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electroplated Ni substrates
Microelectronics Reliability 51 (2011) 636–641
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Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effect of Cr additions on interfacial reaction between the Sn–Zn–Bi solder and Cu/electroplated Ni substrates Jinglin Bi, Anmin Hu ⇑, Jing Hu, Tingbi Luo, Ming Li, Dali Mao State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 17 September 2010 Accepted 17 September 2010 Available online 20 October 2010
a b s t r a c t Intermetallic compounds (IMCs) growth on the Sn–8Zn–3Bi (–Cr) solder joints with Cu and electroplated Ni substrates was investigated after aging at 150 °C. It was found that the IMCs were the Cu5Zn8 and Ni5Zn21 at the solder/Cu and solder/Ni interface, respectively. The IMCs growth rate at the Sn–8Zn– 3Bi–Cr/Cu and Ni interface was slower than that at Sn–8Zn–3Bi/Cu interface (about 1/2 times) and Sn– 8Zn–3Bi/Ni interface (about 1/4 times) during aging. The reason may be that Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing concern on the toxicity of lead in the eutectic Sn–Pb solder, lead-free soldering has emerged as one of the critical technologies in electronic packaging industry. Compared with many other promising lead-free solders, such as Sn–Ag, Sn–Ag–Cu, Sn–Cu, Sn–Bi, the eutectic Sn–Zn solder is widely recommended because of its low melting point, good mechanical properties and low cost. The nature of the interfacial products formed between the solder and the substrate greatly influences the thermomechanical behavior of the solder joint [1–4]. In particular, the mechanical behavior of solder joints is very sensitive to the thickness of the IMC layer between the solder and the Cu substrate [5]. The growth of interfacial reaction products through solid-state aging in solder joints is of particular interest to the electronics industry. A suitable thickness of the IMC between a solder and a substrate offers high interfacial adhesion strength and excellent solder joint reliability, but excessively thick reaction layers, which grow between the solder and substrate, can significantly degrade the physical and mechanical properties of the solder joints, particularly in high impact load environment [6–9]. Thus, how to slow the growth of interfacial IMC is very important. The growth rates of IMC and the interdiffusion coefficients of atomic species in them have been widely determined by estimating the thickness change of the IMC layer. Some authors [10–12] reported that instead of Cu–Sn, the Cu–Zn intermetallics, mainly Cu5Zn8, formed at Sn–Zn/Cu interfaces. However, metallurgical behaviors in Sn–9Zn-based solder alloy with Cu substrate during aging have not yet been sufficiently studied. According to some
⇑ Corresponding author. Tel./fax: +86 10 21 34202748. E-mail address: [email protected] (A. Hu). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.09.028
previous research about the effect of different alloying elements, the Sn–9Zn–Cr alloy has finer microstructure and better oxidation resistant and plasticity than Sn–9Zn alloy while maintaining the same level of thermal and mechanical properties [13]. When Cr addition reach to 0.1 wt.%, oxidation resistance of Sn–9Zn alloy is the best [14]. The objective of this work was to study the effect of 0.3 wt.% Cr content on growth of IMC layer between Sn–8Zn– 3Bi lead-free solder and Cu/electroplated Ni substrates during aging at 150 °C. The average thickness of the IMC was determined by dividing the overall area of the IMC region by the length of the region parallel to the interface on the cross-section.
2. Experimental Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3 wt.%Cr alloys were prepared from pure (>99.99 wt.%) metals. Because of conflict between high melting temperature of Cr (1863 °C) and high vapor pressure of Zn, we firstly prepared SnCr master alloy using medium frequency induction furnace. SnCr master alloy was melted by pure Sn and Cr fragment, and preserved for 30 min at 1300 °C in low vacuum condition. Then it was melted with pure Sn, Zn and Bi, and homogenized at 650 °C for over 1 h. During homogenizing treatment, KCl/LiCl eutectic mixture served as a barrier layer for oxidation. The compositions of alloys were investigated by inductivelycoupled plasma emission spectrometry (ICP-AES). Solder spheres, which weigh 0.1 g, were placed on 10 mm 10 mm Cu and electroplated Ni substrates, the solder-Cu/electroplated Ni couples were reflow soldered at peak temperature 230 °C. The substrates were sequentially degreased in acetone, cleaned by de-ionized water, dipped in 10% HCl solution, and rinsed by de-ionized water again. After the reflow soldering, aging treatments were carried out in chamber at 150 °C for 1, 4, 9, 16 and
J. Bi et al. / Microelectronics Reliability 51 (2011) 636–641
25 days, respectively. To examine IMC growth rate during solid/solid interfacial reactions, the cross-sections of all specimens were observed with SEM. EDX analysis was carried out to identify the
637
elemental compositions of IMC, and the line scanning of elements across the interface for the aged solder joints was also conducted.
