- Email: [email protected]
Study of electrical fatigue test in gold-coated siliver-4 wt.% palladium bonding wire
Microelectronics Reliability 103 (2019) 113502
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Opinion paper
Study of electrical fatigue test in gold-coated siliver-4 wt.% palladium bonding wire
T
Chen-Chin Hoa, Kuan-Jen Chenb, Fei-Yi Hunga, , Truan-Sheng Luia ⁎
a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Instrument Center, National Cheng Kung University, Tainan 701, Taiwan
ARTICLE INFO
ABSTRACT
Keywords: Electrical fatigue test Au-coated Ag-4Pd Poisson's effect Ag2Al intermetallic compound
Present study investigates the effects of the electrical fatigue test on the microstructures, electrical properties and elemental distribution of gold (Au)-coated Ag-4Pd (ACA4P) alloy wire. The electrical fatigue test is an important method for estimating the reliability of electronic devices to cyclic thermal stress caused by bias-induced Joule heat. In the bias test, the thermal diffusion behavior of Au atoms on the wire surface was more intense with increasing current density. The Au atoms diffused along the grain boundary of the wire and finally distributed throughout the wire matrix. In the bias fatigue test, the electrical resistances of the wire and ball bond decreased, and then increased as the bias cycle increased to 6000 cycles. An increase in grain size of the wire and ball bonded wires by bias-induced thermal energy caused their electrical resistances to decrease. Poisson's effect caused the diameter of the wire to decrease, and thus increased the resistance of the wire. The growth of Ag2Al intermetallic compounds (IMCs) at the bonding interface was responsible for the increase in the resistance of the ball bonded wire.
1. Introduction Thermosonic ball bonding is one of the common processes for electronic packaging to make interconnections between chips and lead frames by a thin metal wire [1]. Gold (Au) wire has been used in wire bonding process for many years due to its good ductility, excellent in electrical conductivity, electromigration resistance, oxidation resistance and corrosion resistance. However, the cost of Au is high in the packaging industry, and has serious intermetallic compounds (IMCs) at the bonding interface between Au wire and aluminum (Al) substrate, decreasing the wire conductivity and bonding reliability [2–3]. Therefore, Au wires are gradually replaced by copper (Cu)-based wires and silver (Ag)-based wires to reduce the costs and electronic component failure in bonding ruptures [4–5]. Compared with Au wire, Cu wire has low cost, and excellent tensile strength, electrical conductivity and thermal conductivity. However, some problems that existed in Cu wire applications must be overcome, such as cratering on the bond pad and corrosion problems [4,6]. Ag wire possesses the best electrical and thermal conductivity and is compatible with Au wire-bonding machine [7]. Nevertheless, Ag wires have serious electromigration effect after a long duration of use [8]. The Ag2Al and Ag3Al IMCs formed at the Ag/Al interface, decreasing the conductivity and reliability of the wire [9–10], as well as difference ⁎
of diffusivity between Ag wire and Al pad that may cause Kirkendall Voids [10]. To solve these problems of Ag-based wire, the addition of palladium (Pd) alloy element in the Ag wire can enhance the electromigration resistance and inhibit the formation of IMCs [11]. Therefore, an Ag-4 wt% Pd (Ag-4Pd) alloy wire is used, and coating an Au layer onto the wire surface provided a buffer layer to anti-oxidation and improves the corrosion resistance of the wire in humid environment [12–14]. Bias-induced thermomechanical fatigue was used to estimate the bond strength of heterogeneous metals interface [15]. A previous report proposed that the effect of electromigration on Ag wire was investigated by a dynamic current tensile test [16]. Combining a thermal stress with a bias effects conduct to a fatigue test in the Ag-4Pd alloy wire, that is the pioneering work in bonding wire research. Therefore, the present study investigates the reliability to Au-coated Ag-4Pd alloy wire by the electrical fatigue test. The mechanism of structural evolution and diffusion behavior of Au atoms in the wire are examined to provide a reference for packaging industry. 2. Experimental procedure In present study, an Ag-4Pd wire of 18 μm in diameter is employed, and is coated with 65 nm-thick Au film by an electro-plating, namely
Corresponding author. E-mail address: [email protected] (F.-Y. Hung).
https://doi.org/10.1016/j.microrel.2019.113502 Received 19 February 2019; Received in revised form 19 August 2019; Accepted 10 September 2019 0026-2714/ © 2019 Published by Elsevier Ltd.
