Effect of adding 1 wt% Bi into the Sn 2.8Ag 0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging

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1 Journal of Alloys and Compounds 407 (2006) Effect of adding 1 wt% Bi into the Sn 2.8Ag 0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging M.J. Rizvi a,b, Y.C. Chan a,, C. Bailey b,h.lu b, M.N. Islam a a Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong b School of Computing and Mathematical Sciences, University of Greenwich, 30 Park Row, London SE10 9LS, UK Received 6 May 2005; accepted 22 June 2005 Available online 10 August 2005 Abstract Intermetallic compound (IMC) formations of Sn 2.8Ag 0.5Cu solder with additional 1 wt% Bi were studied for Cu-substrate during soldering at 255 C and isothermal aging at 150 C. It was found that addition of 1 wt% Bi into the Sn 2.8Ag 0.5Cu solder inhibits the excessive formation of intermetallic compounds during the soldering reaction and thereafter in aging condition. Though the intermetallic compound layer was Cu 6 Sn 5, after 14 days of aging a thin Cu 3 Sn layer was also observed for both solders. A significant increase of intermetallic layer thickness was observed for both solders where the increasing tendency was lower for Bi-containing solder. After various days of aging, Sn 2.8Ag 0.5Cu 1.0Bi solder gives comparatively planar intermetallic layer at the solder substrate interface than that of the Sn 2.8Ag 0.5Cu solder. The formation of intermetallic compounds during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for Sn 2.8Ag 0.5Cu solder. Intermetallic growth rate constants for Sn 2.8Ag 0.5Cu and Sn 2.8Ag 0.5Cu 1.0Bi solders were calculated as and m 2 /s, respectively, which had significant effect on the growth behavior of intermetallic compounds during aging Published by Elsevier B.V. Keywords: Soldering; Interfacial reactions; Intermetallic compounds; Growth rate; Aging 1. Introduction Sn lead (Sn Pb) solders are most prominent interconnect materials for soldering the electronic components with the circuit boards [1]. However, nowadays, the use of this solder is so far restricted due to the environmental and health concern [2 4]. These issues had led towards extensive researches to develop suitable lead-free solders with low cost, good wetting properties, and good physical, mechanical, metallurgical, and fatigue resistant properties [5]. The development of lead-free solders such as Sn Ag, Sn Cu, Sn Bi, Sn Ag Cu without a clear and thorough understanding of their structural properties and without sufficient reliability data is crucial [6] Corresponding author. Tel.: ; fax: address: EEYCCHAN@cityu.edu.hk (Y.C. Chan). since the soldering reaction between solders and substrates dominate the joint performance. During the soldering process, the solder alloy melts and reacts with the substrates resulting in the formation of intermetallic compounds (IMC) in between the solder and substrate. Though the presence of intermetallic compound is a sign of good wetting, excessive intermetallic layer formations can be detrimental for the solder joints due to the brittleness of this layer. Solders are normally worked at high homologous temperature even at room temperature condition [7] due to their low melting temperatures. During the aging condition, the formation of thick intermetallic compound layer degrades the solder joint lifetime [8]. Therefore, it is a matter of interest to develop a solder that meets the properties like Sn Pb solders and forms reasonable intermetallic layer. Though National Electronics Manufacturing Initiative (NEMI) has recommended Sn Ag Cu solder alloy for reflow soldering /$ see front matter 2005 Published by Elsevier B.V. doi: /j.jallcom

2 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) and Sn Cu solder alloy for wave soldering as a replacement of eutectic Sn Pb solder [9], their melting temperatures and intermetallic growth behavior are higher than that of Sn Pb solder alloy which in turn influence the joint strength as well as long-term reliability. From this point of view, bismuth (Bi) is a potential candidate that lowers the melting temperature [5]. Several researchers [7,10,11] have already investigated the joint strength and fatigue property of SnAgCu and SnAgCuBi solder alloys and found that the addition of a certain amount of Bi increases the tensile strength and the hardness of the solders. But the behavior of intermetallic compound formations during the solid-state isothermal aging of these two solders on Cu-substrate is not elaborately discussed yet. Due to