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Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads
Microelectronics Reliability 51 (2011) 1393–1397
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads Li-Lan Gao a,b, Lei Wang a, Hong Gao a, Gang Chen a, Xu Chen a,⇑ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Mechanical Engineering, Tianjin University of Technology, Tianjin 300191, PR China
a r t i c l e
i n f o
Article history: Received 27 September 2010 Received in revised form 20 December 2010 Accepted 18 March 2011 Available online 11 April 2011
a b s t r a c t The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Flip chip assembly technologies using anisotropic conductive adhesive film (ACF) joints have been paid more attentions because they are not only making electronic products thinner and smaller but also offering reduction in interconnecting distance and inductance. The ACFs used to make these interconnections are often subjected to cyclic loading resulting from repeated mechanical actions or thermal cycling, leading to fatigue failures of the ACF and the joints. With increasing device miniaturization and power consumption the ACF joints must withstand higher fatigue loading and temperatures, and are also often subjected to hygrothermal environments accelerating its fatigue failures. Hence the fatigue reliability of the ACF joints becomes a critical issue in determining the lifetime of a functional device. Many reliability studies have been done on the ACF joints. The contact resistances of the ACF joints with different bonding parameters (including bonding temperature, bonding pressure, etc.) were evaluated to improve its bonding reliability [1–5]. The thermal cycling and high temperature/humidity aging tests were conducted to investigate the effects of thermal and hygrothermal aging on the reliability performance of the ACF joints [6–15]. However there are few investigations on the fatigue reliability of the ACF joints. Some life predictive models for conductive adhesives have been proposed. Abdel Wahab et al. [16] proposed a model for predicting the life of adhesive joints by using the concept of continuum damage mechanics (CDM) and fracture mechanics (FM). Gomatam and ⇑ Corresponding author. Tel.: +86 22 27408399; fax: +86 22 27403389. E-mail address: [email protected] (X. Chen). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.03.032
Sancaktar [17,18] constructed the fatigue life predictive models under constant cyclic loading or varying cyclic loading conditions for electronically conductive adhesive based on the experimental results. The relationship between the number of thermal or mechanical cycles to failure and ‘engineering’ parameters such as average shear stress at the adhesive/adherent interfaces was established to predict fatigue failure [19,20]. However further investigations on fatigue life model considering different stress states or environmental factors are greatly needed. In this study, the fatigue properties of the ACF joints with different loading amplitudes were investigated by shear fatigue tests. The hygrothermal aging (85 °C/85% RH) and thermal cycling (40 to 150 °C) were used as accelerators for the degradation of ACF joints so as to investigate the effects of thermal cycling and hygrothermal aging on the shear strength and fatigue life of ACF joints. The life predictive model considering hygrothermal aging was proposed and evaluated with experimental data. 2. Experiments The sample is composed of the chip, ACF and ITO (Indium Tin Oxides) glass substrate in this study. Fig. 1 shows the geometric structure of the three-layer sample. The middle ACF is Type cp6920F thermosetting conductive adhesive supplied by SONY Inc. The bonding process of the sample includes two main steps. The pre-bonding process was carried out at a temperature of 100 °C with 0.1 MPa pressure for 10 s and followed by final bonding at 190 °C and 15.7 MPa for 30 s. The shear tests were conducted for six ACF bonding specimens at room temperature and the maximum shear force was
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3. Fatigue life model The Basquin’s equation is well known for the stress-based fatigue models. A shear mode Basquin’s equation is shown in Eq. (1) for shear fatigue. The total number of cycles to failure, Nf, is related to the shear stress amplitude, Ds/2,
Ds ¼ s0f ð2Nf Þb 2
Fig. 1. The geometric structure of the three-layer sample (mm).
