- Email: [email protected]
Strength of bonding interface in lead-free Sn alloy solders
Materials Science and Engineering A319– 321 (2001) 475– 479 www.elsevier.com/locate/msea
Strength of bonding interface in lead-free Sn alloy solders S. Kikuchi a,*, M. Nishimura a, K. Suetsugu b, T. Ikari b, K. Matsushige c a
Department of Materials Science, The Uni6ersity of Shiga Prefecture, Hassaka-chou, Hikone, Shiga 2500, Japan Circuit Manufacturing Technology Laboratory, Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan c Venture Business Laboratory, Kyoto Uni6ersity, Sakyo-ku, Kyoto, Japan
b
Abstract The effect of applying ultrasonic wave to soldering on the strength of the bonding interface in lead-free Sn – 3.5%Ag, Sn–57%Bi and Sn–57%Bi–1%Ag solder joints has been investigated at various test temperatures and strain rates. The bonding strength of the solder joints depends on the test temperature and the strain rate. The bonding strength of the Sn – Bi solder joints near room temperature is much higher than that of the Sn–3.5Ag solder, but the ductility is lower. The relation between the maximum stress in the stress–strain curve and the strain rate is indicated by the same power law type equation as high temperature creep. Applying ultrasonic wave to soldering influences the microstructure near the bonding interface. As a result, the fine microstructure layer is formed near the interface between the solder and the copper and it increases the strength of the bonding interface in the solder joints. © 2001 Elsevier Science B.V. All rights reserved. Keywords: lead-free solders; Sn alloys; bonding strength; structure of bonding interface; ultrasonic bonding
1. Introduction The solder used in electronics industry serves not only to provide electrical continuity but also to support the stresses experienced during service. The strength of the solder joints is important for this latter role. Tin – lead solders have long been used to have the advantage of mechanical properties, low melting temperatures and excellent wetting of copper. However, recently much attention is focused on the environmental problems caused by lead [1]. The development of lead-free solders is urgently needed, but there are few investigations on the mechanical properties of the joints by them. In this paper the effect of applying ultrasonic wave to soldering on the strength of the bonding interface between the lead-free solder and the copper has been investigated for Sn – 3.5%Ag, Sn – 57%Bi and Sn – 57%Bi –1%Ag alloy solders. We demonstrate the solidification microstructure of solders plays an important role for the strengthening of joints. Ultrasonic waves were used during solidification for the suitable microstructure to strengthen the bonding interface. The * Corresponding author. Fax: + 81-749-288486. E-mail address: [email protected] (S. Kikuchi).
effect of the solidification microstructure in the leadfree solders on the strength and the role of ultrasonic waves during solidification are discussed.
2. Experimental procedure Sn–3.5%Ag, Sn –57%Bi and Sn–57%Bi – 1%Ag alloys were prepared from the pure (\ 99.99%) metals. The alloys were melted into 300-g ingots within a metal mold (15× 80 mm inside cross section) and then cut into small pieces of solders. Solder joints were made with a method that is conventionally used in this laboratory to make tensile test specimens for estimating the bonding strength. Two 80× 20-mm pure Cu plates with 2 mm thickness were separated by a distance of 1 mm in a long direction on an aluminum alloy plate. The joint section of the copper plates was mechanically polished with a 1000-grit silicon carbide paper. Soldering was made on the heat plate at 443°K for the Sn–Bi solders and 523°K for the Sn–Ag solder using fluxing solution for about 2 to 3 min. The solder was solidified on the heat plate or on the plate applied by ultrasonic wave with a frequency of 36 kHz and a output power of 100 W. The soldered copper plates
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 1 - 0
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
476
were mechanically polished to remove excess solder and sliced to make the specimens for tensile test as shown in Fig. 1. A tensile test was performed at room temperatures of 313 and 373°K., and at a strain rate range of 8.3×10 − 5 to 8.3 ×10 − 3 s − 1. The cross section of specimens was 2× 3 mm and the length of soldered part was 1 mm. The microstructure of the fracture surface and the interface jointed between the solder and the copper was studied with a scanning electron microscopy (SEM). The compositional profiles near the interface were also analyzed by energy dispersive X-ray diffraction (EDX).