the high concentration of Sn, SnAgCu lead-free solder possesses a high rate of interfacial reaction with Cu and forms Cu Sn intermetallic compound. As a result, the Cu (supplied from the substrate) is consumed rapidly by the molten SnAgCu solder. After the consumption of the whole Cu of the substrate, the Cu Sn compound tends to transform from a scallop-like morphology to spheroid-type morphology [12] and disjoints the solder from the substrate. Therefore, the addition of a fourth-element into the SnAgCu solder alloy to reduce the melting temperature, growth rate of intermetallic compounds and to improve its properties, requires substantial researches. This study focuses on the effect of adding 1 wt% Bi into the well-known SnAgCu solder on the reaction with Cu-substrate and formation of intermetallic compounds in between the solder substrate during the soldering and solid-state isothermal aging. A comparative study of SnAgCu and SnAgCuBi is carried out based on the intermetallic growth behavior. 2. Experimental procedure Pure (99.99%) Cu-plate was used in this experiment as the substrate. Before conducting the experiment, the Cu-plate was cut into a rectangular (10 mm 2mm 0.22 mm) shape and degreased with a 1:1 solution of HCL and deionized water for 1 3 min. They were then washed by ethanol and finally dried. In order to form the solder Cu reaction couples, the Cu substrates were vertically immersed into a commercial No- Fig. 1. Schematic diagram showing the soldering procedure. clean flux (surface tension = 370 mn/mm) and immediately dipped into lead-free Sn 2.8Ag 0.5Cu (eutectic temperature 218 C) and Sn 2.8Ag 0.5Cu 1.0Bi (eutectic temperature 214 C) solders baths as shown in the Fig. 1. For dipping purposes, the solderability tester (MENISCO ST 50) was used with a speed of 21 mm/s. The dipping depth and dipping time were set as 3 mm and 10 s, respectively. The soldering temperature was set as 255 C. The soldered Cu-plates were then aged isothermally into a high-temperature oven at 150 C with a temperature stability of ±1 C for 2, 6, 10 and 14 days. As-dipped and aged-samples were then mounted with rosin, cured at room temperature, mechanically ground and polished for having the cross-sections. Finally, the samples were gold-coated as a preparation for the microstructural investigations of the solder Cu interfaces. The morphologies were examined by scanning electron microscopy (SEM) with a voltage of 20 kv. Energy dispersive X-ray (EDX) analysis was carried out to identify the elemental compositions of the compounds formed due to soldering and aging. 3. Results and discussions The backscattered electron microscope image of Fig. 2 shows the intermetallic layers formed between the solder and Fig. 2. SEM images of solder Cu interfaces after soldering at 255 C for (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders.

3 210 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) Fig. 3. Formation of intermetallic compound in the bulk of the solder after soldering at 255 C for (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders. Cu interfaces just after the soldering reaction. EDX analysis confirmed that Ag and Bi did not contribute to form the intermetallic compounds at the solder Cu interface and therefore, this intermetallic layer is only Cu 6 Sn 5 in which the atomic percentages of Cu and Sn are 55 and 45, respectively. During the dipping of the Cu-substrate into the molten solder, the growth of scallop-like Cu 6 Sn 5 intermetallic depends on the coarsening among the Cu 6 Sn 5 scallops and on the interfacial reaction [6]. At the time of soldering reaction, Cu atoms of the substrate diffuse into the solder matrix through the Cu 6 Sn 5 layer. When the concentration of Cu in the solder exceeds the threshold limit, the Cu 6 Sn 5 compounds also form in the bulk of the solder as shown in the Fig. 3. The thickness of Cu 6 Sn 5 intermetallic layer for the interfacial reaction of Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/ Cu soldering systems were 1.52 and 1.01 m, respectively. Also the Cu 6 Sn 5 intermetallic formation in the bulk of the solder is higher for Sn2.8Ag0.5Cu/Cu soldering system than that of the Sn2.8Ag0.5Cu1.0Bi/Cu soldering system (Fig. 3). However, after 2 days of solid-state aging, the thickness of Cu 6 Sn 5 intermetallic layer for Sn2.8Ag0.5Cu/Cu (Fig. 4(a)) and Sn2.8Ag0.5Cu1.0Bi/Cu (Fig. 4(b)) soldering systems were increased as 3.47 and 3.17 m, respectively. In the solidstate aging, the intermetallic growth at the valleys between two adjacent scallops is faster than at