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
atoms by an electron probe X-ray microanalyzer (EPMA, JEOL/JXA8900R). Observations of surface morphology and interfacial characteristics are examined using a focused ion beam combined with scanning electron microscopy (FIB-SEM, FEI/Helios G3CX). 3. Results and discussions In order to understand the effects of the bias on the diffusion behavior of the coated-Au layer in the ACA4P wire, the EPMA was used to examine the wire structure (Fig. 2). As shown in Fig. 2(a), Au atoms distributed on the wire surface after 50% of CFC bias for 1000 cycles. Under the bias of 70% CFC, the Au atoms diffused along the grain boundaries into the wire matrix (Fig. 2(b)). As the current increased to 90% of CFC (Fig. 2(c)), the Au atoms have been distributed throughout the wire matrix. This result may attribute to the bias-induced thermal diffusion behavior. According to the coefficient of linear thermal expansion [17], the temperature of the wire under different testing current was estimated, as shown in Table 1. Under 50% of CFC, the biasinduced Joule heat was close to the recrystallization temperature of the wire [18], causing the wire structure recrystallized. At the bias of 70% CFC, the phenomenon of grain growth slowed down, and thus the Au atoms on the wire surface diffused along the grain boundaries into the wire matrix. As the current reached 90% CFC (495 °C), the higher biasinduced Joule heat caused the thermal diffusion more serious, mainly based on volume diffusion, which promoted the Au atoms to distribute throughout the wire. A schematic diagram of the diffusion mechanism of the Au atoms under different testing current is expressed as shown in Fig. 3. The bias fatigue test was performed to estimate the effects of intermittent current cycles on the electrical resistance of the ACA4P wire (Table 2). From the bias fatigue result of 2000 and 4000 cycles, the resistance of the biased wire is lower than that of the unbiased wire. This result is attributed to the bias-induced Joule heat promotes the growth of grain to reduce the grain boundary, decreasing the barriers of electron movement (Fig. 4(b) and (c)). Note that the resistance of the wire increased after the bias fatigue of 6000 cycles (Table 2). The microstructure of the wires shows that the grain size increased with increasing bias cycles, and some areas of the wire diameter decreased due to Poisson's effect [19], which is the reason for the increase in the resistance of the wire. From the microstructure observation of the wire after the bias for 6000 cycles (Fig. 4(d)), it found that the grain size at the smaller diameter of the wire is close to the wire diameter. Note that the phenomenon of single crystal was observed at the fractural position (Fig. 4(e)). It can be inferred that the grain size of the wire grew to the wire diameter by the bias-induced Joule heat, causing a lower grain boundary area and thus deteriorated the strength of the wire. I-V measurement of the ACA4P bonded wire after different bias fatigue cycles was also performed (Table 2). The trend in the resistance change of the bonded wire is similar to the measurement result of the wire resistance. Fig. 5(a) shows the cross-section image of the unbiased ACA4P bonded wire. No obvious IMCs were generated at the Ag/Al interface, and the Al elements were distributed along the Al pad and did not diffuse into the ball bond. After the bias fatigue of 6000 cycles, the Al atoms diffused into the ball bond along the direction of electron flow. The EDS measurement confirmed that Ag2Al IMCs formed at the bonding interface. This indicates that the formation of the Ag2Al IMCs is main reason of the increase in the resistance of the ACA4P bonded wire after the bias fatigue for a long duration.
Fig. 1. Instrument of directing current test of bonded wire. (b) Cycle period of electrical fatigue test.