3. Results and discussion
3.1. Bonding strength The specimens with the joints bonded by Sn– 3.5%Ag, Sn–57%Bi and Sn – 57%Bi – 1%Ag solders were deformed in tension. Plastic deformation occurred only in part of the solder at all the deformation conditions. The copper part did not deform plastically. The representative stress– strain curves obtained at 313°K are shown in Fig. 2. The fracture of the Sn– Bi alloys occurred within the elastic strain range or at small plastic strain, whereas the Sn– 3.5Ag specimens frac-
Fig. 1. Dimension of specimen.
Fig. 3. SEM micrographs of fractured surface at 313$K and at a strain rate of 8.3 × 10 − 4 s − 1 in (a) Sn – 3.5%Ag, (b) Sn –57%Bi and (c) Sn – 57%Bi– 1%Ag alloys.
Fig. 2. Stress – strain curves of solder joints without applying the ultrasonic wave to soldering at 313$K. and at a strain rate of 8.3× 10 − 4 s − 1.
tured after lager plastic strain. The strength of the Sn–Bi solders is much higher than that of the Sn– 3.5%Ag solder. This is the reason why the volume fraction of the Bi-rich phase is stronger than the Sn-rich
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
phase and is large in the Sn– 57%Bi alloys. However, the Sn– Bi solders are often brittle. The amount of plastic strain in all the specimens increased with an increase in the test temperature and a decrease in the strain rate.
477
The fracture apparently occurred at the interface between the solder and the copper even in cases when the specimen deformed plastically because of a large strain. The SEM microstructures of fractured surfaces of the specimens soldered without ultrasonic waves are
Fig. 4. Relationship between maximum flow stress and strain rate in (a) Sn – 3.5%Ag, (b) Sn – 57%Bi and (c) Sn – 57%Bi– 1%Ag alloys.
478
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
Fig. 5. Microstructure and concentration profiles near the interface between copper and solder for the specimens of Sn – 57%Bi alloy solidified (a) without and (b) with ultrasonic wave.
shown in Fig. 3a, b and c. These microstructures imply that the Sn–Bi alloys show brittle fracture and the Sn –3.5%Ag is ductile as predicted by the stress–strain curves in Fig. 2. The Sn– 57%Bi – 1%Ag alloy shows very fine microstructure compared with the Sn– 57%Bi and Sn –3.5%Ag eutectic alloys. The addition of silver to the Sn–Bi system promotes the refinement of the microstructure of solder in solidification. The fine microstructure is effective in improving the strength of the solder joints.
3.2. Strain rate dependence of maximum flow stress The stress–strain behavior is dependent on the solder type, the test temperature and the strain rate. The strength of the bonding interface in solders is defined in this paper as the maximum flow stress of the specimen. The strength does not correspond to fracture stress because it becomes very small in the case of work softening. The strain rate dependence of maximum flow stress of all the specimens tested is shown in Fig. 4 a, b and c. The maximum flow stress of the Sn– Bi solders at room temperature and 313°K is much higher than that
of the Sn–3.5%Ag solder. At high temperatures the strength decreased rapidly and was close to that of the Sn–Ag solder. This is because that the Sn– 57%Bi eutectic alloy has a low melting temperature (412°K) compared with the Sn–3.5%Ag alloy (494°K). The maximum flow stress, |max , depends on the strain rate, m, following a power type equation like high temperature creep [2], |max Am m where A and m are constants. The values of m obtained from Fig. 4 are about 0.1 to 0.25 for all the solders. This value is small compared with m= 0.2 for pure metals, m= 0.3 for alloys and m= 0.5 for a Sn–Pb eutectic alloy [3]. The low m values may be caused by the precipitates in the matrix of the solders and the Cu6Sn5 phase near the interface between the copper and the solder. In the Sn–3.5%Ag alloy, the eutectic dispersion of Ag3Sn precipitates is contained within a b–Sn matrix during solidification. In the case of the Sn–Bi alloys, the precipitates of Bi phase are formed in the Sn matrix during the period at room temperature. The same low m values (mB0.1)
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
have been reported for the dispersion-hardened alloys in high temperature creep [2].
