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Whisker growth behaviors in POSS-silanol modified Sn3.0Ag0.5Cu composite solders
Journal of Alloys and Compounds 657 (2016) 400e407
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Whisker growth behaviors in POSS-silanol modified Sn3.0Ag0.5Cu composite solders Limin Ma a, Yong Zuo a, Sihan Liu a, Fu Guo a, *, Andre Lee b, **, K.N. Subramanian b a b
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Chemical Engineering and Materials Science Department, Michigan State University, East Lansing, MI 48824, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 29 September 2015 Accepted 30 September 2015 Available online 9 October 2015
As an ideal modifier, the cage-type polyhedral oligomeric silsesquioxane (POSS)-silanol creates a strong bonding with the solder without agglomeration. It has been shown that the addition of POSS-silanol improves both the mechanical performance and the solder's resistance to electromigration. This study further examined the effect of the addition of POSS-silanol on whisker growth in a Sn3.0Ag0.5Cu (SAC305) solder. The whisker growth was studied for pure SAC305 and SAC305 modified with POSSsilanol under isothermal aging and thermal cycling conditions. The results indicate that the added POSS-silanol not only reinforces the solder joint, but also inhibits whisker formation under both thermal aging and thermal cycling conditions. POSS-silanol can therefore be considered a promising modifier material which enhances the reliability of solders. During the thermal aging process, the added POSSsilanol was found to inhibit the interdiffusion of Sn and Cu atoms at the interface region, thereby preventing the formation of Cu6Sn5 but promoting the conversion of Cu6Sn5 into Cu3Sn. During the thermal cycling process, the added POSS-silanol significantly inhibited whisker formation and the deformation of the interface by stiffening the solder matrix and decreasing the driving force for whisker formation. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microstructure POSS-silanol Whisker formation Composite solder
1. Introduction Whisker formation is a frequent problem in the electronics industry, causing damage to fine-pitch electronic components. The transition to Pb-free electronics renders this issue even more important since Pb very effectively restrained whisker growth in tin [1]. Compressive stresses are considered causing the whisker growth, which can result from residual stresses in electrodeposits [2e4], the growth of intermetallic compounds (IMCs) [5e7], thermal cycling [8] or corrosion [9]. Strategies to mitigate the whisker growth generally aim to release or eliminate the compressive stresses by various means. Because the residual stress in electrodeposits depends on the texture of the deposited coatings, Ashworth et al. [2] varied the electroplating parameters to create a microstructure and grain orientation in the tin deposit that is inherently resistant to whisker growth. Using the same method, Eckold et al. [3,4] reported that (211)-orientated grains exhibit a low surface energy, resulting in a
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Guo), [email protected] (A. Lee). http://dx.doi.org/10.1016/j.jallcom.2015.09.266 0925-8388/© 2015 Elsevier B.V. All rights reserved.
lower susceptibility to corrosion and tin whisker formation. However, Pei et al. [10] argued that strategies focusing mainly on the structure of the as-deposited film may not be sufficient to prevent whisker growth because the initial sites may form after the film has been grown. It is believed that the formation of IMCs due to CueSn interdiffusion is one of most likely reasons for the whisker growth because of the volume expansion effect [11,12]. Therefore, a barrier layer between Cu and Sn is considered a viable option to inhibit whisker formation. Horvath et al. [5] compared the growth of IMCs after the introduction of Ni or Ag as an intermediate layer and found that the Ag layer was easily consumed by both Sn and Cu and rapidly became ineffective. The addition of metals (i.e., Ag, Bi, Ni and Au) was also considered as a potential solution in other studies, but none of these proved as effective as Pb [13e15]. Therefore, much more effort is required to find a feasible solution for the inhibition of whisker growth. Recently, cage-type polyhedral oligomeric silsesquioxane (POSS) trisilanol has been considered a suitable candidate for improving the service performance of Sn-based Pb-free solder materials. POSS-silanol can react with the metallic elements in the solder matrix without forming any new IMCs. The three surfaceactive silanol (SieOH) groups in POSS-silanol can react and form
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a strong bonding with the metal matrix; while the seven inert organic groups ensure a stable SieO-M bonding during changes of the service environment over a wide temperature range. In addition, because it is not a metallic material, electric and magnetic field will not affect the POSS modifier. Its unique structure makes it appealing for solder modification applications [16,17]. Previous studies [16e18] suggested that the addition of POSS-silanol significantly improves the overall performance of Sn-based Pbfree solders, e.g., their shear strength, resistance to electromigration, as well as the material's resistance to thermal fatigue. However, the effect of POSS-silanol on whisker formation has been rarely studied, and therefore this study focused on investigating the effect of the addition of POSS-silanol on whisker formation in the Sn3.0Ag0.5Cu (SAC305) solder material. To improve the understanding of the whisker formation process, comparative experiments were performed, in which pure SAC305 solders and SAC305 solders modified with POSS-silanol were subjected to the same thermal aging and thermal cycling conditions. 2. Experimental procedures
Fig. 2. Temperature profile recorded during the thermal cycling process.