determined as 41.28 N. The shear cyclic fatigue tests were conducted for the ACF bonding samples with different loading amplitudes using the minitype fatigue testing machine with the control style of triangular load pulsation at room temperature. The cyclic peak loading was set as the 50–70% of the maximum shear force and the cycling period was 4 s. For each condition six samples were tested considering random error and the average cycle prior to fracture was defined as the fatigue life. The experimental setup and loading path are shown in Fig. 2. Elevated temperature and high humidity environment will accelerate failure of ACF joints. In order to investigate the effects of hygrothermal aging on the shear strength and fatigue life, a typical hygrothermal environment, 85 °C/85% RH, was used as an accelerator for the degradation of ACF joints and the shear tests and fatigue tests with constant loading amplitude of 10.5 N at a cycle of 4 s were carried out on the specimens with aging time of 0, 24, 96, 168, 500 and 1000 h at room temperature respectively. For each condition six samples were tested. Samples were also subjected to the thermal cycling test with the temperature range of 40 °C to 150 °C according to JEDEC Standard No. 22 Method A104-C. Fig. 3 shows the temperature profile of tested thermal cycling. The test pauses were selected as 0, 25, 50, 100, 200, 500 and 1000 h and the shear strength and fatigue lives of the samples were determined by conducting shear tests and fatigue tests with constant loading amplitude of 10.5 N at a cycle of 4 s at room temperature respectively. For each condition six samples were tested.
ð1Þ
where the s0f is shear fatigue strength coefficient and the b is fatigue strength exponent. The constitutive model describing the shear property of the hygrothermal aged ACF joints is derived as the following based on the nonlinear viscoelastic creep model of the ACF [21] by introducing a damage parameter D,
sth ¼
1
l1 þ x1 tn
eðtÞ ð1 DÞ ¼ s ð1 DÞ
ð2Þ
where the l1, x1 and n are material parameters. The interfacial damage factor, D, describes the damage extent of bonding interface and reflects the macroscopic effect of the microcracks or microdefects on the bonding interface. D = 0 means that the bonding interface is not damaged, however D = 1 means the interface is completely damaged. It also means that the bonding sample is delaminated completely. The fatigue life model considering aging damage is derived as the following based on the aging constitutive Eq. (2) of ACF joints and the Basquin’s Eq. (1) in order to further investigate the effect of hygrothermal aging on the fatigue property of ACF joints.
Ds ¼ s0fth ð2Nf Þbth 2
ð3Þ
where s0fth means the aging fatigue strength coefficient that is expressed as the Eq. (4).
s0fth ¼ s0f ð1 DÞ
ð4Þ
The D is fitted by the shear test data of ACF joints with different hygrothermal aging time based on the Eq. (2) and the relation between the damage factor D and aging time t can be obtained as the following.
D¼
1 e0:00103t e0:00086t
Fig. 2. The apparatus and loading path for shear cyclic fatigue test.
ð5Þ
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Fig. 4. The S–N curve of the ACF joints.
Fig. 3. Temperature profile of thermal cycling.
The parameter bth means the fatigue strength exponent with different aging time. The relationship between the bth and the aging time t is determined by fitting the fatigue test data with different aging time.
bth ¼ 0:04264 þ
0:014 1 þ e0:026t þ3:9
ð6Þ
4. Results and discussion 4.1. Effect of loading amplitude on fatigue life of ACF joints The fatigue lives of the ACF joints were investigated by shear fatigue tests with different loading amplitudes at room temperature and the results are shown in Table 1. It is shown that the fatigue lives of the ACF joints decrease with the increasing loading amplitudes. The experimental data with different loading amplitudes were fitted according to Basquin’s equation and the constants of s0f = 6.6442 MPa and b = 0.0426 were gained from the stress-lifetime curve fitted as shown in Fig. 4. It is found that the predictions of Basquin’s equation agree with the experimental results for the shear fatigue lives of ACF joints. Fig. 5 shows the relationship of maximum displacement with cycles under different loading amplitudes. Due to the maximum displacement under 11.9 N changes very gently after 1500 cycles, only the data of former 1500 cycles is displayed in the figure. The maximum displacement of each cycle increases with increasing cycles under different loading amplitudes, and increases rapidly in the first 200 cycles and more and more slowly in the following cycles. In other words, the growth rate of the maximum displacement is reduced in the initial period and then gradually stabilized to a fix value. We can also found that, the greater the loading amplitude, the larger the maximum displacement.
Fig. 5. The maximum displacement with increasing cycles under different loading amplitudes.
Table 1 The fatigue lives of the ACF joints with different loading amplitudes. Loading amplitude (N)
Shear stress amplitude D2s (MPa)
Life Nf (cycles)
10.5 11.9 12.25 12.9 13.125 14
4.12 4.67 4.80 5.06 5.15 5.49
36,821 6838 4897 844 357 59
Fig. 6. Shear strength of ACF joints with increasing hygrothermal aging time.
