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Cu soldering couples
Journal of Alloys and Compounds 473 (2009) 382–388
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Kinetics of intermetallic compound layers and shear strength in Bi-bearing SnAgCu/Cu soldering couples Jie Zhao ∗ , Cong-qian Cheng, Lin Qi, Cheng-yu Chi National Key Laboratory of Materials Modification, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 16 May 2008 Accepted 25 May 2008 Available online 11 July 2008 Keywords: Kinetics Intermetallic Shear strength Solder
a b s t r a c t Three lead-free solder alloys, Sn–3Ag–0.5Cu, Sn–3Ag–0.5Cu–1Bi and Sn–3Ag–0.5Cu–3Bi, have been used to solder with Cu substrate in the current experiments to investigate the influence of Bi on the kinetics of intermetallic compound (IMC) layer growth and shear strength of the soldering couples. The experimental results indicate that the thickness of IMC layer at the solder/Cu interface increases with aging process. The addition of Bi delays the propagation rate of total IMC layer. For the samples aged at 140 ◦ C up to 500 h, their shear strengths keep at a relative stable level where the fracture occurs in bulk solder side and the addition of Bi improve the strength level, while for the samples aged at 195 ◦ C, the shear strengths decrease continuously with aging time where the fracture occurs along interface between IMC layers and solders and the influence of Bi can be negligible. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, as the use of Pb-bearing solder is restricted due to the environmental and health concern [1–3], significant pressure has been put forward to the development of lead-free solder. As for achieving higher circuit board component densities, package dimensions have been shrinking with decreasing solder bump sizes. Therefore, reduction of solder bump size has resulted in severe requirements for mechanical reliability of solder joint. Among the new developed lead-free solders, Sn–Ag–Cu–Bi is regarded as one with high quality and performance and can meet the requirement of electronic packaging. Researches have been reported on their property and microstructure evolution [1,2]. Normally, solders in joints are already worked at high homologous temperature even at room temperature because of their low melting temperatures. Therefore, the microstructures are unstable during applications. It is believed that the microstructure evolution can significantly influence the reliability of solder joints, therefore the knowledge of microstructure change in solders as well as in joints is required [4]. The study of the effect of aging time on interfacial microstructure and growth of intermetallic compound (IMC) was performed in Sn–3.5Ag, Sn–Bi, Sn–Zn–Bi and Sn–Ag–Bi solder joints [5–7]. In our previous work, the influence of Bi on microstructure evolution and mechanical properties in bulk Sn–Ag–Cu solders was reported [8]. Relative stable mechanical properties with aging
∗ Corresponding author. Tel.: +86 411 84707636. E-mail address: [email protected] (J. Zhao). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.05.082
procedure were observed in Bi-bearing solders which attributed to the strengthening effect of Bi from solid solution strengthening to precipitating strengthening. Recently, effect of Bi on the IMC layer formations in Sn–Ag–Cu/Cu solder joints during soldering and isothermal aging has been reported [9,10], it is demonstrated that Bi addition delay the growth tendency of IMC layer. Other researchers suggest that the growth mechanism of interfacial IMC layers will change when the Sn/Cu and Sn58Bi/Cu joints are aged in different temperature ranges [11,12]. And we also have found the similar result in the Sn–Ag–Cu–Bi/Cu joints [13]. However, the effect of Bi on the growth activation energies in variant temperature ranges has not been described in detail. From the interfacial reliability of solder joints during service, it is necessary to study the mechanical properties of Sn–Ag–Cu–Bi solder joints. To ensure the reliability of the joints, knowledge is required of the shear strength of the solders joints during aging. The previous study shows that addition of Bi increases the shear strength in Cu/Sn–Ag–Bi/Cu joints and the tensile strength of Sn–8Zn–3Bi solder during soldering [14,15]. However, the presence of Cu in the solder can affect the growth kinetics of interfacial IMC layer during the soldering [16]. As described above, the mechanical properties of the substrate/solder joint are greatly influenced by the growth kinetics of interface IMC layer because of the intrinsic brittleness. And the change in the fracture mode of solder joint is also closely related to the interfacial IMC thickness [17]. So it is of particular important to investigate the shear strength of Sn–Ag–Cu–Bi/Cu solder during aging. Unfortunately, scarce work has been published on the influence of Bi addition on the shear strength of solder joints during aging process.
J. Zhao et al. / Journal of Alloys and Compounds 473 (2009) 382–388
In order to apply a fully understanding of Bi effect on the growth activation energies of IMC layers in various temperature ranges and the mechanical properties in Sn–Ag–Cu solder joints, the current work focused on the kinetics of IMC layer growth in Sn–3Ag–0.5Cu–xBi/Cu (x = 0, 1, 3) solder joints in a very large temperature range, as well as the shear strength of the couples. 2. Experimental procedures Three lead-free solder alloys, Sn–3Ag–0.5Cu, Sn–3Ag–0.5Cu–1Bi and Sn–3Ag–0.5Cu–3Bi, were used in the current experiments. They were remelted and cast into bars in a copper mould, then cut into plates with diameter of 6 mm, and grinded to weight of 0.38 ± 0.005 g. The melting temperature of these solders are measured to be 219–220 ◦ C. The substrates used in this study were pure Cu sheets. These substrates as well as the solder plates were grinded with fine sandpaper and cleaned with acetone before re-flow process to remove surface oxide and contaminates. The Sn–3Ag–0.5Cu–xBi/Cu couple samples were prepared by reflowing solder on Cu substrates at peak temperature of 270 ◦ C and spent 30 s at molten state. Rosin mildly activated (RMA) flux was used. To investigate the growth kinetics of IMC layers, some of the soldered specimens were aged up to 1000 h at temperatures of 70 ◦ C, 120 ◦ C, 150 ◦ C and 170 ◦ C, the others were aged up to 600 h at temperatures of 140 ◦ C, 160 ◦ C, 180 ◦ C and 195 ◦ C after soldering process. The temperature fluctuation during aging process was kept within ±1 ◦ C. The aged samples were then mounted in epoxy and grinded. Finally the samples were polished by using 0.3 m diamond paste and etched with a solution of 5%HNO3 –2%HCl–93% methanol for several seconds. In order to examine the microstructure evolution of the joints and growth kinetics of IMC layers, all the samples were observed using JEOL JSM-5600LV scanning electron microscopy (SEM). The components of interfacial IMC layers were identified using Energy Dispersive of X-ray (EDX). The measurements of interfacial IMC layer thickness were done by dividing the area of IMC layer by its length using Q500IW image analyzer. The elemental distribution near jointed interfaces was analyzed by using Shimadzu EPMA-1600. In this study, the soldering couple samples for shear testing were designed based on Chinese National Standard GB/T15111-94 with a soldering area of 10 mm × 5 mm, as shown in Fig. 1. Following similar soldering process as described above, some of the specimens were thermally aged at temperatures of 140 ◦ C and 195 ◦ C up to 600 h.
