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A study on the tensile fracture mechanism of 15 μm copper wire after EFO process
Microelectronics Reliability 51 (2011) 25–29
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
A study on the tensile fracture mechanism of 15 lm copper wire after EFO process I-Ting Huang a, Fei-Yi Hung b,*, Truan-Sheng Lui a,**, Li-Hui Chen a, Hao-Wen Hsueh a a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 9 April 2010 Accepted 9 April 2010 Available online 6 May 2010
a b s t r a c t In this study, 15 lm copper wires were bonded on substrates with thermosonic process, and the tensile fracture characteristics of FAB, as well as bonded samples, were investigated. For electronic packaging applications, all 15 lm wires were fully annealed, and the microstructures consisted of equiaxed grains. After EFO (electric flame-off) process, the microstructure of wire can be divided into three parts: (1) free air ball (FAB) with columnar grains, (2) heat-affected zone (HAZ) with equal-diameter grains, (3) annealing zone with equiaxed grains. According to tensile test results, EFO process simultaneously reduced UTS and elongation of the wire. For both FAB and bonded samples, the tensile fracture zones were either in the region of equal-diameter grains or in coarse grains located within 100 lm from the ball. And it was observed that the breakage sites appeared near the twins and the columnar grains when tensile fracture happened. Meanwhile, the relationship between hardness and microstructure of wires after EFO process were analyzed with nano-indentation. The nano-hardness value of 15 lm wire was 1.2–1.45 GPa. Ó 2010 Published by Elsevier Ltd.
1. Introduction
2. Experiment
Now, the gold wire is the most popular interconnection material in the wire bonding processes [1]. Compared to the gold wire, the copper wire has the inherent properties of higher stiffness, tensile strength, electrical conductivity and thermal conductivity as well as its lower cost, so it is getting well accepted as a reliable design alternative to the gold wire. When applied in electronic packaging processes, the copper wires are fully annealed to attend lower hardness and higher elongation. And the weaker strength after EFO process made the wire easily broken in the region of HAZ [2,3]. However, the fracture mechanism of copper wire has not been investigated. In many literatures [1–3], the pull test has been applied to determine the interfacial strength between bonding face and substrate. In other words, the strength of bond was even stronger than the wire itself under a pulling force. Notably, for the need of higher I/O numbers and the finer pitch areas, the copper can prove a promising material with its remarkable performances [4,5]. This present paper used advanced 15 lm copper wires, not only use the nano-indentation analysis to understand the nano-hardness of ‘‘equal-diameter” grains and microstructure characteristics, but also discuss the mechanical property and the pull fracture mechanism before and after bonding.
Copper ingot of 99.99% purity was drawn into thin copper wire with a diameter of 15 lm. The wires were produced by TA-YA company in Taiwan. And the wires were annealed in a continuous heat treatment process of 445 °C for 0.3 s, while the 20 lm copper wire for comparison was 610 °C for 0.02 s for compare. For the 20 lm wires, the annealing needs a higher temperature than that of the 15 lm wires. Notably, two kinds of wires are fully annealed structures that are selected for comparison. The bonding process was carried out on a thermosonic wire bonder. To prevent FABs from oxidation during EFO process, a 95% nitrogen and 5% hydrogen gas mixture was applied to the work zone, and the flow rate maintained at 1 L/min all the time [2,3]. To understand the effects of EFO process on the mechanical properties of the Cu wire, both wires and FAB samples were subjected to tensile test, so the correlation between the mechanical properties and recrystallization behavior could be analyzed. The testing method used a plate clamp to fix the FAB, while the wire tip was fixed using a tongs-like clamp. A schematic illustration of the tensile test is shown in Fig. 1. And the whole test was carried out under a constant strain rate of 2.54 cm/min. For a better understanding of breaking mechanism, the fractures of bonded samples were extensively observed after pull test. A schematic illustration of bond pull test is shown in Fig. 2. Micro-hardness measurements were performed on the cross section of the annealed copper wire, the ball and the HAZ. The test load was 5 g while the holding time was 10 s. An illustration
* Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. ** Corresponding author. Tel.: +886 6 2757575x62931; fax: +886 6 2745885. E-mail addresses: [email protected] (F.-Y. Hung), z7408020@ email.ncku.edu.tw (T.-S. Lui). 0026-2714/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.microrel.2010.04.007
26
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
a
Tensile Direction
b
Fig. 1. The schematic illustration of the tensile test for the FAB samples.
