- Email: [email protected]
Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire
Microelectronics Reliability xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire Hao-Wen Hsueh, Fei-Yi Hung ⇑, Truan-Sheng Lui, Li-Hui Chen, Jun-Kai Chang Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 7 February 2014 Received in revised form 8 June 2014 Accepted 8 June 2014 Available online xxxx Keywords: Ag–La Wire bounding Free air ball Tensile properties
a b s t r a c t Silver has potential for application in the electronic packaging industry because of its great electrical and thermal properties and lower price compared to that of gold. Silver oxidizes easily, so doping lanthanum to form Ag–La alloy improves its anti-oxidation capacity. In this study, the microstructure, tensile properties, electronic flame-off (EFO) characteristics, and fusing current of Ag–La alloy wire (u = 20 lm) are studied. Samples annealed at three temperatures (325 °C, 375 °C, and 425 °C) are analyzed. According to the experimental results, after annealing at 425 °C, Ag–La alloy wire recrystallized, giving it a tensile strength similar to that of pure silver wire and a uniform structure. Doping lanthanum reduced the diameter of free air balls (FABs) in the EFO process. The fusing current of Ag–La wire was about 0.45 A, and the grains of Ag–La wire grew to the size of the wire diameter when a 0.4 A current (90% fusing current) was applied for a long time. Ag–La alloy wire can be used in the electronic packaging industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Wire bonding is used to connect a semiconductor chip and a lead frame. Thermosonic ball bonding is the most common process used for this purpose in the electronic packaging industry [1]. Considering the reliability and ductility of wire, gold is extensively applied in wire bonding [1,2]. With the recent rise in gold prices, researchers have looked for alternative metals for wire bonding, with copper [1–3] and silver [4] being the two main candidates. Copper is commonly used in the electronic packaging industry due to its low cost and excellent electrical and thermal properties [5,6]. However, the reliability of copper wire is poor because of its easy oxidation and poor ductility. Pd has been coated on the surface of copper wire to effectively avoid oxidation, but it causes the segregation of free air balls (FABs) after the electronic flameoff (EFO) process, lowering the reliability of wire bonding [7]. Silver wire also has great electrical and thermal properties. Silver is more expensive than copper but cheaper than gold. Silver has the following advantages: (1) it can use the same wire bonder machines as gold because of its similar hardness, ductility, elastic modulus, and tensile stress compared to those of gold, (2) it leads to spherical FABs in the shield gas process, and (3) it has bondability with Al pads. To inhibit oxidation and improve the size of FABs, anti-oxidation elements can be added into the silver matrix. ⇑ Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. E-mail address: [email protected] (F.-Y. Hung).
Ag alloys containing a rare earth (RE) element have excellent mechanical properties and anti-oxidation properties (oxidation reaction: 4Ag + O2 ? 2Ag2O) [8,9]. In this study [10], 0.05 wt.% lanthanum is doped into the silver matrix to form Ag–La alloy wire. The microstructure, tensile properties, and electrical properties in the wire bonding process are investigated. The end of the metal wire was fused into a ball-shaped liquid and then quickly solidified to form FABs. In the EFO process, the heat causes fine wire to form three zones, namely a FAB zone, a heat-affected zone (HAZ), and a non-affected zone [5,6]. The size and shape of FABs are important to avoid short-circuiting and directly affects reliability [11,12]. No previous studies have investigated wires doped with rare earth elements for electronic packaging. In this study, the effects of doping lanthanum into silver on recrystallization, tensile properties, conductivity, FAB characteristics after the EFO process, and fusing current are determined to provide a reference for electronic packaging applications.