383
Fig. 1. Schematic diagram of solder joint. All the shear tests were performed at a strain rate of 10 mm/min. After shear tests, the fracture surface was observed using SEM.
3. Experimental results and discussions 3.1. Microstructural evolution Fig. 2 chooses Sn–3Ag–0.5Cu/Cu and Sn–3Ag–0.5Cu–3Bi/Cu joint samples as examples to demonstrate their microstructure evolution during aging process. From the observation on the as-soldered interfacial microstructures, it can be seen that the IMC layers after reflow for all the types of joints have round scallop-type morphology. The interfacial IMC layers are mainly composed of Cu6 Sn5 . The interfacial Cu3 Sn layers are too thin to be observed. After aging treatment, the interfacial Cu6 Sn5 and Cu3 Sn layers appear in the solder joints. The round scallop-type morphology gradually disappears and changes to planar-type morphology, and the thickness of IMC layer increases significantly as shown in Fig. 2(c) and (d). Comparing the microstructure at solder material side, it can be seen that Sn–3Ag–0.5Cu–3Bi has obvious as-soldered eutectic structure (Fig. 2(b)), while it changes to particle structure with the aging process as shown in Fig. 2(d). With regard to the IMC layer, the thickness increases with increasing aging temperature and aging time. The Sn–3Ag–0.5Cu/Cu layer exhibits rapid growth rate, and moreover cavities can be observed in the long-term
Fig. 2. The microstructure evolution of Sn–3Ag–0.5Cu/Cu and Sn–3Ag–0.5Cu–3Bi/Cu joint samples during aging process: (a) 0Bi, as soldered; (b) 3Bi, as soldered; (c) 0Bi, aged at 150 ◦ C for 1000 h; (d) 3Bi, aged at 150 ◦ C for 1000 h.
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Fig. 3. The growth kinetics of interfacial IMC layers versus the aging time at two temperatures: (a) 140 ◦ C and (b) 180 ◦ C. The solid squares, circles and triangle represent the relative thickness of total interfacial layers in Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu, and Sn–3Ag–0.5Cu–3Bi/Cu, respectively. The hollow ones in the bottom of (b) represent the thickness of Cu3 Sn layer.
aged layer. In the case of Sn–3Ag–0.5Cu–3Bi/Cu joint as shown in Fig. 2(d), it can be seen that the thickness of IMC layer as well as the density of cavities in the layer decreases. It implies that the IMC layer grows slowly and becomes more compact with the addition of Bi.
From the growth kinetics curve, one of the important factors to characterize the interfacial IMC growth, the apparent growth activation energies, can be obtained. The following Arrhenius relationship is often used to determine the activation energy for a layer growth:
3.2. The growth kinetics of IMC layer
D = D0 exp −
Generally, the isothermal growth kinetics of continuous intermetallic layer can be expressed using an empirical power law:
where D0 is the growth rate constant, Q is the activation energy, R is the gas constant and T is the aging temperature in absolute unit. Therefore, the activation energy can be calculated from the slope of the plot between ln D and 1/T by using linear regression model. Fig. 4 shows the Arrhenius plots for the IMC layer growth of the three solder joints. It is noted that the low temperature data (<140 ◦ C) exhibits a different values of apparent activation energies comparing with the high temperature data (140–195 ◦ C). The values of activation energy at high temperature (140–195 ◦ C) are 83.2, 86.9 and 95.9 kJ mol−1 for Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu and Sn–3Ag–0.5Cu–3Bi/Cu couples respectively. While the values at lower temperature (<140 ◦ C) are among 25–30 kJ mol−1 for the couples. Vianco [11] and Yoon [12] considered that the different activation energies are related to two mechanisms over the temperature ranges of the analysis. At relatively low temperatures with respect to the melting points of the corresponding materials (with metals 0.3–0.5 Tm), grain boundary diffusion becomes an important mechanism [4]. Since the melting temperature of these solders
d − d0 = Dt n
Q
(1)
where d0 is the as-soldered thickness of reaction layer, d is the aged thickness after time t, D is the growth rate coefficient, t is the reaction time and n is the time exponent. It has been recognized that a diffusion process appears to be largely responsible for IMC layer growth of many solder joints where the time exponents were approximately equal to 0.5 [9,12,18]. Fig. 3 shows the influence of aging temperature on the growth kinetics of IMC layers of the joints by taking the results at 140 ◦ C and 180 ◦ C as examples. The solid squares, circles and triangle in Fig. 3(a) and (b) represent the relative thickness of total interfacial layers in Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu, and Sn–3Ag–0.5Cu–3Bi/Cu, respectively. In the bottom of Fig. 3(b), the hollow symbols represent the growth kinetics of Cu3 Sn layer at 180 ◦ C. The relative thickness means the thickness of IMC layer (d) at aging time t subtracting the initial thickness (d0 ). It is found that the relative thickness of the total layer increases linearly with the square root of aging time. Moreover, it is clear that the addition of Bi reduces the growth rate of total IMC layer at the tested temperature region. Generally, a Cu6 Sn5 compound layer was observed after soldering, and then a Cu3 Sn compound layer as well as the Cu6 Sn5 layer was recognized after aging, but Cu3 Sn layer could not be measured reliably due to its small absolute values at low temperature [19]. In this experiment, only the Cu3 Sn layer at 180 ◦ C and 195 ◦ C were examined. The thickness of Cu3 Sn layer also increased linearly with the square root of aging time, as shown by Fig. 3(b). However, the addition of Bi had slight effect on the growth rate of Cu3 Sn layer. And the same results were found in the solder joints annealed at 195 ◦ C. These results suggest that the growth of interfacial Cu6 Sn5 layers has been inhibited by adding Bi into the solder. Because Sn is the main diffusion species at this temperature range [4], it can be considered that the addition of Bi decreased the Sn flux in Cu6 Sn5 layer.