ACA4P wire. A thermosonic bonder machine was used to conduct an electronic flame off (EFO) process under a shield gas (95%N2 + 5%H2) to create a free air ball (FAB), and then is bonded on Al thin film (400 nm). The ACA4P wire 50 mm in length is applied a direct current until the wire fused to determine the maximum fusing current. The electrical resistance of the wire is obtained from the currentvoltage (I-V) curve, the circuit mechanism of which is shown in Fig. 1(a). To understand the effects of an intermittent current for a long duration on the wire, 90% of the critical fusing current (CFC) is chosen to test the effect of bias fatigue (2000–8000 cycles) on the ACA4P wire. A current 5 s on and 5 s off is defined as one cycle period in the bias fatigue test (Fig. 1(b)). The ACA4P wire is biased with various CFC (50%–90%) for 1000 cycles to examine the diffusion behavior of the Au
4. Conclusions The results of various CFC bias test show that the Au layer still existed on the wire surface under 50% of CFC bias, indicating that 180 °C Joule heat was not enough to drive the Au atoms into the wire matrix. As the bias increased to 70% CFC, the Au atoms diffused along the grain boundary and distributed throughout the wire. 2
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Fig. 2. Elemental distribution of ACA4P wire under different testing current of electrical fatigue test: (a) 50% CFC; (b) 70% CFC; and (c) 90% CFC. 3
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Table 1 Bias-induced temperature of wire under different testing current. CFC (%)
Wire temperature (°C)
50 70 90
180 370 495
Table 2 I-V curves of ACA4P wire obtained under different intermittent current cycles. Bias fatigue (cycles)
Resistance (Ω)
ACA4P wire ACA4P bonded wire
0
2000
4000
6000
12.0 13.9
10.4 10.6
10.6 10.9
14.1 14.4
resistance was due to the reduction in the wire diameter. However, the increase in the bonded wire resistance was related to the formation of the Ag2Al IMC at the bonding interface.
In the bias fatigue test, the grain size of the wire gradually grew up by the bias-induced Joule heat with increasing bias cycles. But, the diameter of the wire became smaller due to the Poisson's effect. As the grain size grew to the wire diameter, the grain boundary area of the wire decreased, and eventually caused the wire to break by the cyclic thermal stress. The electrical resistance of the wire and bonded wire first decreased, and then raised with increasing bias fatigue cycles. The decrease in the wire resistance after the bias fatigue cycles was due to an increase in the grain size of the wire; and the increase in the wire
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 3. Schematic diagram of the diffusion mechanism of Au atoms under different testing current: (a) 50% CFC, (b) 70% CFC, and (c) 90% CFC. 4
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
(caption on next page) 5
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Fig. 4. Surface morphology and microstructure of ACA4P wire under different bias fatigue cycles: (a) 0 cycle, (b) 2000 cycles, (c) 4000 cycles, (d) 6000 cycles, and (e) 8000 cycles.
Fig. 5. Interfacial characteristics of ACA4P ball bonded wire obtained after bias fatigue test: (a) 0 cycle, and (b) 6000 cycles.
Acknowledgements [4]
The authors are grateful to the Instrument Center of National Cheng Kung University and the Ministry of Science and Technology, Taiwan (MOST) (MOST 107-2221-E-006-012-MY2), for their financial support.
[5] [6]
References
[7]
[1] G.G. Harman, Wire Bonding in Microelectronics, 3rd ed., McGraw-Hill, New York, 2010. [2] H. Xu, C. Liu, V.V. Silberschmidtb, S.S. Pramanac, T.J. White, Z. Chen, V.L. Acoff, Intermetallic phase transformations in Au–Al wire bonds, Intermetallic 19 (2011) 1808–1816. [3] C.D. Breach, F. Wulff, New observations on intermetallic compound formation in
[8] [9]
6
gold ball bonds: general growth patterns and identification of two forms of Au4Al, Microelectron. Reliab. 44 (2004) 973–981. Z.W. Zhong, Wire bonding using copper wire, Microelectron. International. 26 (1) (2009) 10–16. Y.W. Tseng, F.Y. Hung, T.S. Lui, M.Y. Chen, H.W. Hsueh, Effect of annealing on the microstructure and bonding interface properties of Ag-2Pd alloy wire, Microelectron. Reliab. 55 (2015) 1256–1261. C. Lu, The challenges of copper wire bonding, International Microsystems Packaging Assembly and Circuits Technology Conference (IMPACT) on IEEE, 2010, pp. 1–4. C.D. Breach, What is the future of bonding wire? Will copper entirely replace gold? Gold Bull. 43 (3) (2010) 150–168. E. Sancaktar, P. Rajput, A. Khanolkar, Correlation of silver migration to the pull out strength of silver wire embedded in an adhesive matrix, IEEE Trans. Compon. Packag. Technol. 28 (4) (2005) 771–780. H.W. Hsueh, F.Y. Hung, T.S. Lui, L.H. Chen, K.J. Chen, Intermetallic phase on the interface of Ag-Au-Pd/Al structure, Adv. Mater. Sci. Eng. 2014 (2014) 925768.
