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Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 803–809
www.materialstoday.com/proceedings
RAMM 2018
Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder A. Nabihah1 and M.S Nurulakmal1* 1
School of Materials and Mineral Resources Engineering, USM Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
Abstract Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: SnCu solder, In addition, microstructure _____________ *Corresponding author. Tel.: +6-04-599-6180; fax: +6-04-594-1011 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
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A. Nabihah and M.S. Nurulakmal / Materials Today: Proceedings 17 (2019) 803–809
1. Introduction Tin-lead (Sn-Pb) solders have been widely used in electronic industry due to a combination of many advantages, such as low melting temperature, economically affordable and excellent wettability. Despite all of these advantages, a rapid switching to Pb-free solders has occurred to replace the toxic Pb-based solder in the packaging process of electronic device and components. The toxicity of lead has been the focus of many discussion since the 1930s. Various published researches have revealed that lead is hazardous not only to the enviroment, but also to human health [1-3]. Driven by these concerns and international legislation, electronic manufacturing companies and researchers have since concentrating their efforts in fabricating lead-free solder to replace the Sn-Pb solder. Indeed, many research groups have focused on developing new Pb-free solders [4-5]. The new composition solder alloy must comply to these requirements such as; economically affordable material, good wettability, suitable melting temperature, excellent mechanical and electrical properties, high corrosion resistence and non-toxic for human health and enviroment [6]. Several tin-based lead-free solder alloys such as Sn-Ag, Sn-Cu, Sn-Au, Sn-Ag-Cu, and Sn-Zn have been developed and applied in the electronic packaging industries [7-8] but none of them meets all standards, which are required material properties (e.g. low melting temperature, wettability, and mechanical properties), good manufacturing, and yet affordable. Processing issue and condition (involving fluxes) are also need to be considered for development of proper alloy composition of lead-free solder. Sn-Cu alloy has currently been proposed as most promising substitute for lead-containing solder for electronic packaging industries especially in wave soldering [9]. This alloy has many benefits compared with Sn-Pb eutectic alloy; it has good manufacturability, good mechanical properties, and stable interfaces with most metallic substrates and surface finishes, as well as nontoxic. However, fabricating lead-free solder that have properties and melting point close to eutectic tin-lead and cost effectiveness has been a great challenge to researchers. The Sn-Cu solder alloys has higher melting point and higher wetting angle compared to Sn-Pb solder. The higher melting point caused higher soldering temperature. Most of the sensitive components and substrate cannot withstand high temperature and pose a risk to the polymer substrate and under fill material. Studies have shown that a potentially and economically affordable approch to improve the properties of Sn-Cu solder alloys is by addition of small amount of low melting alloying element such as Ag and In. Recently, Ag and In-containing Sn-Cu alloys have been reported to demonstrate superior performance, at least in its mechanical strength [10-11]. Sn-0.7Cu solder has been proposed as one of the most promising substitutes for lead-containing solder wave, dip and iron soldering process with inexpensive and good electrical conductivity along with melting temperature of 227°C. However, the main disadvantages of high melting temperature, insufficient oxidation resistance characteristic and short lifetime under several thermal cycling compared to Sn-Pb solder joints prevent its wide use in microelectronic packaging industry. Generally, mechanical creep of soldering material occurs more readily when high temperature are involved. At high temperature the mechanical properties of metal are the result of simultaneous process of strain hardening, due to plastic deformation and softening effect of recovert and recrystallization [12]. Thus, knowledge about the creep behavior of Sn-Cu solder is very important for successful electronic product development. Studies have shown that creep behavior of eutectic Sn-Cu alloys solder with small amount of In and Ag addition potentially improved the creep resistance and lifetime of solder [11]. In, with relatively low melting temperature, is one of the alloying elements used to reduce the melting point of eutectic Sn-0.7Cu. Addition of alloying element for a specific purpose however, should not result in reduction of other properties or give detrimental effect to performance. This paper reports on the teffect of adding In to the properties of Sn-Cu solder. A successful deveopment of solder alloy requires the solder to have good combination of wettability and mechanical properties as these contribute to high relibility of the solder joint. Therefore, this project focuses on various percentage of indium (In) addition to the Sn-Cu solder to investigate the effect on melting behavior, microstructure of the bulk solder, wettability and shear strength of Sn–Cu solders alloy.