RT
Fig. 4. Arrhenius plot of the IMC’s layer growth for the three kinds of couples.
(2)
J. Zhao et al. / Journal of Alloys and Compounds 473 (2009) 382–388
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Fig. 5. EPMA analysis of Sn–3Ag–0.5Cu–3Bi/Cu joint aged at 150 ◦ C for 1000 h. The line scanning results for each element are based on analyzing the area between two white lines: (a) BEI; (b) Cu distribution; (c) Bi distribution; (d) Ag distribution.
Fig. 6. SEM micrographs of Sn–3Ag–0.5Cu–3Bi/Cu couple aged at 170 ◦ C for 250 h: (a) near IMC layer and (b) bulk solder far from IMC layer.
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Fig. 7. Shear strength versus aging time for the couples aged at: (a) 140 ◦ C and (b) 195 ◦ C.
are 219–220 ◦ C, the lower activation energy in lower temperature range means that the growth is controlled by the grain boundary diffusion [12]. And the higher activation energy in higher temperature is attributed to the bulk diffusion through the IMC layer. It appears that the addition of Bi can enhance the activation energy and thus reduce the growth rate of IMC layer. The inhibition effect of Bi addition was interpreted by the following considerations: firstly, Bi can affect the driving force of Sn diffusion through Cu6 Sn5 layer and also the driving force of the formation of Cu6 Sn5 layer [4,20]. Ma [21] discussed the effect of adding La on the growth of interfacial IMC layers by means of thermodynamic analysis, and found that adding a small amount of La can reduce the driving force for Cu6 Sn5 IMC formation. And then
the Cu6 Sn5 layer was reduced. Their observations are similar to the present experimental results. Secondly, Bi retards Sn diffusion from the solder to IMC. When Bi has been added into the solder, the solid-solution of Bi leads to the lattice distortion in Sn-rich phase. This phenomenon has been confirmed by the observation of the enlarged lattice plane spacing in Sn–Ag–Bi alloy [18]. And the lattice distortion affects the atomic self-diffusion [22] and the mobility of interstitial impurities [23]. The supply flux of Sn from Sn substrate may be decreased by this effect. Fig. 5 shows the morphology and X-ray mapping of a Sn–3Ag–0.5Cu–3Bi/Cu joint after aging at 150 ◦ C for 1000 h. The line scanning result for each element is based on analyzing the area between two white lines as shown in the figure. It is clear
Fig. 8. Shear fracture morphology of the couples aged at 140 ◦ C: (a) 360 h, aged at 140 ◦ C, Sn3Ag0.5Cu/Cu; (b) 600 h, aged at 140 ◦ C, Sn3Ag0.5Cu/Cu; (c) 360 h, aged at 140 ◦ C, Sn3Ag0.5Cu3Bi/Cu; (d) 600 h, aged at 140 ◦ C, Sn3Ag0.5Cu3Bi/Cu.
J. Zhao et al. / Journal of Alloys and Compounds 473 (2009) 382–388
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Fig. 9. Shear fracture morphology of the couples aged at 195 ◦ C: (a) 120 h, aged at 195 ◦ C, Sn3Ag0.5Cu1Bi/Cu; (b) 600 h, aged at 195 ◦ C, Sn3Ag0.5Cu1Bi/Cu; (c) 120 h, aged at 195 ◦ C, Sn3Ag0.5Cu3Bi/Cu; (d) 600 h, aged at 195 ◦ C, Sn3Ag0.5Cu3Bi/Cu.