Pull Direction
20µm Fig. 5. The cross section of the FABs and HAZ: (a) 20 lm, (b) 15 lm.
Substrate
a
Fig. 2. The schematic illustration of the pull test for the bonded samples.
40µm Fig. 3. The schematic illustration of the micro-hardness measuring.
20µm
b
20µm
40µm
Fig. 4. The schematic illustration of the nano-indentation.
Fig. 6. Equal-diameter grains in 15 lm Cu wire: (a) structure, (b) illustration.
27
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
3. Result and discussion 3.1. Microstructures and mechanical properties The microstructures of fully annealed 15 lm wires consisted of equiaxed grains. After EFO process, the microstructure of the wire could be divided into three parts: (1) free air ball (FAB), (2) heat-affected zone (HAZ), (3) annealing zone with equiaxed grains, as shown in Fig. 5. The FAB section comprised a few columnar grains, and a continuous structure without void or boundary between FAB and HAZ could be seen. So the solidification direction was from the un-melted wire toward the melted ball. In HAZ area, the coarse grains and some annealing twins could be found. It should be notified that within 100 lm from the balls existed some ‘‘equal-diameter” grains, as we called them, which are illustrated in Fig. 6. In addition, the average micro-hardness value of 20 lm and 15 lm copper wires were 79 Hv and 72 Hv respectively. Micro-hardness analysis of 20 lm and 15 lm wires after EFO revealed that the neck of the FAB (i.e., HAZ) was the
a
weakest in strength (Fig. 7). Also, it could be seen that the further away from the ball, the hardness was higher, until it reached the average value before EFO. For the present columnar grains (within Cu ball), they were rapid solidification structure and the hardness was detected in the direction of long-axis. This is the main reason that the larger columnar grain near neck zone had higher hardness than the smaller equiaxed grain. Fig. 8 compared the tensile test results of wires with and without EFO process. The strength of the samples dropped after EFO, the heat effect induced by the EFO process lowered the deformation resistance of the HAZ. According to the test results, EFO process reduced the UTS of 15 lm wire (Fig. 8a–c) shows that elongation drastically decreased after EFO. This is because the gauge length included both HAZ and non-HAZ, but deformation was almost in HAZ [2,3].
Before EFO
a
120
80
40
100
0
80
b
60
20µm
15µm
300
40
UTS (MPa)
Hardness (Hv)
After EFO 200
160
YS (MPa)
(including the electric flame-off wire) of the hardness test setup is shown in Fig. 3. Due to the micro-hardness testing cannot obtain the modulus of grains, so the nano-indentation had to be used. The nano-indentation setup is illustrated in Fig. 4. A MTS XP system and a diamond Berkovich pyramid indentor with a tip radius of 100–200 nm were used and the indentation depth was 600 nm. Each data is the average value of 10 test results (including the FAB samples).
HAZ 20
0 0
200
400
600
200
100
800
Distance (µm)
b
100
0
c
20
16
60
40
Elongation (%)
Hardness (Hv)
80
HAZ
20
12
8
4
0 0
200
400
600
800
Distance (µm) Fig. 7. Micro-hardness of FAB samples (the origin of the cross axis is the center of FAB): (a) 20 lm, (b) 15 lm.