The composite solder used for the experiments consisted of the SAC305 solder mixed with 3 wt% POSS-silanol. The addition of this proportion of POSS-silanol has been reported in literature to effectively enhance various properties of the solder material [16e18]. The composite solder was fabricated employing the mechanical mixing method. Firstly, the commercial SAC305 paste and the POSS-silanol particles were mixed proportionately in a ceramic crucible and stirred for 30 min in order to obtain a uniform mixture. Then, the mixture was subjected to a reflowing process and fabricated into a solid ingot. Finally, the as-casted ingot was rolled into a thin sheet with a thickness of 20 mm. The specimen preparation process can be briefly summarized as follows: First, a solder sheet was placed on a clean Cu substrate (10 mm 5 mm 0.5 mm). Then, a custom-made aluminum fixture was used to hold the substrate and the solder sheet, as shown in Fig. 1, which allowed to control the thickness of the solder coating. Finally, the sample was soldered in a reflow oven. The peak temperature during the reflow soldering was 300C and the dwell time at the peak temperature was 150s. After reflow soldering, the specimens were cooled to room temperature (20C) on an aluminum heat sink. Thermal aging and thermal cycling tests were conducted to investigate the effect of the addition of POSS-silanol on the whisker growth. The thermal aging experiments were performed at a temperature of 200C. For the thermal cycling tests, the temperature was varied between 40 and 85C. The dwell time was 10 min at both the maximum and minimum temperature. Fig. 2 shows the real temperature profile recorded by the thermocouple attached to the specimen's surface during the thermal cycling process. The microstructure and grain orientation of the specimens was studied by scanning electron microscopy (SEM, Hitachi S-3400
operated at 20 kV) and electron backscatter diffraction (EBSD), respectively. The specimens for the EBSD investigations were prepared using a precision etching-coating system to remove the strained layer. Energy dispersive X-Ray spectroscopy (EDX) was performed to determine the phase composition of the specimens.
3. Results and discussion 3.1. EBSD analysis of the specimens after reflow soldering Previous studies suggested that the addition of POSS-silanol could alter the microstructure of the solder matrix and influence their performance [16e18]. However, considering the small size of POSS-silanol and the small volume percentage, such changes might occur on a grain-scale level. Consequently, it is difficult to identify the POSS-silanol particles on the SEM images. Therefore, an EBSD analysis was conducted to compare the microstructure of the SAC305 solder material with and without POSS-silanol. The orientation images obtained for the specimens after reflow soldering are shown in Fig. 3. Comparing Fig. 3(a) and (d), the modified SAC305 solder matrix shows a more random distribution of orientations and a finer grain size of the Sn phases. The distribution of orientations could be better characterized using the (001) pole figures. As shown in Fig. 3(c) and (f), the solder modified with POSS-silanol features a more discrete pattern of orientations. Fig. 3(b) and (e) show the orientation images obtained for the Cu6Sn5 phase in the solder joints. Comparing Fig. 3(b) and (e), both the pure SAC305 and the modified SAC305 feature the typical scallop-shaped Cu6Sn5 layers after reflow soldering. However, the
Fig. 1. Schematic illustration of the test specimen and the custom-made aluminum fixture.