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al. [10] K.A. Yoo, C. Uhm, T.J. Kwon, J.S. Cho, J.T. Moon, Reliability study of low cost alternative Ag bonding wire with various bond pad materials, Electronics Packaging Technology Conference (EPTC) on IEEE (2009) 851–857. [11] J.S. Cho, K.A. Yoo, J.T. Moon, S.B. Son, S.H. Lee, K.H. Oh, Pd effect on reliability of Ag bonding wires in microelectronic devices in high-humidity environments, Metal. Mater. International. 18 (5) (2012) 881–885. [12] Y.W. Tseng, F.Y. Hung, T.S. Lui, Microstructure, tensile and electrical properties of gold-coated silver bonding wire, Microelectron. Reliab. 55 (3–4) (2015) 608–612. [13] Y.W. Tseng, F.Y. Hung, T.S. Lui, Microstructure, mechanical and high-temperature electrical properties of cyanide-free Au-coated Ag wire (ACA), Mater. Trans. 56 (3) (2015) 441–444. [14] Y.W. Tseng, F.Y. Hung, T.S. Lui, Thermoelectric mechanism and interface characteristics of cyanide-free nanogold-coated silver wire, J. Electron. Mater. 45 (1) (2016) 624–630.
[15] R.R. Keller, R.H. Geiss, Y.W. Cheng, D.T. Read, Microstructure evolution during electric current induced thermomechanical fatigue of interconnects, Materials Research Society Conference, 2005, pp. 295–300. [16] H.W. Hsueh, F.Y. Hung, T.S. Lui, A study on electromigration-inducing intergranular fracture of fine silver alloy wires, Appl. Phys. Lett. 110 (2017) 031902. [17] J.D. James, J.A. Spittle, S.G.R. Brown, R.W. Evans, A review of measurement techniques for the thermal expansion coefficient of metals and alloys at elevated temperatures, Meas. Sci. Technol. 12 (2001) R1–R15. [18] H.W. Hsueh, F.Y. Hung, T.S. Lui, L.H. Chen, Annealing effect and tensile interface fracture mechanism of pure silver bonding wires, EPD Congress 2012, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2012, pp. 142–152. [19] S. Dong, B. Han, X. Yu, J. Ou, Constitutive model and reinforcing mechanisms of uniaxial compressive property for reactive powder concrete with super-fine stainless wire, Compos. Part B 166 (2019) 298–309.
7
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Opinion paper
Study of electrical fatigue test in gold-coated siliver-4 wt.% palladium bonding wire
T
Chen-Chin Hoa, Kuan-Jen Chenb, Fei-Yi Hunga, , Truan-Sheng Luia ⁎
a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Instrument Center, National Cheng Kung University, Tainan 701, Taiwan
ARTICLE INFO
ABSTRACT
Keywords: Electrical fatigue test Au-coated Ag-4Pd Poisson's effect Ag2Al intermetallic compound
Present study investigates the effects of the electrical fatigue test on the microstructures, electrical properties and elemental distribution of gold (Au)-coated Ag-4Pd (ACA4P) alloy wire. The electrical fatigue test is an important method for estimating the reliability of electronic devices to cyclic thermal stress caused by bias-induced Joule heat. In the bias test, the thermal diffusion behavior of Au atoms on the wire surface was more intense with increasing current density. The Au atoms diffused along the grain boundary of the wire and finally distributed throughout the wire matrix. In the bias fatigue test, the electrical resistances of the wire and ball bond decreased, and then increased as the bias cycle increased to 6000 cycles. An increase in grain size of the wire and ball bonded wires by bias-induced thermal energy caused their electrical resistances to decrease. Poisson's effect caused the diameter of the wire to decrease, and thus increased the resistance of the wire. The growth of Ag2Al intermetallic compounds (IMCs) at the bonding interface was responsible for the increase in the resistance of the ball bonded wire.