that Ag is dominantly distributed in the particles in the bulk solder side; Cu is mainly distributed in the particles in the bulk solder side as well as in the IMC layer. For the distribution of Bi, it is found that Bi is primarily distributed in the bulk solder side with an obvious accumulation near the IMC layer. It has been indicated that Bi normally dissolves in Sn matrix in the Sn–Ag–Cu-based solders at high temperature, while it precipitates from Sn matrix at low temperature [20]. Fig. 6 compares the fine Bi precipitates at different location. Fig. 6(a) is near IMC interface and Fig. 6(b) is the bulk solder region far from IMC layer. Besides Ag3 Sn particles, very fine precipitates can also be observed in the solder matrix, and this kind of fine precipitation is more significant near the IMC layer than the region far from the IMC layer. It is corresponding with the continuous decrease of Bi concentration from the IMC layer as shown in Fig. 5(c). A hypothesis is proposed to explain why the element Bi accumulated adjacent to the joint: according to the Sn–Bi phase diagrams, the Sn-rich phase can dissolve substantial amount of Bi at higher temperature. During the aging process, the initial formed IMC layers continuously grow to bulk solder side. The reaction between Sn and Cu normally form Cu6 Sn5 compound. As Bi does not dissolve in Cu6 Sn5 compound, more Bi might move to bulk solder side. As a result, Bi accumulates in the Sn matrix near the joint and Bi particles will precipitate from Sn-rich phase. It is supposed that the precipitation and accumulation of Bi also act as obstacles to suppress the growth rate of IMC layer. 3.3. Shear property The shear tests were performed for the samples aged at 140 ◦ C and 195 ◦ C for 120 h, 240 h, 360 h, 480 h and 600 h. Fig. 7 presents the shear strength results obtained for the couples. In general, the
shear strengths of the couples aged at 140 ◦ C are higher than those aged at 195 ◦ C. With the addition of Bi, an obvious improvement in shear strength is obtained for the samples aged at 140 ◦ C rather than the samples aged at 195 ◦ C. In the case of 140 ◦ C aging process as shown in Fig. 7(a), the shear strength remained almost constant with aging time up to 500 h, and there is a remarkably drop occurred for Bi-bearing samples aged to 600 h. While for the samples aged at 195 ◦ C as shown in Fig. 7(b), there has a continuously decrease of shear strength with aging time for three kinds of couples. Fig. 8 shows the shear fracture morphology of specimens aged at 140 ◦ C. Typical ductile shear mode is observed on the fracture surface aged up to 500 h as shown in Fig. 8(a) and (c). However, for the sample aged to 600 h, the morphology transforms to mixture or brittle fracture modes which can be seen in Fig. 8(b) and (d). Comparatively, brittle fracture is the dominant mode for the shear specimens aged at 195 ◦ C as shown in Fig. 9. It is supposed that the ductile fracture occurred in the bulk solder region, while the mixture and brittle fracture happened in the region near solder/intermetallics interface. The change of fracture mode in the solder joints is dependent on the thickness of interfacial IMC layers [18]. In the stage without serious IMC propagation, the round scallop-type morphology still remains, therefore the couples have high adhesive strength and the fracture is controlled by bulk solder alloys. Solid-solution and precipitation strengthening mechanisms in the solder alloys are contributing factors in the strength properties [2,14,24]. The data in Fig. 7(a) has good coincidence with the results obtained in the bulk alloys in our previous report [8], where it has been indicated that the Sn–Ag–Cu–xBi solders exhibited relative stable property with aging process and the Bi-bearing solders have higher strength. With the significant growth of IMC layers, because of the difference of lattice constant and property between IMC layer and solder alloys, it
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might develop serious residual stress around solder/IMC interface [25]. Therefore the strength near solder/IMC interface becomes a weak factor for fracture and it leads to a decrease of shear strength with aging time. 4. Conclusions From the foregoing results about Bi influence on the IMC growth kinetics and shear property in Sn–3Ag–0.5Cu–xBi/Cu joints, it is concluded that: (1) The thickness of intermetallic compound (IMC) layer at the solder/Cu interface increases with aging temperature and aging time, and the addition of Bi delays the propagation rate of IMC layer. (2) From Arrhenius plots on the IMC layer growth of the three solder joints, the IMC layer growth behavior exhibits two regions with temperature: low temperature region (<140 ◦ C) where the apparent activation energies are among 25–30 kJ mol−1 , and high temperature region (140–195 ◦ C) where the values of activation energy are among 83–96 kJ mol−1 . The difference of activation energy at two temperature regions is attributed to the change of diffusion mechanism from bulk diffusion at higher temperature to grain boundary diffusion at low temperature. (3) For the couples aged at 140 ◦ C, their shear strength remained almost constant with aging time up to 500 h and the addition of Bi exhibits an improvement in the strength. Ductile fracture in solder matrix is the dominant mode. While for the samples aged at 195 ◦ C, there has a continuously decrease in shear strength with aging time and brittle fracture along solder/IMC interface becomes the dominant mode.