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Fig. 3. Orientation images and (001) pole figures: (a), (b) and (c) SAC305; (d), (e) and (f) SAC305 modified with POSS-silanol.
morphology of the Cu6Sn5 layer was thicker and sharper in the pure SAC305 than in the SAC305 modified with POSS-silanol. In addition, numerous Cu6Sn5 IMC phases were found dispersed in the SAC305 solder matrix, but only a low amount of Cu6Sn5 was observed in the modified solder. These results demonstrate that the addition of POSS-silanol results in a finer microstructure with random Sn grain orientations, which inhibits the growth of interfacial Cu6Sn5 as well as the migration of Cu6Sn5 into the solder matrix during the solidification process. These microstructure characteristics are beneficial to reducing the stress level and mitigating whisker growth. Firstly, the growth of Cu6Sn5 was identified as the driving force for whisker formation because of the volume expansion effect [11,12]. Therefore, inhibiting the growth of the interfacial Cu6Sn5 phase is expected to reduce the driving force for whisker formation. Secondly, the migrated Cu6Sn5 usually precipitates at the Sn grain boundaries [19]. Although its presence at the grain boundaries can enhance the mechanical performance, it would negatively affect the stress relief along the grain boundaries and induce a build-up of stress around those IMCs. This indicates that preventing the migration of Cu6Sn5 also results in an inhibition of whisker growth. The inhibitory effect of the added POSS-silanol on the formation of Cu6Sn5 and its contribution to a finer and stable microstructure have not been well explained in literature. A possible explanation for the effect might be as follows: The active eOH groups of the POSS-silanol strongly bonded with the metallic matrix via the formation of SieOeSn bonds within and at the grain boundary regions of the solder alloy. At the same time, the inert groups on the POSS-silanol prevented an agglomeration of the POSS-silanol particles [17]. Therefore, POSS-silanol might act as a barrier, effectively
blocking the grain boundary diffusion pathways of the atoms and inhibiting the solid state reaction between Cu and Sn which drives the formation of IMCs at the grain boundaries. 3.2. Evolution of the microstructure under isothermal aging conditions Fig. 4 shows the evolution of the microstructure at the interface under isothermal aging conditions. As demonstrated in Fig. 4(a) and (e), the solder matrix of the modified SAC305 exhibits a finer microstructure and a thinner interfacial IMCs layer after reflow soldering than the pure SAC305 specimen. This result is consistent with the results of the EBSD analysis mentioned above. After aging at 200C for 16 h, the scallop-shaped interfacial IMC layer was transformed and featured a planar morphology after aging, as shown in Fig. 4(b) and (f). This interfacial IMC layer continued to coarsen with increasing aging time. In addition, a continuous Cu3Sn layer appeared in-between the Cu6Sn5 and the Cu substrate. However, it is worth noting that the total IMC layer (counting both Cu3Sn and Cu6Sn5) in the pure SAC305 solder was much thicker than the IMC layer in the solder modified with POSS-silanol. In contrast, the Cu3Sn layer in the modified solder grew much faster than in the pure SAC305 solder. Fig. 5 shows the thickness of the combined IMC layer and the Cu3Sn layer as a function of the square root of the aging time. The thickness of the IMC layer was found to increase proportional to the square root of the aging time. These results are consistent with the results published by Li et al., but the growth rate was higher than the rate reported by Li et al. because the aging temperature used in this study was higher [20]. Fig. 5 shows that the total IMC layer
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Fig. 4. Evolution of the microstructure under different aging conditions: (a)e(d) pure SAC305, and (d)e(h) SAC305 modified with POSS-silanol.
