180 C, isothermal curing time should be about 25 s.
4. ACF flip chip bonding processes and electrical monitoring 4.1. Process sequences Fig. 3 shows schematic ACF flip chip bonding processes. An FR4 substrate was held on the bonding stage of a flip chip bonder heated at 80 C. ACFs were then pre-bonded on an FR4 substrate. Bumps of a test chip on the heating tool heated at 80 C were aligned to pads on an FR4 substrate. After chip to substrate alignment, bonding force was applied. ACFs placed between a test chip and an FR4 substrate flow and fill the gap between a test chip and an FR4 substrate, because ACF is still the uncured and low viscosity state. At this process stage, electrical contact is established and maintained by the external bonding force with 3N/ bump. Right after bonding force was applied, the temperature of the heating tool ramped up from 80 C to 200 C and bonding stage from 80 C to 190 C, resulting in ACF temperature of 180 C. At this stage, ACF resin starts curing reaction, which increases viscosity and modulus due to the development of cross-linking structure. Both bonding force and temperature were continuously applied for ACFs to be fully cured. After ACF resin was fully cured, bonding force was removed, and assembly was cooled to room temperature. During the cooling process, shrinkage stress can be developed due to thermal shrinkage and build-up stiffness of ACFs. Although the external bonding force was removed after bonding process, the electrical conduction of ACF joint could be mechanically maintained by compressive force due to thermal shrinkage of ACF. 4.2. In situ ACF temperature and connection resistance monitoring
40 30 20 10 0 -10 0
(b)
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30
40
50
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70
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90
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110
120
Isothermal curing time (seconds)
Fig. 2. DSC study of the ACF: (a) dynamic scan at 10 C/min ramp rate and (b) isothermal scan at different temperatures.
Connection resistances between a test chip and a substrate and ACF temperature were monitored during bonding process window at various main bonding times (30, 40, 60 and 90 s). The changes of ACF temperature, measured by a specially designed thermocouple embedded in ACF resin, was shown in Fig. 4. ACFs temperature was initially about 70 C because of a substrate temperature
ARTICLE IN PRESS W.-S. Kwon, K.-W. Paik / International Journal of Adhesion & Adhesives 24 (2004) 135–142
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80oC
Heating tool Die
Bump
ACF
ACF pre-bonding & Align
Substrate Conductive filler
80oC
80oC 200oC
Heating tool Die
ACF
Thermo-compression bonding
Substrate 80oC
190oC Die
ACF
Substrate
Assembly cooling
Fig. 3. Schematics of ACF flip chip bonding process.
80oC
200oC
0.40
Die Substrate 80oC
190oC
220
Transient temperature response
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Main bonding time 30 sec 40 sec 60 sec 90 sec
ACF Temperature (°C)
180 160 o
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2.238 C/sec o =134 C/min
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Connection resistance (Ω)
Heating tool ACF
In-situ resistance monitoring
0.35
30 sec 40 sec 60 sec 90 sec
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Time (sec)
Fig. 5. In situ monitoring of connection resistance at various bonding process condition.
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Time (sec) Fig. 4. In situ monitoring of actual transient temperature for ACFs at various bonding process condition. (Temperature monitoring point is at the center of ACFs.)
of 80 C. During ACF bonding, the ACF temperature increased linearly up to 180 C. In situ monitoring result of interconnect resistance during bonding process was shown in Fig. 5. The measured connection resistance is sum of electrical
resistance of PCB Cu traces, Al metallization lines and Au bumps. The electrical connection between FR4 substrate and silicon chip is established through ACF contacts between Au bumps and substrate pads. The connection resistance rapidly decreased and stayed at about 0.22 O, when electrical conduction was formed due to ACF contact by external bonding force of heating tool. As a heating tool was heated up to 200 C, connection resistance increased due to temperaturedependent resistivity of metal. After the external bonding force and heat were removed, connection resistance decreased during the cooling of assembly. However, for the bonding time of 30 s, the connection
ARTICLE IN PRESS W.-S. Kwon, K.-W. Paik / International Journal of Adhesion & Adhesives 24 (2004) 135–142
resistance rapidly became back to high-resistance values, after the bonding force and heat are removed. This is because the degree of ACF cure is not sufficient enough to maintain good ACF contacts between bumps and substrate pads. 4.3. Relationship between in situ connection resistance and degree of ACF cure
Degree of cure (%)
The in situ measurement of both degrees of ACFs cure and connection resistance changes during bonding process windows is very important to optimize real bonding condition and to obtain reliable ACF joint. The degree of ACF cure at various bonding times was shown in Fig. 6. In order to calculate the degree of ACFs cure after specific assembly bonding time, exothermic heats of ACFs that undergo the same bonding profile as the bonding process window were measured with DSC. As shown in Fig. 6, cross-linking reaction of epoxy chains rapidly occurs during 25–40 s bonding time. And then after 40 s, curing reaction was almost ended and degree of ACFs cure reached almost more than 90%. In situ measurement results of interconnect resistance and degree of cure during ACF bonding process showed that degree of ACFs cure significantly affected conduction establishment of ACF joints. When the degree of cure was below 60%, the connection resistance bounce up phenomena was observed during the cooling of assembly. However, as the degree of cure increased more than 90%, stable ACF interconnect was established. It is presumably because high degree of ACFs cure may result in sufficient shrinkage stress for the stable electrical conduction between chip and substrate. And the degree of ACFs cure is also closely related with the evolution of shrinkage strain and mechanical stiffness of ACF. Therefore, the degree of ACFs cure should be
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sufficiently high in order to obtain both ultimate ACF material properties and compressive force for a stable ACF electrical conduction.
