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The influence of copper nanopowders on microstructure and hardness of lead–tin solder
April 2002
Materials Letters 53 (2002) 333 – 338 www.elsevier.com/locate/matlet
The inf luence of copper nanopowders on microstructure and hardness of lead–tin solder D. Lin a, G.X. Wang a, T.S. Srivatsan a,*, Meslet Al-Hajri a, M. Petraroli b a Department of Mechanical Engineering, The University of Akron, Akron, OH 44325-3903, USA Timken Research, The Timken Company, 1835, Dueber Avenue, S.W., P.O. Box 6930, Canton, OH 44706-0930, USA
b
Received 16 June 2001; accepted 2 July 2001
Abstract This paper presents the microstructure and hardness of composite solders obtained by the addition of nanopowders of copper to a conventional solder. Copper powders-reinforced lead (Pb) – tin (Sn) composite solders were prepared by thoroughly blending nano-sized copper powders (average powder particle size 100 nm) with a powder of a eutectic solder and using a water-soluble flux. The blended solder paste was melted and allowed to re-solidify in a crucible placed on a hot plate and maintained at a constant temperature. Optical microscopy observations revealed the as-solidified microstructure of the composite solder to be altered by the addition of nanopowders to the eutectic Sn – Pb solder. The copper powders precipitated as intermetallic compounds that were non-uniformly distributed through the microstructure. Microhardness measurements revealed a 30 – 40% increase in hardness of the composite solder over the conventional unreinforced eutectic counterpart. D 2002 Published by Elsevier Science B.V. Keywords: Sn – Pb eutectic; Copper nanopowders; Composite solder; Microstructure; Microhardness
1. Introduction The technique of soldering finds extensive use in the electronics industry for purposes of providing mechanical support and electrical connection for electronic components on a printed circuit board. In particular, the use of solders in optoelectronic packaging offers advantages over the user of adhesives, specifically, (a) the passive alignment of components using surface tension forces of the molten solder, (b) high thermal conduction, and (c) superior dimensional
*
Corresponding author. Tel.: +1-330-972-7731; fax: +1-330972-6027. E-mail address: [email protected] (T.S. Srivatsan).
stability at elevated temperatures [1]. Through the years, the materials that have been widely used for conventional soldering are metal alloy mixtures of lead – tin (Pb:Sn), silver – tin (Ag:Sn) and tin – zinc (Sn:Zn), to name a few [2]. For surface mount technology, especially for optical and opto-electronic devices, solders having improved mechanical properties are required for application that demand high reliability and dimensional stability [3,4]. Also, progressive technological demands in the area of device packaging have created a need to move beyond conventional solders. An attractive and potentially viable method of enhancing the mechanical properties of a solder is by using composite solders formed by the addition of reinforcement particles to a conventional solder alloy [5,6].