Acknowledgement This work has been supported by NSFC (National Nature Science Foundation of China) Funding (Project No. 50471068). References [1] K. Doi, H. Ohtani, M. Hasebe, Mater. Trans. 45 (2004) 380–383. [2] J. Zhao, Y. Mutoh, Y. Miyashita, S.L. Mannan, J. Electron. Mater. 31 (2002) 879–886. [3] B.T. Lampe, Weld. Res. Supp. 55 (1976) 330–334. [4] T. Laurila, V. Vuorinen, J.K. Kivilahti, Mater. Sci. Eng. R 49 (2005) 1–60. [5] X.Q. Shi, H.F. Kwan, S.M.L. Nai, G.H. Lim, J. Mater. Sci. 39 (2004) 1095–1099. [6] Y. Takaku, X.J. Liu, I. Ohnuma, R. Kainuma, K. Ishida, Mater. Trans. 45 (2004) 646–651. [7] A. Hirose, H. Yanagawa, E. Ide, K. Kobayashi, Sci. Tech. Adv. Mater. 5 (2004) 267–276. [8] J. Zhao, L. Qi, X.M. Wang, L. Wang, J. Alloys Compd. 375 (2004) 196–201. [9] M.J. Rizvi, Y.C. Chan, C. Bailey, H. Lu, M.N. Islam, J. Alloys Compd. 407 (2006) 208–214. [10] G.Y. Li, X.Q. Shi, Trans. Nonferrous Met. SOC. China 16 (2006) s739–s743. [11] P.T. Vianco, P.F. Hlava, A.C. Kilgo, J. Electron Mater. 23 (1994) 583–594. [12] J.W. Yoon, C.B. Lee, S.B. Jung, Mater. Trans. 43 (2002) 1821–1826. [13] J. Zhao, L. Qi, L. Wang, 2006 Conference on High Density Microsystem Design and Packaging and Component Failure Analysis HDP’06, Shanghai, 2006, 59 pp. [14] P.T. Vianco, J.A. Rejent, J. Electron. Mater. 28 (1999) 1138–1143. [15] R. Mayappan, A.B. Ismail, Z.A. Ahmad, T. Ariga, L.B. Hussain, J. Alloys Compd. 436 (2007) 112–117. [16] M.L. Huang, T. Loeher, A. Ostmann, H. Reichi, Appl. Phys. Lett. 86 (2005) 181908. [17] C.K. Shin, Y.J. Balk, J.Y. Huh, J. Electron Mater. 30 (2001) 1323–1331. [18] P.T. Vianco, J.A. Rejent, J. Electron Mater. 28 (1999) 1127–1137. [19] T. Takenaka, S. Kano, M. Kajihara, N. Kurokawa, K. Sakamoto, Mater. Sci. Eng. A 396 (2005) 115–123. [20] F.J.J. van Loo, Prog. Solid State Chem. 20 (1990) 47–99. [21] X. Ma, Y. Qian, F. Yoshida, J. Alloys Compd. 334 (2002) 224–227. [22] L.S. Perkins, A.E. DePristo, Surf. Sci. 325 (1995) 169–176. [23] V.N. Shershnev, P.V. Volobuev, Sov. Phys. Solid State 26 (1984) 440–442. [24] M. He, V.L. Acoff, J. Electron Mater. 35 (2006) 2098–2106. [25] H.T. Lee, M.H. Chen, Mater. Sci. Eng. A 333 (2002) 24–34.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Kinetics of intermetallic compound layers and shear strength in Bi-bearing SnAgCu/Cu soldering couples Jie Zhao ∗ , Cong-qian Cheng, Lin Qi, Cheng-yu Chi National Key Laboratory of Materials Modification, School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2008 Received in revised form 16 May 2008 Accepted 25 May 2008 Available online 11 July 2008 Keywords: Kinetics Intermetallic Shear strength Solder
a b s t r a c t Three lead-free solder alloys, Sn–3Ag–0.5Cu, Sn–3Ag–0.5Cu–1Bi and Sn–3Ag–0.5Cu–3Bi, have been used to solder with Cu substrate in the current experiments to investigate the influence of Bi on the kinetics of intermetallic compound (IMC) layer growth and shear strength of the soldering couples. The experimental results indicate that the thickness of IMC layer at the solder/Cu interface increases with aging process. The addition of Bi delays the propagation rate of total IMC layer. For the samples aged at 140 ◦ C up to 500 h, their shear strengths keep at a relative stable level where the fracture occurs in bulk solder side and the addition of Bi improve the strength level, while for the samples aged at 195 ◦ C, the shear strengths decrease continuously with aging time where the fracture occurs along interface between IMC layers and solders and the influence of Bi can be negligible. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, as the use of Pb-bearing solder is restricted due to the environmental and health concern [1–3], significant pressure has been put forward to the development of lead-free solder. As for achieving higher circuit board component densities, package dimensions have been shrinking with decreasing solder bump sizes. Therefore, reduction of solder bump size has resulted in severe requirements for mechanical reliability of solder joint. Among the new developed lead-free solders, Sn–Ag–Cu–Bi is regarded as one with high quality and performance and can meet the requirement of electronic packaging. Researches have been reported on their property and microstructure evolution [1,2]. Normally, solders in joints are already worked at high homologous temperature even at room temperature because of their low melting temperatures. Therefore, the microstructures are unstable during applications. It is believed that the microstructure evolution can significantly influence the reliability of solder joints, therefore the knowledge of microstructure change in solders as well as in joints is required [4]. The study of the effect of aging time on interfacial microstructure and growth of intermetallic compound (IMC) was performed in Sn–3.5Ag, Sn–Bi, Sn–Zn–Bi and Sn–Ag–Bi solder joints [5–7]. In our previous work, the influence of Bi on microstructure evolution and mechanical properties in bulk Sn–Ag–Cu solders was reported [8]. Relative stable mechanical properties with aging
∗ Corresponding author. Tel.: +86 411 84707636. E-mail address: [email protected] (J. Zhao). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.05.082
procedure were observed in Bi-bearing solders which attributed to the strengthening effect of Bi from solid solution strengthening to precipitating strengthening. Recently, effect of Bi on the IMC layer formations in Sn–Ag–Cu/Cu solder joints during soldering and isothermal aging has been reported [9,10], it is demonstrated that Bi addition delay the growth tendency of IMC layer. Other researchers suggest that the growth mechanism of interfacial IMC layers will change when the Sn/Cu and Sn58Bi/Cu joints are aged in different temperature ranges [11,12]. And we also have found the similar result in the Sn–Ag–Cu–Bi/Cu joints [13]. However, the effect of Bi on the growth activation energies in variant temperature ranges has not been described in detail. From the interfacial reliability of solder joints during service, it is necessary to study the mechanical properties of Sn–Ag–Cu–Bi solder joints. To ensure the reliability of the joints, knowledge is required of the shear strength of the solders joints during aging. The previous study shows that addition of Bi increases the shear strength in Cu/Sn–Ag–Bi/Cu joints and the tensile strength of Sn–8Zn–3Bi solder during soldering [14,15]. However, the presence of Cu in the solder can affect the growth kinetics of interfacial IMC layer during the soldering [16]. As described above, the mechanical properties of the substrate/solder joint are greatly influenced by the growth kinetics of interface IMC layer because of the intrinsic brittleness. And the change in the fracture mode of solder joint is also closely related to the interfacial IMC thickness [17]. So it is of particular important to investigate the shear strength of Sn–Ag–Cu–Bi/Cu solder during aging. Unfortunately, scarce work has been published on the influence of Bi addition on the shear strength of solder joints during aging process.