the peaks of the scallops as reported by Lee et al. [6]. As a result, Cu 6 Sn 5 intermetallic layer is transformed from the scallop-like shape (Fig. 2) to layer-type shape (Fig. 4). The heterogeneous nucleation of Ag 3 Sn compound is also noticed for both solders after 2 days of aging (Fig. 4). It is stated earlier that the formation of Cu 6 Sn 5 is due to the diffusion of Cu atoms from the substrate into the solder matrix. This diffusion is much faster during the soldering reaction than at the solid-state aging. Moreover, the diffusivity of Cu in Sn is also faster than the diffusivity of Ag in Sn [6]. Therefore, only Cu 6 Sn 5 intermetallic was detected for as-dipped soldered condition. On the other hand, Ag 3 Sn compounds were also formed after 2 days of aging due to the selective formation of Cu 6 Sn 5 compounds in the solder matrix. After 6 days of aging, a bit more planar intermetallic layer was found for Sn2.8Ag0.5Cu1.0Bi solder (Fig. 5(b)) than Sn2.8Ag0.5Cu solder (Fig. 5(a)). At this stage, Cu 6 Sn 5 intermetallic layer thickness for Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems were measured as 5.65 m and 4.50 m, respectively. In this case, the sizes of Cu 6 Sn 5 and Ag 3 Sn intermetallics are bigger in Sn2.8Ag0.5Cu solder matrix than those in the Sn2.8Ag0.5Cu1.0Bi solder matrix. Therefore, addition of 1 wt% Bi into the Sn2.8Ag0.5Cu solder controls higher nucleation of Cu 6 Sn 5 and Ag 3 Sn intermetallics in the solder matrix. The similar trend of intermetallic formations was documented for both solders even after 10 days of aging. The thickness of intermetallic layer after 14 days of aging for Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems were measured as 7.50 and 6.20 m, respectively. At this stage, an additional thin layer was detected beneath the Cu 6 Sn 5 layer for both solders (Fig. 6(a and b)). EDX analysis confirmed that this layer is Cu 3 Sn with a composition of Fig. 4. SEM images showing the solder Cu interfaces after 2 days of aging for (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders.

4 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) Fig. 5. SEM images showing the solder Cu interfaces after 6 days of aging for (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders. Fig. 6. SEM images showing the solder Cu interfaces after 14 days of aging for (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders. 76 at% Cu and 24 at% Sn. When the supply of Sn through the intermetallic layer is limited, the diffused-cu from substrate reacts with Cu 6 Sn 5 and forms Cu 3 Sn underneath the thick Cu 6 Sn 5 layer [13]. The thickness of the intermetallic layer formed during the aging condition can be expressed by the simple parabolic equation: Y = kt n (1) where Y is the thickness of the intermetallic layer at time t, k the intermetallic growth rate constant and n the time exponent. Kim and Jung [2] reported that during the solid-state aging, the growth of intermetallic compounds generally follows linear or parabolic kinetics. The growth rate is controlled by the reaction rate at the growth site when the intermetallic growth follows the linear kinetics, whereas parabolic growth kinetics implies that the intermetallic growth is controlled by volume diffusion. For the diffusion-controlled mechanism, the growth of intermetallic compound layer thickness after aging should follow the square root of time power law relationship that can be expressed as Y = kt 1/2, where the value of the time exponent n equals to 0.5 [14]. Fig. 7 shows the thickness of the Cu 6 Sn 5 intermetallic layer as a function of square root of the aging time. It is seen that the intermetallic layer thickness increases almost linearly over the aging time and the growth is faster for the Sn2.8Ag0.5Cu/Cu soldering system than for the Sn2.8Ag0.5Cu1.0Bi/Cu soldering system. From this figure, it is evident that the growth of Cu 6 Sn 5 intermetallic layer for both soldering systems follows the parabolic growth kinetics. Therefore, the Cu Sn growth behavior for both soldering systems is diffusion-controlled. Other researchers [15,16] also noticed that Cu 6 Sn 5 intermetallic formation follows the diffusion-controlled mechanism. To identify the growth behavior of Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems, the growth rate constant k was determined by the multivariable linear regression analysis of Y versus t 0.5.The slope of the curves shown in Fig. 7 gives the k values for the Sn2.8Ag0.5Cu/Cu and Fig. 7. Thickness of intermetallic compound layer as a function of square root of aging time.