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 0 3 - 1
334
D. Lin et al. / Materials Letters 53 (2002) 333–338
In the time spanning the most recent decade a variety of particle reinforcements have been tried in an attempt to engineer composite solders [5,6]. A large majority of the reinforcements are either pure metallic or intermetallic compounds. In a recent study, researchers at International Business Machine Company (IBM) developed composite solders by blending together molybdenum (average size 200 mesh) or tantalum powders (average size 200 mesh) with lead (50%) – tin (50%) solder powder mixture. The resultant composite solder was found to exhibit increased bonding strength with the metal substrate [5]. Around the same time Marshal et al. [6] developed a composite solder by mixing fine particles of the powdered intermetallic (type: Cu6Sn5) with a lead – tin solder [60/40] mixture. Sastry et al. [7] produced composite solder powders using the technique of hot gas atomization of a melt of eutectic Sn– Pb solder mixed with trace amount of copper. The resultant dispersion-strengthened composite solder was found to exhibit improved mechanical properties over the conventional (Sn – Pb) eutectic counterpart. It was observed that the copper reacted with the tin in the eutectic solder paste to form an intermetallic compound that was dispersed randomly through the microstructure [7]. The dispersoids altered the mechanical properties by: (a) pinning grain boundaries and thus impeding grain boundary sliding, and (b) restricting the movement of dislocations. To achieve this the reinforcing particles must be sufficiently fine, stable with respect to size and interparticle spacing, are undeformable, and resistant to fracture. The dispersoids also altered the recrystallization kinetics such that the improved mechanical properties were maintained during thermal cycle. Varying the amount of the reinforcement (copper nanopowders) altered the mechanical properties of the resultant composite solder [2]. However, beyond a certain percentage, the endurance and reliability of the composite solder will begin to decrease even though the strength and stiffness show marginal improvement over the unreinforced counterpart [5,6]. Reno and Panumo [8] analyzed the phases and relative amounts of submicron size dispersoid particles within bulk dispersion-strengthened composite solder powder mixture. They found for additions up to 7.5 wt.% Cu to the eutectic melt, all of copper was converted to the intermetallic compound Cu6Sn5 [9,10]. Marshall and Calderon [11] evaluated a series of composite solders
based on 63%Sn/37%Pb mixture, and identified the second phase particles in the composite microstructure for 10 , 20 and 30 wt.% copper to be primarily Cu6Sn5 and Cu3Sn. They also found the second-phase particles to exhibit excellent binding with the solder matrix. For the pure copper powder reinforcing Sn –Pb solder, the copper particles precipitated as rod-like lamellae, which served to modify the microstructure of the composite solder. This paper presents the outcome of a study aimed at mixing nano-size copper powders with eutectic tin/ lead solder powders to form a composite solder. The influence of weight percent of nanopowder reinforcements on microstructural development and hardness of the composite solder is detailed.
2. Experimental procedures 2.1. Material and sample preparation A commercial water soluble RMA flux and solder powders (average diameter of 74 Am) having the eutectic (63%Sn – 37%Pb) composition were thoroughly blended with pure copper nanopowders (average powder particle size 100 nm). The composite solder mixture was prepared by blending pre-weighed solder (Sn –Pb) powders with pure copper nanopowders. Three different weight percentages, i.e., 1 wt.% Cu, 2 wt.% Cu, and 5 wt.% Cu, were tried. To the composite powder mixture was added a water-soluble RMA flux (about 10 wt.%). The resultant mixture was stirred in a ceramic crucible for a full 30 min, so as to ensure a homogeneous distribution of the reinforcing nanopowders in the eutectic. The composite solder paste was taken in a crucible, which is placed on an aluminum plate. The aluminum plate is placed over a hot plate maintained at a temperature of 250 BC. Once the solder mixture begins to melt the power to the hot plate was turned off. The composite mixture was cooled to room temperature. To minimize oxidation, argon was used as shroud gas and allowed to flow over the aluminum crucible. The entire system was encased in a box-like unit. The composite samples were cleaned in aqueous hydrochloric acid solution so as to remove flux, surface oxide and contaminates. Ultrasonic cleaning in isopropanol and drying in ambient air followed this.