A. Nabihah and M.S. Nurulakmal / Materials Today: Proceedings 17 (2019) 803–809
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2. Methodology In this project, commercial Sn-0.7Cu solder alloy patented by Nihon Superior under code of SN100C (Sn0.7Cu-0.05Ni-0.01Ge) was used. Indium (In) was added into the SN100C at 0.5, 1.0, 1.5 and 2.0 wt.%. After weighting according to composition, the solder alloys were then melted in VISTEC VT-MF01-1000 muffle furnace preparing them via casting and then poured into steel mould. Solder samples were then characterized in terms of microstructure by using FESEM equipped with EDX, melting behavior using Differential Scanning Calorimetry (DSC), spreading and contact angle test, and single lap joint shear test. For bulk solder sample preparation, the solder alloys were ground with SiC abrasive paper grit 100 until 2000 before polished using 1 µm alumina powder. Once mirror-like surface was obtained, the samples were chemically etched with 5% HNO3-2% HCl-93% CH4O etching solution to reveal the microstructure before observed using field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDX). Single lap joint shear test is used to measure the shear strength of the solder joints. In this project, pure Cu sheet was cut into a pair of half-dumbbell shape and rolled solder alloy sheet was reflowed in between the gauge-end to make the joint. The Cu strips have thickness of 1.0 mm and the solder was 0.3 mm thick. The Cu substrates were ground using SiC grid paper from 800 and 1000 to remove any oxide layer. Then, the Cu substrates were immersed in acetone for 2 minutes, rinsed using distilled water and dried using an air gun. Solder reflow was then performed using lead free reflow oven at 280°C. Prior to reflow, activated resin (RA) flux was carefully applied on the Cu substrate to assist wetting. After reflowed, lap joint samples were tested using Instron Universal Testing Machine with displacement rate of 1.3 mm/min. 3.
Result And Discussion
3.1 Differential Scanning Calorimetry (DSC) Analysis Table 1 lists the peak temperature for each solder alloy and the melting temperature for SN100C solder alloy was found to be 229.64 °C. The value is slightly higher compared to the conventional Sn-0.7Cu solder alloy which is 227°C [13]. The cause of this difference in melting temperature could be due to the presence of Ni and Ge in SN100C solder. The addition of indium lowered the onset melting point of solder alloy due to the low melting point of pure In, which melts around 157°C. The reduction of onset melting temperature could also be attributed to the surface instability and the variation in physical properties of the grain boundary/interfacial characteristics rendered by the alteration in phases present. Moreover, the addition of In lower the degree of undercooling as the In atoms act as additional nucleating agents during crystallization process, leading to faster solidification of the molten solder. The melting temperature of In-added solders are slightly lower than Sn-Cu and the degree of undercooling (25.12 °C to 22.28 °C for In-added solder) are smaller than that of reported Sn-Cu solders [14] which is (28.5 °C). Therefore, the results of thermal analysis showed positive influence of In on the Sn-Cu solder. Table 1: DSC table for melting temperature, crystallization temperature onset melting temperature, onset crystallization temperature and the degree of undercooling.
Solder alloys
SN100C SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C.2.0In
Onset melting temperature, Tom 229.64 229.16 229.09 228.97 225.40
Onset crystallization temperature, Toc 202.35 204.04 203.49 203.17 203.12
Temperature (°C) Crystallization Melting temperature, Tc temperature, Tm 235.32 234.60 234.38 234.01 232.52
200.68 201.42 201.41 201.33 201.36
Degree of undercooling, Tom-Toc 27.29 25.12 25.60 25.80 22.28
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3.2 Bulk Microstructure The microstructure of (a) SN100C, (b) SN100C-0.5In (c) SN100C-1.0In, (d) SN100C-1.5In and (e) SN100C2.0In bulk solder alloys are presented in Fig. 1. The commercial SN100C consisted of primary β-Sn dendrites and a two-phase eutectic of Cu6Sn5 and (Cu, Ni)6Sn5 phases. The (Cu, Ni)6Sn5 phase is most likely formed due to the presence of Ni, and the Ni atoms doped into Cu6Sn5 forming (Cu, Ni)6Sn5 phases. Cu and Ni atoms are completely soluble, and thus, Ni could easily substitute Cu in Cu6Sn5 compound. Meanwhile, the addition of In can be observed to refine β-Sn grains and the intermetallic compounds are more uniformly distributed. This is due to the adsorption phenomenon during solidification process of an alloy as reported by [15] in which heterogeneous catalysis that take place on the interface