Fig. 1. Microstructure evolution in Sn–8Zn–3Bi solder joints with Cu substrates during aging at 150 °C for (a) 4, (c) 9, (e) 25 days and in Sn–8Zn–3Bi–0.3Cr solder joints during aging at 150 °C for (b) 4, (d) 9, (f) 25 days.
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3. Results and discussion 3.1. Microstructure evolution of solder/Cu joint Fig. 1a, c, e and g shows the microstructure evolution of Sn– 8Zn–3Bi solder joints with Cu substrates during isothermal aging at 150 °C for 0, 4, 9, 16, 25 days, and Fig. 1b, d, f and h shows the microstructure evolution in Sn–8Zn–3Bi–0.3Cr solder joints with Cu substrates during long-term aging at 150 °C isothermal aging at 150 °C for 0, 4, 9,16, 25 days, respectively. Before the aging process, the microstructure of the Sn–8Zn–3Bi solder consists of a typical Sn–Zn eutectic region with a primary Sn-phase, some needle-like Zn-rich phase. Very fine Bi precipitates (white dot) were found in the b-Sn matrix. After aging, the Zn-rich phase grew coarse obviously, and Bi was dissolved into b-Sn matrix. In the Sn–8Zn–3Bi–0.3Cr solder, the Zn-rich phase was finer than in the Sn–8Zn–3Bi solder, which indicated Cr could refine the microstructure of the Sn–8Zn–3Bi solders. After reflowed, the IMCs were found in the interface of the Sn– 8Zn–3Bi–(Cr)/Cu, and the IMCs grew and increased in thickness with aging time. Fig. 2a shows the EDS line analysis of the Sn– 8Zn–3Bi/Cu solder joints after aging at 150 °C for 25 days. It was found that the Cu–Zn IMC at the interface was determined to be c-Cu5Zn8. Since the Gibbs-free energy of formation for c-Cu5Zn8 is 12.345 kJ/mol, which is much lower than those of e-Cu3Sn (DG = 7.78 kJ/mol)and g0 -Cu6Sn5(DG = 7.42 kJ/mol) at 250 °C [15,16], respectively. The formation of c-Cu5Zn8 is predominant instead of Cu–Sn compounds [17]. The thickness of c-Cu5Zn8 IMCs in Cr-containing solder joints was much thinner than that in the Sn–8Zn–3Bi joints, which indicated that Cr could significantly restrain the IMC growth in Sn–Zn– Bi (–Cr) solder joints during aging. After 25 days aging, the IMC thickness at the Sn–8Zn–3Bi/Cu interface was about 16.2 lm and Cu5Zn8 compound was disrupted at the interface. While in Cr-containing solder joint, the IMC thickness was only about 11.8 lm and the IMC layer has a more uniform and compact morphology. A
layer of Zn-depletion zone (Zn-poor layer) formed near the interface in the Sn–8Zn–3Bi/Cu joint, and this Zn-depletion zone expands with the aging time. The thickness of the Zn-depletion zone reached 170 lm after 25 days aging. This means Zn atoms continuously diffuse to the interface and react with Cu atoms to form the Cu–Zn IMC at the interface during aging. 3.2. Microstructure evolution in solder joints with electroplated Ni substrates Fig. 3a and c present the microstructure evolution in the Sn– 8Zn–3Bi solder joints with electroplated Ni substrates during long-term aging treatment at 150 °C for 16, 25 days, respectively. The IMCs layer at the interface was identified as c-Ni5Zn21 [18,19]. When Sn based solder reacted with the Ni metallization, Ni3Sn4 will be formed, but because of the presence of the Zn which is having the higher affinity towards the formation of IMC with Ni resulted in the formation of Ni–Zn based intermetallic compound. [20] Thus the Ni5Zn21 layer was predominately formed at the interface. The Zn-depletion zone was also observed near the Sn–8Zn–3Bi/ electroplated Ni interface. After 25 days aging, the thickness of Zn-depletion zone was about 60 lm, much thinner than that in the Sn–8Zn–3Bi/Cu joint. Moreover, the c-Ni5Zn21 IMC did not scatter into the bulk solder, which was different from the c-Cu5Zn8 IMC in the Sn–8Zn–3Bi–(Cr) solder/Cu joint. Fig. 3b and d shows the microstructure evolution in the Sn– 8Zn–3Bi–0.3Cr solder joints with electroplated Ni substrates during long-term aging at 150 °C for 16, 25 days, respectively. The microstructure in the bulk solders was consistent with that of the Sn–8Zn–3Bi solder. After 25 days aging, it was also observed that the petal-like Zn–Cr phases were scattered evenly in the bulk solders. The thickness of c-Ni5Zn21 IMCs in Cr-containing solders was thinner than that in the Sn–8Zn–3Bi solder joints. After 25 days aging, the IMC thickness at the Sn–8Zn–3Bi/electroplated Ni and the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface were
Fig. 2. EDS line analysis of solder joint interface during aging at 150 °C for 25 days: (a) Sn–8Zn–3Bi/Cu interface; (b) Sn–8Zn–3Bi/electroplated Ni interface.