2. Experimental procedure 2.1. Annealing treatment and EFO process 0.05 wt.% (500 ppm) lanthanum was doped into silver to form Ag–La alloy wire. Pure silver wire and Ag–La alloy wire were drawn from ingots, whose diameter was 20 lm. Pure Ag wire was annealed at 275 °C for 30 min. Ag–La wire was annealed at 325 °C, 375 °C, and 425 °C, respectively, for 30 min. The four kinds
http://dx.doi.org/10.1016/j.microrel.2014.06.004 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
2
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
of silver wire were denoted as Ag, Ag–La325, Ag–La375, and Ag– La425, respectively. A thermosonic bonder machine (SPB-TS668) was used in the EFO process. A shield gas of 95%N2 + 5%H2 was used (1 L/min) to prevent oxidation. The EFO current was 51 mA and the EFO firing time was 1.2 ms. 2.2. Material characterization To compare pure Ag wire and Ag–La alloy wires, the tensile properties and micro-hardness were determined. The tensile machine was JOBHO TD-121. The strain rate was 5 10 3 s 1 and the wire length was 5 cm. A Vicker’s hardness tester (Shimadzu HMV-2000L) was used for the mirco-hardness test. A 5 g loading force was applied for 10 s (Hv: 5 g/10 s). The tensile test and micro-hardness data are reported as averages of five results. To determine the effects of EFO on the strength of the neck zone, Ag and Ag–La425 were subjected to the tensile test after
the EFO process. A plate clamp was used to fix the FAB, and then testing was performed. The test conditions were the same as wire. In addition, a nano-indentation test was used to determine the nano-hardness after EFO (MTS Nano Indenter G200). The first test point was at the center of the FAB, the second point was at the neck, and the third test point was 40 lm along the wire, until the hardness was similar to the wire before EFO. To determine the maximum fusing current that the wire can endure, Ag and Ag–La425 wires were chosen for the current– voltage (I–V) test (the wire length was 5 cm) [13,14]. The fusing current density and resistance were obtained. 3. Results and discussion 3.1. Microstructure and tensile properties Microstructure observations of pure Ag and Ag–La alloy wires are shown in Fig. 1. For Ag (Fig. 1a) and Ag–La325 (Fig. 1b),
Fig. 1. Microstructures of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Fig. 2. Grain sizes of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
3
Fig. 3. Tensile properties of pure Ag wire and three annealed Ag–La wires. (a) yielding stress, (b) ultimate tensile stress, and (c) elongation. Fig. 5. Images of FAB for (a) Ag and (b) Ag–La425.
Fig. 4. Micro-hardness of pure Ag wire and annealed Ag–La wires.
fiber-shaped grains appear in the middle of the wire because of the drawing process. The microstructure of Ag–La375 (Fig. 1c) shows that increasing the annealing temperature led to recrystallization.
Fig. 6. Cross-section microstructure of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
4
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
Fig. 7. Comparison of ultimate tensile strength of Ag and Ag–La425 wires before and after EFO process. Fig. 9. I–V curves of Ag and Ag–La425 wires.
Fig. 8. Hardness distribution of FAB and wire (distance = 0–600 lm from FAB) for (a) Ag and (b) Ag–La425 wires.
For Ag–La425 (Fig. 1d), the grains recrystallized to form equiaxed grains, whose matrix is similar to that of annealed pure Ag wire. Fig. 2 shows the grain sizes of the samples. The grain size of Ag–La wire tended to be stable with annealing temperature. Ag–La375 (Fig. 2c) and Ag–La425 (Fig. 2d) had similar matrices, which indicates that higher annealing temperature made the Ag– La wires stable. The tensile properties of the samples are shown in Fig. 3. From Fig. 3(a and b), the tensile strength of Ag–La wire decreases with increasing annealed temperature. Ag–La375 and Ag–La425 had similar yield strength (YS) and ultimate tensile strength (UTS), indicating that the annealing conditions were stable. Ag–La325 (Fig. 3c) had the lowest elongation due to insufficient annealing. The retained fiber-shaped grains (not equiaxed grains) in the middle of wire from the drawing were the main reason for the low elongation.
Fig. 10. Microstructure of Ag–La wire with 90% fusing current (0.4 A, current density: 1.27 10 9 A/mm2) applied for (a) 0 min, (b) 1 min, (c) 5 min, (d) 30 min, and (e) 120 min.
Fig. 4 shows the micro-hardness results. The micro-hardness of Ag–La wires decreased with increasing annealed temperature. The micro-hardness values of Ag–La wires are Hv70.2 (5 g/10 s),
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
5
Fig. 11. Grain size of Ag–La wire for current times of (a) 0 min, (b) 1 min, (c) 5 min, (d) 30 min, and (e) 120 min.