between elements. On the other hand, compound InSn4 phase could not be observed in the bulk of In-added solder, despite the addition of up to 2.0 wt%. Based on the Sn-In phase diagram, the solubility limit of In in Sn is about 7 wt%. Therefore, low amount of In added into the solder is expected to be fully dissolved in Snmatrix [16]. In could also be found doped into Cu-Sn IMC, based on EDX analysis which showed Cu-Sn-Ni-In particles (see Fig. 2 (c)). The In-doped Cu-Sn IMC particles were found to be more equiaxed compared to Cu6Sn5 which are more elongated or needle-like. This is shown in Fig. 2 (a) and (b) where higher magnification of IMC particles on SN100C and SN100C added with 2 wt% In samples. The In-doped Cu-Sn IMC particles were smaller compared to Cu-Sn, and addition of In also seems to affect the Cu-Sn-Ni particles, where they are similarly more equiaxed. The more equiaxed IMC particles could positively influence the strength of solder joint as they are better able to impede dislocation motion. Finer particles could also make them more evenly distributed within the bulk solder. The refinement of β-Sn grains along with more equiaxed and evenly distributed In-doped Cu-Sn IMCs are expected to improve the strength of solder joint. c)
b)
a)
d
e
Fig. 1: The microstructure of (a) SN100C, (b) SN100C-0.5In (c) SN100C-1.0In, (d) SN100C-1.5In and (e) SN100C-2.0In bulk solder alloys
A. Nabihah and M.S. Nurulakmal / Materials Today: Proceedings 17 (2019) 803–809
a)
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b)
c) Element NiL InL SnL CuK
Wt% 02.05 01.24 91.23 05.49
At% 03.87 01.20 85.34 09.59
Fig. 2 : (a) Microstructure of SN100C, (b) SN100C with 2 wt% In, (c) EDX point analysis on Cu-Sn-Ni-In particle, 3000x magnification
3.3 Spreading and Wetting Angle From the result, wetting angle and spreading area decreased and increased respectively with addition of In. It is clear that the increasing amount of In gave lower wetting angle and increased spreading area. When the indium content is 2.0 wt.%, the spreading area of solder is 55.12 mm2, about 24.1% larger than that of the Sn-Cu solder without indium. This suggests that indium addition can effectively improve the wettability of SN100C solder. Table 2 shows the spreading area of the solder alloy increased with increasing In content, suggesting that the solders with a lower melting temperature may experience a higher superheating, resulting in the increase of fluidity at the same soldering temperature. Adding In lowered the melting temperature, allowing better fluidity at the soldering temperature, and In could also influence the surface tension enabling better wetting on the surface of Cu substrate. Therefore, adding In can accelerate spreading of the solder on the substrate and improve the wettability. Liu et al. [13] also reported similar trend in their work on spreading area of Sn–0.7Cu–0.2Ni–In solders on copper with increasing addition of indium. Table 2: Wetting angle and spreading test of solder alloys.
Type of solder alloy SN100C SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C.2.0In
Wetting angle, ી (o) 29.58 27.82 27.52 26.01 22.45
Spreading area, A(mm2) 35.94 42.70 43.94 45.97 55.12
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3.4 Single Lap Joint Shear Strength Single lap joint test was performed to reflowed and aged samples (aging at 150°C for 100 hours) under crosshead speed 1.3 mm/min according to ASTM D1002. The results showed that all samples failed by cohesive type of failure at solder joint area. This indicates that good adhesion and complete wetting of the solders to the copper substrate was achieved. From Fig. 3, the shear strength SN100C solder increased as indium content increases from 0.5 to 2.0 wt.%, with the shear strength ranged from 72.07 MPa to 79.09 MPa. This increment although only slightly (increased by 9.7%), was due to the grain structure that was substantially refined with increasing amount of indium addition. Grain refinement that formed with addition of indium promoted higher shear strength of SN100C as higher grain boundary density that can impede the dislocation motion. Referring to Fig. 1, the size of β-Sn in SN100C was small after addition of indium. Similar result was reported by [10] and the In were located either in the eutectic regions or at the boundaries of the β-Sn phases. The In-doped Cu-Sn IMC particles which are finer and more evenly distributed play an important role in pinning the movement of dislocations, resulting in the increase of mechanical properties of solder joint. It is interesting to note that although the increment is only slight with addition of In up to 2 wt%, the ability to maintain the strength after thermal exposure (aging) is obvious. As can be seen in Fig. 3, the strength of 2 wt% In-added sample was only slightly reduced after aging compared to that of in solder without In. This improvement could be the result of better distribution of IMC particles which hold the matrix allowing strength to be maintained after long period of thermal exposure. It also might indicates better stability or slower growth of Indoped Cu-Sn IMC particles during thermal aging. 100