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Fig. 7. The Nf of ACF joints with increasing hygrothermal aging time at constant loading amplitude of 10.5 N.
Fig. 9. The Nf of ACF joints with the increasing thermal cycling time at constant loading amplitude of 10.5.
4.2. Effects of hygrothermal aging on shear strength and fatigue life of ACF joints
because of the steady damage of ACF joints, and the fatigue life is very short and only 150 cycles for ACF joints with aging time of 1000 h. In previous investigations [22–24], the effects of the hygrothermal aging on epoxy system of ACF have been partly investigated by the uniaxial tensile test, scanning electron microscopy (SEM) analysis, computer simulation and molecular dynamics method. Results show that, due to the plasticization effect of absorbed moisture, both the tensile elastic module and tensile strength of the studied epoxy system have decreased, that is, the absorbed moisture has deleterious effects on the physical properties of epoxies and can, therefore, greatly compromise the performance of an epoxy-based component. Furthermore, the absorbed moisture can attack the adhesive/inorganic substrates interfaces and the hydrolysis occurs in the interfacial region. Generally, the adhesion between polymers and adherents in electronic packages is primarily controlled by coupling agents at the interfaces. The coupling action is dependent on the formation of stable covalent bonds between the polymer adhesive and the adherent. Due to the effect of the moisture, the stable covalent bonds were damaged so that the bonding strength between the adhesive and adherent and fatigue life of the adhesive assembly decrease. Additionally the SEM micrographs of fracture surfaces of ACF joints [25] show that the fracture mechanism of ACF joints gradually changes with hygrothermal aging. ACF joints lose their ductility gradually and yield a relatively flat fracture surface with increasing aging time, and brittle fracture takes place for ACF joints with rapid crack propagation. Thus the bonding strength between the ACF and chip or glass substrate decreases gradually.
Fig. 6 and 7 show the effects of hygrothermal aging on shear strength and fatigue life of the ACF joints by shear tests and shear fatigue tests with constant loading amplitude of 10.5 N respectively. Simultaneously the fatigue lives of the samples with different aging time were predicted by the fatigue model considering hygrothermal aging as shown in the Fig. 7. It shows agreement between the experimental results and predictions for the fatigue lives of ACF joints with different hygrothermal aging time. It is found that the shear strength and fatigue life of ACF joints decrease gradually with the increasing aging time because of the action of moisture and temperature and the decreasing process can be approximately divided into three phases. The primary phase is with slow decrease of the shear strength and fatigue life because hygrothermal aging results in irreversible damage in the ACF, physically (plasticization, as well as the formation of cracks and crazes) and/or chemically (hydrolysis). The second phase is with fast decrease of the strength and life due to the combined effects of the degradation of the ACF material and the interface delamination because of different hygrothermal expansion coefficients of each element under the hygrothermal condition. In the tertiary phase with aging time of 500–1000 h, they tend to steady decrease
4.3. Effects of thermal cycling on shear strength and fatigue life of ACF joints
Fig. 8. Shear strength of ACF joints with increasing thermal cycling time.