grew faster in the pure SAC305 solder. However, the growth rate of the Cu3Sn layer was higher in the modified solder. These results suggest that the added POSS-silanol inhibited the growth of the Cu6Sn5 layer but promoted the growth of the Cu3Sn layer. This phenomenon can be explained as follows: The formation and growth of Cu6Sn5 and Cu3Sn depend on the interdiffusion of Sn and Cu, as illustrated in Fig. 6(a). In the SAC305 modified with POSS-silanol, the modifier is considered to be bonded to the Sn atoms. Therefore, the Sn atoms bonded to the POSS-silanol diffuse slower, which inhibits the growth of the Cu6Sn5 layer, as shown in Fig. 6(b). However, the Cu atoms from the substrate continue to diffuse into the already formed Cu6Sn5 layer. Therefore, the Cu3Sn layer grows at the expense of Cu6Sn5. Although this reaction also occurs in SAC305, the solder matrix can supply sufficient Sn atoms to react with the Cu atoms from the substrate to preferentially form
Cu6Sn5. In summary, the modifier POSS-silanol reduces the Cu6Sn5 formation rate by inhibiting the interdiffusion of Sn and Cu in the solder/interfacial IMC boundary region. However, in the Cu6Sn5 region, due to the continued supply of Cu atoms, Cu6Sn5 can easily be converted into Cu3Sn, resulting in a more rapid growth of the Cu3Sn layer, as shown in Fig. 6(c). This special IMC growth behavior facilitated the inhibition of the volume expansion effect. For the formation of Cu6Sn5, Cu atoms must first diffuse into the Sn lattice and occupy the sites of the Sn atoms. The reaction then generates a molar volume expansion of 36.56 cm3/mol (DV ¼ VCu6Sn5 e 5 VSn, molar volume of the different phases is listed in Table 1). Concerning the formation of Cu3Sn5, the reaction between Cu and Cu6Sn5 generates a molar volume expansion of 18.49 cm3/mol (DV ¼ 5 VCu3Sn 5 VCu6Sn5) [11,12]. On the one hand, the added POSS-silanol inhibited the
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L. Ma et al. / Journal of Alloys and Compounds 657 (2016) 400e407 Table 1 Comparison of selected properties of the materials involved in this study. Materials
3
Density (g/cm ) Molar volume (cm3/mol) CTE (ppm/ C) Young's Modulus (GPa)
Fig. 5. Thickness of the IMC layer as a function of the square root of the aging time.
growth of Cu6Sn5. On the other hand, it promoted the transition of Cu6Sn5 into Cu3Sn. These two aspects both helped to account for the volume expansion, thereby reducing the compressive stress. It must be pointed out that no whisker growth was observed even in the unmodified SAC305 specimens during and after thermal aging. This may be attributed to the high aging temperature. A high annealing temperature and a post-annealing treatment at 150C for 1 h are effective ways to reduce whisker formation, as previously reported in literature [21,22]. Hence, the stress concentration due to the growth of the IMCs may be offset by the relaxation effect at elevated temperatures. Although no whisker growth was observed during the thermal aging experiments in this study, there is a possibility of whisker formation at high temperature. However, whisker growth was observed in Sn-based solders even at temperatures higher than 150C in another study [23]. Therefore, the current study might still increase understanding of the whisker growth mechanism and might help to find means to inhibit their formation. 3.3. Evolution of the microstructure under thermal cycling conditions Fig. 7 compares the whiskers growth at the surface of the pure SAC305 and the modified SAC305 solder after thermal cycling. For the pure SAC305 solder, whiskers were observed after 625 cycles, and most of them emerged along the crack, as shown in Fig. 7(b). The roughness of the microstructure of the pure SAC305 increased
Cu
Sn
Cu6Sn5
Cu3Sn
8.95 7.10 17.30 117.00
7.286 16.29 22.00 46.46
8.26 118.01 16.30 85.56
11.33 27.30 19.00 108.30