Shear Strength (MPa)
80 60 40 20 0 SN100C
SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C2.0In Reflow Aging
Fig. 3: Comparison of shear strength between reflowed and aged (150°C, 100 hours) solder joint
4. Conclusion This study found that the increase amount of In addition is observed to induce grain refinement of SN100C solder with better wettability and higher shear strength of solder joint. Addition of In had reduce the melting temperature and lower the degree of undercooling in SN100C solder. Also, increasing amount of In gave lower wetting angle and increased spreading area. From the FESEM and EDX analysis, the grain refinement can be observed and In was detected within the β-Sn matrix or doped into the Cu6Sn5 forming Cu-Sn-Ni-In particles. The In-doped IMC particles are finer and more equiaxed, with better distribution within the matrix. The finer β-Sn grains along with finer and more evenly distributed In-doped IMC particles led to higher joint strength as the grain boundaries and IMC particles provided more effective barrier to impede dislocation motions. Acknowledgements The authors would like to acknowledge the funding provided by USM RUI grant (grant no : 1001.PBAHAN.814264).
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D. A. Cory-slechta, B. Weiss, and C. Cox, “Delayed behavioral toxicity of lead with increasing exposure concentration,” Toxicol. Appl. Pharmacol., vol. 71, no. 3, p. Pages 342-352, 1983. N. F. S. P.H. Davies, J.P. Goettl Jr., J.R. Sinley, “Acute and chronic toxicity of lead to rainbow trout salmo gairdneri, in hard and soft water,” Water Res., vol. 10, no. 3, pp. 199–206, 1976. R. Wassink, Lead-Free Soldering in electronics. 1989. A. A. El-Daly, F. El-Tantawy, A. E. Hammad, M. S. Gaafar, E. H. El-Mossalamy, and A. A. Al-Ghamdi, “Structural and elastic properties of eutectic Sn-Cu lead-free solder alloy containing small amount of Ag and in,” J. Alloys Compd., vol. 509, no. 26, pp. 7238–7246, 2011. S. G. Keller J., Baither D., Wilke U., “Mechanical properties of Pb-free SnAg solder joints,” Acta Mater., vol. 59, no. 7, pp. 2731–2741, 2011. A. K. Gain, T. Fouzder, Y. C. Chan, A. Sharif, and W. K. C. Yung, “Investigation of small Sn-3.5Ag-0.5Cu additions on the microstructure and properties of Sn-8Zn-3Bi solder on Au/Ni/Cu pads,” J. Alloys Compd., vol. 489, no. 2, pp. 678–684, 2010. S. Wiese and K. J. Wolter, “Microstructure and creep behaviour of eutectic SnAg and SnAgCu solders,” Microelectron. Reliab., vol. 44, no. 12, pp. 1923–1931, 2004. L. C. Tsao, “Suppressing effect of 0.5wt.% nano-TiO2 addition into Sn–3.5Ag–0.5Cu solder alloy on the intermetallic growth with Cu substrate during isothermal aging,” J. Alloys Compd., vol. 509, no. 33, pp. 8441–8448, 2011. X. Li, F. Zhang, F. Zu, X. Lv, Z. Zhao, and D. Yang, “Effect of liquid-liquid structure transition on solidification and wettability of Sn0.7Cu solder,” J. Alloys Compd., vol. 505, no. 2, pp. 472–475, 2010. A. A. El-Daly and A. E. Hammad, “Development of high strength Sn-0.7Cu solders with the addition of small amount of Ag and in,” J. Alloys Compd., vol. 509, no. 34, pp. 8554–8560, 2011. A. A. El-Daly and A. E. Hammad, “Enhancement of creep resistance and thermal behavior of eutectic Sn-Cu lead-free solder alloy by Ag and In-additions,” Mater. Des., vol. 40, pp. 292–298, 2012. S. E. Negm, H. Mady, and A. A. Bahgat, “Influence of the addition of indium on the mechanical creep of Sn-3.5%Ag alloy,” J. Alloys Compd., vol. 503, no. 1, pp. 65–70, 2010. J. C. Liu, G. Zhang, Z. H. Wang, J. S. Ma, and K. Suganuma, “Thermal property, wettability and interfacial characterization of novel SnZn-Bi-In alloys as low-temperature lead-free solders,” Mater. Des., vol. 84, pp. 331–339, 2015. X. Hu, Y. Li, Y. Liu, and Z. Min, “Developments of high strength Bi-containing Sn0.7Cu lead-free solder alloys prepared by directional solidification,” J. Alloys Compd., 2014. K. Kanlayasiri, M. Mongkolwongrojn, and T. Ariga, “Influence of indium addition on characteristics of Sn-0.3Ag-0.7Cu solder alloy,” J. Alloys Compd., vol. 485, no. 1–2, pp. 225–230, 2009. F. Rodrigues, J. Watson, S. Liu, and J. C. Madeni, “Study of intermetallic compounds ( IMC ) that form between indium-enriched SAC solder alloys and copper substrate,” Welding in the World, Le Soudage Dans Le Monde 61(9):1-9, pp. 603–611, 2017.