Figs. 8 and 9 present the effects of thermal cycling with the temperature range of 40 °C to 150 °C on the shear strength and fatigue life of the ACF joints. The shear strength and fatigue lives of the samples present the increasing trend at aging time between 0 and 25 h, however decrease fast at aging time between 25 and 500 h and then decrease gently with increasing thermal cycling time. The fatigue life is 650 cycles for ACF joints when aging time is 1000 h. The interfacial residual stresses are produced due to the CTE mismatch of the connected components for layered electronic assemblies during manufacturing process. It was found that as a
L.-L. Gao et al. / Microelectronics Reliability 51 (2011) 1393–1397
result of temperature cycles the local yielding and creep deformation of the solders would uniform the residual stresses distribution introduced in manufacturing process and the solder joint strength increased during a certain thermal cycling time [26]. Thus the increases of the shear strength and fatigue lives of ACF joints at initial thermal cycling as shown in Figs. 8 and 9 may be due to the redistribution of the interfacial residual stresses along the assembly length because of ACF’s creep deformation. Then the shear strength and fatigue lives of ACF joints decrease with the increasing aging time due to the combined effects of the thermal degradation of the ACFs and the damage on the interfaces under the thermal cycling. 5. Conclusions The shear fatigue tests were conducted for ACF joints under different testing conditions. The results show that the fatigue lives of ACF joints decrease with increasing loading amplitudes and the maximum displacement of each cycle increases with increasing cycles under different loading amplitudes. It is found that Basquin’s equation can predict the fatigue lives of ACF joints. The shear strength and lives of ACF joints gradually decrease at first and then quickly decrease, and finally the decreasing rates slow down again with increasing hygrothermal aging time. However the shear strength and lives of ACF joints increase firstly and then decrease with increasing thermal cycling. The fatigue life model considering aging damage is proposed based on the aging constitutive model of ACF joints and the Basquin’s equation, and it is found that the predictions of the fatigue model agree with the experimental results at different aging time for ACF joints. Acknowledgements The Project was supported by NSFC (Nos. 10672118, 11072171 and 31000422) and Tianjin Natural Science Foundation (No. 09JCYBJC03100). References [1] Chen X, Zhang J, Jiao CL, Liu YM. Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection. Microelectro Reliab 2006;46:774–85. [2] Chan YC, Luk DY. Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly I. Different bonding temperature. Microelectro Reliab 2002;42:1185–94. [3] Chan YC, Luk DY. Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly II. Different bonding pressure. Microelectro Reliab 2002;42:1195–204. [4] Yim MJ, Chung CK, Paik KW. Effect of conductive particle properties on the reliability of anisotropic conductive film for chip-on-glass applications. IEEE Trans Electr Pack Manufacturing 2007;30:306–12.
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[5] Hwang JS, Yim MJ, Paik KW. Effects of bonding temperature on the properties and reliabilities of anisotropic conductive films (ACFs) for flip chip on organic substrate application. Microelectro Reliab 2008;48:293–9. [6] Jokinen E, Ristolainen E. Anisotropic conductive film flip chip joining using thin chips. Microelectro Reliab 2002;42(12):1913–20. [7] Murray CT, Hogerton PB, Chheang T, et al. Reliability studies of anisotropic conductive adhesives in flex to LCD applications. Inter Symp Microelectro 2000:820–5. [8] Chen X, Zhang J, Wang ZP. Microscopic observation of failure mechanism of anisotropic conductive film for flip-chip joining. Inter Soc Conf Therm Phenom 2004:453–7. [9] Zhang JH, Chan YC. Research on the contact resistance, reliability, and degradation mechanisms of anisotropically conductive film interconnection for flip-chip-on-flex applications. J Electron Mater 2003;32(4):228–34. [10] Cao LQ, Lai ZH, Liu J. In: 3rd International IEEE Conference on Polymers and adhesives in Microelectronics and Photonics; 2003. p. 309–13. [11] Zhang JH, Chan YC, Alam MO, et al. Contact resistance and adhesion performance of ACF interconnections to aluminum metallization. Microelectro Reliab 2003;43:1303–10. [12] Tan CW, Chan YC, Yeung NH. Effect of autoclave test on anisotropic conductive joints. Microelectro Reliab 2003;43:279–85. [13] Mercado LL, White J, Sarihan V, et al. Failure mechanism study of anisotropic conductive film (ACF) packages. IEEE Trans Compo Packaging Technol 2003;26(3):509–16. [14] Wang ZP. Challenges in the reliability study of chip-on-glass (COG) technology for mobile display applications. In: Proceedings of the 5th Electronics Packaging Technology Conference 2003: 595–9. [15] Wu CML, Chau ML. Degradation of flip-chip-on-glass interconnection