D. Lin et al. / Materials Letters 53 (2002) 333–338
2.2. Microstructural characterization and indentation testing The as-solidified samples were prepared for micrographic examination in a light optical microscope, so as to: (a) identify the presence and distribution of second phases in the eutectic microstructure, (b) identify the nature, presence and distribution of microscopic defects. Typically, these defect features include (a) microscopic surface cracks, and (b) small cavities such as fine microscopic pores and voids. Sample preparation involved an initial wet grind and coarse polish on progressively finer grades of silicon carbide impregnated emery paper using copious amounts of water both as a coolant and lubricant. The samples were then fine polished using 5- and 1-Am alumina suspended in distilled water. Finish polishing to mirror finish was achieved using 3- and 1-Am diamond paste with water as the lubricant. Polishing aids in reducing both the size and abundance of surface flaws, but does not remove any of the flaws. The as-polished samples were chemically etched using a solution mixture of nitric acid (2 ml) and ethanol (98 ml). The polished and etched samples were observed in an optical microscope with the objective of determining (a) the presence, distribution and morphology of the second-phase particles, (b) size and morphology of the grains, and (c) location of microscopic pores in the microstructure. These features were recorded by photographing using a standard bright-field illumination technique. The mechanical property of a metallic material is frequently quantified by its hardness. The value of
335
hardness provides a measure of the resistance of the material to deformation, densification, and cracking. The Knoop microhardness (Hk) of the composite solder samples was measured using a Buehler Micromet II Microhardness tester. The machine makes an indent or impression whose diagonal size is measured with an attached optical microscope. The indentation load used was 10 g. Higher loads can: (a) cause problems as a direct result of load dependence of hardness, and (b) promote the occurrence of localized microscopic cracking. To minimize uncertainties from measurement and cracking of the as-consolidated composite solder samples, an indentation load of 10 g for a dwell time of 10 s was chosen. At least four indents were made on polished surfaces of each composite sample, and the result is reported as the average value in units of kg/mm2 and GPa.
3. Results and discussion Fig. 1 is an optical micrograph showing the microstructure of the 63%Sn –37%Pb solders in the as-cast condition. The features observed are quite typical of a eutectic tin –lead solder and consist of fine alternating lamellae of the two constituent phases. In this micrographs, the light regions are representative of the tin-rich phase, while the dark regions represent the lead-rich phase. The grains were near spherical in morphology with an average size of about 100 Am. Addition of a small percentage of copper nanopowders was observed to alter the optical microstruc-
Fig. 1. Optical micrograph showing the microstructure of the as-cast 63%Sn – 37%Pb solder.
336
D. Lin et al. / Materials Letters 53 (2002) 333–338
ture of the eutectic solder, in the as-solidified condition, and is shown in Fig. 2. The second-phase particles have a rod-like morphology and were identified in earlier studies to be tin – copper intermetallic compound (type: Cu6Sn5) [9,10]. The amount (i.e., volume fraction) of the second-phase particles (Cu6Sn5) increases with an increase in the addition of copper nanopowders to the eutectic solder. Also observed was the size of the second-phase particles decreased with an increase in concentration of the reinforcing copper nanopowders. The average length of the second-phase particles was: (a) 50 Am for 1wt.% Cu, (b) 35 Am for 2-wt.% Cu, and (c) 10 Am for 5-wt.% Cu. Also, the reinforcing second phase particles revealed a non-uniform dispersion through the solder microstructure with degree of randomness
increasing with an increase in the amount of copper nanopowders (Fig. 3). The introduction of copper nanopowders as a reinforcing phase to a eutectic solder mixture also results in the formation and presence of fine microscopic voids in the solidified end product. Formation and presence of a large amount of a microscopic voids in the solidified composite solder is attributed to be due to the gas bubbles being entrapped in the bulk solder mixture during solidification. Marshall and Calderon [11] in their study on micro-sized copper powder-reinforced composite solders also observed a similar phenomenon. Efforts are being made to alter the cooling kinetics so as to either totally eliminate or at best minimize the microscopic voids and pores in the solidified end product.
Fig. 2. Influence of weight percent of copper nanopowders on the microstructure of the 63%Sn – 37%Pb composite solder.
D. Lin et al. / Materials Letters 53 (2002) 333–338
337
Fig. 3. A comparison of the size of the second-phase intermetallic particle (Cu6Sn5) in the microstructure of the composite solder.
The microhardness of the samples is summarized in Table 1 and compared in the bar graph of Fig. 4. Results reveal the microhardness of the solidified
composite solder to increase by about 40% for 5wt.% addition of copper nanopowders to a eutectic solder mixture. The observed increase in hardness of
Table 1 Knoop Hardness measurements Sample #
Addition
Hardness indentation resultsa
1
Nil
2
1.0% Cu
3
2.0% Cu
4
5.0% Cu
D (Am) Hk D (Am) Hk D (Am) Hk D (Am) Hk
a
Values at 10 g load and dwell time of 10 s.