ScienceDirect Materials Today: Proceedings 17 (2019) 803–809
www.materialstoday.com/proceedings
RAMM 2018
Effect of In Addition on Microstructure, Wettability and Strength of SnCu Solder A. Nabihah1 and M.S Nurulakmal1* 1
School of Materials and Mineral Resources Engineering, USM Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
Abstract Since the restriction on the use of Pb in solders, several Pb-free solders have been developed to replace the traditional Sn-Pb eutectic solder. The Sn-Ag-Cu series are the most commonly used lead-free solders in the electronics industry. However, during isothermal aging, the brittle Ag3Sn phase can strongly reduce the reliability of solder joints. In addition, the relatively high cost of Sn-Ag-Cu solder limited its application in the electronics industry. An attractive alternative is the Sn-0.7Cu alloy, which has shown good wettability and reasonable cost advantage compared to the Sn-Ag-Cu alloy. Despite the advantages, the melting temperature of Sn-0.7Cu alloy is approximately 10°C higher than that of Sn-Ag-Cu alloy and coarse precipitation of Cu6Sn5 during isothermal aging has been observed. This paper reports the investigation on indium addition into commercial Sn-Cu solder (SN100C) to improve its microstructure, wettability and strength performance. The solder alloys used in this work were SN100C (0.7 wt% Cu, 0.05 wt% Ni, 0.01 wt% Ge and bal. Sn) and SN100C added with 0.5, 1.0, 1.5 and 2.0wt% indium. The addition of indium is expected to refine the β-Sn grains and contribute to higher solder strength. The microstructure of bulk solder was observed using SEM equipped with EDX, while single lap joint shear strength is performed to evaluate joint strength. The wettability of solder alloy was improved with addition of In, observed via higher spreading diameter and lower wetting angle. Microstructure observation showed that In addition refined β-Sn structure, and doping of In into Cu-Sn IMC resulted in fine and rounded shape Cu-Sn-Ni-In particles. Shear strength of Sn-Cu solder joints were increased with increasing In content. The increase could be attributed to the grain structure which was substantially refined with increasing amount of In addition. The smaller grains that formed with addition of In lead to higher grain boundary density that can impede dislocation motion resulting in higher shear strength. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018. Keywords: SnCu solder, In addition, microstructure _____________ *Corresponding author. Tel.: +6-04-599-6180; fax: +6-04-594-1011 E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the 6th International Conference on Recent Advances in Materials, Minerals & Environment (RAMM) 2018.