Hv60.1 (5 g/10 s), and Hv55.4 (5 g/10 s), respectively. Micro-hardness of gold is about Hv50. The micro-hardness of Ag–La425 is similar to that of gold wire. Therefore, it can be applied directly in the traditional wire bonding process. 3.2. EFO wire tensile test and current test The microstructures of Ag and Ag–La425 after the EFO process are shown in Fig. 5. The FAB diameter of pure Ag wire after EFO was 85 lm, which is too large for bonding. The wire consisted of many columnar grains, as shown in Fig. 5a. In fact, the lowest current was used for EFO, the FAB of pure silver was still larger. So we fixed EFO parameters to compare the two wires to stress this phenomenon. Under the same conditions, the FAB diameter of Ag–La425 after EFO was 55 lm, as shown in Fig. 5b. This shows that doping lanthanum can change the thermal properties (reducing the thermal coefficient). Fig. 6a shows the cross-section microstructure of Ag wire. Some columnar grains can be seen in the FAB. The grains grow in the neck due to the heat-inducing EFO process. Fig. 6(b–d) shows the cross-section microstructures of Ag–La wires, respectively. There are few columnar grains in the FABs, and there is no evidence of
grain growth due to the effect of heat. During the EFO process, doping lanthanum reduces the diameter of FABs. The neck tensile test results of Ag and Ag–La425 wire before and after the EFO process are shown in Fig. 7. The strength of Ag–La425 wire after the EFO process is slightly lower than that of Ag wire, so there was no evidence of stress concentration in the neck of Ag–La wire. The hardness distributions obtained using the nano-indenter of Ag wire and Ag–La425 wire after the EFO process are shown in Fig. 8. The HAZ of both Ag and Ag–La425 was about 350–400 lm from the neck (AgHAZ: 350 lm, Ag–LaHAZ: 400 lm). The nano-hardness of Ag–La425 was higher than that of pure Ag, which indicates that alloying increased the hardness of the matrix. Notably, the hardness profile of Ag–LaHAZ is a rising curve but that of AgHAZ is a parabolic curve. This is due to La doping (solid-solution strengthening) improving the broken problems and reliability of the HAZ [5,6,15,16]. Fig. 9 shows the I–V curves for pure Ag wire and Ag–La425 wire. Both wires were fused at 0.45 A, but the voltage of Ag–La425 was higher than that of pure Ag [6,13]. The slope in the figure indicates the conductivity value (the inverse of electric resistance). Ag–La425 wire had a lower conductivity value (its resistance was higher) than that of Ag wire because La doping increased electric
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
6
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
4. Conclusions (1) Ag–La wire and pure Ag wire have equiaxed grains after annealing treatment. Adding La reduced the FAB diameter and increased the hardness of the matrix. (2) The neck strength of Ag–La wire was similar to that of pure Ag wire. The HAZ of Ag–La425 wire was about 400 lm from the neck and the hardness profile of Ag–LaHAZ was a rising curve. Ag–La425 wire had a lower conductivity than that of Ag wire, but its fusing current was the same. (3) With increasing current duration, the equiaxed grains of Ag–La wire transformed into unusually coarse grains. Due to the solid-solution effect, the hardness of the matrix did not significantly change.
Acknowledgements The authors are grateful to the Instrument Center of National Cheng Kung University and to the National Science Council of Taiwan for financially supporting this research under Grant NSC 1022221-E-006-061. Fig. 12. Nano-hardness of Ag–La wire with 90% fusing current (0.4 A, current density: 1.27 10 9 A/mm2) applied for various times.
resistance. Fig. 10 shows the microstructure of Ag–La wire with 90% fusing current (0.4 A, current density of 1.27 10 9 A/ mm2) applied for various durations. The grains continued growing to the size of the wire diameter, and even became equiaxed grains. Fig. 11 shows the grain size of Ag–La wire versus current for various durations. The grain size tended to be stable for a long time. Fig. 12 shows the nano-hardness results. The hardness of the wire decreased with increasing current time. The grains of the matrix kept growing even after 30 min, and superheating occurred (grain boundary not obvious; equiaxed grains transformed into unusually coarse grains), but the hardness of the matrix did not significantly change. It is main reason that trace La had the solid-solution strengthening in Ag–La matrix. The results show that Ag–La alloy wire can be used in the electronic packaging industry.