3.3. Effect of ultrasonic wa6e Applying the ultrasonic wave to soldering can be expected to increase the bonding strength at the interface between the copper and the solder as shown in Fig. 4 a, b and c. The maximum stress increases with application of ultrasonic wave, except for the case of Sn – Ag solder at 373 K. Especially, it seems that the effect of ultrasonic wave on the strength of the bond increases at lower temperatures in the Sn– 57Bi alloy. Fig. 5a and b show the microstructures and the concentration profile of elements near the bonding interface solidified without and with the ultrasonic wave in the Sn–57%Bi alloy. The microstructure of the solder solidified with the ultrasonic wave is very fine near the interface compared with that without the ultrasonic wave. The layer that Cu concentration is detectable in is in the region of solder near the interface becomes thick in the specimen applied by ultrasonic wave. This result means that the liquid of solder attacks the surface of the copper by vibration from the ultrasonic wave and scales off the intermetallic phase of Cu6Sn5 made on the copper surface [4]. The refinement of microstructure is achieved by the existence of small intermetallic particles that are scaled off in solder and cause the increase in bond strength. The effect of ultrasonic waves on the strength of bonding interface of the Sn– Ag solder is small because the interface bonded is stronger than the solder matrix in which the fracture occurs after the large plastic deformation.
479
4. Conclusion The effect of applying ultrasonic waves to soldering on the strength of the lead-free Sn–Ag and Sn – Bi solder joints has been investigated at various test temperatures and strain rates. Applying ultrasonic waves to soldering influences the microstructure near the bonding interface and, as a result, the bonding stress of the solder joints increases. The fracture of the Sn– Bi solder joint occurs in the elastic strain region or after a small plastic strain in the solder part. The Sn–Ag solder joints fracture after a large plastic strain in solder because the Sn–Ag solder matrix is ductile. The strength of the Sn– Bi solders at room temperature and at 313 K is much higher than that of the Sn–Ag solder. At 373 K, the strength of the Sn–Bi solders appears similar to that of the Sn– Ag solder. The strength of the bonding interface in the solder joints is dependent on the test conditions (temperature and strain rate) and the microstructures. The relation between the maximum stress in the stress–strain curve and the strain rate is indicated by the same power law equation as for high temperature creep.
References [1] J.S. Hwang, Z. Guo, Circuit World 20 (1994) 19. [2] J. Cadek, Creep in Metallic Materials, Academia, Prague, 1988, p. 51. [3] H.J. Frost, M.F. Ashby, in: J.C. Li, A.K. Mukherjee (Eds.), Rate Processes in Plastic Deformation of Materials, ASM, 1975, p. 70. [4] A.J. Sunwood, J.W. Morris Jr, G.K. Lucey Jr, Metal, Trans. 23A (1991) 1323.
Strength of bonding interface in lead-free Sn alloy solders S. Kikuchi a,*, M. Nishimura a, K. Suetsugu b, T. Ikari b, K. Matsushige c a
Department of Materials Science, The Uni6ersity of Shiga Prefecture, Hassaka-chou, Hikone, Shiga 2500, Japan Circuit Manufacturing Technology Laboratory, Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan c Venture Business Laboratory, Kyoto Uni6ersity, Sakyo-ku, Kyoto, Japan
b
Abstract The effect of applying ultrasonic wave to soldering on the strength of the bonding interface in lead-free Sn – 3.5%Ag, Sn–57%Bi and Sn–57%Bi–1%Ag solder joints has been investigated at various test temperatures and strain rates. The bonding strength of the solder joints depends on the test temperature and the strain rate. The bonding strength of the Sn – Bi solder joints near room temperature is much higher than that of the Sn–3.5Ag solder, but the ductility is lower. The relation between the maximum stress in the stress–strain curve and the strain rate is indicated by the same power law type equation as high temperature creep. Applying ultrasonic wave to soldering influences the microstructure near the bonding interface. As a result, the fine microstructure layer is formed near the interface between the solder and the copper and it increases the strength of the bonding interface in the solder joints. © 2001 Elsevier Science B.V. All rights reserved. Keywords: lead-free solders; Sn alloys; bonding strength; structure of bonding interface; ultrasonic bonding
1. Introduction The solder used in electronics industry serves not only to provide electrical continuity but also to support the stresses experienced during service. The strength of the solder joints is important for this latter role. Tin – lead solders have long been used to have the advantage of mechanical properties, low melting temperatures and excellent wetting of copper. However, recently much attention is focused on the environmental problems caused by lead [1]. The development of lead-free solders is urgently needed, but there are few investigations on the mechanical properties of the joints by them. In this paper the effect of applying ultrasonic wave to soldering on the strength of the bonding interface between the lead-free solder and the copper has been investigated for Sn – 3.5%Ag, Sn – 57%Bi and Sn – 57%Bi –1%Ag alloy solders. We demonstrate the solidification microstructure of solders plays an important role for the strengthening of joints. Ultrasonic waves were used during solidification for the suitable microstructure to strengthen the bonding interface. The * Corresponding author. Fax: + 81-749-288486. E-mail address: [email protected] (S. Kikuchi).
effect of the solidification microstructure in the leadfree solders on the strength and the role of ultrasonic waves during solidification are discussed.