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A. Nabihah and M.S. Nurulakmal / Materials Today: Proceedings 17 (2019) 803–809
1. Introduction Tin-lead (Sn-Pb) solders have been widely used in electronic industry due to a combination of many advantages, such as low melting temperature, economically affordable and excellent wettability. Despite all of these advantages, a rapid switching to Pb-free solders has occurred to replace the toxic Pb-based solder in the packaging process of electronic device and components. The toxicity of lead has been the focus of many discussion since the 1930s. Various published researches have revealed that lead is hazardous not only to the enviroment, but also to human health [1-3]. Driven by these concerns and international legislation, electronic manufacturing companies and researchers have since concentrating their efforts in fabricating lead-free solder to replace the Sn-Pb solder. Indeed, many research groups have focused on developing new Pb-free solders [4-5]. The new composition solder alloy must comply to these requirements such as; economically affordable material, good wettability, suitable melting temperature, excellent mechanical and electrical properties, high corrosion resistence and non-toxic for human health and enviroment [6]. Several tin-based lead-free solder alloys such as Sn-Ag, Sn-Cu, Sn-Au, Sn-Ag-Cu, and Sn-Zn have been developed and applied in the electronic packaging industries [7-8] but none of them meets all standards, which are required material properties (e.g. low melting temperature, wettability, and mechanical properties), good manufacturing, and yet affordable. Processing issue and condition (involving fluxes) are also need to be considered for development of proper alloy composition of lead-free solder. Sn-Cu alloy has currently been proposed as most promising substitute for lead-containing solder for electronic packaging industries especially in wave soldering [9]. This alloy has many benefits compared with Sn-Pb eutectic alloy; it has good manufacturability, good mechanical properties, and stable interfaces with most metallic substrates and surface finishes, as well as nontoxic. However, fabricating lead-free solder that have properties and melting point close to eutectic tin-lead and cost effectiveness has been a great challenge to researchers. The Sn-Cu solder alloys has higher melting point and higher wetting angle compared to Sn-Pb solder. The higher melting point caused higher soldering temperature. Most of the sensitive components and substrate cannot withstand high temperature and pose a risk to the polymer substrate and under fill material. Studies have shown that a potentially and economically affordable approch to improve the properties of Sn-Cu solder alloys is by addition of small amount of low melting alloying element such as Ag and In. Recently, Ag and In-containing Sn-Cu alloys have been reported to demonstrate superior performance, at least in its mechanical strength [10-11]. Sn-0.7Cu solder has been proposed as one of the most promising substitutes for lead-containing solder wave, dip and iron soldering process with inexpensive and good electrical conductivity along with melting temperature of 227°C. However, the main disadvantages of high melting temperature, insufficient oxidation resistance characteristic and short lifetime under several thermal cycling compared to Sn-Pb solder joints prevent its wide use in microelectronic packaging industry. Generally, mechanical creep of soldering material occurs more readily when high temperature are involved. At high temperature the mechanical properties of metal are the result of simultaneous process of strain hardening, due to plastic deformation and softening effect of recovert and recrystallization [12]. Thus, knowledge about the creep behavior of Sn-Cu solder is very important for successful electronic product development. Studies have shown that creep behavior of eutectic Sn-Cu alloys solder with small amount of In and Ag addition potentially improved the creep resistance and lifetime of solder [11]. In, with relatively low melting temperature, is one of the alloying elements used to reduce the melting point of eutectic Sn-0.7Cu. Addition of alloying element for a specific purpose however, should not result in reduction of other properties or give detrimental effect to performance. This paper reports on the teffect of adding In to the properties of Sn-Cu solder. A successful deveopment of solder alloy requires the solder to have good combination of wettability and mechanical properties as these contribute to high relibility of the solder joint. Therefore, this project focuses on various percentage of indium (In) addition to the Sn-Cu solder to investigate the effect on melting behavior, microstructure of the bulk solder, wettability and shear strength of Sn–Cu solders alloy.
A. Nabihah and M.S. Nurulakmal / Materials Today: Proceedings 17 (2019) 803–809
805
2. Methodology In this project, commercial Sn-0.7Cu solder alloy patented by Nihon Superior under code of SN100C (Sn0.7Cu-0.05Ni-0.01Ge) was used. Indium (In) was added into the SN100C at 0.5, 1.0, 1.5 and 2.0 wt.%. After weighting according to composition, the solder alloys were then melted in VISTEC VT-MF01-1000 muffle furnace preparing them via casting and then poured into steel mould. Solder samples were then characterized in terms of microstructure by using FESEM equipped with EDX, melting behavior using Differential Scanning Calorimetry (DSC), spreading and contact angle test, and single lap joint shear test. For bulk solder sample preparation, the solder alloys were ground with SiC abrasive paper grit 100 until 2000 before polished using 1 µm alumina powder. Once mirror-like surface was obtained, the samples were chemically etched with 5% HNO3-2% HCl-93% CH4O etching solution to reveal the microstructure before observed using field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray (EDX). Single lap joint shear test is used to measure the shear strength of the solder joints. In this project, pure Cu sheet was cut into a pair of half-dumbbell shape and rolled solder alloy sheet was reflowed in between the gauge-end to make the joint. The Cu strips have thickness of 1.0 mm and the solder was 0.3 mm thick. The Cu substrates were ground using SiC grid paper from 800 and 1000 to remove any oxide layer. Then, the Cu substrates were immersed in acetone for 2 minutes, rinsed using distilled water and dried using an air gun. Solder reflow was then performed using lead free reflow oven at 280°C. Prior to reflow, activated resin (RA) flux was carefully applied on the Cu substrate to assist wetting. After reflowed, lap joint samples were tested using Instron Universal Testing Machine with displacement rate of 1.3 mm/min. 3.