0 Fig. 8. The comparison of the tensile properties before and after EFO: (a) YS, (b) UTS, (c) elongation.
28
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
Taking 15 lm wire for example, the breakage site of the EFO wires occurred in the HAZ, and mostly within 100–150 lm from
the balls (Fig. 9a) while a small number near the balls (Fig. 9b). The literatures [2,3] of 20–25 lm copper wires also show the fracture site of HAZ. The main reason is the effect of equal-diameter grain to cause the difference of breakage sites. Notably, the 15 lm wire after EFO was also measured by nano-indentation as shown in Fig. 10. It is reported that the nano-hardness value of 15 lm wire was 1.2–1.45 GPa. The nano-hardness value of ball and neck were lower than the wire in non-HAZ, and it was similar to micro-hardness value (Fig. 7). However, compared the ball with the neck, the nano-hardness value of ball was not always higher than neck, so the result of micro-hardness which showed ball was always harder than neck was affected by the matrix of sample. The difference of nano-hardness in ball showed that the columnar grains had an effect to enhance hardness. Affected by directions of columnar grains, the modulus of ball were higher than neck as shown in Fig. 10b. Notably, If the indented points approach and far away the grain boundary, the data were scattered. The modulus of the region within 100 lm from ball was lower, it showed the effects of equal-diameter grains and coarse grains were more obvious than that of wire beyond 100 lm from ball. According to observations of FABs, the columnar grains (inside Cu ball) had higher hardness and modulus. This is main reason that the columnar grains were rapid solidification structure and the hardness was detected in the direction of longaxis. 3.2. Fracture characteristics
Fig. 9. The breakage site of FAB samples of 15 lm Cu wire after tensile test (SEM): (a) far from ball, (b) close to ball.
a
According to literature [6–9], when single crystals (equal-diameter grains) and polycrystals existed in fine copper wire at the same time, deformation almost occurred in single crystals. In this study, the breakage sites of FAB tensile test lay in 100–150 lm
1.6
Hardness (GPa)
1.4
1.2
1
0.8 0
100
200
300
400
500
Distance (µm)
b
100
Modulus (GPa)
80
60
40
20
0
0
100
200 300 400 Distance (µm)
500
Fig. 10. Nano-indentation of 15 lm FAB wire: (a) hardness, (b) Young’s modulus.
Fig. 11. The breakage site of bonded sample of 15 lm Cu wire after pull testing: (a) at HAZ (b) at neck near ball.
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
a
b
29
The breakage sites can divide into two groups: one is far from bonds, and another is close to bonds. Fig. 12 shows that slip lines appeared locally on the surface of two groups after pull test, which may be the breakage site [9]. Fig. 12a shows the sample broke far from bonds; moreover, slips of equal-diameter grains or coarse grains can be seen, and localization deformation happened. Fig. 12b shows that slip lines was close to bonds and then wire fracture. And it was observed that the breakage sites appeared near the twins and the columnar grains. Because columnar grains and twins have higher deformation resistance [6,7], they would be obstacles when equal-diameter grains or coarse grains deformed and then fracture happened nearby. To summarize, the process of bonded in pull test shows that equal-diameter grains and coarse grains were the main deformation mechanism. 4. Conclusions EFO process caused a reduction of UTS in 15 lm Cu wire, and elongation drastically decreased after EFO because of localization deformation. Nano-hardness value of 15 lm Cu wire was 1.2– 1.45 GPa. The modulus of ball was higher than neck. Nanohardness and modulus were lower within 100 lm from ball. Equal-diameter grains were found within 100 lm from ball. After pull test, fracture occurred in the region of equal-diameter grains or coarse grains within 100 lm. Acknowledgements
Fig. 12. The slip lines of 15 lm Cu wire after pull test: (a) breakage site far from bonds, (b) breakage site close to bonds (at neck zone).
The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Technology (NCKU Project of Promoting Academic Excellence & Developing World Class Research Center: D98-2700) and NSC 98-2221-E-006-068/NSC 982622-E-006-024-CC3 for the financial support. References
from ball, not always within 100 lm where equal-diameter grains were found. So it was supposed that equal-diameter grains or coarse grains experienced more deformation, but the non-homogeneous deformation between coarse grains and smaller grains affected the breakage site eventually. Some samples broke within 100 lm, it was supposed that the strong h1 0 0i//AD (as draw direction) texture resulted in lower deformation resistance. After bonding, the breakage samples of tensile test were also investigated (Fig. 11). Pull fracture took place at equal-diameter grains and coarse grains within or around 100 lm away from bonds. The breakage sites of bonds tensile test were a little different from those of FAB tensile test. Because the gauge length of bonds in pull test (15 mm) was shorter than FAB tensile test (150 mm), it may cause deformation localize in the region of equal-diameter grains and coarse grains more easily.