with ACF under high humidity and thermal aging. Solder Surf Mount Technol 2002;14(2):51–8. [16] Abdel Wahab MM, Ashcroft IA, Crocombe AD, Shaw SJ. Prediction of fatigue thresholds in adhesively bonded joints using damage mechanics and fracture mechanics. J Adhesion Sci Technol 2001;15:763–81. [17] Gomatam RR, Sancaktar E. A comprehensive fatigue life predictive model for electronically conductive adhesive joints under constant-cycle loading. J Adhesion Sci Technol 2006;20(1):87–104. [18] Gomatam RR, Sancaktar E. A novel cumulative fatigue damage model for electronically-conductive adhesive joints under variable loading. J Adhesion Sci Technol 2006;20(1):69–86. [19] Suhir E. Die attachment and its influence on thermal stresses in the die and the attachment. In: Proceedings of the 37th IEEE Electronic Components Conference 1987:143–148. [20] Gladkov A, Bar-Cohen A. Parametric dependence of fatigue of electronic adhesives. In: Proc. 3rd Int. Conference on Adhesive Joining and Coating Technology in Electronic Manufacturing, Binghampton, NY, J. H. Constable (Ed.), 1998:116–124. [21] Gao LL, Chen X, Gao H, Zhang SB. Description of nonlinear viscoelastic behavior and creep rupture time of anisotropic conductive film. Mater Sci Eng A 2010;527:5115–21. [22] Lin YC, Chen X. Moisture sorption-desorption-resorption characteristics and its effect on the mechanical behavior of the epoxy system. Polymer 2005;46(25):11994–2003. [23] Lin YC, Chen X. Investigation of the effect of hygrothermal conditions on epoxy system by fractography and computer simulation. Mater Lett 2005;59:3831–6. [24] Lin YC, Chen X. Investigation of moisture diffusion in epoxy system: experiments and molecular dynamics simulations. Chem Phys Lett 2005;412:322–6. [25] Lin YC, Chen X, Wang ZP. Effects of hygrothermal aging on anisotropic conductive joints: experiments and theoretical analyses. J Adhesion Sci Technol 2006;20:1383–99. [26] Kuang JH, Sheen MT, Chang CFH, Chen CC, Wang GL, Cheng WH. Member, IEEE. Effect of temperature cycling on joint strength of PbSn and AuSn solders in laser packages.. IEEE Trans Adv Pack 2001;24:563–8.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Fatigue life evaluation of anisotropic conductive adhesive film joints under mechanical and hygrothermal loads Li-Lan Gao a,b, Lei Wang a, Hong Gao a, Gang Chen a, Xu Chen a,⇑ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China School of Mechanical Engineering, Tianjin University of Technology, Tianjin 300191, PR China
a r t i c l e
i n f o
Article history: Received 27 September 2010 Received in revised form 20 December 2010 Accepted 18 March 2011 Available online 11 April 2011
a b s t r a c t The shear fatigue lives of Anisotropic Conductive Adhesive Film (ACF) joints were evaluated experimentally and theoretically under different testing conditions. The shear fatigue tests of ACF joints were performed with different loading amplitudes. It is found that the fatigue lives of ACF joints decrease with increasing loading amplitudes and Basquin’s equation is fit to predict the fatigue lives of ACF joints. Hygrothermal aging and thermal cycling tests were conducted to investigate the shear strength and lives of ACF joints. The results show that the shear strength and lives of ACF joints decrease with increasing hygrothermal aging time, however increase firstly and then decrease with increasing thermal cycling time. The fatigue life model considering aging damage is proposed and the predictions of the fatigue life agree with the experimental results at different aging time for ACF joints. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Flip chip assembly technologies using anisotropic conductive adhesive film (ACF) joints have been paid more attentions because they are not only making electronic products thinner and smaller but also offering reduction in interconnecting distance and inductance. The ACFs used to make these interconnections are often subjected to cyclic loading resulting from repeated mechanical actions or thermal cycling, leading to fatigue failures of the ACF and the joints. With increasing device miniaturization and power consumption the ACF joints must withstand higher fatigue loading and temperatures, and are also often subjected to hygrothermal environments accelerating its fatigue failures. Hence the fatigue reliability of the ACF joints becomes a critical issue in determining the lifetime of a functional device. Many reliability studies have been done on the ACF joints. The contact resistances of the ACF joints with different bonding parameters (including bonding temperature, bonding pressure, etc.) were evaluated to improve its bonding reliability [1–5]. The thermal cycling and high temperature/humidity aging tests were conducted to investigate the effects of thermal and hygrothermal aging on the reliability performance of the ACF joints [6–15]. However there are few investigations on the fatigue reliability of the ACF joints. Some life predictive models for conductive adhesives have been proposed. Abdel Wahab et al. [16] proposed a model for predicting the life of adhesive joints by using the concept of continuum damage mechanics (CDM) and fracture mechanics (FM). Gomatam and ⇑ Corresponding author. Tel.: +86 22 27408399; fax: +86 22 27403389. E-mail address: [email protected] (X. Chen). 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.03.032