Trial 1
Trial 2
Trial 3
Trial 4
Average
90.8 17.3 86.7 18.9 86.4 19.1 76.9 24.1
100.9 14.0 87.2 18.7 82.5 20.9 82.5 20.9
93.8 16.2 93.3 16.3 83.0 20.7 82.4 21.0
90.8 14.8 84.2 20.1 85.8 19.3 78.6 23.0
0.153 GPa 15.6 kg/mm2 0.181 GPa 18.5 kg/mm2 0.196 GPa 20.0 kg/mm2 0.219 GPa 22.3 kg/mm2
338
D. Lin et al. / Materials Letters 53 (2002) 333–338
Acknowledgements Dr. G.X. Wang graciously acknowledges the financial support provided by the Goodyear Research Initiative Fellowship, the University of Akron (Summer Faculty Research Grant) and the National Science Foundation (Grant Number: NSF 01).
References
Fig. 4. Influence of weight percent of copper nanopowders on the hardness of the composite solder.
the composite solder is attributed to the presence of intermetallic phases (type: Cu6Sn5) in the microstructure.
4. Conclusion Optical microscopy observations and microhardness measurements reveal that the addition of copper nanopowders to a eutectic Pb – Sn is beneficial in enhancing strength. This is essentially due to the precipitation and presence of the second-phase intermetallic particles (type: Cu6Sn5). An increase in the weight percentage of copper nanopowders in the eutectic solder causes the size of the second-phase intermetallic (Cu6Sn5) particle to become smaller with a concurrent increase in the amount and resulting in a non-uniform dispersion through the as-solidified microstructure.
[1] R.A. Boudreau, J. Met. 46 (6) (1994) 41. [2] S.-P. Yu, H.-C. Wang, M.-H. Hon, M.-C. Wang, J. Met. (2000) 36 – 38, June. [3] H. Mavoori, Dimensionally stable solders for optoelectronics and microelectronics, J. Met. (2000) 30 – 32, June. [4] Y. Wu, J.A. Sees, C. Pouraghabagher, L.A. Foster, J.L. Marshall, E.G. Jacobs, R.F. Pinizzotto, The formation and growth of intermetallics in composite solder. J. Electron. Mater. 22 (7) (1993) 769 – 777. [5] Composite solders, IBM Tech. Discl. Bull. 29 (4) (1986) 1573 – 1574. [6] J.L. Marshall, J. Calderon, J. Sees, G. Lucey, J.S. Hwang, Composite Solders. IEEE Trans. Compon., Hybrids, Manuf. Technol. 14 (4) (1991) 698 – 702. [7] S.M.L. Sastry, D.R. Frear, G. Kuo, K.L. Jerina, The properties of composite solders, in: F.G. Yost, F.M. Hosking, D.R. Frear (Eds.), The Mechanics of Solder Wetting and Spreading. Van Nostrand Reinhold, New York, 1993. [8] C. Reno, M.J. Panunto, A mossbauer study of tin-based intermetallics formed during the manufacture of dispersionstrengthened composite solders. J. Electron. Mater. 26 (1) (1997) 12. [9] J.A. Wasynczuk, G.K. Lucey, Proceedings NEPCON West. Cahners Exhibition Group, Des Plains, IL, 1992, pp. 1245 – 1255. [10] R.B. Clough, Proceedings NEPCON West. Cahners Exhibition Group, Des Plains, IL, 1992, pp. 1256 – 1265. [11] J.L. Marshall, J. Calderon, Hard-particle reinforced composite solders: Part 1. Micro-characterization. Soldering Surf. Mount Technol. 26 (1997) 22 – 28.