5. Experimental analysis for the development of shrinkage stress As described above, shrinkage stress in a joint resulting from full-cure of ACF is important because it establishes reliable ACF flip chip joint. The development of shrinkage stress is generally governed by the evolution of thickness contraction and mechanical stiffness during curing and cooling-down processes. Typical ACF bonding processes involve two steps in viewpoint of thermal and mechanical properties: (1) curing at an elevated bonding temperature where ACFs chemically shrink and build up stiffness; (2) cooling from elevated bonding temperature to room temperature where ACFs physically shrink and become stiff. Shrinkage stress developed along the thickness direction during these bonding steps is the source of joint clamping force which builds a stable ACF electrical conduction. The shrinkage stress along the z-axis can be expressed as an integral of the thickness contraction and elastic modulus as shown below: Z sz ¼ EdEz ; ð1Þ T
where E is the temperature-dependent modulus. Ez denotes the thickness change of ACFs. The shrinkage stress calculated with Eq. (1) has an analogy to the compressive force of ACF layer along the thickness direction, which supports the compressed state of conductive particles at ACF flip chip joint. In other words, both thickness shrinkage and build-up stiffness of the ACF layer cause the ACF flip chip joint to be compressed state but the ACF matrix be stretched state. In this study, the compressive force acted on the interconnect due to thickness shrinkage of ACF is called ‘‘shrinkage stress’’. Experimental analysis of shrinkage stress estimated from Eq. (1) will be helpful to understand the ACF electrical conduction established by the z-axis mechanical clamping during curing and cooling down. 5.1. Evolution of shrinkage and mechanical stiffness
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Assembly bonding time (sec)
Fig. 6. Degree of ACF cure as a function of assembly bonding times in bonding process window. Assembly bonding time is not equivalent to isothermal ACF cure time.
The shrinkage stress along the z-axis was obtained in terms of the thickness contraction and modulus. Using thermo-mechanical analysis, these polymeric properties were monitored during curing and cooling-down processes of ACF bonding. The z-directional dimension changes of ACFs as a function of temperature were shown in Fig. 7. As shown
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Dimensional change (%)
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Fig. 7. z-directional dimension changes of ACFs during curing and cooling down.
5.2. Shrinkage stress build-up Elastic modulus change and thickness contraction during the temperature history can be simultaneously plotted with changing temperature. Shear modulus in
Storage modulus (GPa)
in Fig. 7, with increasing temperature, ACF started to expand because of polymer chain movement. This early expansion of ACF has no significant contribution to the evolution of stress in ACFs. When ACFs acquired enough thermal energy for curing reaction to start, ACFs started cross-linking reaction to form a polymer network structure. When curing reaction started at about 100 C, cure shrinkage became detectable as shown in Fig. 7. Finally as the cure reaction finished, cure shrinkage due to cross-linking reaction diminished. And cured ACFs started to expand again at about 130 C. During cooling down to room temperature, dimensional changes exhibited two different slopes differentiated by glass transition temperature at about 110 C. It is important to recognize that during ACFs bonding process, ACFs experience both cure shrinkage due to epoxy cross-linking and thermal shrinkage due to cooling down from bonding temperature to room temperature. Fig. 8(a) shows how G 0 (elastic shear modulus) and G 00 (viscous shear modulus) of ACFs change as a function of temperature during ACFs curing. As temperature increased, ACFs started softening and moduli decreased up to 80 C. After ACFs acquired enough thermal energy for cross-linking reaction to take place, modulus started to rise rapidly in the B-stage region. Fig. 8(b) shows the elastic modulus changes of fully cured ACF during subsequent cooling down. Upon cooling below Tg ; ACFs changed from rubbery materials to glassy solid and shrinkage stress may be primarily developed during cooling down below 120 C.
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Fig. 8. ACF modulus as a function of temperature: (a) modulus changes of uncured ACFs during ACF curing and (b) modulus changes of cured ACFs during cooling down to room temperature.