[1] Herman. Wire bonding in microelectronics, materials, processes reliability and yield. 2nd ed. McGraw-Hill; 1997. p. 1–10. [2] Hung FY, Lui TS, Chen LH, Wang YT. Recrystallization and fracture characteristics of thin copper wire. J Mater Sci 2007;42:5476–82. [3] Hung FY, Wang YT, Chen LH, Lui TS. Recrystallization effect and electric flameoff characteristic of thin copper wire. Mater Trans 2006;47:1776–81. [4] Hang CJ, Wang CQ, Mayer M, Tian YH, Zhou Y, Wang HH. growth behavior of Cu/ Al intermetallic compounds and cracks in copper ball bonds during isothermal aging. Microelectron Reliab 2008;48:416–24. [5] Srikanth N, Murali S, Wong YM, Vath III Charles J. Critical study of thermosonic copper ball bonding. Thin Solid Films 2004;462-463:339–45. [6] Hung FY, Lui TS, Chen LH, Lin YC. Electric flame-off characteristics and fracture properties of 20 um thin copper bonding wire. Mater Trans 2009;50:293–8. [7] Baudin T, Etter AL, Penelle R. Annealing twin formation and recrystallization study of cold-drawn copper wires from EBSD measurements. Mater Charact 2007;58:947–52. [8] Chin GY, Mammel WL. Computer solutions of taylor analysis for axisymmetric flow. T Metall Soc AIME 1967;239:1400. [9] Khatibi G, Stickler R, Gröger V, Weiss B. Tensile properties of thin Cu-wires with a bamboo microstructure. J Alloy Compd 2004;378:326–8.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
A study on the tensile fracture mechanism of 15 lm copper wire after EFO process I-Ting Huang a, Fei-Yi Hung b,*, Truan-Sheng Lui a,**, Li-Hui Chen a, Hao-Wen Hsueh a a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Institute of Nanotechnology and Microsystems Engineering, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 31 January 2010 Received in revised form 9 April 2010 Accepted 9 April 2010 Available online 6 May 2010
a b s t r a c t In this study, 15 lm copper wires were bonded on substrates with thermosonic process, and the tensile fracture characteristics of FAB, as well as bonded samples, were investigated. For electronic packaging applications, all 15 lm wires were fully annealed, and the microstructures consisted of equiaxed grains. After EFO (electric flame-off) process, the microstructure of wire can be divided into three parts: (1) free air ball (FAB) with columnar grains, (2) heat-affected zone (HAZ) with equal-diameter grains, (3) annealing zone with equiaxed grains. According to tensile test results, EFO process simultaneously reduced UTS and elongation of the wire. For both FAB and bonded samples, the tensile fracture zones were either in the region of equal-diameter grains or in coarse grains located within 100 lm from the ball. And it was observed that the breakage sites appeared near the twins and the columnar grains when tensile fracture happened. Meanwhile, the relationship between hardness and microstructure of wires after EFO process were analyzed with nano-indentation. The nano-hardness value of 15 lm wire was 1.2–1.45 GPa. Ó 2010 Published by Elsevier Ltd.
1. Introduction
2. Experiment
Now, the gold wire is the most popular interconnection material in the wire bonding processes [1]. Compared to the gold wire, the copper wire has the inherent properties of higher stiffness, tensile strength, electrical conductivity and thermal conductivity as well as its lower cost, so it is getting well accepted as a reliable design alternative to the gold wire. When applied in electronic packaging processes, the copper wires are fully annealed to attend lower hardness and higher elongation. And the weaker strength after EFO process made the wire easily broken in the region of HAZ [2,3]. However, the fracture mechanism of copper wire has not been investigated. In many literatures [1–3], the pull test has been applied to determine the interfacial strength between bonding face and substrate. In other words, the strength of bond was even stronger than the wire itself under a pulling force. Notably, for the need of higher I/O numbers and the finer pitch areas, the copper can prove a promising material with its remarkable performances [4,5]. This present paper used advanced 15 lm copper wires, not only use the nano-indentation analysis to understand the nano-hardness of ‘‘equal-diameter” grains and microstructure characteristics, but also discuss the mechanical property and the pull fracture mechanism before and after bonding.