Materials Letters 53 (2002) 333 – 338 www.elsevier.com/locate/matlet
The inf luence of copper nanopowders on microstructure and hardness of lead–tin solder D. Lin a, G.X. Wang a, T.S. Srivatsan a,*, Meslet Al-Hajri a, M. Petraroli b a Department of Mechanical Engineering, The University of Akron, Akron, OH 44325-3903, USA Timken Research, The Timken Company, 1835, Dueber Avenue, S.W., P.O. Box 6930, Canton, OH 44706-0930, USA
b
Received 16 June 2001; accepted 2 July 2001
Abstract This paper presents the microstructure and hardness of composite solders obtained by the addition of nanopowders of copper to a conventional solder. Copper powders-reinforced lead (Pb) – tin (Sn) composite solders were prepared by thoroughly blending nano-sized copper powders (average powder particle size 100 nm) with a powder of a eutectic solder and using a water-soluble flux. The blended solder paste was melted and allowed to re-solidify in a crucible placed on a hot plate and maintained at a constant temperature. Optical microscopy observations revealed the as-solidified microstructure of the composite solder to be altered by the addition of nanopowders to the eutectic Sn – Pb solder. The copper powders precipitated as intermetallic compounds that were non-uniformly distributed through the microstructure. Microhardness measurements revealed a 30 – 40% increase in hardness of the composite solder over the conventional unreinforced eutectic counterpart. D 2002 Published by Elsevier Science B.V. Keywords: Sn – Pb eutectic; Copper nanopowders; Composite solder; Microstructure; Microhardness
1. Introduction The technique of soldering finds extensive use in the electronics industry for purposes of providing mechanical support and electrical connection for electronic components on a printed circuit board. In particular, the use of solders in optoelectronic packaging offers advantages over the user of adhesives, specifically, (a) the passive alignment of components using surface tension forces of the molten solder, (b) high thermal conduction, and (c) superior dimensional
*
Corresponding author. Tel.: +1-330-972-7731; fax: +1-330972-6027. E-mail address: [email protected] (T.S. Srivatsan).
stability at elevated temperatures [1]. Through the years, the materials that have been widely used for conventional soldering are metal alloy mixtures of lead – tin (Pb:Sn), silver – tin (Ag:Sn) and tin – zinc (Sn:Zn), to name a few [2]. For surface mount technology, especially for optical and opto-electronic devices, solders having improved mechanical properties are required for application that demand high reliability and dimensional stability [3,4]. Also, progressive technological demands in the area of device packaging have created a need to move beyond conventional solders. An attractive and potentially viable method of enhancing the mechanical properties of a solder is by using composite solders formed by the addition of reinforcement particles to a conventional solder alloy [5,6].
0167-577X/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 5 7 7 X ( 0 1 ) 0 0 5 0 3 - 1
334
D. Lin et al. / Materials Letters 53 (2002) 333–338
In the time spanning the most recent decade a variety of particle reinforcements have been tried in an attempt to engineer composite solders [5,6]. A large majority of the reinforcements are either pure metallic or intermetallic compounds. In a recent study, researchers at International Business Machine Company (IBM) developed composite solders by blending together molybdenum (average size 200 mesh) or tantalum powders (average size 200 mesh) with lead (50%) – tin (50%) solder powder mixture. The resultant composite solder was found to exhibit increased bonding strength with the metal substrate [5]. Around the same time Marshal et al. [6] developed a composite solder by mixing fine particles of the powdered intermetallic (type: Cu6Sn5) with