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Fig. 3. Microstructure evolution in Sn–8Zn–3Bi solder joints with Ni substrates during aging at 150 °C for (a) 16, (c) 25 days and in Sn–8Zn–3Bi–0.3Cr solder joints during aging at 150 °C for (b) 16, (d) 25 days.
about 9.2 lm and 5.1 lm, respectively. Compared with the cCu5Zn8 IMC, the thickness of the c-Ni5Zn21 IMC was much thinner after aging at 150 °C for 25 days. The thickness of c-Cu5Zn8 IMC was about 16.2 lm while the c-Ni5Zn21 was nearly half the thickness. The possible reason is that the dissolution and diffusion rate of Ni to the Sn–8Zn–3Bi (0.3Cr) solders were slower.
where t is the aging time, X(t) is the IMCs thickness at t time, X0 is the initial IMCs thickness, Q is the activity energy, A is a constant, n is the time constant. Since the growth is diffusion-controlled mechanism, n is taken as 0.5 as reported by many researchers [22]. Our results are quite consistent with the results reported by them. So the Eq. (1) can be simplified as follows:
3.3. IMCs growth on solder joints during aging
X X0 ¼ The relationship between the IMC thickness and aging time can be represented by the following equation [21]:
pffiffiffiffiffi Kt
ð2Þ
ð1Þ
where X is the IMCs thickness at t time, X0 is the initial as-soldered IMCs thickness, K is the growth rate constant of IMC growth rate which can be determined by the slope.
Fig. 4. c-Cu5Zn8 IMCs growth with increased aging time (day1/2): (a) Sn–8Zn–3Bi/ Cu interface; (b) Sn–8Zn–3Bi–03Cr/Cu interface.
Fig. 5. c-Ni5Zn21 IMCs growth with increased aging time (day1/2): (a) Sn–8Zn–3Bi/ electroplated Ni interface; (b) Sn–8Zn–3Bi–03Cr/electroplated Ni interface.
X ðtÞ ¼ X 0 þ At n expðQ=RtÞ
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J. Bi et al. / Microelectronics Reliability 51 (2011) 636–641
Fig. 6. EDS analysis of Cr contained phase: (a) as-reflowed, (b) after aging for 25 days.
Fig. 4 presents the linear relationship between the thickness of
c-Cu5Zn8 IMC and the square root of aging time (day1/2). K values of Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3Cr solder joints with Cu substrates at 150 °C were 1.9 1016 m2/s and 9.6 1017 m2/s, respectively. This result indicated that total IMC growth at the Sn–8Zn–3Bi–0.3Cr/Cu interface was about 50% slower than that at the Sn–8Zn–3Bi/Cu interface. Fig. 5 presents the linear relationship between the thickness of c-Ni5Zn21 IMC and the square root of aging time (day1/2). K values of Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3Cr solder joints with electroplated Ni substrates at 150 °C were 6.4 1017 m2/s and 1.7 1017 m2/s, respectively. This result indicated that total IMC growth at the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface was about 25% slower than that at the Sn–8Zn–3Bi/electroplated Ni interface. This relationship indicates that the formation of IMC is a diffusion-controlled process [23]. The presence of the 0.3 wt.%Cr in the solder matrix may have retarded the growth of IMC. The reason may be that Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate. At the Sn–8Zn–3Bi–Cr/Cu system, some phases containing Cr were observed in the bulk solders and the number increased greatly during aging. Fig. 6a shows the EDS analysis of phase containing Cr in as-reflowed solders, it has a composition of 68–70Sn and 30–32Cr (at.%). After aging for 25 days, EDS showed the Cr contained phase had a composition of 20–22Sn, 66–68Zn, 10–12Cr (at.%) (Fig. 6b). From the Zn–Cr phase diagram [13], these IMCs may be Zn17Cr and/or Zn13Cr. It implied that during aging, Zn prefer to combine with Cr forming Zn–Cr phase. Since the growth of Cu–Zn IMC is controlled by diffusion of Zn atom, the preference of Zn–Cr phase formation may delay the Cu–Zn IMC growth.