J. Zhao et al. / Journal of Alloys and Compounds 473 (2009) 382–388
In order to apply a fully understanding of Bi effect on the growth activation energies of IMC layers in various temperature ranges and the mechanical properties in Sn–Ag–Cu solder joints, the current work focused on the kinetics of IMC layer growth in Sn–3Ag–0.5Cu–xBi/Cu (x = 0, 1, 3) solder joints in a very large temperature range, as well as the shear strength of the couples. 2. Experimental procedures Three lead-free solder alloys, Sn–3Ag–0.5Cu, Sn–3Ag–0.5Cu–1Bi and Sn–3Ag–0.5Cu–3Bi, were used in the current experiments. They were remelted and cast into bars in a copper mould, then cut into plates with diameter of 6 mm, and grinded to weight of 0.38 ± 0.005 g. The melting temperature of these solders are measured to be 219–220 ◦ C. The substrates used in this study were pure Cu sheets. These substrates as well as the solder plates were grinded with fine sandpaper and cleaned with acetone before re-flow process to remove surface oxide and contaminates. The Sn–3Ag–0.5Cu–xBi/Cu couple samples were prepared by reflowing solder on Cu substrates at peak temperature of 270 ◦ C and spent 30 s at molten state. Rosin mildly activated (RMA) flux was used. To investigate the growth kinetics of IMC layers, some of the soldered specimens were aged up to 1000 h at temperatures of 70 ◦ C, 120 ◦ C, 150 ◦ C and 170 ◦ C, the others were aged up to 600 h at temperatures of 140 ◦ C, 160 ◦ C, 180 ◦ C and 195 ◦ C after soldering process. The temperature fluctuation during aging process was kept within ±1 ◦ C. The aged samples were then mounted in epoxy and grinded. Finally the samples were polished by using 0.3 m diamond paste and etched with a solution of 5%HNO3 –2%HCl–93% methanol for several seconds. In order to examine the microstructure evolution of the joints and growth kinetics of IMC layers, all the samples were observed using JEOL JSM-5600LV scanning electron microscopy (SEM). The components of interfacial IMC layers were identified using Energy Dispersive of X-ray (EDX). The measurements of interfacial IMC layer thickness were done by dividing the area of IMC layer by its length using Q500IW image analyzer. The elemental distribution near jointed interfaces was analyzed by using Shimadzu EPMA-1600. In this study, the soldering couple samples for shear testing were designed based on Chinese National Standard GB/T15111-94 with a soldering area of 10 mm × 5 mm, as shown in Fig. 1. Following similar soldering process as described above, some of the specimens were thermally aged at temperatures of 140 ◦ C and 195 ◦ C up to 600 h.
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Fig. 1. Schematic diagram of solder joint. All the shear tests were performed at a strain rate of 10 mm/min. After shear tests, the fracture surface was observed using SEM.
3. Experimental results and discussions 3.1. Microstructural evolution Fig. 2 chooses Sn–3Ag–0.5Cu/Cu and Sn–3Ag–0.5Cu–3Bi/Cu joint samples as examples to demonstrate their microstructure evolution during aging process. From the observation on the as-soldered interfacial microstructures, it can be seen that the IMC layers after reflow for all the types of joints have round scallop-type morphology. The interfacial IMC layers are mainly composed of Cu6 Sn5 . The interfacial Cu3 Sn layers are too thin to be observed. After aging treatment, the interfacial Cu6 Sn5 and Cu3 Sn layers appear in the solder joints. The round scallop-type morphology gradually disappears and changes to planar-type morphology, and the thickness of IMC layer increases significantly as shown in Fig. 2(c) and (d). Comparing the microstructure at solder material side, it can be seen that Sn–3Ag–0.5Cu–3Bi has obvious as-soldered eutectic structure (Fig. 2(b)), while it changes to particle structure with the aging process as shown in Fig. 2(d). With regard to the IMC layer, the thickness increases with increasing aging temperature and aging time. The Sn–3Ag–0.5Cu/Cu layer exhibits rapid growth rate, and moreover cavities can be observed in the long-term
Fig. 2. The microstructure evolution of Sn–3Ag–0.5Cu/Cu and Sn–3Ag–0.5Cu–3Bi/Cu joint samples during aging process: (a) 0Bi, as soldered; (b) 3Bi, as soldered; (c) 0Bi, aged at 150 ◦ C for 1000 h; (d) 3Bi, aged at 150 ◦ C for 1000 h.
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Fig. 3. The growth kinetics of interfacial IMC layers versus the aging time at two temperatures: (a) 140 ◦ C and (b) 180 ◦ C. The solid squares, circles and triangle represent the relative thickness of total interfacial layers in Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu, and Sn–3Ag–0.5Cu–3Bi/Cu, respectively. The hollow ones in the bottom of (b) represent the thickness of Cu3 Sn layer.
aged layer. In the case of Sn–3Ag–0.5Cu–3Bi/Cu joint as shown in Fig. 2(d), it can be seen that the thickness of IMC layer as well as the density of cavities in the layer decreases. It implies that the IMC layer grows slowly and becomes more compact with the addition of Bi.