Sancaktar [17,18] constructed the fatigue life predictive models under constant cyclic loading or varying cyclic loading conditions for electronically conductive adhesive based on the experimental results. The relationship between the number of thermal or mechanical cycles to failure and ‘engineering’ parameters such as average shear stress at the adhesive/adherent interfaces was established to predict fatigue failure [19,20]. However further investigations on fatigue life model considering different stress states or environmental factors are greatly needed. In this study, the fatigue properties of the ACF joints with different loading amplitudes were investigated by shear fatigue tests. The hygrothermal aging (85 °C/85% RH) and thermal cycling (40 to 150 °C) were used as accelerators for the degradation of ACF joints so as to investigate the effects of thermal cycling and hygrothermal aging on the shear strength and fatigue life of ACF joints. The life predictive model considering hygrothermal aging was proposed and evaluated with experimental data. 2. Experiments The sample is composed of the chip, ACF and ITO (Indium Tin Oxides) glass substrate in this study. Fig. 1 shows the geometric structure of the three-layer sample. The middle ACF is Type cp6920F thermosetting conductive adhesive supplied by SONY Inc. The bonding process of the sample includes two main steps. The pre-bonding process was carried out at a temperature of 100 °C with 0.1 MPa pressure for 10 s and followed by final bonding at 190 °C and 15.7 MPa for 30 s. The shear tests were conducted for six ACF bonding specimens at room temperature and the maximum shear force was
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3. Fatigue life model The Basquin’s equation is well known for the stress-based fatigue models. A shear mode Basquin’s equation is shown in Eq. (1) for shear fatigue. The total number of cycles to failure, Nf, is related to the shear stress amplitude, Ds/2,
Ds ¼ s0f ð2Nf Þb 2
Fig. 1. The geometric structure of the three-layer sample (mm).
determined as 41.28 N. The shear cyclic fatigue tests were conducted for the ACF bonding samples with different loading amplitudes using the minitype fatigue testing machine with the control style of triangular load pulsation at room temperature. The cyclic peak loading was set as the 50–70% of the maximum shear force and the cycling period was 4 s. For each condition six samples were tested considering random error and the average cycle prior to fracture was defined as the fatigue life. The experimental setup and loading path are shown in Fig. 2. Elevated temperature and high humidity environment will accelerate failure of ACF joints. In order to investigate the effects of hygrothermal aging on the shear strength and fatigue life, a typical hygrothermal environment, 85 °C/85% RH, was used as an accelerator for the degradation of ACF joints and the shear tests and fatigue tests with constant loading amplitude of 10.5 N at a cycle of 4 s were carried out on the specimens with aging time of 0, 24, 96, 168, 500 and 1000 h at room temperature respectively. For each condition six samples were tested. Samples were also subjected to the thermal cycling test with the temperature range of 40 °C to 150 °C according to JEDEC Standard No. 22 Method A104-C. Fig. 3 shows the temperature profile of tested thermal cycling. The test pauses were selected as 0, 25, 50, 100, 200, 500 and 1000 h and the shear strength and fatigue lives of the samples were determined by conducting shear tests and fatigue tests with constant loading amplitude of 10.5 N at a cycle of 4 s at room temperature respectively. For each condition six samples were tested.
ð1Þ
where the s0f is shear fatigue strength coefficient and the b is fatigue strength exponent. The constitutive model describing the shear property of the hygrothermal aged ACF joints is derived as the following based on the nonlinear viscoelastic creep model of the ACF [21] by introducing a damage parameter D,
sth ¼
1
l1 þ x1 tn
eðtÞ ð1 DÞ ¼ s ð1 DÞ
ð2Þ
where the l1, x1 and n are material parameters. The interfacial damage factor, D, describes the damage extent of bonding interface and reflects the macroscopic effect of the microcracks or microdefects on the bonding interface. D = 0 means that the bonding interface is not damaged, however D = 1 means the interface is completely damaged. It also means that the bonding sample is delaminated completely. The fatigue life model considering aging damage is derived as the following based on the aging constitutive Eq. (2) of ACF joints and the Basquin’s Eq. (1) in order to further investigate the effect of hygrothermal aging on the fatigue property of ACF joints.