1. Introduction Thermosonic ball bonding is one of the common processes for electronic packaging to make interconnections between chips and lead frames by a thin metal wire [1]. Gold (Au) wire has been used in wire bonding process for many years due to its good ductility, excellent in electrical conductivity, electromigration resistance, oxidation resistance and corrosion resistance. However, the cost of Au is high in the packaging industry, and has serious intermetallic compounds (IMCs) at the bonding interface between Au wire and aluminum (Al) substrate, decreasing the wire conductivity and bonding reliability [2–3]. Therefore, Au wires are gradually replaced by copper (Cu)-based wires and silver (Ag)-based wires to reduce the costs and electronic component failure in bonding ruptures [4–5]. Compared with Au wire, Cu wire has low cost, and excellent tensile strength, electrical conductivity and thermal conductivity. However, some problems that existed in Cu wire applications must be overcome, such as cratering on the bond pad and corrosion problems [4,6]. Ag wire possesses the best electrical and thermal conductivity and is compatible with Au wire-bonding machine [7]. Nevertheless, Ag wires have serious electromigration effect after a long duration of use [8]. The Ag2Al and Ag3Al IMCs formed at the Ag/Al interface, decreasing the conductivity and reliability of the wire [9–10], as well as difference ⁎
of diffusivity between Ag wire and Al pad that may cause Kirkendall Voids [10]. To solve these problems of Ag-based wire, the addition of palladium (Pd) alloy element in the Ag wire can enhance the electromigration resistance and inhibit the formation of IMCs [11]. Therefore, an Ag-4 wt% Pd (Ag-4Pd) alloy wire is used, and coating an Au layer onto the wire surface provided a buffer layer to anti-oxidation and improves the corrosion resistance of the wire in humid environment [12–14]. Bias-induced thermomechanical fatigue was used to estimate the bond strength of heterogeneous metals interface [15]. A previous report proposed that the effect of electromigration on Ag wire was investigated by a dynamic current tensile test [16]. Combining a thermal stress with a bias effects conduct to a fatigue test in the Ag-4Pd alloy wire, that is the pioneering work in bonding wire research. Therefore, the present study investigates the reliability to Au-coated Ag-4Pd alloy wire by the electrical fatigue test. The mechanism of structural evolution and diffusion behavior of Au atoms in the wire are examined to provide a reference for packaging industry. 2. Experimental procedure In present study, an Ag-4Pd wire of 18 μm in diameter is employed, and is coated with 65 nm-thick Au film by an electro-plating, namely
Corresponding author. E-mail address: [email protected] (F.-Y. Hung).
https://doi.org/10.1016/j.microrel.2019.113502 Received 19 February 2019; Received in revised form 19 August 2019; Accepted 10 September 2019 0026-2714/ © 2019 Published by Elsevier Ltd.
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
atoms by an electron probe X-ray microanalyzer (EPMA, JEOL/JXA8900R). Observations of surface morphology and interfacial characteristics are examined using a focused ion beam combined with scanning electron microscopy (FIB-SEM, FEI/Helios G3CX). 3. Results and discussions In order to understand the effects of the bias on the diffusion behavior of the coated-Au layer in the ACA4P wire, the EPMA was used to examine the wire structure (Fig. 2). As shown in Fig. 2(a), Au atoms distributed on the wire surface after 50% of CFC bias for 1000 cycles. Under the bias of 70% CFC, the Au atoms diffused along the grain boundaries into the wire matrix (Fig. 2(b)). As the current increased to 90% of CFC (Fig. 2(c)), the Au atoms have been distributed throughout the wire matrix. This result may attribute to the bias-induced thermal diffusion behavior. According to the coefficient of linear thermal expansion [17], the temperature of the wire under different testing current was estimated, as shown in Table 1. Under 50% of CFC, the biasinduced Joule heat was close to the recrystallization temperature of the wire [18], causing the wire structure recrystallized. At the bias of 70% CFC, the phenomenon of grain growth slowed down, and thus the Au atoms on the wire surface diffused along the grain boundaries into the wire matrix. As the current reached 90% CFC (495 °C), the higher biasinduced Joule heat caused the thermal diffusion more serious, mainly based on volume diffusion, which promoted the Au atoms to distribute throughout the wire. A schematic diagram of the diffusion mechanism of the Au atoms under different testing current is expressed as shown in Fig. 3. The bias fatigue test was performed to estimate the effects of intermittent current cycles on the electrical resistance of the ACA4P wire (Table 2). From the bias fatigue result of 2000 and 4000 cycles, the resistance of the biased wire is lower than that of the unbiased wire. This result is attributed to the bias-induced Joule heat promotes the growth of grain to reduce the grain boundary, decreasing the barriers of electron movement (Fig. 4(b) and (c)). Note that the resistance of the wire increased after the bias fatigue of 6000 cycles (Table 2). The microstructure of the wires shows that the grain size increased with increasing bias cycles, and some areas of the wire diameter decreased due to Poisson's effect [19], which is the reason for the increase in the resistance of the wire. From the microstructure observation of the wire after the bias for 6000 cycles (Fig. 4(d)), it found that the grain size at the smaller diameter of the wire is close to the wire diameter. Note that the phenomenon of single crystal was observed at the fractural position (Fig. 4(e)). It can be inferred that the grain size of the wire grew to the wire diameter by the bias-induced Joule heat, causing a lower grain boundary area and thus deteriorated the strength of the wire. I-V measurement of the ACA4P bonded wire after different bias fatigue cycles was also performed (Table 2). The trend in the resistance change of the bonded wire is similar to the measurement result of the wire resistance. Fig. 5(a) shows the cross-section image of the unbiased ACA4P bonded wire. No obvious IMCs were generated at the Ag/Al interface, and the Al elements were distributed along the Al pad and did not diffuse into the ball bond. After the bias fatigue of 6000 cycles, the Al atoms diffused into the ball bond along the direction of electron flow. The EDS measurement confirmed that Ag2Al IMCs formed at the bonding interface. This indicates that the formation of the Ag2Al IMCs is main reason of the increase in the resistance of the ACA4P bonded wire after the bias fatigue for a long duration.