4. Conclusion It was found that the c-Cu5Zn8/c-Ni5Zn21 IMCs were formed at the Cu/electroplated Ni substrates after aging. The growth rate of IMCs at the Sn–8Zn–3Bi–0.3Cr/Cu system was about 50% slower
than that at the Sn–8Zn–3Bi/Cu interface, and the growth rate of IMCs at the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface was 25% slower than that at the Sn–8Zn–3Bi/electroplated Ni interface during aging at 150 °C. The Zn–Cr phase (Zn17Cr and/or Zn13Cr) was formed in the solders during aging. It indirectly controlled the diffusion of Zn atom to the interface, which delayed the IMCs growth rate in the Cr-containing solders during aging.
Acknowledgement This work is sponsored by International Science and Technology Cooperation of China (No. 2008DFA51680) and National Natural Science foundation of China (60876071).
References [1] Rosalbino F, Angelini E, Zanicchi G, Marazza R. Corrosion behaviour assessment of lead-free Sn–Ag–M (M = In, Bi, Cu) solder alloys. Mater Chem Phys 2008;109:386–91. [2] Sun P, Andersson C, Wei XC, Cheng ZN, Shangguan DK, Liu J, et al. Sn–Ag–Cu and eutectic Sn–Cu solder joints on electroless Ni (P) immersion Au surface finish after reflow soldering. Mater Sci Eng B 2006;135:134–40. [3] Chen CM, Hung YM, Lin CP, Su WC. Effect of temperature on microstructural changes of Sn–9 wt.%Zn lead-free solder stripe under current stressing. Mater Chem Phys 2009;115:367–70. [4] Lee CY, Yoon JW, Kim YJ, Jung SB. Interfacial reactions and joint reliability of Sn–9Zn solder on Cu or electrolytic Au/Ni/Cu BGA substrate. Microelectron Eng 2005;82:561–8. [5] Yao Y, Keerb LM, Finec ME. Modeling the failure of intermetallic/solder interfaces. Intermetallics 2010;18:1603–61. [6] Wang FJ, Yu ZS, Qi K. Intermetallic compound formation at Sn–3.0Ag–0.5Cu– 1.0Zn lead-free solder alloy/Cu interface during as-soldered and as-aged condition. J Alloys Compd 2007;438:110–5. [7] Li JF, Mannan SH, Clode MP, Whalley DC, Hutt DA. Interfacial reactions between molten Sn–Bi–X solders and Cu substrate for liquid solder interconnects. Acta Mater 2006;54:2907–22. [8] Sun P, Andersson C, Wei XC, Cheng Z, Shang DK, Liu J, et al. J Alloys Compd 2007;437:169–79. [9] Chang TC, Hon MH, Wang MC. Intermetallic compounds formation and interfacial adhesion strength of Sn–9Zn–0.5Ag solder alloy hot-dipped on Cu substrate. J Alloys Compd 2003;352:168–74. [10] Suganuma K, Niihara K, Shoutoku T, Nakamura Y. Wetting and interface microstructure between Sn–Zn binary alloys and Cu. J Mater Res 1998;13:2859–65.