Ds ¼ s0fth ð2Nf Þbth 2
ð3Þ
where s0fth means the aging fatigue strength coefficient that is expressed as the Eq. (4).
s0fth ¼ s0f ð1 DÞ
ð4Þ
The D is fitted by the shear test data of ACF joints with different hygrothermal aging time based on the Eq. (2) and the relation between the damage factor D and aging time t can be obtained as the following.
D¼
1 e0:00103t e0:00086t
Fig. 2. The apparatus and loading path for shear cyclic fatigue test.
ð5Þ
L.-L. Gao et al. / Microelectronics Reliability 51 (2011) 1393–1397
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Fig. 4. The S–N curve of the ACF joints.
Fig. 3. Temperature profile of thermal cycling.
The parameter bth means the fatigue strength exponent with different aging time. The relationship between the bth and the aging time t is determined by fitting the fatigue test data with different aging time.
bth ¼ 0:04264 þ
0:014 1 þ e0:026t þ3:9
ð6Þ
4. Results and discussion 4.1. Effect of loading amplitude on fatigue life of ACF joints The fatigue lives of the ACF joints were investigated by shear fatigue tests with different loading amplitudes at room temperature and the results are shown in Table 1. It is shown that the fatigue lives of the ACF joints decrease with the increasing loading amplitudes. The experimental data with different loading amplitudes were fitted according to Basquin’s equation and the constants of s0f = 6.6442 MPa and b = 0.0426 were gained from the stress-lifetime curve fitted as shown in Fig. 4. It is found that the predictions of Basquin’s equation agree with the experimental results for the shear fatigue lives of ACF joints. Fig. 5 shows the relationship of maximum displacement with cycles under different loading amplitudes. Due to the maximum displacement under 11.9 N changes very gently after 1500 cycles, only the data of former 1500 cycles is displayed in the figure. The maximum displacement of each cycle increases with increasing cycles under different loading amplitudes, and increases rapidly in the first 200 cycles and more and more slowly in the following cycles. In other words, the growth rate of the maximum displacement is reduced in the initial period and then gradually stabilized to a fix value. We can also found that, the greater the loading amplitude, the larger the maximum displacement.
Fig. 5. The maximum displacement with increasing cycles under different loading amplitudes.
Table 1 The fatigue lives of the ACF joints with different loading amplitudes. Loading amplitude (N)
Shear stress amplitude D2s (MPa)
Life Nf (cycles)
10.5 11.9 12.25 12.9 13.125 14
4.12 4.67 4.80 5.06 5.15 5.49
36,821 6838 4897 844 357 59
Fig. 6. Shear strength of ACF joints with increasing hygrothermal aging time.
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Fig. 7. The Nf of ACF joints with increasing hygrothermal aging time at constant loading amplitude of 10.5 N.
Fig. 9. The Nf of ACF joints with the increasing thermal cycling time at constant loading amplitude of 10.5.
4.2. Effects of hygrothermal aging on shear strength and fatigue life of ACF joints
because of the steady damage of ACF joints, and the fatigue life is very short and only 150 cycles for ACF joints with aging time of 1000 h. In previous investigations [22–24], the effects of the hygrothermal aging on epoxy system of ACF have been partly investigated by the uniaxial tensile test, scanning electron microscopy (SEM) analysis, computer simulation and molecular dynamics method. Results show that, due to the plasticization effect of absorbed moisture, both the tensile elastic module and tensile strength of the studied epoxy system have decreased, that is, the absorbed moisture has deleterious effects on the physical properties of epoxies and can, therefore, greatly compromise the performance of an epoxy-based component. Furthermore, the absorbed moisture can attack the adhesive/inorganic substrates interfaces and the hydrolysis occurs in the interfacial region. Generally, the adhesion between polymers and adherents in electronic packages is primarily controlled by coupling agents at the interfaces. The coupling action is dependent on the formation of stable covalent bonds between the polymer adhesive and the adherent. Due to the effect of the moisture, the stable covalent bonds were damaged so that the bonding strength between the adhesive and adherent and fatigue life of the adhesive assembly decrease. Additionally the SEM micrographs of fracture surfaces of ACF joints [25] show that the fracture mechanism of ACF joints gradually changes with hygrothermal aging. ACF joints lose their ductility gradually and yield a relatively flat fracture surface with increasing aging time, and brittle fracture takes place for ACF joints with rapid crack propagation. Thus the bonding strength between the ACF and chip or glass substrate decreases gradually.