From the growth kinetics curve, one of the important factors to characterize the interfacial IMC growth, the apparent growth activation energies, can be obtained. The following Arrhenius relationship is often used to determine the activation energy for a layer growth:
3.2. The growth kinetics of IMC layer
D = D0 exp −
Generally, the isothermal growth kinetics of continuous intermetallic layer can be expressed using an empirical power law:
where D0 is the growth rate constant, Q is the activation energy, R is the gas constant and T is the aging temperature in absolute unit. Therefore, the activation energy can be calculated from the slope of the plot between ln D and 1/T by using linear regression model. Fig. 4 shows the Arrhenius plots for the IMC layer growth of the three solder joints. It is noted that the low temperature data (<140 ◦ C) exhibits a different values of apparent activation energies comparing with the high temperature data (140–195 ◦ C). The values of activation energy at high temperature (140–195 ◦ C) are 83.2, 86.9 and 95.9 kJ mol−1 for Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu and Sn–3Ag–0.5Cu–3Bi/Cu couples respectively. While the values at lower temperature (<140 ◦ C) are among 25–30 kJ mol−1 for the couples. Vianco [11] and Yoon [12] considered that the different activation energies are related to two mechanisms over the temperature ranges of the analysis. At relatively low temperatures with respect to the melting points of the corresponding materials (with metals 0.3–0.5 Tm), grain boundary diffusion becomes an important mechanism [4]. Since the melting temperature of these solders
d − d0 = Dt n
Q
(1)
where d0 is the as-soldered thickness of reaction layer, d is the aged thickness after time t, D is the growth rate coefficient, t is the reaction time and n is the time exponent. It has been recognized that a diffusion process appears to be largely responsible for IMC layer growth of many solder joints where the time exponents were approximately equal to 0.5 [9,12,18]. Fig. 3 shows the influence of aging temperature on the growth kinetics of IMC layers of the joints by taking the results at 140 ◦ C and 180 ◦ C as examples. The solid squares, circles and triangle in Fig. 3(a) and (b) represent the relative thickness of total interfacial layers in Sn–3Ag–0.5Cu/Cu, Sn–3Ag–0.5Cu–1Bi/Cu, and Sn–3Ag–0.5Cu–3Bi/Cu, respectively. In the bottom of Fig. 3(b), the hollow symbols represent the growth kinetics of Cu3 Sn layer at 180 ◦ C. The relative thickness means the thickness of IMC layer (d) at aging time t subtracting the initial thickness (d0 ). It is found that the relative thickness of the total layer increases linearly with the square root of aging time. Moreover, it is clear that the addition of Bi reduces the growth rate of total IMC layer at the tested temperature region. Generally, a Cu6 Sn5 compound layer was observed after soldering, and then a Cu3 Sn compound layer as well as the Cu6 Sn5 layer was recognized after aging, but Cu3 Sn layer could not be measured reliably due to its small absolute values at low temperature [19]. In this experiment, only the Cu3 Sn layer at 180 ◦ C and 195 ◦ C were examined. The thickness of Cu3 Sn layer also increased linearly with the square root of aging time, as shown by Fig. 3(b). However, the addition of Bi had slight effect on the growth rate of Cu3 Sn layer. And the same results were found in the solder joints annealed at 195 ◦ C. These results suggest that the growth of interfacial Cu6 Sn5 layers has been inhibited by adding Bi into the solder. Because Sn is the main diffusion species at this temperature range [4], it can be considered that the addition of Bi decreased the Sn flux in Cu6 Sn5 layer.
RT
Fig. 4. Arrhenius plot of the IMC’s layer growth for the three kinds of couples.
(2)
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Fig. 5. EPMA analysis of Sn–3Ag–0.5Cu–3Bi/Cu joint aged at 150 ◦ C for 1000 h. The line scanning results for each element are based on analyzing the area between two white lines: (a) BEI; (b) Cu distribution; (c) Bi distribution; (d) Ag distribution.
Fig. 6. SEM micrographs of Sn–3Ag–0.5Cu–3Bi/Cu couple aged at 170 ◦ C for 250 h: (a) near IMC layer and (b) bulk solder far from IMC layer.
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Fig. 7. Shear strength versus aging time for the couples aged at: (a) 140 ◦ C and (b) 195 ◦ C.
are 219–220 ◦ C, the lower activation energy in lower temperature range means that the growth is controlled by the grain boundary diffusion [12]. And the higher activation energy in higher temperature is attributed to the bulk diffusion through the IMC layer. It appears that the addition of Bi can enhance the activation energy and thus reduce the growth rate of IMC layer. The inhibition effect of Bi addition was interpreted by the following considerations: firstly, Bi can affect the driving force of Sn diffusion through Cu6 Sn5 layer and also the driving force of the formation of Cu6 Sn5 layer [4,20]. Ma [21] discussed the effect of adding La on the growth of interfacial IMC layers by means of thermodynamic analysis, and found that adding a small amount of La can reduce the driving force for Cu6 Sn5 IMC formation. And then
the Cu6 Sn5 layer was reduced. Their observations are similar to the present experimental results. Secondly, Bi retards Sn diffusion from the solder to IMC. When Bi has been added into the solder, the solid-solution of Bi leads to the lattice distortion in Sn-rich phase. This phenomenon has been confirmed by the observation of the enlarged lattice plane spacing in Sn–Ag–Bi alloy [18]. And the lattice distortion affects the atomic self-diffusion [22] and the mobility of interstitial impurities [23]. The supply flux of Sn from Sn substrate may be decreased by this effect. Fig. 5 shows the morphology and X-ray mapping of a Sn–3Ag–0.5Cu–3Bi/Cu joint after aging at 150 ◦ C for 1000 h. The line scanning result for each element is based on analyzing the area between two white lines as shown in the figure. It is clear
Fig. 8. Shear fracture morphology of the couples aged at 140 ◦ C: (a) 360 h, aged at 140 ◦ C, Sn3Ag0.5Cu/Cu; (b) 600 h, aged at 140 ◦ C, Sn3Ag0.5Cu/Cu; (c) 360 h, aged at 140 ◦ C, Sn3Ag0.5Cu3Bi/Cu; (d) 600 h, aged at 140 ◦ C, Sn3Ag0.5Cu3Bi/Cu.