with the number of thermal cycles. In Fig. 7(c), the whiskers are larger, and some new whiskers with a smaller size appeared close to other microcracks. No further changes were observed when the total number of thermal cycles was increased to 3910 (Fig. 7(d)), which suggests that the internal stress had reached an equilibrium state. For the modified solder, a few tiny whiskers grew along a crack after 625 cycles, as shown in Fig. 7(f). No new whiskers formed when the number of thermal cycles was increased, but the crack became wider (Fig. 7(g) and (h)). Fig. 8 compares the interface region between the solder matrix and the Cu substrate for two different specimens during thermal cycling. As shown in Fig. 8(a) and (d), the microstructures were similar after reflow soldering. However, a serious deformation of the solder could be observed along the interface in the pure SAC305 solder, as shown in Fig. 8(b). With increasing number of thermal cycles, a crack appeared along the interface. For the modified solder, there was no obvious deformation or no cracks occurred at the interface region. The results shown in Figs. 7 and 8 demonstrate that the addition of POSS-silanol could effectively inhibit whisker formation and relieve the damage to the interfacial regions during thermal cycling. The reason for this effect is analyzed and discussed below. The whisker and damage formation mechanism in the pure SAC305 and the SAC305 modified with POSS-silanol is illustrated in Fig. 9. A mismatch in the coefficient of thermal expansion (CTE) between the different phases is considered the main cause for whisker growth and interfacial damage [24]. The CTE values of the different materials are compared in Table 1. There are obvious differences in the CTE between the Sn matrix and the CuSn IMCs or the Cu substrate. Therefore, an alternating stress is generated at the interface regions during thermal cycling, as illustrated in Fig. 9(a). It was noting that the CTE of Sn listed in Table 1 was in average. Sn features a body-centered tetragonal (BCT) structure, and the CTE of the c-axis is almost twice the CTE of the a-axis [25,26]. Therefore, in addition to the mismatch between the Sn matrix and the CuSn IMCs or the Cu substrate, a CTE mismatch between Sn grains with different orientations exists as well. Hence, in the pure SAC305 solder joint, cracks are very likely to occur among the Sn grains with the maximum divergence in orientation. Subsequently, with the crack representing a weak point, the growth of Sn whiskers was promoted at this site due to
Fig. 6. Evolution of the microstructure at the interface of SAC305 due to the addition of POSS-silanol during thermal aging: (a) early stage, (b) intermediate stage, and (c) advanced stage of diffusion.
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Fig. 7. Whisker growth at the surface of the solder during thermal cycling: (a)e(d) pure SAC305; (e)e(h) SAC305 modified with POSS-silanol.
the compressive stress originating from the bottom of the crack. The compressive stress was caused either by the misorientation of the Sn grains or the mismatch between the Sn matrix and the CuSn IMCs or the Cu substrate at the bottom of the crack, as shown in Fig. 9(b). However, when POSS-silanol was introduced, the SieOeSn bonds within and at the grain boundary regions reinforced the solder matrix. Previous studies [17,18] reported that the shear strength and micro hardness of SAC305 solders improved by about 42.86% and 11.18%, respectively, when 3 wt% POSS-silanol was
added to the SAC305 solder. Therefore, the movement and distortion of metal atoms caused by the CTE mismatch was limited by the pinning effect of the POSS-silanol. Furthermore, the POSS-silanol inhibited the growth of interfacial IMCs. This means that the compressive stress at the bottom was reduced and was not sufficient to drive the whisker out of the surface even when the crack appeared, as shown in Fig. 9(c). The combination of these two effects significantly inhibited whisker formation and interfacial deformation by stiffening the solder matrix and decreasing the driving force for whisker formation.
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Fig. 8. Evolution of the microstructure at the interface during thermal cycling: (a)e(c) pure SAC305, and (d)e(f) SAC305 modified with POSS-silanol.
Fig. 9. Schematic illustration of the whisker formation process: (a) stress analysis, (b) whisker formation in pure SAC305, and (c) whisker formation in SAC305 modified with POSSsilanol.