Fig. 1. Instrument of directing current test of bonded wire. (b) Cycle period of electrical fatigue test.
ACA4P wire. A thermosonic bonder machine was used to conduct an electronic flame off (EFO) process under a shield gas (95%N2 + 5%H2) to create a free air ball (FAB), and then is bonded on Al thin film (400 nm). The ACA4P wire 50 mm in length is applied a direct current until the wire fused to determine the maximum fusing current. The electrical resistance of the wire is obtained from the currentvoltage (I-V) curve, the circuit mechanism of which is shown in Fig. 1(a). To understand the effects of an intermittent current for a long duration on the wire, 90% of the critical fusing current (CFC) is chosen to test the effect of bias fatigue (2000–8000 cycles) on the ACA4P wire. A current 5 s on and 5 s off is defined as one cycle period in the bias fatigue test (Fig. 1(b)). The ACA4P wire is biased with various CFC (50%–90%) for 1000 cycles to examine the diffusion behavior of the Au
4. Conclusions The results of various CFC bias test show that the Au layer still existed on the wire surface under 50% of CFC bias, indicating that 180 °C Joule heat was not enough to drive the Au atoms into the wire matrix. As the bias increased to 70% CFC, the Au atoms diffused along the grain boundary and distributed throughout the wire. 2
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Fig. 2. Elemental distribution of ACA4P wire under different testing current of electrical fatigue test: (a) 50% CFC; (b) 70% CFC; and (c) 90% CFC. 3
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Table 1 Bias-induced temperature of wire under different testing current. CFC (%)
Wire temperature (°C)
50 70 90
180 370 495
Table 2 I-V curves of ACA4P wire obtained under different intermittent current cycles. Bias fatigue (cycles)
Resistance (Ω)
ACA4P wire ACA4P bonded wire
0
2000
4000
6000
12.0 13.9
10.4 10.6
10.6 10.9
14.1 14.4
resistance was due to the reduction in the wire diameter. However, the increase in the bonded wire resistance was related to the formation of the Ag2Al IMC at the bonding interface.
In the bias fatigue test, the grain size of the wire gradually grew up by the bias-induced Joule heat with increasing bias cycles. But, the diameter of the wire became smaller due to the Poisson's effect. As the grain size grew to the wire diameter, the grain boundary area of the wire decreased, and eventually caused the wire to break by the cyclic thermal stress. The electrical resistance of the wire and bonded wire first decreased, and then raised with increasing bias fatigue cycles. The decrease in the wire resistance after the bias fatigue cycles was due to an increase in the grain size of the wire; and the increase in the wire
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 3. Schematic diagram of the diffusion mechanism of Au atoms under different testing current: (a) 50% CFC, (b) 70% CFC, and (c) 90% CFC. 4
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
(caption on next page) 5
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al.
Fig. 4. Surface morphology and microstructure of ACA4P wire under different bias fatigue cycles: (a) 0 cycle, (b) 2000 cycles, (c) 4000 cycles, (d) 6000 cycles, and (e) 8000 cycles.
Fig. 5. Interfacial characteristics of ACA4P ball bonded wire obtained after bias fatigue test: (a) 0 cycle, and (b) 6000 cycles.
Acknowledgements [4]
The authors are grateful to the Instrument Center of National Cheng Kung University and the Ministry of Science and Technology, Taiwan (MOST) (MOST 107-2221-E-006-012-MY2), for their financial support.