J. Bi et al. / Microelectronics Reliability 51 (2011) 636–641 [11] Wanga F, Mab X, Qian YY. Improvement of microstructure and interface structure of eutectic Sn–0.7Cu solder with small amount of Zn addition. Scripta Matetialia 2005;53:699–702. [12] Kivilahti JK. Modeling joining materials for microelectronics packing. IEEE Trans Compd Pack Manuf Trans – Part B 1995;18:326–33. [13] Chen X, Li M, Ren XX, Hu AM, Mao DL. Effect of small additions of alloying elements on the properties of Sn–Zn eutectic alloy. J Electron Mater 2006;35:1734–9. [14] Chen X, Hu AM, Li M, Mao DL. Study on the properties of Sn–9Zn–x Cr lead-free solder. J Alloys Compd 2008;460:78–484. [15] Hultgren R, Desai PD, Hawkins DT, Gileiser M, Kelley KK. Selected values of the thermodynamic properties of binary alloy. Metals Park (OH): ASM; 1973. [16] Chang TC, Hsu YT, Hon MH, Wang MC. Enhancement of the wettability and solder joint reliability at the Sn–9Zn–0.5Ag lead-free solder alloy–Cu interface by Ag precoating. J Alloys Compd 2003;360:217–24. [17] Hon MH, Chang TC, Wang MC. Phase transformation and morphology of the intermetallic compounds formed at the Sn–9Zn–3.5Ag/Cu interface in aging. J Alloys Compd 2008;458:189–99.
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[18] Kim KS, Yang JM, Yu CH, Jung IO, Kim HH. Analysis on interfacial reactions between Sn–Zn solders and the Au/Ni electrolytic-plated Cu pad. J Alloys Compd 2004;379:314–8. [19] Lin CT, Hsi CS, Wang MC, Chang TC, Liang MK. Interfacial microstructures and solder joint strength of the Sn–9Zn–3Bi and Sn–9Zn–1Al Pb-free solder pastes on OSP finished printed circuit boards. J Alloys Compd 2008;459:225–31. [20] Kumar KM, Kripeshb V, Shenc L, Zeng KY, Tay AAO. Nanoindentation study of Zn-based Pb free solders used in fine pitch interconnect applications. Mater Sci Eng A 2006;423:57–63. [21] Kim DG, Jung SB. Interfacial reactions and growth kinetics for intermetallic compound layer between In–48Sn solder and bare Cu substrate. J Alloys Compd 2005;386:151–6. [22] Liu NS, Lin KL. Evolution of interfacial morphology of Sn–8.5Zn–0.5Ag–0.1Al– xGa/Cu system during isothermal aging. J Alloys Compd 2008;456:466–73. [23] Mayappan R, Ahmad ZA. Effect of Bi addition on the activation energy for the growth of Cu5Zn8 intermetallic in the Sn–Zn lead-free solder. Intermetallics 2010;18:730–5.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Effect of Cr additions on interfacial reaction between the Sn–Zn–Bi solder and Cu/electroplated Ni substrates Jinglin Bi, Anmin Hu ⇑, Jing Hu, Tingbi Luo, Ming Li, Dali Mao State Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 17 September 2010 Accepted 17 September 2010 Available online 20 October 2010
a b s t r a c t Intermetallic compounds (IMCs) growth on the Sn–8Zn–3Bi (–Cr) solder joints with Cu and electroplated Ni substrates was investigated after aging at 150 °C. It was found that the IMCs were the Cu5Zn8 and Ni5Zn21 at the solder/Cu and solder/Ni interface, respectively. The IMCs growth rate at the Sn–8Zn– 3Bi–Cr/Cu and Ni interface was slower than that at Sn–8Zn–3Bi/Cu interface (about 1/2 times) and Sn– 8Zn–3Bi/Ni interface (about 1/4 times) during aging. The reason may be that Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction With the increasing concern on the toxicity of lead in the eutectic Sn–Pb solder, lead-free soldering has emerged as one of the critical technologies in electronic packaging industry. Compared with many other promising lead-free solders, such as Sn–Ag, Sn–Ag–Cu, Sn–Cu, Sn–Bi, the eutectic Sn–Zn solder is widely recommended because of its low melting point, good mechanical properties and low cost. The nature of the interfacial products formed between the solder and the substrate greatly influences the thermomechanical behavior of the solder joint [1–4]. In particular, the mechanical behavior of solder joints is very sensitive to the thickness of the IMC layer between the solder and the Cu substrate [5]. The growth of interfacial reaction products through solid-state aging in solder joints is of particular interest to the electronics industry. A suitable thickness of the IMC between a solder and a substrate offers high interfacial adhesion strength and excellent solder joint reliability, but excessively thick reaction layers, which grow between the solder and substrate, can significantly degrade the physical and mechanical properties of the solder joints, particularly in high impact load environment [6–9]. Thus, how to slow the growth of interfacial IMC is very important. The growth rates of IMC and the interdiffusion coefficients of atomic species in them have been widely determined by estimating the thickness change of the IMC layer. Some authors [10–12] reported that instead of Cu–Sn, the Cu–Zn intermetallics, mainly Cu5Zn8, formed at Sn–Zn/Cu interfaces. However, metallurgical behaviors in Sn–9Zn-based solder alloy with Cu substrate during aging have not yet been sufficiently studied. According to some
⇑ Corresponding author. Tel./fax: +86 10 21 34202748. E-mail address: [email protected] (A. Hu). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.09.028
previous research about the effect of different alloying elements, the Sn–9Zn–Cr alloy has finer microstructure and better oxidation resistant and plasticity than Sn–9Zn alloy while maintaining the same level of thermal and mechanical properties [13]. When Cr addition reach to 0.1 wt.%, oxidation resistance of Sn–9Zn alloy is the best [14]. The objective of this work was to study the effect of 0.3 wt.% Cr content on growth of IMC layer between Sn–8Zn– 3Bi lead-free solder and Cu/electroplated Ni substrates during aging at 150 °C. The average thickness of the IMC was determined by dividing the overall area of the IMC region by the length of the region parallel to the interface on the cross-section.
2. Experimental Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3 wt.%Cr alloys were prepared from pure (>99.99 wt.%) metals. Because of conflict between high melting temperature of Cr (1863 °C) and high vapor pressure of Zn, we firstly prepared SnCr master alloy using medium frequency induction furnace. SnCr master alloy was melted by pure Sn and Cr fragment, and preserved for 30 min at 1300 °C in low vacuum condition. Then it was melted with pure Sn, Zn and Bi, and homogenized at 650 °C for over 1 h. During homogenizing treatment, KCl/LiCl eutectic mixture served as a barrier layer for oxidation. The compositions of alloys were investigated by inductivelycoupled plasma emission spectrometry (ICP-AES). Solder spheres, which weigh 0.1 g, were placed on 10 mm 10 mm Cu and electroplated Ni substrates, the solder-Cu/electroplated Ni couples were reflow soldered at peak temperature 230 °C. The substrates were sequentially degreased in acetone, cleaned by de-ionized water, dipped in 10% HCl solution, and rinsed by de-ionized water again. After the reflow soldering, aging treatments were carried out in chamber at 150 °C for 1, 4, 9, 16 and
J. Bi et al. / Microelectronics Reliability 51 (2011) 636–641
25 days, respectively. To examine IMC growth rate during solid/solid interfacial reactions, the cross-sections of all specimens were observed with SEM. EDX analysis was carried out to identify the
637
elemental compositions of IMC, and the line scanning of elements across the interface for the aged solder joints was also conducted.
Fig. 1. Microstructure evolution in Sn–8Zn–3Bi solder joints with Cu substrates during aging at 150 °C for (a) 4, (c) 9, (e) 25 days and in Sn–8Zn–3Bi–0.3Cr solder joints during aging at 150 °C for (b) 4, (d) 9, (f) 25 days.