4. Conclusions This study aimed to investigate the whisker growth in SAC305 and POSS-silanol-modified SAC305 under either high isothermal temperature aging or thermal cycling conditions and to determine the effect of the addition of POSS-silanol on the whisker growth. In summary, the following conclusions can be drawn: 1. During the thermal aging process, the added POSS-silanol inhibited the interdiffusion of Sn and Cu atoms at the interface region, thereby preventing the formation of Cu6Sn5 but promoting the conversion of Cu6Sn5 into Cu3Sn. 2. During the thermal cycling process, the added POSS-silanol significantly inhibited whisker formation and the deformation of the interface by stiffening the solder matrix and decreasing the driving force for whisker formation. 3. The added POSS-silanol not only reinforced the solder joint, but also reduced the risk of whisker formation under both the thermal aging and the thermal cycling conditions. POSS-silanol can therefore be considered a promising modifier material which enhances the reliability of solders. Acknowledgments This work was supported by the National Natural Science
Foundation of China (Grant No. 51301007), the Beijing Municipal Natural Science Foundation (Grant No. 2144044) and the Joint Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20131103120030). References [1] K.N. Tu, J.C.M. Li, Mater. Sci. Eng. A 409 (2005) 131e139. [2] M.A. Ashworth, G.D. Wilcox, R.L. Higginson, R.J. Heath, C. Liu, R.J. Mortimer, Microelectron. Reliab. 55 (2015) 180e191. [3] P. Eckold, R. Niewa, W. Hugel, Microelectron. Reliab. 54 (2014) 2578e2585. [4] P. Eckold, M.S. Sellers, R. Niewa, W. Hügel, Microelectronics Reliab. (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.018 (in press). [5] B. Horvath, B. Illes, T. Shinohara, Thin Solid Films 556 (2014) 345e353. [6] C.-F. Li, Z.-Q. Liu, J.-K. Shang, J. Alloys Compd. 550 (2013) 231e238. [7] K.N. Tu, C. Chen, A. Wu, J. Mater. Sci. Mater. Electron. 18 (2007) 269e281. [8] Y. Wang, J.E. Blendell, C.A. Handwerker, J. Mater. Sci. 49 (2014) 1099e1113. [9] B. Illes, B. Horvath, J. Alloys Compd. 616 (2014) 116e121. [10] F. Pei, C.L. Briant, H. Kesari, A.F. Bower, E. Chason, Scr. Mater. 93 (2014) 16e19. [11] G.T. Galyon, L. Palmer, Electron. Packag. Manuf. IEEE Trans. 28 (2005) 17e30. [12] J.Y. Song, J. Yu, T.Y. Lee, Scr. Mater. 51 (2004) 167e170. [13] J.W. Cahn, Y. Mishin, A. Suzuki, Acta Mater 54 (2006) 4953e4975. [14] A. Baated, K. Hamasaki, S. Kim, K.-S. Kim, K. Suganuma, J. Electron. Mater. 40 (2011) 2278e2289. [15] K.S. Kim, C.H. Yu, J.M. Yang, Microelectron. Reliab. 46 (2006) 1080e1086. [16] R. Zhang, G. Xu, X. Wang, F. Guo, A. Lee, K.N. Subramanian, J. Electron. Mater 39 (2010) 2513e2521. [17] A. Lee, K.N. Subramanian, J. Electron. Mater 34 (2005) 1399e1407. [18] J. Shen, C. Peng, H. Yin, J. Chen, J. Mater. Sci. Mater. Electron. 23 (2012) 1640e1646.
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C.-F. Yu, C.-M. Chan, K.-C. Hsieh, Microelectron. Reliab. 50 (2010) 1146e1151. H.L. Li, R. An, C.Q. Wang, Z. Jiang, J. Alloys Compd. 634 (2015) 94e98. E. Shaffir, I. Riess, W.D. Kaplan, Acta Mater 57 (2009) 248e256. L. Sauter, A. Seekamp, Y. Shibata, Y. Kanameda, H. Yamashita, Microelectron. Reliab. 50 (2010) 1631e1635. [23] C.C. Hu, Y.D. Tsai, C.C. Lin, G.L. Lee, S.W. Chen, T.C. Lee, T.C. Wen, J. Alloys
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Whisker growth behaviors in POSS-silanol modified Sn3.0Ag0.5Cu composite solders Limin Ma a, Yong Zuo a, Sihan Liu a, Fu Guo a, *, Andre Lee b, **, K.N. Subramanian b a b
College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China Chemical Engineering and Materials Science Department, Michigan State University, East Lansing, MI 48824, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2015 Received in revised form 29 September 2015 Accepted 30 September 2015 Available online 9 October 2015
As an ideal modifier, the cage-type polyhedral oligomeric silsesquioxane (POSS)-silanol creates a strong bonding with the solder without agglomeration. It has been shown that the addition of POSS-silanol improves both the mechanical performance and the solder's resistance to electromigration. This study further examined the effect of the addition of POSS-silanol on whisker growth in a Sn3.0Ag0.5Cu (SAC305) solder. The whisker growth was studied for pure SAC305 and SAC305 modified with POSSsilanol under isothermal aging and thermal cycling conditions. The results indicate that the added POSS-silanol not only reinforces the solder joint, but also inhibits whisker formation under both thermal aging and thermal cycling conditions. POSS-silanol can therefore be considered a promising modifier material which enhances the reliability of solders. During the thermal aging process, the added POSSsilanol was found to inhibit the interdiffusion of Sn and Cu atoms at the interface region, thereby preventing the formation of Cu6Sn5 but promoting the conversion of Cu6Sn5 into Cu3Sn. During the thermal cycling process, the added POSS-silanol significantly inhibited whisker formation and the deformation of the interface by stiffening the solder matrix and decreasing the driving force for whisker formation. © 2015 Elsevier B.V. All rights reserved.