2. Experimental procedure Sn–3.5%Ag, Sn –57%Bi and Sn–57%Bi – 1%Ag alloys were prepared from the pure (\ 99.99%) metals. The alloys were melted into 300-g ingots within a metal mold (15× 80 mm inside cross section) and then cut into small pieces of solders. Solder joints were made with a method that is conventionally used in this laboratory to make tensile test specimens for estimating the bonding strength. Two 80× 20-mm pure Cu plates with 2 mm thickness were separated by a distance of 1 mm in a long direction on an aluminum alloy plate. The joint section of the copper plates was mechanically polished with a 1000-grit silicon carbide paper. Soldering was made on the heat plate at 443°K for the Sn–Bi solders and 523°K for the Sn–Ag solder using fluxing solution for about 2 to 3 min. The solder was solidified on the heat plate or on the plate applied by ultrasonic wave with a frequency of 36 kHz and a output power of 100 W. The soldered copper plates
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 0 3 1 - 0
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
476
were mechanically polished to remove excess solder and sliced to make the specimens for tensile test as shown in Fig. 1. A tensile test was performed at room temperatures of 313 and 373°K., and at a strain rate range of 8.3×10 − 5 to 8.3 ×10 − 3 s − 1. The cross section of specimens was 2× 3 mm and the length of soldered part was 1 mm. The microstructure of the fracture surface and the interface jointed between the solder and the copper was studied with a scanning electron microscopy (SEM). The compositional profiles near the interface were also analyzed by energy dispersive X-ray diffraction (EDX).
3. Results and discussion
3.1. Bonding strength The specimens with the joints bonded by Sn– 3.5%Ag, Sn–57%Bi and Sn – 57%Bi – 1%Ag solders were deformed in tension. Plastic deformation occurred only in part of the solder at all the deformation conditions. The copper part did not deform plastically. The representative stress– strain curves obtained at 313°K are shown in Fig. 2. The fracture of the Sn– Bi alloys occurred within the elastic strain range or at small plastic strain, whereas the Sn– 3.5Ag specimens frac-
Fig. 1. Dimension of specimen.
Fig. 3. SEM micrographs of fractured surface at 313$K and at a strain rate of 8.3 × 10 − 4 s − 1 in (a) Sn – 3.5%Ag, (b) Sn –57%Bi and (c) Sn – 57%Bi– 1%Ag alloys.
Fig. 2. Stress – strain curves of solder joints without applying the ultrasonic wave to soldering at 313$K. and at a strain rate of 8.3× 10 − 4 s − 1.
tured after lager plastic strain. The strength of the Sn–Bi solders is much higher than that of the Sn– 3.5%Ag solder. This is the reason why the volume fraction of the Bi-rich phase is stronger than the Sn-rich
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
phase and is large in the Sn– 57%Bi alloys. However, the Sn– Bi solders are often brittle. The amount of plastic strain in all the specimens increased with an increase in the test temperature and a decrease in the strain rate.
477
The fracture apparently occurred at the interface between the solder and the copper even in cases when the specimen deformed plastically because of a large strain. The SEM microstructures of fractured surfaces of the specimens soldered without ultrasonic waves are
Fig. 4. Relationship between maximum flow stress and strain rate in (a) Sn – 3.5%Ag, (b) Sn – 57%Bi and (c) Sn – 57%Bi– 1%Ag alloys.
478
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
Fig. 5. Microstructure and concentration profiles near the interface between copper and solder for the specimens of Sn – 57%Bi alloy solidified (a) without and (b) with ultrasonic wave.
shown in Fig. 3a, b and c. These microstructures imply that the Sn–Bi alloys show brittle fracture and the Sn –3.5%Ag is ductile as predicted by the stress–strain curves in Fig. 2. The Sn– 57%Bi – 1%Ag alloy shows very fine microstructure compared with the Sn– 57%Bi and Sn –3.5%Ag eutectic alloys. The addition of silver to the Sn–Bi system promotes the refinement of the microstructure of solder in solidification. The fine microstructure is effective in improving the strength of the solder joints.