5 212 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems as and m 2 /s, respectively. Therefore, the growth rate of intermetallic compounds during the isothermal aging is higher for the Sn2.8Ag0.5Cu/Cu soldering system than for the Sn2.8Ag0.5Cu1.0Bi/Cu soldering system and is clearly visible in the SEM images shown in Figs The linear regression coefficients denoted by R 2, which is a ratio of the total sum of squares to the regression sum of squares, are and for Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems, respectively. This implies that the intermetallic formations for both the soldering systems follow a similar diffusion mechanism. The time exponent n for the growth of intermetallic compounds during the isothermal aging was calculated using the following equation [17]: Y t = At n + Y 0 (2) where Y t is the thickness of intermetallic compound layer at time t, A the constant, n the time exponent, Y 0 the initial thickness of intermetallic compound layer. The logarithmic expression of Eq. (2) is ln(y t Y 0 ) = n lnt + lna (3) Fig. 8 represents the plot of ln (Y t Y 0 ) versus ln t. From the slope of this graph, the n values for Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems were obtained as 0.57 and 0.45, respectively, and these n values greatly influence the diffusion process as well as the intermetallic layer formations of Sn2.8Ag0.5Cu/Cu and Sn2.8Ag0.5Cu1.0Bi/Cu soldering systems. From the above discussions, it is clear that the intermetallic layer thickness increases due to the Cu-diffusion process, which is higher for Sn2.8Ag0.5Cu solder than for Sn2.8Ag0.5Cu1.0Bi solder. During the soldering reaction, Cu atoms come from the substrate and react with Sn atoms to form intermetallic at the solder substrate interface and within the bulk of the solder. This Cu-diffusion process continues through the channels of the intermetallic layer. However, the interfacial reaction is not too fast to consume all the diffused Cu atoms. Therefore, unreacted Cu atoms further diffuse into the bulk of the solder and react with Sn to form Cu 6 Sn 5 compounds. As a result, the concentration of Cu in the bulk of the solder decreases which encourages further diffusion of Cu from the substrate. When the solubility of Cu in the bulk of the solder decreases, Cu 6 Sn 5 compounds deposit on the existing intermetallic layer. As a consequence, the thickness of the intermetallic layer increases. Similarly, the intermetallic layer thickness increases during the solid-state aging with the gradual diffusion of Cu into the solder. A comparatively rougher intermetallic layer was observed for the Sn2.8Ag0.5Cu/Cu soldering system than for the Sn2.8Ag0.5Cu1.0Bi/Cu soldering system even after 14 days of aging (Fig. 6) and this is because of the higher consumption of Cu by the Sn2.8Ag0.5Cu solder than for the Bi-containing solder (Fig. 9). Comparative images of forming the intermetallic compounds in the bulk of Sn2.8Ag0.5Cu and Sn2.8Ag0.5Cu1.0Bi solders are shown in Fig. 10. InFig. 10(a(i) and b(i)), it was found that selective Cu 6 Sn 5 and Ag 3 Sn were formed after 2 days of aging, whereas the sizes of these intermetallics were increased with the gradual increase of aging time (Fig. 10(a(i iv) and b(i iv))). Moreover, the sizes of intermetallic nuclei are bigger for Sn2.8Ag0.5Cu solder than for Sn2.8Ag0.5Cu1.0Bi solder. Therefore, Bi plays a strong role on the diffusion process as well as in the formation and growth of intermetallics. According to the Sn Bi binary phase diagram, the solid solubility of Bi at room temperature is about 1 wt%. At high temperature, this solubility limit increases to 11 wt% at 120 C and 21 wt% at 139 C [7]. Vianco and Rejent [18] had found Bi-precipitations on the Cu 6 Sn 5 intermetallic layer and this was because of the Bi-rejection from the Sn-matrix. However, in our study, we did not find any precipitation of Bi due to addition of lower amount (1 wt%) of Bi into the solder. Therefore, the presence of Bi in the Sn-matrix lowers the Cu-consumption and hampers the higher-growth of intermetallics. Fig. 8. Plot of ln (Y t Y 0 ) vs. ln t to obtain the time exponent n. Fig. 9. Cu-consumption by solder as a function of aging time.