References [1] Zhong ZW. Microelectron Reliab 2011;51:4–12. [2] Khoury SL, Burkhard DJ. IEEE Trans Compon Hybrids Manuf Technol 1990;13:673–81. [3] Liu P, Tong L, Wanga J, Shi L, Tang H. Microelectron Reliab 2012;52:1092–8. [4] Hsueh HW, Hung FY, Lui TS, Chen LH. Microelectron Reliab 2011;51:2243–9. [5] Hung FY, Wang YT, Chen LH, Lui TS. Mater Trans 2006;47:1776–81. [6] Hsueh HW, Hung FY, Lui TS, Chen LH. Microelectron Reliab 2013;53:1159–63. [7] Uno T. Microelectron Reliab 2011;51:88–96. [8] Castor SB, Hedrick JB. Rare earth elements, industrial minerals volume, 7th ed., 2006, p. 769–92. [9] Hung FY, Lui TS, Chen LH, Lan KA. Mater Trans 2008;49:1491–5. [10] Hsueh HW, Hung FY, Lui TS. Adv Mater Res 2013;804:151–7. [11] Misra E, Alford TL. Appl Phys Lett 2005;87:172111. [12] Murali S, Srikanth N, Vath CJ. Grains. Mater Charact 2003;50:39–50. [13] Hung FY, Liao JD, Lui TS, Chen LH. Curr Appl Phys 2011;11:1269–73. [14] Alford TL, Misra E, Bhagat SK, Mayer JW. Thin Solid Films 2009;517:1833–6. [15] Chen CM, Chen SW. Acta Mater 2002;50:2461–9. [16] Du MY, Chen CM, Chen SW. Mater Chem Phys 2003;82:818–25.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire Hao-Wen Hsueh, Fei-Yi Hung ⇑, Truan-Sheng Lui, Li-Hui Chen, Jun-Kai Chang Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 7 February 2014 Received in revised form 8 June 2014 Accepted 8 June 2014 Available online xxxx Keywords: Ag–La Wire bounding Free air ball Tensile properties
a b s t r a c t Silver has potential for application in the electronic packaging industry because of its great electrical and thermal properties and lower price compared to that of gold. Silver oxidizes easily, so doping lanthanum to form Ag–La alloy improves its anti-oxidation capacity. In this study, the microstructure, tensile properties, electronic flame-off (EFO) characteristics, and fusing current of Ag–La alloy wire (u = 20 lm) are studied. Samples annealed at three temperatures (325 °C, 375 °C, and 425 °C) are analyzed. According to the experimental results, after annealing at 425 °C, Ag–La alloy wire recrystallized, giving it a tensile strength similar to that of pure silver wire and a uniform structure. Doping lanthanum reduced the diameter of free air balls (FABs) in the EFO process. The fusing current of Ag–La wire was about 0.45 A, and the grains of Ag–La wire grew to the size of the wire diameter when a 0.4 A current (90% fusing current) was applied for a long time. Ag–La alloy wire can be used in the electronic packaging industry. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Wire bonding is used to connect a semiconductor chip and a lead frame. Thermosonic ball bonding is the most common process used for this purpose in the electronic packaging industry [1]. Considering the reliability and ductility of wire, gold is extensively applied in wire bonding [1,2]. With the recent rise in gold prices, researchers have looked for alternative metals for wire bonding, with copper [1–3] and silver [4] being the two main candidates. Copper is commonly used in the electronic packaging industry due to its low cost and excellent electrical and thermal properties [5,6]. However, the reliability of copper wire is poor because of its easy oxidation and poor ductility. Pd has been coated on the surface of copper wire to effectively avoid oxidation, but it causes the segregation of free air balls (FABs) after the electronic flameoff (EFO) process, lowering the reliability of wire bonding [7]. Silver wire also has great electrical and thermal properties. Silver is more expensive than copper but cheaper than gold. Silver has the following advantages: (1) it can use the same wire bonder machines as gold because of its similar hardness, ductility, elastic modulus, and tensile stress compared to those of gold, (2) it leads to spherical FABs in the shield gas process, and (3) it has bondability with Al pads. To inhibit oxidation and improve the size of FABs, anti-oxidation elements can be added into the silver matrix. ⇑ Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. E-mail address: [email protected] (F.-Y. Hung).