3.2. Strain rate dependence of maximum flow stress The stress–strain behavior is dependent on the solder type, the test temperature and the strain rate. The strength of the bonding interface in solders is defined in this paper as the maximum flow stress of the specimen. The strength does not correspond to fracture stress because it becomes very small in the case of work softening. The strain rate dependence of maximum flow stress of all the specimens tested is shown in Fig. 4 a, b and c. The maximum flow stress of the Sn– Bi solders at room temperature and 313°K is much higher than that
of the Sn–3.5%Ag solder. At high temperatures the strength decreased rapidly and was close to that of the Sn–Ag solder. This is because that the Sn– 57%Bi eutectic alloy has a low melting temperature (412°K) compared with the Sn–3.5%Ag alloy (494°K). The maximum flow stress, |max , depends on the strain rate, m, following a power type equation like high temperature creep [2], |max Am m where A and m are constants. The values of m obtained from Fig. 4 are about 0.1 to 0.25 for all the solders. This value is small compared with m= 0.2 for pure metals, m= 0.3 for alloys and m= 0.5 for a Sn–Pb eutectic alloy [3]. The low m values may be caused by the precipitates in the matrix of the solders and the Cu6Sn5 phase near the interface between the copper and the solder. In the Sn–3.5%Ag alloy, the eutectic dispersion of Ag3Sn precipitates is contained within a b–Sn matrix during solidification. In the case of the Sn–Bi alloys, the precipitates of Bi phase are formed in the Sn matrix during the period at room temperature. The same low m values (mB0.1)
S. Kikuchi et al. / Materials Science and Engineering A319–321 (2001) 475–479
have been reported for the dispersion-hardened alloys in high temperature creep [2].
3.3. Effect of ultrasonic wa6e Applying the ultrasonic wave to soldering can be expected to increase the bonding strength at the interface between the copper and the solder as shown in Fig. 4 a, b and c. The maximum stress increases with application of ultrasonic wave, except for the case of Sn – Ag solder at 373 K. Especially, it seems that the effect of ultrasonic wave on the strength of the bond increases at lower temperatures in the Sn– 57Bi alloy. Fig. 5a and b show the microstructures and the concentration profile of elements near the bonding interface solidified without and with the ultrasonic wave in the Sn–57%Bi alloy. The microstructure of the solder solidified with the ultrasonic wave is very fine near the interface compared with that without the ultrasonic wave. The layer that Cu concentration is detectable in is in the region of solder near the interface becomes thick in the specimen applied by ultrasonic wave. This result means that the liquid of solder attacks the surface of the copper by vibration from the ultrasonic wave and scales off the intermetallic phase of Cu6Sn5 made on the copper surface [4]. The refinement of microstructure is achieved by the existence of small intermetallic particles that are scaled off in solder and cause the increase in bond strength. The effect of ultrasonic waves on the strength of bonding interface of the Sn– Ag solder is small because the interface bonded is stronger than the solder matrix in which the fracture occurs after the large plastic deformation.
479
4. Conclusion The effect of applying ultrasonic waves to soldering on the strength of the lead-free Sn–Ag and Sn – Bi solder joints has been investigated at various test temperatures and strain rates. Applying ultrasonic waves to soldering influences the microstructure near the bonding interface and, as a result, the bonding stress of the solder joints increases. The fracture of the Sn– Bi solder joint occurs in the elastic strain region or after a small plastic strain in the solder part. The Sn–Ag solder joints fracture after a large plastic strain in solder because the Sn–Ag solder matrix is ductile. The strength of the Sn– Bi solders at room temperature and at 313 K is much higher than that of the Sn–Ag solder. At 373 K, the strength of the Sn–Bi solders appears similar to that of the Sn– Ag solder. The strength of the bonding interface in the solder joints is dependent on the test conditions (temperature and strain rate) and the microstructures. The relation between the maximum stress in the stress–strain curve and the strain rate is indicated by the same power law equation as for high temperature creep.
References [1] J.S. Hwang, Z. Guo, Circuit World 20 (1994) 19. [2] J. Cadek, Creep in Metallic Materials, Academia, Prague, 1988, p. 51. [3] H.J. Frost, M.F. Ashby, in: J.C. Li, A.K. Mukherjee (Eds.), Rate Processes in Plastic Deformation of Materials, ASM, 1975, p. 70. [4] A.J. Sunwood, J.W. Morris Jr, G.K. Lucey Jr, Metal, Trans. 23A (1991) 1323.