Result And Discussion
3.1 Differential Scanning Calorimetry (DSC) Analysis Table 1 lists the peak temperature for each solder alloy and the melting temperature for SN100C solder alloy was found to be 229.64 °C. The value is slightly higher compared to the conventional Sn-0.7Cu solder alloy which is 227°C [13]. The cause of this difference in melting temperature could be due to the presence of Ni and Ge in SN100C solder. The addition of indium lowered the onset melting point of solder alloy due to the low melting point of pure In, which melts around 157°C. The reduction of onset melting temperature could also be attributed to the surface instability and the variation in physical properties of the grain boundary/interfacial characteristics rendered by the alteration in phases present. Moreover, the addition of In lower the degree of undercooling as the In atoms act as additional nucleating agents during crystallization process, leading to faster solidification of the molten solder. The melting temperature of In-added solders are slightly lower than Sn-Cu and the degree of undercooling (25.12 °C to 22.28 °C for In-added solder) are smaller than that of reported Sn-Cu solders [14] which is (28.5 °C). Therefore, the results of thermal analysis showed positive influence of In on the Sn-Cu solder. Table 1: DSC table for melting temperature, crystallization temperature onset melting temperature, onset crystallization temperature and the degree of undercooling.
Solder alloys
SN100C SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C.2.0In
Onset melting temperature, Tom 229.64 229.16 229.09 228.97 225.40
Onset crystallization temperature, Toc 202.35 204.04 203.49 203.17 203.12
Temperature (°C) Crystallization Melting temperature, Tc temperature, Tm 235.32 234.60 234.38 234.01 232.52
200.68 201.42 201.41 201.33 201.36
Degree of undercooling, Tom-Toc 27.29 25.12 25.60 25.80 22.28
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3.2 Bulk Microstructure The microstructure of (a) SN100C, (b) SN100C-0.5In (c) SN100C-1.0In, (d) SN100C-1.5In and (e) SN100C2.0In bulk solder alloys are presented in Fig. 1. The commercial SN100C consisted of primary β-Sn dendrites and a two-phase eutectic of Cu6Sn5 and (Cu, Ni)6Sn5 phases. The (Cu, Ni)6Sn5 phase is most likely formed due to the presence of Ni, and the Ni atoms doped into Cu6Sn5 forming (Cu, Ni)6Sn5 phases. Cu and Ni atoms are completely soluble, and thus, Ni could easily substitute Cu in Cu6Sn5 compound. Meanwhile, the addition of In can be observed to refine β-Sn grains and the intermetallic compounds are more uniformly distributed. This is due to the adsorption phenomenon during solidification process of an alloy as reported by [15] in which heterogeneous catalysis that take place on the interface between elements. On the other hand, compound InSn4 phase could not be observed in the bulk of In-added solder, despite the addition of up to 2.0 wt%. Based on the Sn-In phase diagram, the solubility limit of In in Sn is about 7 wt%. Therefore, low amount of In added into the solder is expected to be fully dissolved in Snmatrix [16]. In could also be found doped into Cu-Sn IMC, based on EDX analysis which showed Cu-Sn-Ni-In particles (see Fig. 2 (c)). The In-doped Cu-Sn IMC particles were found to be more equiaxed compared to Cu6Sn5 which are more elongated or needle-like. This is shown in Fig. 2 (a) and (b) where higher magnification of IMC particles on SN100C and SN100C added with 2 wt% In samples. The In-doped Cu-Sn IMC particles were smaller compared to Cu-Sn, and addition of In also seems to affect the Cu-Sn-Ni particles, where they are similarly more equiaxed. The more equiaxed IMC particles could positively influence the strength of solder joint as they are better able to impede dislocation motion. Finer particles could also make them more evenly distributed within the bulk solder. The refinement of β-Sn grains along with more equiaxed and evenly distributed In-doped Cu-Sn IMCs are expected to improve the strength of solder joint. c)
b)
a)
d
e
Fig. 1: The microstructure of (a) SN100C, (b) SN100C-0.5In (c) SN100C-1.0In, (d) SN100C-1.5In and (e) SN100C-2.0In bulk solder alloys
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a)
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b)
c) Element NiL InL SnL CuK
Wt% 02.05 01.24 91.23 05.49
At% 03.87 01.20 85.34 09.59
Fig. 2 : (a) Microstructure of SN100C, (b) SN100C with 2 wt% In, (c) EDX point analysis on Cu-Sn-Ni-In particle, 3000x magnification