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J. Bi et al. / Microelectronics Reliability 51 (2011) 636–641
3. Results and discussion 3.1. Microstructure evolution of solder/Cu joint Fig. 1a, c, e and g shows the microstructure evolution of Sn– 8Zn–3Bi solder joints with Cu substrates during isothermal aging at 150 °C for 0, 4, 9, 16, 25 days, and Fig. 1b, d, f and h shows the microstructure evolution in Sn–8Zn–3Bi–0.3Cr solder joints with Cu substrates during long-term aging at 150 °C isothermal aging at 150 °C for 0, 4, 9,16, 25 days, respectively. Before the aging process, the microstructure of the Sn–8Zn–3Bi solder consists of a typical Sn–Zn eutectic region with a primary Sn-phase, some needle-like Zn-rich phase. Very fine Bi precipitates (white dot) were found in the b-Sn matrix. After aging, the Zn-rich phase grew coarse obviously, and Bi was dissolved into b-Sn matrix. In the Sn–8Zn–3Bi–0.3Cr solder, the Zn-rich phase was finer than in the Sn–8Zn–3Bi solder, which indicated Cr could refine the microstructure of the Sn–8Zn–3Bi solders. After reflowed, the IMCs were found in the interface of the Sn– 8Zn–3Bi–(Cr)/Cu, and the IMCs grew and increased in thickness with aging time. Fig. 2a shows the EDS line analysis of the Sn– 8Zn–3Bi/Cu solder joints after aging at 150 °C for 25 days. It was found that the Cu–Zn IMC at the interface was determined to be c-Cu5Zn8. Since the Gibbs-free energy of formation for c-Cu5Zn8 is 12.345 kJ/mol, which is much lower than those of e-Cu3Sn (DG = 7.78 kJ/mol)and g0 -Cu6Sn5(DG = 7.42 kJ/mol) at 250 °C [15,16], respectively. The formation of c-Cu5Zn8 is predominant instead of Cu–Sn compounds [17]. The thickness of c-Cu5Zn8 IMCs in Cr-containing solder joints was much thinner than that in the Sn–8Zn–3Bi joints, which indicated that Cr could significantly restrain the IMC growth in Sn–Zn– Bi (–Cr) solder joints during aging. After 25 days aging, the IMC thickness at the Sn–8Zn–3Bi/Cu interface was about 16.2 lm and Cu5Zn8 compound was disrupted at the interface. While in Cr-containing solder joint, the IMC thickness was only about 11.8 lm and the IMC layer has a more uniform and compact morphology. A
layer of Zn-depletion zone (Zn-poor layer) formed near the interface in the Sn–8Zn–3Bi/Cu joint, and this Zn-depletion zone expands with the aging time. The thickness of the Zn-depletion zone reached 170 lm after 25 days aging. This means Zn atoms continuously diffuse to the interface and react with Cu atoms to form the Cu–Zn IMC at the interface during aging. 3.2. Microstructure evolution in solder joints with electroplated Ni substrates Fig. 3a and c present the microstructure evolution in the Sn– 8Zn–3Bi solder joints with electroplated Ni substrates during long-term aging treatment at 150 °C for 16, 25 days, respectively. The IMCs layer at the interface was identified as c-Ni5Zn21 [18,19]. When Sn based solder reacted with the Ni metallization, Ni3Sn4 will be formed, but because of the presence of the Zn which is having the higher affinity towards the formation of IMC with Ni resulted in the formation of Ni–Zn based intermetallic compound. [20] Thus the Ni5Zn21 layer was predominately formed at the interface. The Zn-depletion zone was also observed near the Sn–8Zn–3Bi/ electroplated Ni interface. After 25 days aging, the thickness of Zn-depletion zone was about 60 lm, much thinner than that in the Sn–8Zn–3Bi/Cu joint. Moreover, the c-Ni5Zn21 IMC did not scatter into the bulk solder, which was different from the c-Cu5Zn8 IMC in the Sn–8Zn–3Bi–(Cr) solder/Cu joint. Fig. 3b and d shows the microstructure evolution in the Sn– 8Zn–3Bi–0.3Cr solder joints with electroplated Ni substrates during long-term aging at 150 °C for 16, 25 days, respectively. The microstructure in the bulk solders was consistent with that of the Sn–8Zn–3Bi solder. After 25 days aging, it was also observed that the petal-like Zn–Cr phases were scattered evenly in the bulk solders. The thickness of c-Ni5Zn21 IMCs in Cr-containing solders was thinner than that in the Sn–8Zn–3Bi solder joints. After 25 days aging, the IMC thickness at the Sn–8Zn–3Bi/electroplated Ni and the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface were
Fig. 2. EDS line analysis of solder joint interface during aging at 150 °C for 25 days: (a) Sn–8Zn–3Bi/Cu interface; (b) Sn–8Zn–3Bi/electroplated Ni interface.
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Fig. 3. Microstructure evolution in Sn–8Zn–3Bi solder joints with Ni substrates during aging at 150 °C for (a) 16, (c) 25 days and in Sn–8Zn–3Bi–0.3Cr solder joints during aging at 150 °C for (b) 16, (d) 25 days.
about 9.2 lm and 5.1 lm, respectively. Compared with the cCu5Zn8 IMC, the thickness of the c-Ni5Zn21 IMC was much thinner after aging at 150 °C for 25 days. The thickness of c-Cu5Zn8 IMC was about 16.2 lm while the c-Ni5Zn21 was nearly half the thickness. The possible reason is that the dissolution and diffusion rate of Ni to the Sn–8Zn–3Bi (0.3Cr) solders were slower.