Ag alloys containing a rare earth (RE) element have excellent mechanical properties and anti-oxidation properties (oxidation reaction: 4Ag + O2 ? 2Ag2O) [8,9]. In this study [10], 0.05 wt.% lanthanum is doped into the silver matrix to form Ag–La alloy wire. The microstructure, tensile properties, and electrical properties in the wire bonding process are investigated. The end of the metal wire was fused into a ball-shaped liquid and then quickly solidified to form FABs. In the EFO process, the heat causes fine wire to form three zones, namely a FAB zone, a heat-affected zone (HAZ), and a non-affected zone [5,6]. The size and shape of FABs are important to avoid short-circuiting and directly affects reliability [11,12]. No previous studies have investigated wires doped with rare earth elements for electronic packaging. In this study, the effects of doping lanthanum into silver on recrystallization, tensile properties, conductivity, FAB characteristics after the EFO process, and fusing current are determined to provide a reference for electronic packaging applications.
2. Experimental procedure 2.1. Annealing treatment and EFO process 0.05 wt.% (500 ppm) lanthanum was doped into silver to form Ag–La alloy wire. Pure silver wire and Ag–La alloy wire were drawn from ingots, whose diameter was 20 lm. Pure Ag wire was annealed at 275 °C for 30 min. Ag–La wire was annealed at 325 °C, 375 °C, and 425 °C, respectively, for 30 min. The four kinds
http://dx.doi.org/10.1016/j.microrel.2014.06.004 0026-2714/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
2
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
of silver wire were denoted as Ag, Ag–La325, Ag–La375, and Ag– La425, respectively. A thermosonic bonder machine (SPB-TS668) was used in the EFO process. A shield gas of 95%N2 + 5%H2 was used (1 L/min) to prevent oxidation. The EFO current was 51 mA and the EFO firing time was 1.2 ms. 2.2. Material characterization To compare pure Ag wire and Ag–La alloy wires, the tensile properties and micro-hardness were determined. The tensile machine was JOBHO TD-121. The strain rate was 5 10 3 s 1 and the wire length was 5 cm. A Vicker’s hardness tester (Shimadzu HMV-2000L) was used for the mirco-hardness test. A 5 g loading force was applied for 10 s (Hv: 5 g/10 s). The tensile test and micro-hardness data are reported as averages of five results. To determine the effects of EFO on the strength of the neck zone, Ag and Ag–La425 were subjected to the tensile test after
the EFO process. A plate clamp was used to fix the FAB, and then testing was performed. The test conditions were the same as wire. In addition, a nano-indentation test was used to determine the nano-hardness after EFO (MTS Nano Indenter G200). The first test point was at the center of the FAB, the second point was at the neck, and the third test point was 40 lm along the wire, until the hardness was similar to the wire before EFO. To determine the maximum fusing current that the wire can endure, Ag and Ag–La425 wires were chosen for the current– voltage (I–V) test (the wire length was 5 cm) [13,14]. The fusing current density and resistance were obtained. 3. Results and discussion 3.1. Microstructure and tensile properties Microstructure observations of pure Ag and Ag–La alloy wires are shown in Fig. 1. For Ag (Fig. 1a) and Ag–La325 (Fig. 1b),
Fig. 1. Microstructures of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Fig. 2. Grain sizes of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
3
Fig. 3. Tensile properties of pure Ag wire and three annealed Ag–La wires. (a) yielding stress, (b) ultimate tensile stress, and (c) elongation. Fig. 5. Images of FAB for (a) Ag and (b) Ag–La425.
Fig. 4. Micro-hardness of pure Ag wire and annealed Ag–La wires.
fiber-shaped grains appear in the middle of the wire because of the drawing process. The microstructure of Ag–La375 (Fig. 1c) shows that increasing the annealing temperature led to recrystallization.