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Fig. 9. Shear fracture morphology of the couples aged at 195 ◦ C: (a) 120 h, aged at 195 ◦ C, Sn3Ag0.5Cu1Bi/Cu; (b) 600 h, aged at 195 ◦ C, Sn3Ag0.5Cu1Bi/Cu; (c) 120 h, aged at 195 ◦ C, Sn3Ag0.5Cu3Bi/Cu; (d) 600 h, aged at 195 ◦ C, Sn3Ag0.5Cu3Bi/Cu.
that Ag is dominantly distributed in the particles in the bulk solder side; Cu is mainly distributed in the particles in the bulk solder side as well as in the IMC layer. For the distribution of Bi, it is found that Bi is primarily distributed in the bulk solder side with an obvious accumulation near the IMC layer. It has been indicated that Bi normally dissolves in Sn matrix in the Sn–Ag–Cu-based solders at high temperature, while it precipitates from Sn matrix at low temperature [20]. Fig. 6 compares the fine Bi precipitates at different location. Fig. 6(a) is near IMC interface and Fig. 6(b) is the bulk solder region far from IMC layer. Besides Ag3 Sn particles, very fine precipitates can also be observed in the solder matrix, and this kind of fine precipitation is more significant near the IMC layer than the region far from the IMC layer. It is corresponding with the continuous decrease of Bi concentration from the IMC layer as shown in Fig. 5(c). A hypothesis is proposed to explain why the element Bi accumulated adjacent to the joint: according to the Sn–Bi phase diagrams, the Sn-rich phase can dissolve substantial amount of Bi at higher temperature. During the aging process, the initial formed IMC layers continuously grow to bulk solder side. The reaction between Sn and Cu normally form Cu6 Sn5 compound. As Bi does not dissolve in Cu6 Sn5 compound, more Bi might move to bulk solder side. As a result, Bi accumulates in the Sn matrix near the joint and Bi particles will precipitate from Sn-rich phase. It is supposed that the precipitation and accumulation of Bi also act as obstacles to suppress the growth rate of IMC layer. 3.3. Shear property The shear tests were performed for the samples aged at 140 ◦ C and 195 ◦ C for 120 h, 240 h, 360 h, 480 h and 600 h. Fig. 7 presents the shear strength results obtained for the couples. In general, the
shear strengths of the couples aged at 140 ◦ C are higher than those aged at 195 ◦ C. With the addition of Bi, an obvious improvement in shear strength is obtained for the samples aged at 140 ◦ C rather than the samples aged at 195 ◦ C. In the case of 140 ◦ C aging process as shown in Fig. 7(a), the shear strength remained almost constant with aging time up to 500 h, and there is a remarkably drop occurred for Bi-bearing samples aged to 600 h. While for the samples aged at 195 ◦ C as shown in Fig. 7(b), there has a continuously decrease of shear strength with aging time for three kinds of couples. Fig. 8 shows the shear fracture morphology of specimens aged at 140 ◦ C. Typical ductile shear mode is observed on the fracture surface aged up to 500 h as shown in Fig. 8(a) and (c). However, for the sample aged to 600 h, the morphology transforms to mixture or brittle fracture modes which can be seen in Fig. 8(b) and (d). Comparatively, brittle fracture is the dominant mode for the shear specimens aged at 195 ◦ C as shown in Fig. 9. It is supposed that the ductile fracture occurred in the bulk solder region, while the mixture and brittle fracture happened in the region near solder/intermetallics interface. The change of fracture mode in the solder joints is dependent on the thickness of interfacial IMC layers [18]. In the stage without serious IMC propagation, the round scallop-type morphology still remains, therefore the couples have high adhesive strength and the fracture is controlled by bulk solder alloys. Solid-solution and precipitation strengthening mechanisms in the solder alloys are contributing factors in the strength properties [2,14,24]. The data in Fig. 7(a) has good coincidence with the results obtained in the bulk alloys in our previous report [8], where it has been indicated that the Sn–Ag–Cu–xBi solders exhibited relative stable property with aging process and the Bi-bearing solders have higher strength. With the significant growth of IMC layers, because of the difference of lattice constant and property between IMC layer and solder alloys, it
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might develop serious residual stress around solder/IMC interface [25]. Therefore the strength near solder/IMC interface becomes a weak factor for fracture and it leads to a decrease of shear strength with aging time. 4. Conclusions From the foregoing results about Bi influence on the IMC growth kinetics and shear property in Sn–3Ag–0.5Cu–xBi/Cu joints, it is concluded that: (1) The thickness of intermetallic compound (IMC) layer at the solder/Cu interface increases with aging temperature and aging time, and the addition of Bi delays the propagation rate of IMC layer. (2) From Arrhenius plots on the IMC layer growth of the three solder joints, the IMC layer growth behavior exhibits two regions with temperature: low temperature region (<140 ◦ C) where the apparent activation energies are among 25–30 kJ mol−1 , and high temperature region (140–195 ◦ C) where the values of activation energy are among 83–96 kJ mol−1 . The difference of activation energy at two temperature regions is attributed to the change of diffusion mechanism from bulk diffusion at higher temperature to grain boundary diffusion at low temperature. (3) For the couples aged at 140 ◦ C, their shear strength remained almost constant with aging time up to 500 h and the addition of Bi exhibits an improvement in the strength. Ductile fracture in solder matrix is the dominant mode. While for the samples aged at 195 ◦ C, there has a continuously decrease in shear strength with aging time and brittle fracture along solder/IMC interface becomes the dominant mode.
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