Fig. 6 and 7 show the effects of hygrothermal aging on shear strength and fatigue life of the ACF joints by shear tests and shear fatigue tests with constant loading amplitude of 10.5 N respectively. Simultaneously the fatigue lives of the samples with different aging time were predicted by the fatigue model considering hygrothermal aging as shown in the Fig. 7. It shows agreement between the experimental results and predictions for the fatigue lives of ACF joints with different hygrothermal aging time. It is found that the shear strength and fatigue life of ACF joints decrease gradually with the increasing aging time because of the action of moisture and temperature and the decreasing process can be approximately divided into three phases. The primary phase is with slow decrease of the shear strength and fatigue life because hygrothermal aging results in irreversible damage in the ACF, physically (plasticization, as well as the formation of cracks and crazes) and/or chemically (hydrolysis). The second phase is with fast decrease of the strength and life due to the combined effects of the degradation of the ACF material and the interface delamination because of different hygrothermal expansion coefficients of each element under the hygrothermal condition. In the tertiary phase with aging time of 500–1000 h, they tend to steady decrease
4.3. Effects of thermal cycling on shear strength and fatigue life of ACF joints
Fig. 8. Shear strength of ACF joints with increasing thermal cycling time.
Figs. 8 and 9 present the effects of thermal cycling with the temperature range of 40 °C to 150 °C on the shear strength and fatigue life of the ACF joints. The shear strength and fatigue lives of the samples present the increasing trend at aging time between 0 and 25 h, however decrease fast at aging time between 25 and 500 h and then decrease gently with increasing thermal cycling time. The fatigue life is 650 cycles for ACF joints when aging time is 1000 h. The interfacial residual stresses are produced due to the CTE mismatch of the connected components for layered electronic assemblies during manufacturing process. It was found that as a
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result of temperature cycles the local yielding and creep deformation of the solders would uniform the residual stresses distribution introduced in manufacturing process and the solder joint strength increased during a certain thermal cycling time [26]. Thus the increases of the shear strength and fatigue lives of ACF joints at initial thermal cycling as shown in Figs. 8 and 9 may be due to the redistribution of the interfacial residual stresses along the assembly length because of ACF’s creep deformation. Then the shear strength and fatigue lives of ACF joints decrease with the increasing aging time due to the combined effects of the thermal degradation of the ACFs and the damage on the interfaces under the thermal cycling. 5. Conclusions The shear fatigue tests were conducted for ACF joints under different testing conditions. The results show that the fatigue lives of ACF joints decrease with increasing loading amplitudes and the maximum displacement of each cycle increases with increasing cycles under different loading amplitudes. It is found that Basquin’s equation can predict the fatigue lives of ACF joints. The shear strength and lives of ACF joints gradually decrease at first and then quickly decrease, and finally the decreasing rates slow down again with increasing hygrothermal aging time. However the shear strength and lives of ACF joints increase firstly and then decrease with increasing thermal cycling. The fatigue life model considering aging damage is proposed based on the aging constitutive model of ACF joints and the Basquin’s equation, and it is found that the predictions of the fatigue model agree with the experimental results at different aging time for ACF joints. Acknowledgements The Project was supported by NSFC (Nos. 10672118, 11072171 and 31000422) and Tianjin Natural Science Foundation (No. 09JCYBJC03100). References [1] Chen X, Zhang J, Jiao CL, Liu YM. Effects of different bonding parameters on the electrical performance and peeling strengths of ACF interconnection. Microelectro Reliab 2006;46:774–85. [2] Chan YC, Luk DY. Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly I. Different bonding temperature. Microelectro Reliab 2002;42:1185–94. [3] Chan YC, Luk DY. Effects of bonding parameters on the reliability performance of anisotropic conductive adhesive interconnects for flip-chip-on-flex packages assembly II. Different bonding pressure. Microelectro Reliab 2002;42:1195–204. [4] Yim MJ, Chung CK, Paik KW. Effect of conductive particle properties on the reliability of anisotropic conductive film for chip-on-glass applications. IEEE Trans Electr Pack Manufacturing 2007;30:306–12.
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