6 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) Fig. 10. Formation of intermetallic compounds in the bulk of (a) Sn 2.8Ag 0.5Cu and (b) Sn 2.8Ag 0.5Cu 1.0Bi solders after aging at 150 C for (i) 2 days, (ii) 6 days, (iii) 10 days and (iv) 14 days. 4. Conclusions Intermetallic formations of Sn 2.8Ag 0.5Cu solder alloy with additional 1 wt% Bi were investigated for Cu-substrate during soldering at 255 C and isothermal aging at 150 C. It was found that just after the soldering reaction, the intermetallic layer thickness was comparatively higher for Sn 2.8Ag 0.5Cu solder than for Sn 2.8Ag 0.5Cu 1.0Bi solder. For both solders, the Cu Sn intermetallic forms and increases due to continuous diffusion of Cu from the substrate into the solder matrix. Addition of 1 wt% Bi into the Sn2.8Ag0.5Cu solder controls the nucleation of Cu 6 Sn 5 and Ag 3 Sn intermetallics in the solder matrix as well as in the solder substrate interface during the aging condition. A significant increase of the intermetallic layer thickness during isothermal aging was noticed for both solders where the increasing tendency was lower for the Bi containing Sn 2.8Ag 0.5Cu solder. A comparatively rougher intermetallic layer after various days of aging was observed for Sn 2.8Ag 0.5Cu solder than

7 214 M.J. Rizvi et al. / Journal of Alloys and Compounds 407 (2006) for Sn 2.8Ag 0.5Cu 1.0Bi solder. Higher interfacial reaction and higher Cu-consumption from the Cu-substrate by Sn 2.8Ag 0.5Cu solder was thought to be the reason to obtain this kind of rough intermetallic layer. The growth rate constant of intermetallic layer formation was calculated for Sn 2.8Ag 0.5Cu solder as m 2 /s, whereas the addition of 1 wt% Bi into Sn 2.8Ag 0.5Cu solder lowers this value to m 2 /s. A small amount (like 1 wt%) of Bi can be added into the Sn Ag Cu solder to reduce the melting temperature, growth rate of intermetallics and consumption of Cu. Acknowledgement The authors would like to acknowledge the financial support provided by Research Grant Council of Hong Kong for the project Wetting Kinetics and Interfacial Interaction Behavior between lead-free solders and Electroless Nickel Metallizations, CERG project no. CityU 1187/01E (CityU Project no ). References [1] C.C. Young, J.G. Duh, S.Y. Tsai, J. Electron. Mater. 30 (2001) [2] D.G. Kim, S.B. Jung, J. Alloys Compd. 386 (2005) [3] J.W. Yoon, S.W. Kim, S.B. Jung, J. Alloys Compd. 391 (2005) [4] D.Q. Yu, L. Wang, C.M.L. Wu, C.M.T. Law, J. Alloys Compd. 389 (2005) [5] J.W. Yoon, C.B. Lee, S.B. Jung, Mater. Sci. Technol. 19 (2003) [6] T.Y. Lee, W.J. Choi, K.N. Tu, J. Mater. Res. 17 (2002) [7] J. Zhao, L. Qi, X.-M. Wang, L. Wang, J. Alloys Compd. 375 (2004) [8] P.L. Tu, Y.C. Chan, K.C. Hung, J.K.L. Lai, Scripta Mater. 44 (2001) [9] K. Zeng, K.N. Tu, Mater. Sci. Eng., R 38 (2002) [10] J. Zhao, Y. Mutoh, Y. Miyasuita, S.L. Mannan, J. Electron. Mater. 31 (2002) [11] C.M.L. Wu, M.L. Huang, J.K.L. Lai, Y.C. Chan, J. Electron. Mater. 29 (2000) [12] K.N. Tu, F. Ku, T.Y. Lee, J. Electron. Mater. 30 (2001) [13] L. Qi, J. Zhao, X.M. Wang, L. Wang, Proceedings of 2004 International Conference on the Business of Electronic Product Reliability and Liability, Shanghai, China, April 27 30, 2004, pp [14] C.Y. Lee, K.L. Lin, J. Mater. Sci. Mater. Electron. 8 (1997) [15] P.T. Vianco, A.C. Kilgo, R. Grant, J. Electron. Mater. 24 (1995) [16] C. Chen, C.E. Ho, A.H. Lin, G.L. Luo, C.R. Kao, J. Electron. Mater. 29 (2000) [17] J.W. Yoon, C.B. Lee, S.B. Jung, Mater. Trans. 43 (2002) [18] P.T. Vianco, J.A. Rejent, J. Electron. Mater. 28 (1999)