Fig. 6. Cross-section microstructure of (a) Ag, (b) Ag–La325, (c) Ag–La375, and (d) Ag–La425 wires.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
4
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
Fig. 7. Comparison of ultimate tensile strength of Ag and Ag–La425 wires before and after EFO process. Fig. 9. I–V curves of Ag and Ag–La425 wires.
Fig. 8. Hardness distribution of FAB and wire (distance = 0–600 lm from FAB) for (a) Ag and (b) Ag–La425 wires.
For Ag–La425 (Fig. 1d), the grains recrystallized to form equiaxed grains, whose matrix is similar to that of annealed pure Ag wire. Fig. 2 shows the grain sizes of the samples. The grain size of Ag–La wire tended to be stable with annealing temperature. Ag–La375 (Fig. 2c) and Ag–La425 (Fig. 2d) had similar matrices, which indicates that higher annealing temperature made the Ag– La wires stable. The tensile properties of the samples are shown in Fig. 3. From Fig. 3(a and b), the tensile strength of Ag–La wire decreases with increasing annealed temperature. Ag–La375 and Ag–La425 had similar yield strength (YS) and ultimate tensile strength (UTS), indicating that the annealing conditions were stable. Ag–La325 (Fig. 3c) had the lowest elongation due to insufficient annealing. The retained fiber-shaped grains (not equiaxed grains) in the middle of wire from the drawing were the main reason for the low elongation.
Fig. 10. Microstructure of Ag–La wire with 90% fusing current (0.4 A, current density: 1.27 10 9 A/mm2) applied for (a) 0 min, (b) 1 min, (c) 5 min, (d) 30 min, and (e) 120 min.
Fig. 4 shows the micro-hardness results. The micro-hardness of Ag–La wires decreased with increasing annealed temperature. The micro-hardness values of Ag–La wires are Hv70.2 (5 g/10 s),
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
5
Fig. 11. Grain size of Ag–La wire for current times of (a) 0 min, (b) 1 min, (c) 5 min, (d) 30 min, and (e) 120 min.
Hv60.1 (5 g/10 s), and Hv55.4 (5 g/10 s), respectively. Micro-hardness of gold is about Hv50. The micro-hardness of Ag–La425 is similar to that of gold wire. Therefore, it can be applied directly in the traditional wire bonding process. 3.2. EFO wire tensile test and current test The microstructures of Ag and Ag–La425 after the EFO process are shown in Fig. 5. The FAB diameter of pure Ag wire after EFO was 85 lm, which is too large for bonding. The wire consisted of many columnar grains, as shown in Fig. 5a. In fact, the lowest current was used for EFO, the FAB of pure silver was still larger. So we fixed EFO parameters to compare the two wires to stress this phenomenon. Under the same conditions, the FAB diameter of Ag–La425 after EFO was 55 lm, as shown in Fig. 5b. This shows that doping lanthanum can change the thermal properties (reducing the thermal coefficient). Fig. 6a shows the cross-section microstructure of Ag wire. Some columnar grains can be seen in the FAB. The grains grow in the neck due to the heat-inducing EFO process. Fig. 6(b–d) shows the cross-section microstructures of Ag–La wires, respectively. There are few columnar grains in the FABs, and there is no evidence of
grain growth due to the effect of heat. During the EFO process, doping lanthanum reduces the diameter of FABs. The neck tensile test results of Ag and Ag–La425 wire before and after the EFO process are shown in Fig. 7. The strength of Ag–La425 wire after the EFO process is slightly lower than that of Ag wire, so there was no evidence of stress concentration in the neck of Ag–La wire. The hardness distributions obtained using the nano-indenter of Ag wire and Ag–La425 wire after the EFO process are shown in Fig. 8. The HAZ of both Ag and Ag–La425 was about 350–400 lm from the neck (AgHAZ: 350 lm, Ag–LaHAZ: 400 lm). The nano-hardness of Ag–La425 was higher than that of pure Ag, which indicates that alloying increased the hardness of the