Fig. 8(a) was converted to Young’s modulus in order to calculate the shrinkage stress as expressed in Eq. (1). The uncured ACF exhibits the liquid-like behavior. Moreover, the fully cured ACF exhibits the elastomericlike behavior above its glass transition temperature. Therefore, the polymeric ACF which exhibits the liquidlike and elastomeric behavior can be regarded as the elastic incompressibility material, and consequently Poisson’s ratio approaches a value of 0.5. In the result, the shear modulus in Fig. 8(a) was converted to Young’s modulus as shown below: G¼
E E ¼ ; therefore E ¼ 3G: 2ð1 þ nÞ 3
ð2Þ
Young’s modulus versus thickness contraction curves during curing and cooling-down processes were presented in Fig. 9. Each starting points in these curves (Fig. 9(a) and (b)) individually correspond to the shrinkage onset temperature (100 C) and bonding temperature (180 C). The progress of shrinkage stresses with increasing temperature is proportional to the area
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Shrinkage stress (MPa)
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5
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Thickness contraction (%)
Fig. 9. Modulus (E) versus thickness contraction (de) during (a) curing and (b) cooling down. The area under the curve corresponds to the shrinkage stress.
of each curve for the curing and cooling-down processes. Fig. 10 shows shrinkage stresses developed during ACF curing and cooling-down processes, estimated by numerical integration according to Eq. (1), as a function of temperatures. As shown in Fig. 10(a), in early heating stage up to 110 C, total stress could be negligible because the modulus of uncured ACFs was virtually zero. As curing reaction was proceeded above 100 C, ACFs started to chemically shrink and build up stiffness, resulting in cure shrinkage stress build-up as shown in Fig. 10(a). Upon cooling ACF from bonding temperature to Tg ; the shrinkage stress was almost negligible because rubbery ACF had an extremely low modulus. At lower than Tg of ACFs, shrinkage stress was considerably developed as a result of ACF shrinkage and significant modulus increase. It is evident that shrinkage stress during ACF bonding process is mainly due to the cooling-down process below Tg : Therefore, glass transi-
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Temperature (°C)
(b)
Fig. 10. Calculated shrinkage stresses along the z-axis during (a) ACF curing and (b) cooling down. Shrinkage stresses were calculated by numerical integration according to Eq. (1).
Table 1 Comparison of shrinkage stresses during the curing and cooling-down processes of ACF Process step
Young’s modulusa (MPa)
Thickness contractiona (%)
Shrinkage stressb (MPa)
Cure Cooling below Tg
70 2600
1.46 2.2
0.2 35.1
Total a b
35.3
Measured. Numerical integration according to Eq. (1).
tion temperature is the important factor to determine the shrinkage stress level in ACF joint. Shrinkage stresses from the different parts of curing and cooling-down processes were summarized in Table 1. The cure shrinkage stress, 0.2 MPa, was very
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small when compared with shrinkage stress during cooling down, 35.1 MPa.
6. Conclusions Conduction establishment of flip chip joint using ACFs with emphasis on thermal and mechanical behaviors was experimentally investigated. Measurement of degree of cure for ACF materials during bonding process is important for choosing proper bonding condition and ensuring optimal performance. Moreover, the degree of cure of ACF has an effect on the evolution of shrinkage strain and mechanical stiffness associated with the shrinkage stress. The relationship of connect resistance with degree of cure during ACF bonding revealed that conduction establishment in ACF joint was greatly affected by the degree of cure of ACF resulting in the development of shrinkage stress. Based on the thermal shrinkage and thermomechanical properties of ACF, the build-up of z-axis shrinkage stresses during cure and cool-down process of ACF was experimentally investigated. The considerable development of shrinkage stress was due to the cooldown process below Tg : Therefore, it is considered that
glass transition temperature is the important onset point for the development of shrinkage stress resulting in reliable ACF joint. Consequently, fundamental understanding about ACF flip chip joint in viewpoint of thermal and mechanical behaviors is important for the conduction establishment and optimal ACF joint.
References [1] Liu J. An overview of advances of conductive adhesive joining technology in electronics applications. Mater Technol 1995;10: 247–52. [2] Yim MJ, Paik KW, Kim YK, Hwang HN. A study on the electrical conduction mechanism of anisotropically conductive film for LCD packaging application. In: Advances in electronic packaging, International Electronic Packaging Technical Conference and Exhibition (Inter PACK), vol. 19-1. New York: EEP; 1997. p. 65–72. [3] Fu Y, Wang Y, Wang X, Liu J, Lai Z. Experimental characterization and theoretical characterization of electrical contact in anisotropically conductive adhesives. IEEE Trans CPMT B: Adv Packaging 2000;23(1):15–21. [4] Lai Z, Liu J. Anisotropically conductive adhesive flip-chip bonding on rigid and flexible printed circuit substrates. IEEE Trans CPMT B: Adv Packaging 1996;19(3):644–60.