3.3 Spreading and Wetting Angle From the result, wetting angle and spreading area decreased and increased respectively with addition of In. It is clear that the increasing amount of In gave lower wetting angle and increased spreading area. When the indium content is 2.0 wt.%, the spreading area of solder is 55.12 mm2, about 24.1% larger than that of the Sn-Cu solder without indium. This suggests that indium addition can effectively improve the wettability of SN100C solder. Table 2 shows the spreading area of the solder alloy increased with increasing In content, suggesting that the solders with a lower melting temperature may experience a higher superheating, resulting in the increase of fluidity at the same soldering temperature. Adding In lowered the melting temperature, allowing better fluidity at the soldering temperature, and In could also influence the surface tension enabling better wetting on the surface of Cu substrate. Therefore, adding In can accelerate spreading of the solder on the substrate and improve the wettability. Liu et al. [13] also reported similar trend in their work on spreading area of Sn–0.7Cu–0.2Ni–In solders on copper with increasing addition of indium. Table 2: Wetting angle and spreading test of solder alloys.
Type of solder alloy SN100C SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C.2.0In
Wetting angle, ી (o) 29.58 27.82 27.52 26.01 22.45
Spreading area, A(mm2) 35.94 42.70 43.94 45.97 55.12
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3.4 Single Lap Joint Shear Strength Single lap joint test was performed to reflowed and aged samples (aging at 150°C for 100 hours) under crosshead speed 1.3 mm/min according to ASTM D1002. The results showed that all samples failed by cohesive type of failure at solder joint area. This indicates that good adhesion and complete wetting of the solders to the copper substrate was achieved. From Fig. 3, the shear strength SN100C solder increased as indium content increases from 0.5 to 2.0 wt.%, with the shear strength ranged from 72.07 MPa to 79.09 MPa. This increment although only slightly (increased by 9.7%), was due to the grain structure that was substantially refined with increasing amount of indium addition. Grain refinement that formed with addition of indium promoted higher shear strength of SN100C as higher grain boundary density that can impede the dislocation motion. Referring to Fig. 1, the size of β-Sn in SN100C was small after addition of indium. Similar result was reported by [10] and the In were located either in the eutectic regions or at the boundaries of the β-Sn phases. The In-doped Cu-Sn IMC particles which are finer and more evenly distributed play an important role in pinning the movement of dislocations, resulting in the increase of mechanical properties of solder joint. It is interesting to note that although the increment is only slight with addition of In up to 2 wt%, the ability to maintain the strength after thermal exposure (aging) is obvious. As can be seen in Fig. 3, the strength of 2 wt% In-added sample was only slightly reduced after aging compared to that of in solder without In. This improvement could be the result of better distribution of IMC particles which hold the matrix allowing strength to be maintained after long period of thermal exposure. It also might indicates better stability or slower growth of Indoped Cu-Sn IMC particles during thermal aging. 100
Shear Strength (MPa)
80 60 40 20 0 SN100C
SN100C.0.5In SN100C.1.0In SN100C.1.5In SN100C2.0In Reflow Aging
Fig. 3: Comparison of shear strength between reflowed and aged (150°C, 100 hours) solder joint
4. Conclusion This study found that the increase amount of In addition is observed to induce grain refinement of SN100C solder with better wettability and higher shear strength of solder joint. Addition of In had reduce the melting temperature and lower the degree of undercooling in SN100C solder. Also, increasing amount of In gave lower wetting angle and increased spreading area. From the FESEM and EDX analysis, the grain refinement can be observed and In was detected within the β-Sn matrix or doped into the Cu6Sn5 forming Cu-Sn-Ni-In particles. The In-doped IMC particles are finer and more equiaxed, with better distribution within the matrix. The finer β-Sn grains along with finer and more evenly distributed In-doped IMC particles led to higher joint strength as the grain boundaries and IMC particles provided more effective barrier to impede dislocation motions. Acknowledgements The authors would like to acknowledge the funding provided by USM RUI grant (grant no : 1001.PBAHAN.814264).
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