matrix. Notably, the hardness profile of Ag–LaHAZ is a rising curve but that of AgHAZ is a parabolic curve. This is due to La doping (solid-solution strengthening) improving the broken problems and reliability of the HAZ [5,6,15,16]. Fig. 9 shows the I–V curves for pure Ag wire and Ag–La425 wire. Both wires were fused at 0.45 A, but the voltage of Ag–La425 was higher than that of pure Ag [6,13]. The slope in the figure indicates the conductivity value (the inverse of electric resistance). Ag–La425 wire had a lower conductivity value (its resistance was higher) than that of Ag wire because La doping increased electric
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004
6
H.-W. Hsueh et al. / Microelectronics Reliability xxx (2014) xxx–xxx
4. Conclusions (1) Ag–La wire and pure Ag wire have equiaxed grains after annealing treatment. Adding La reduced the FAB diameter and increased the hardness of the matrix. (2) The neck strength of Ag–La wire was similar to that of pure Ag wire. The HAZ of Ag–La425 wire was about 400 lm from the neck and the hardness profile of Ag–LaHAZ was a rising curve. Ag–La425 wire had a lower conductivity than that of Ag wire, but its fusing current was the same. (3) With increasing current duration, the equiaxed grains of Ag–La wire transformed into unusually coarse grains. Due to the solid-solution effect, the hardness of the matrix did not significantly change.
Acknowledgements The authors are grateful to the Instrument Center of National Cheng Kung University and to the National Science Council of Taiwan for financially supporting this research under Grant NSC 1022221-E-006-061. Fig. 12. Nano-hardness of Ag–La wire with 90% fusing current (0.4 A, current density: 1.27 10 9 A/mm2) applied for various times.
resistance. Fig. 10 shows the microstructure of Ag–La wire with 90% fusing current (0.4 A, current density of 1.27 10 9 A/ mm2) applied for various durations. The grains continued growing to the size of the wire diameter, and even became equiaxed grains. Fig. 11 shows the grain size of Ag–La wire versus current for various durations. The grain size tended to be stable for a long time. Fig. 12 shows the nano-hardness results. The hardness of the wire decreased with increasing current time. The grains of the matrix kept growing even after 30 min, and superheating occurred (grain boundary not obvious; equiaxed grains transformed into unusually coarse grains), but the hardness of the matrix did not significantly change. It is main reason that trace La had the solid-solution strengthening in Ag–La matrix. The results show that Ag–La alloy wire can be used in the electronic packaging industry.
References [1] Zhong ZW. Microelectron Reliab 2011;51:4–12. [2] Khoury SL, Burkhard DJ. IEEE Trans Compon Hybrids Manuf Technol 1990;13:673–81. [3] Liu P, Tong L, Wanga J, Shi L, Tang H. Microelectron Reliab 2012;52:1092–8. [4] Hsueh HW, Hung FY, Lui TS, Chen LH. Microelectron Reliab 2011;51:2243–9. [5] Hung FY, Wang YT, Chen LH, Lui TS. Mater Trans 2006;47:1776–81. [6] Hsueh HW, Hung FY, Lui TS, Chen LH. Microelectron Reliab 2013;53:1159–63. [7] Uno T. Microelectron Reliab 2011;51:88–96. [8] Castor SB, Hedrick JB. Rare earth elements, industrial minerals volume, 7th ed., 2006, p. 769–92. [9] Hung FY, Lui TS, Chen LH, Lan KA. Mater Trans 2008;49:1491–5. [10] Hsueh HW, Hung FY, Lui TS. Adv Mater Res 2013;804:151–7. [11] Misra E, Alford TL. Appl Phys Lett 2005;87:172111. [12] Murali S, Srikanth N, Vath CJ. Grains. Mater Charact 2003;50:39–50. [13] Hung FY, Liao JD, Lui TS, Chen LH. Curr Appl Phys 2011;11:1269–73. [14] Alford TL, Misra E, Bhagat SK, Mayer JW. Thin Solid Films 2009;517:1833–6. [15] Chen CM, Chen SW. Acta Mater 2002;50:2461–9. [16] Du MY, Chen CM, Chen SW. Mater Chem Phys 2003;82:818–25.
Please cite this article in press as: Hsueh H-W et al. Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.06.004