a lead – tin solder [60/40] mixture. Sastry et al. [7] produced composite solder powders using the technique of hot gas atomization of a melt of eutectic Sn– Pb solder mixed with trace amount of copper. The resultant dispersion-strengthened composite solder was found to exhibit improved mechanical properties over the conventional (Sn – Pb) eutectic counterpart. It was observed that the copper reacted with the tin in the eutectic solder paste to form an intermetallic compound that was dispersed randomly through the microstructure [7]. The dispersoids altered the mechanical properties by: (a) pinning grain boundaries and thus impeding grain boundary sliding, and (b) restricting the movement of dislocations. To achieve this the reinforcing particles must be sufficiently fine, stable with respect to size and interparticle spacing, are undeformable, and resistant to fracture. The dispersoids also altered the recrystallization kinetics such that the improved mechanical properties were maintained during thermal cycle. Varying the amount of the reinforcement (copper nanopowders) altered the mechanical properties of the resultant composite solder [2]. However, beyond a certain percentage, the endurance and reliability of the composite solder will begin to decrease even though the strength and stiffness show marginal improvement over the unreinforced counterpart [5,6]. Reno and Panumo [8] analyzed the phases and relative amounts of submicron size dispersoid particles within bulk dispersion-strengthened composite solder powder mixture. They found for additions up to 7.5 wt.% Cu to the eutectic melt, all of copper was converted to the intermetallic compound Cu6Sn5 [9,10]. Marshall and Calderon [11] evaluated a series of composite solders
based on 63%Sn/37%Pb mixture, and identified the second phase particles in the composite microstructure for 10 , 20 and 30 wt.% copper to be primarily Cu6Sn5 and Cu3Sn. They also found the second-phase particles to exhibit excellent binding with the solder matrix. For the pure copper powder reinforcing Sn –Pb solder, the copper particles precipitated as rod-like lamellae, which served to modify the microstructure of the composite solder. This paper presents the outcome of a study aimed at mixing nano-size copper powders with eutectic tin/ lead solder powders to form a composite solder. The influence of weight percent of nanopowder reinforcements on microstructural development and hardness of the composite solder is detailed.
2. Experimental procedures 2.1. Material and sample preparation A commercial water soluble RMA flux and solder powders (average diameter of 74 Am) having the eutectic (63%Sn – 37%Pb) composition were thoroughly blended with pure copper nanopowders (average powder particle size 100 nm). The composite solder mixture was prepared by blending pre-weighed solder (Sn –Pb) powders with pure copper nanopowders. Three different weight percentages, i.e., 1 wt.% Cu, 2 wt.% Cu, and 5 wt.% Cu, were tried. To the composite powder mixture was added a water-soluble RMA flux (about 10 wt.%). The resultant mixture was stirred in a ceramic crucible for a full 30 min, so as to ensure a homogeneous distribution of the reinforcing nanopowders in the eutectic. The composite solder paste was taken in a crucible, which is placed on an aluminum plate. The aluminum plate is placed over a hot plate maintained at a temperature of 250 BC. Once the solder mixture begins to melt the power to the hot plate was turned off. The composite mixture was cooled to room temperature. To minimize oxidation, argon was used as shroud gas and allowed to flow over the aluminum crucible. The entire system was encased in a box-like unit. The composite samples were cleaned in aqueous hydrochloric acid solution so as to remove flux, surface oxide and contaminates. Ultrasonic cleaning in isopropanol and drying in ambient air followed this.