Copper ingot of 99.99% purity was drawn into thin copper wire with a diameter of 15 lm. The wires were produced by TA-YA company in Taiwan. And the wires were annealed in a continuous heat treatment process of 445 °C for 0.3 s, while the 20 lm copper wire for comparison was 610 °C for 0.02 s for compare. For the 20 lm wires, the annealing needs a higher temperature than that of the 15 lm wires. Notably, two kinds of wires are fully annealed structures that are selected for comparison. The bonding process was carried out on a thermosonic wire bonder. To prevent FABs from oxidation during EFO process, a 95% nitrogen and 5% hydrogen gas mixture was applied to the work zone, and the flow rate maintained at 1 L/min all the time [2,3]. To understand the effects of EFO process on the mechanical properties of the Cu wire, both wires and FAB samples were subjected to tensile test, so the correlation between the mechanical properties and recrystallization behavior could be analyzed. The testing method used a plate clamp to fix the FAB, while the wire tip was fixed using a tongs-like clamp. A schematic illustration of the tensile test is shown in Fig. 1. And the whole test was carried out under a constant strain rate of 2.54 cm/min. For a better understanding of breaking mechanism, the fractures of bonded samples were extensively observed after pull test. A schematic illustration of bond pull test is shown in Fig. 2. Micro-hardness measurements were performed on the cross section of the annealed copper wire, the ball and the HAZ. The test load was 5 g while the holding time was 10 s. An illustration
* Corresponding author. Tel.: +886 6 2757575x31395; fax: +886 6 2745885. ** Corresponding author. Tel.: +886 6 2757575x62931; fax: +886 6 2745885. E-mail addresses: [email protected] (F.-Y. Hung), z7408020@ email.ncku.edu.tw (T.-S. Lui). 0026-2714/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.microrel.2010.04.007
26
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
a
Tensile Direction
b
Fig. 1. The schematic illustration of the tensile test for the FAB samples.
Pull Direction
20µm Fig. 5. The cross section of the FABs and HAZ: (a) 20 lm, (b) 15 lm.
Substrate
a
Fig. 2. The schematic illustration of the pull test for the bonded samples.
40µm Fig. 3. The schematic illustration of the micro-hardness measuring.
20µm
b
20µm
40µm
Fig. 4. The schematic illustration of the nano-indentation.
Fig. 6. Equal-diameter grains in 15 lm Cu wire: (a) structure, (b) illustration.
27
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
3. Result and discussion 3.1. Microstructures and mechanical properties The microstructures of fully annealed 15 lm wires consisted of equiaxed grains. After EFO process, the microstructure of the wire could be divided into three parts: (1) free air ball (FAB), (2) heat-affected zone (HAZ), (3) annealing zone with equiaxed grains, as shown in Fig. 5. The FAB section comprised a few columnar grains, and a continuous structure without void or boundary between FAB and HAZ could be seen. So the solidification direction was from the un-melted wire toward the melted ball. In HAZ area, the coarse grains and some annealing twins could be found. It should be notified that within 100 lm from the balls existed some ‘‘equal-diameter” grains, as we called them, which are illustrated in Fig. 6. In addition, the average micro-hardness value of 20 lm and 15 lm copper wires were 79 Hv and 72 Hv respectively. Micro-hardness analysis of 20 lm and 15 lm wires after EFO revealed that the neck of the FAB (i.e., HAZ) was the
a
weakest in strength (Fig. 7). Also, it could be seen that the further away from the ball, the hardness was higher, until it reached the average value before EFO. For the present columnar grains (within Cu ball), they were rapid solidification structure and the hardness was detected in the direction of long-axis. This is the main reason that the larger columnar grain near neck zone had higher hardness than the smaller equiaxed grain. Fig. 8 compared the tensile test results of wires with and without EFO process. The strength of the samples dropped after EFO, the heat effect induced by the EFO process lowered the deformation resistance of the HAZ. According to the test results, EFO process reduced the UTS of 15 lm wire (Fig. 8a–c) shows that elongation drastically decreased after EFO. This is because the gauge length included both HAZ and non-HAZ, but deformation was almost in HAZ [2,3].