where t is the aging time, X(t) is the IMCs thickness at t time, X0 is the initial IMCs thickness, Q is the activity energy, A is a constant, n is the time constant. Since the growth is diffusion-controlled mechanism, n is taken as 0.5 as reported by many researchers [22]. Our results are quite consistent with the results reported by them. So the Eq. (1) can be simplified as follows:
3.3. IMCs growth on solder joints during aging
X X0 ¼ The relationship between the IMC thickness and aging time can be represented by the following equation [21]:
pffiffiffiffiffi Kt
ð2Þ
ð1Þ
where X is the IMCs thickness at t time, X0 is the initial as-soldered IMCs thickness, K is the growth rate constant of IMC growth rate which can be determined by the slope.
Fig. 4. c-Cu5Zn8 IMCs growth with increased aging time (day1/2): (a) Sn–8Zn–3Bi/ Cu interface; (b) Sn–8Zn–3Bi–03Cr/Cu interface.
Fig. 5. c-Ni5Zn21 IMCs growth with increased aging time (day1/2): (a) Sn–8Zn–3Bi/ electroplated Ni interface; (b) Sn–8Zn–3Bi–03Cr/electroplated Ni interface.
X ðtÞ ¼ X 0 þ At n expðQ=RtÞ
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Fig. 6. EDS analysis of Cr contained phase: (a) as-reflowed, (b) after aging for 25 days.
Fig. 4 presents the linear relationship between the thickness of
c-Cu5Zn8 IMC and the square root of aging time (day1/2). K values of Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3Cr solder joints with Cu substrates at 150 °C were 1.9 1016 m2/s and 9.6 1017 m2/s, respectively. This result indicated that total IMC growth at the Sn–8Zn–3Bi–0.3Cr/Cu interface was about 50% slower than that at the Sn–8Zn–3Bi/Cu interface. Fig. 5 presents the linear relationship between the thickness of c-Ni5Zn21 IMC and the square root of aging time (day1/2). K values of Sn–8Zn–3Bi and Sn–8Zn–3Bi–0.3Cr solder joints with electroplated Ni substrates at 150 °C were 6.4 1017 m2/s and 1.7 1017 m2/s, respectively. This result indicated that total IMC growth at the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface was about 25% slower than that at the Sn–8Zn–3Bi/electroplated Ni interface. This relationship indicates that the formation of IMC is a diffusion-controlled process [23]. The presence of the 0.3 wt.%Cr in the solder matrix may have retarded the growth of IMC. The reason may be that Cr reacts with Zn and forms the Sn–Zn–Cr phase which block the diffusion of Zn atom to the interface and slow down the IMCs growth rate. At the Sn–8Zn–3Bi–Cr/Cu system, some phases containing Cr were observed in the bulk solders and the number increased greatly during aging. Fig. 6a shows the EDS analysis of phase containing Cr in as-reflowed solders, it has a composition of 68–70Sn and 30–32Cr (at.%). After aging for 25 days, EDS showed the Cr contained phase had a composition of 20–22Sn, 66–68Zn, 10–12Cr (at.%) (Fig. 6b). From the Zn–Cr phase diagram [13], these IMCs may be Zn17Cr and/or Zn13Cr. It implied that during aging, Zn prefer to combine with Cr forming Zn–Cr phase. Since the growth of Cu–Zn IMC is controlled by diffusion of Zn atom, the preference of Zn–Cr phase formation may delay the Cu–Zn IMC growth.
4. Conclusion It was found that the c-Cu5Zn8/c-Ni5Zn21 IMCs were formed at the Cu/electroplated Ni substrates after aging. The growth rate of IMCs at the Sn–8Zn–3Bi–0.3Cr/Cu system was about 50% slower
than that at the Sn–8Zn–3Bi/Cu interface, and the growth rate of IMCs at the Sn–8Zn–3Bi–0.3Cr/electroplated Ni interface was 25% slower than that at the Sn–8Zn–3Bi/electroplated Ni interface during aging at 150 °C. The Zn–Cr phase (Zn17Cr and/or Zn13Cr) was formed in the solders during aging. It indirectly controlled the diffusion of Zn atom to the interface, which delayed the IMCs growth rate in the Cr-containing solders during aging.
Acknowledgement This work is sponsored by International Science and Technology Cooperation of China (No. 2008DFA51680) and National Natural Science foundation of China (60876071).
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