D. Lin et al. / Materials Letters 53 (2002) 333–338
2.2. Microstructural characterization and indentation testing The as-solidified samples were prepared for micrographic examination in a light optical microscope, so as to: (a) identify the presence and distribution of second phases in the eutectic microstructure, (b) identify the nature, presence and distribution of microscopic defects. Typically, these defect features include (a) microscopic surface cracks, and (b) small cavities such as fine microscopic pores and voids. Sample preparation involved an initial wet grind and coarse polish on progressively finer grades of silicon carbide impregnated emery paper using copious amounts of water both as a coolant and lubricant. The samples were then fine polished using 5- and 1-Am alumina suspended in distilled water. Finish polishing to mirror finish was achieved using 3- and 1-Am diamond paste with water as the lubricant. Polishing aids in reducing both the size and abundance of surface flaws, but does not remove any of the flaws. The as-polished samples were chemically etched using a solution mixture of nitric acid (2 ml) and ethanol (98 ml). The polished and etched samples were observed in an optical microscope with the objective of determining (a) the presence, distribution and morphology of the second-phase particles, (b) size and morphology of the grains, and (c) location of microscopic pores in the microstructure. These features were recorded by photographing using a standard bright-field illumination technique. The mechanical property of a metallic material is frequently quantified by its hardness. The value of
335
hardness provides a measure of the resistance of the material to deformation, densification, and cracking. The Knoop microhardness (Hk) of the composite solder samples was measured using a Buehler Micromet II Microhardness tester. The machine makes an indent or impression whose diagonal size is measured with an attached optical microscope. The indentation load used was 10 g. Higher loads can: (a) cause problems as a direct result of load dependence of hardness, and (b) promote the occurrence of localized microscopic cracking. To minimize uncertainties from measurement and cracking of the as-consolidated composite solder samples, an indentation load of 10 g for a dwell time of 10 s was chosen. At least four indents were made on polished surfaces of each composite sample, and the result is reported as the average value in units of kg/mm2 and GPa.
3. Results and discussion Fig. 1 is an optical micrograph showing the microstructure of the 63%Sn –37%Pb solders in the as-cast condition. The features observed are quite typical of a eutectic tin –lead solder and consist of fine alternating lamellae of the two constituent phases. In this micrographs, the light regions are representative of the tin-rich phase, while the dark regions represent the lead-rich phase. The grains were near spherical in morphology with an average size of about 100 Am. Addition of a small percentage of copper nanopowders was observed to alter the optical microstruc-
Fig. 1. Optical micrograph showing the microstructure of the as-cast 63%Sn – 37%Pb solder.
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D. Lin et al. / Materials Letters 53 (2002) 333–338
ture of the eutectic solder, in the as-solidified condition, and is shown in Fig. 2. The second-phase particles have a rod-like morphology and were identified in earlier studies to be tin – copper intermetallic compound (type: Cu6Sn5) [9,10]. The amount (i.e., volume fraction) of the second-phase particles (Cu6Sn5) increases with an increase in the addition of copper nanopowders to the eutectic solder. Also observed was the size of the second-phase particles decreased with an increase in concentration of the reinforcing copper nanopowders. The average length of the second-phase particles was: (a) 50 Am for 1wt.% Cu, (b) 35 Am for 2-wt.% Cu, and (c) 10 Am for 5-wt.% Cu. Also, the reinforcing second phase particles revealed a non-uniform dispersion through the solder microstructure with degree of randomness
increasing with an increase in the amount of copper nanopowders (Fig. 3). The introduction of copper nanopowders as a reinforcing phase to a eutectic solder mixture also results in the formation and presence of fine microscopic voids in the solidified end product. Formation and presence of a large amount of a microscopic voids in the solidified composite solder is attributed to be due to the gas bubbles being entrapped in the bulk solder mixture during solidification. Marshall and Calderon [11] in their study on micro-sized copper powder-reinforced composite solders also observed a similar phenomenon. Efforts are being made to alter the cooling kinetics so as to either totally eliminate or at best minimize the microscopic voids and pores in the solidified end product.
Fig. 2. Influence of weight percent of copper nanopowders on the microstructure of the 63%Sn – 37%Pb composite solder.
D. Lin et al. / Materials Letters 53 (2002) 333–338
337
Fig. 3. A comparison of the size of the second-phase intermetallic particle (Cu6Sn5) in the microstructure of the composite solder.
The microhardness of the samples is summarized in Table 1 and compared in the bar graph of Fig. 4. Results reveal the microhardness of the solidified
composite solder to increase by about 40% for 5wt.% addition of copper nanopowders to a eutectic solder mixture. The observed increase in hardness of
Table 1 Knoop Hardness measurements Sample #
Addition
Hardness indentation resultsa
1
Nil
2
1.0% Cu
3
2.0% Cu
4
5.0% Cu
D (Am) Hk D (Am) Hk D (Am) Hk D (Am) Hk
a
Values at 10 g load and dwell time of 10 s.