Before EFO
a
120
80
40
100
0
80
b
60
20µm
15µm
300
40
UTS (MPa)
Hardness (Hv)
After EFO 200
160
YS (MPa)
(including the electric flame-off wire) of the hardness test setup is shown in Fig. 3. Due to the micro-hardness testing cannot obtain the modulus of grains, so the nano-indentation had to be used. The nano-indentation setup is illustrated in Fig. 4. A MTS XP system and a diamond Berkovich pyramid indentor with a tip radius of 100–200 nm were used and the indentation depth was 600 nm. Each data is the average value of 10 test results (including the FAB samples).
HAZ 20
0 0
200
400
600
200
100
800
Distance (µm)
b
100
0
c
20
16
60
40
Elongation (%)
Hardness (Hv)
80
HAZ
20
12
8
4
0 0
200
400
600
800
Distance (µm) Fig. 7. Micro-hardness of FAB samples (the origin of the cross axis is the center of FAB): (a) 20 lm, (b) 15 lm.
0 Fig. 8. The comparison of the tensile properties before and after EFO: (a) YS, (b) UTS, (c) elongation.
28
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
Taking 15 lm wire for example, the breakage site of the EFO wires occurred in the HAZ, and mostly within 100–150 lm from
the balls (Fig. 9a) while a small number near the balls (Fig. 9b). The literatures [2,3] of 20–25 lm copper wires also show the fracture site of HAZ. The main reason is the effect of equal-diameter grain to cause the difference of breakage sites. Notably, the 15 lm wire after EFO was also measured by nano-indentation as shown in Fig. 10. It is reported that the nano-hardness value of 15 lm wire was 1.2–1.45 GPa. The nano-hardness value of ball and neck were lower than the wire in non-HAZ, and it was similar to micro-hardness value (Fig. 7). However, compared the ball with the neck, the nano-hardness value of ball was not always higher than neck, so the result of micro-hardness which showed ball was always harder than neck was affected by the matrix of sample. The difference of nano-hardness in ball showed that the columnar grains had an effect to enhance hardness. Affected by directions of columnar grains, the modulus of ball were higher than neck as shown in Fig. 10b. Notably, If the indented points approach and far away the grain boundary, the data were scattered. The modulus of the region within 100 lm from ball was lower, it showed the effects of equal-diameter grains and coarse grains were more obvious than that of wire beyond 100 lm from ball. According to observations of FABs, the columnar grains (inside Cu ball) had higher hardness and modulus. This is main reason that the columnar grains were rapid solidification structure and the hardness was detected in the direction of longaxis. 3.2. Fracture characteristics
Fig. 9. The breakage site of FAB samples of 15 lm Cu wire after tensile test (SEM): (a) far from ball, (b) close to ball.
a
According to literature [6–9], when single crystals (equal-diameter grains) and polycrystals existed in fine copper wire at the same time, deformation almost occurred in single crystals. In this study, the breakage sites of FAB tensile test lay in 100–150 lm
1.6
Hardness (GPa)
1.4
1.2
1
0.8 0
100
200
300
400
500
Distance (µm)
b
100
Modulus (GPa)
80
60
40
20
0
0
100
200 300 400 Distance (µm)
500
Fig. 10. Nano-indentation of 15 lm FAB wire: (a) hardness, (b) Young’s modulus.
Fig. 11. The breakage site of bonded sample of 15 lm Cu wire after pull testing: (a) at HAZ (b) at neck near ball.