[5] [6]
References
[7]
[1] G.G. Harman, Wire Bonding in Microelectronics, 3rd ed., McGraw-Hill, New York, 2010. [2] H. Xu, C. Liu, V.V. Silberschmidtb, S.S. Pramanac, T.J. White, Z. Chen, V.L. Acoff, Intermetallic phase transformations in Au–Al wire bonds, Intermetallic 19 (2011) 1808–1816. [3] C.D. Breach, F. Wulff, New observations on intermetallic compound formation in
[8] [9]
6
gold ball bonds: general growth patterns and identification of two forms of Au4Al, Microelectron. Reliab. 44 (2004) 973–981. Z.W. Zhong, Wire bonding using copper wire, Microelectron. International. 26 (1) (2009) 10–16. Y.W. Tseng, F.Y. Hung, T.S. Lui, M.Y. Chen, H.W. Hsueh, Effect of annealing on the microstructure and bonding interface properties of Ag-2Pd alloy wire, Microelectron. Reliab. 55 (2015) 1256–1261. C. Lu, The challenges of copper wire bonding, International Microsystems Packaging Assembly and Circuits Technology Conference (IMPACT) on IEEE, 2010, pp. 1–4. C.D. Breach, What is the future of bonding wire? Will copper entirely replace gold? Gold Bull. 43 (3) (2010) 150–168. E. Sancaktar, P. Rajput, A. Khanolkar, Correlation of silver migration to the pull out strength of silver wire embedded in an adhesive matrix, IEEE Trans. Compon. Packag. Technol. 28 (4) (2005) 771–780. H.W. Hsueh, F.Y. Hung, T.S. Lui, L.H. Chen, K.J. Chen, Intermetallic phase on the interface of Ag-Au-Pd/Al structure, Adv. Mater. Sci. Eng. 2014 (2014) 925768.
Microelectronics Reliability 103 (2019) 113502
C.-C. Ho, et al. [10] K.A. Yoo, C. Uhm, T.J. Kwon, J.S. Cho, J.T. Moon, Reliability study of low cost alternative Ag bonding wire with various bond pad materials, Electronics Packaging Technology Conference (EPTC) on IEEE (2009) 851–857. [11] J.S. Cho, K.A. Yoo, J.T. Moon, S.B. Son, S.H. Lee, K.H. Oh, Pd effect on reliability of Ag bonding wires in microelectronic devices in high-humidity environments, Metal. Mater. International. 18 (5) (2012) 881–885. [12] Y.W. Tseng, F.Y. Hung, T.S. Lui, Microstructure, tensile and electrical properties of gold-coated silver bonding wire, Microelectron. Reliab. 55 (3–4) (2015) 608–612. [13] Y.W. Tseng, F.Y. Hung, T.S. Lui, Microstructure, mechanical and high-temperature electrical properties of cyanide-free Au-coated Ag wire (ACA), Mater. Trans. 56 (3) (2015) 441–444. [14] Y.W. Tseng, F.Y. Hung, T.S. Lui, Thermoelectric mechanism and interface characteristics of cyanide-free nanogold-coated silver wire, J. Electron. Mater. 45 (1) (2016) 624–630.
[15] R.R. Keller, R.H. Geiss, Y.W. Cheng, D.T. Read, Microstructure evolution during electric current induced thermomechanical fatigue of interconnects, Materials Research Society Conference, 2005, pp. 295–300. [16] H.W. Hsueh, F.Y. Hung, T.S. Lui, A study on electromigration-inducing intergranular fracture of fine silver alloy wires, Appl. Phys. Lett. 110 (2017) 031902. [17] J.D. James, J.A. Spittle, S.G.R. Brown, R.W. Evans, A review of measurement techniques for the thermal expansion coefficient of metals and alloys at elevated temperatures, Meas. Sci. Technol. 12 (2001) R1–R15. [18] H.W. Hsueh, F.Y. Hung, T.S. Lui, L.H. Chen, Annealing effect and tensile interface fracture mechanism of pure silver bonding wires, EPD Congress 2012, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2012, pp. 142–152. [19] S. Dong, B. Han, X. Yu, J. Ou, Constitutive model and reinforcing mechanisms of uniaxial compressive property for reactive powder concrete with super-fine stainless wire, Compos. Part B 166 (2019) 298–309.
7