Trial 1
Trial 2
Trial 3
Trial 4
Average
90.8 17.3 86.7 18.9 86.4 19.1 76.9 24.1
100.9 14.0 87.2 18.7 82.5 20.9 82.5 20.9
93.8 16.2 93.3 16.3 83.0 20.7 82.4 21.0
90.8 14.8 84.2 20.1 85.8 19.3 78.6 23.0
0.153 GPa 15.6 kg/mm2 0.181 GPa 18.5 kg/mm2 0.196 GPa 20.0 kg/mm2 0.219 GPa 22.3 kg/mm2
338
D. Lin et al. / Materials Letters 53 (2002) 333–338
Acknowledgements Dr. G.X. Wang graciously acknowledges the financial support provided by the Goodyear Research Initiative Fellowship, the University of Akron (Summer Faculty Research Grant) and the National Science Foundation (Grant Number: NSF 01).
References
Fig. 4. Influence of weight percent of copper nanopowders on the hardness of the composite solder.
the composite solder is attributed to the presence of intermetallic phases (type: Cu6Sn5) in the microstructure.
4. Conclusion Optical microscopy observations and microhardness measurements reveal that the addition of copper nanopowders to a eutectic Pb – Sn is beneficial in enhancing strength. This is essentially due to the precipitation and presence of the second-phase intermetallic particles (type: Cu6Sn5). An increase in the weight percentage of copper nanopowders in the eutectic solder causes the size of the second-phase intermetallic (Cu6Sn5) particle to become smaller with a concurrent increase in the amount and resulting in a non-uniform dispersion through the as-solidified microstructure.
[1] R.A. Boudreau, J. Met. 46 (6) (1994) 41. [2] S.-P. Yu, H.-C. Wang, M.-H. Hon, M.-C. Wang, J. Met. (2000) 36 – 38, June. [3] H. Mavoori, Dimensionally stable solders for optoelectronics and microelectronics, J. Met. (2000) 30 – 32, June. [4] Y. Wu, J.A. Sees, C. Pouraghabagher, L.A. Foster, J.L. Marshall, E.G. Jacobs, R.F. Pinizzotto, The formation and growth of intermetallics in composite solder. J. Electron. Mater. 22 (7) (1993) 769 – 777. [5] Composite solders, IBM Tech. Discl. Bull. 29 (4) (1986) 1573 – 1574. [6] J.L. Marshall, J. Calderon, J. Sees, G. Lucey, J.S. Hwang, Composite Solders. IEEE Trans. Compon., Hybrids, Manuf. Technol. 14 (4) (1991) 698 – 702. [7] S.M.L. Sastry, D.R. Frear, G. Kuo, K.L. Jerina, The properties of composite solders, in: F.G. Yost, F.M. Hosking, D.R. Frear (Eds.), The Mechanics of Solder Wetting and Spreading. Van Nostrand Reinhold, New York, 1993. [8] C. Reno, M.J. Panunto, A mossbauer study of tin-based intermetallics formed during the manufacture of dispersionstrengthened composite solders. J. Electron. Mater. 26 (1) (1997) 12. [9] J.A. Wasynczuk, G.K. Lucey, Proceedings NEPCON West. Cahners Exhibition Group, Des Plains, IL, 1992, pp. 1245 – 1255. [10] R.B. Clough, Proceedings NEPCON West. Cahners Exhibition Group, Des Plains, IL, 1992, pp. 1256 – 1265. [11] J.L. Marshall, J. Calderon, Hard-particle reinforced composite solders: Part 1. Micro-characterization. Soldering Surf. Mount Technol. 26 (1997) 22 – 28.