I-Ting Huang et al. / Microelectronics Reliability 51 (2011) 25–29
a
b
29
The breakage sites can divide into two groups: one is far from bonds, and another is close to bonds. Fig. 12 shows that slip lines appeared locally on the surface of two groups after pull test, which may be the breakage site [9]. Fig. 12a shows the sample broke far from bonds; moreover, slips of equal-diameter grains or coarse grains can be seen, and localization deformation happened. Fig. 12b shows that slip lines was close to bonds and then wire fracture. And it was observed that the breakage sites appeared near the twins and the columnar grains. Because columnar grains and twins have higher deformation resistance [6,7], they would be obstacles when equal-diameter grains or coarse grains deformed and then fracture happened nearby. To summarize, the process of bonded in pull test shows that equal-diameter grains and coarse grains were the main deformation mechanism. 4. Conclusions EFO process caused a reduction of UTS in 15 lm Cu wire, and elongation drastically decreased after EFO because of localization deformation. Nano-hardness value of 15 lm Cu wire was 1.2– 1.45 GPa. The modulus of ball was higher than neck. Nanohardness and modulus were lower within 100 lm from ball. Equal-diameter grains were found within 100 lm from ball. After pull test, fracture occurred in the region of equal-diameter grains or coarse grains within 100 lm. Acknowledgements
Fig. 12. The slip lines of 15 lm Cu wire after pull test: (a) breakage site far from bonds, (b) breakage site close to bonds (at neck zone).
The authors are grateful to National Cheng Kung University, the Center for Micro/Nano Science and Technology (NCKU Project of Promoting Academic Excellence & Developing World Class Research Center: D98-2700) and NSC 98-2221-E-006-068/NSC 982622-E-006-024-CC3 for the financial support. References
from ball, not always within 100 lm where equal-diameter grains were found. So it was supposed that equal-diameter grains or coarse grains experienced more deformation, but the non-homogeneous deformation between coarse grains and smaller grains affected the breakage site eventually. Some samples broke within 100 lm, it was supposed that the strong h1 0 0i//AD (as draw direction) texture resulted in lower deformation resistance. After bonding, the breakage samples of tensile test were also investigated (Fig. 11). Pull fracture took place at equal-diameter grains and coarse grains within or around 100 lm away from bonds. The breakage sites of bonds tensile test were a little different from those of FAB tensile test. Because the gauge length of bonds in pull test (15 mm) was shorter than FAB tensile test (150 mm), it may cause deformation localize in the region of equal-diameter grains and coarse grains more easily.
[1] Herman. Wire bonding in microelectronics, materials, processes reliability and yield. 2nd ed. McGraw-Hill; 1997. p. 1–10. [2] Hung FY, Lui TS, Chen LH, Wang YT. Recrystallization and fracture characteristics of thin copper wire. J Mater Sci 2007;42:5476–82. [3] Hung FY, Wang YT, Chen LH, Lui TS. Recrystallization effect and electric flameoff characteristic of thin copper wire. Mater Trans 2006;47:1776–81. [4] Hang CJ, Wang CQ, Mayer M, Tian YH, Zhou Y, Wang HH. growth behavior of Cu/ Al intermetallic compounds and cracks in copper ball bonds during isothermal aging. Microelectron Reliab 2008;48:416–24. [5] Srikanth N, Murali S, Wong YM, Vath III Charles J. Critical study of thermosonic copper ball bonding. Thin Solid Films 2004;462-463:339–45. [6] Hung FY, Lui TS, Chen LH, Lin YC. Electric flame-off characteristics and fracture properties of 20 um thin copper bonding wire. Mater Trans 2009;50:293–8. [7] Baudin T, Etter AL, Penelle R. Annealing twin formation and recrystallization study of cold-drawn copper wires from EBSD measurements. Mater Charact 2007;58:947–52. [8] Chin GY, Mammel WL. Computer solutions of taylor analysis for axisymmetric flow. T Metall Soc AIME 1967;239:1400. [9] Khatibi G, Stickler R, Gröger V, Weiss B. Tensile properties of thin Cu-wires with a bamboo microstructure. J Alloy Compd 2004;378:326–8.