Keywords: Microstructure POSS-silanol Whisker formation Composite solder
1. Introduction Whisker formation is a frequent problem in the electronics industry, causing damage to fine-pitch electronic components. The transition to Pb-free electronics renders this issue even more important since Pb very effectively restrained whisker growth in tin [1]. Compressive stresses are considered causing the whisker growth, which can result from residual stresses in electrodeposits [2e4], the growth of intermetallic compounds (IMCs) [5e7], thermal cycling [8] or corrosion [9]. Strategies to mitigate the whisker growth generally aim to release or eliminate the compressive stresses by various means. Because the residual stress in electrodeposits depends on the texture of the deposited coatings, Ashworth et al. [2] varied the electroplating parameters to create a microstructure and grain orientation in the tin deposit that is inherently resistant to whisker growth. Using the same method, Eckold et al. [3,4] reported that (211)-orientated grains exhibit a low surface energy, resulting in a
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (F. Guo), [email protected] (A. Lee). http://dx.doi.org/10.1016/j.jallcom.2015.09.266 0925-8388/© 2015 Elsevier B.V. All rights reserved.
lower susceptibility to corrosion and tin whisker formation. However, Pei et al. [10] argued that strategies focusing mainly on the structure of the as-deposited film may not be sufficient to prevent whisker growth because the initial sites may form after the film has been grown. It is believed that the formation of IMCs due to CueSn interdiffusion is one of most likely reasons for the whisker growth because of the volume expansion effect [11,12]. Therefore, a barrier layer between Cu and Sn is considered a viable option to inhibit whisker formation. Horvath et al. [5] compared the growth of IMCs after the introduction of Ni or Ag as an intermediate layer and found that the Ag layer was easily consumed by both Sn and Cu and rapidly became ineffective. The addition of metals (i.e., Ag, Bi, Ni and Au) was also considered as a potential solution in other studies, but none of these proved as effective as Pb [13e15]. Therefore, much more effort is required to find a feasible solution for the inhibition of whisker growth. Recently, cage-type polyhedral oligomeric silsesquioxane (POSS) trisilanol has been considered a suitable candidate for improving the service performance of Sn-based Pb-free solder materials. POSS-silanol can react with the metallic elements in the solder matrix without forming any new IMCs. The three surfaceactive silanol (SieOH) groups in POSS-silanol can react and form
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a strong bonding with the metal matrix; while the seven inert organic groups ensure a stable SieO-M bonding during changes of the service environment over a wide temperature range. In addition, because it is not a metallic material, electric and magnetic field will not affect the POSS modifier. Its unique structure makes it appealing for solder modification applications [16,17]. Previous studies [16e18] suggested that the addition of POSS-silanol significantly improves the overall performance of Sn-based Pbfree solders, e.g., their shear strength, resistance to electromigration, as well as the material's resistance to thermal fatigue. However, the effect of POSS-silanol on whisker formation has been rarely studied, and therefore this study focused on investigating the effect of the addition of POSS-silanol on whisker formation in the Sn3.0Ag0.5Cu (SAC305) solder material. To improve the understanding of the whisker formation process, comparative experiments were performed, in which pure SAC305 solders and SAC305 solders modified with POSS-silanol were subjected to the same thermal aging and thermal cycling conditions. 2. Experimental procedures
Fig. 2. Temperature profile recorded during the thermal cycling process.
The composite solder used for the experiments consisted of the SAC305 solder mixed with 3 wt% POSS-silanol. The addition of this proportion of POSS-silanol has been reported in literature to effectively enhance various properties of the solder material [16e18]. The composite solder was fabricated employing the mechanical mixing method. Firstly, the commercial SAC305 paste and the POSS-silanol particles were mixed proportionately in a ceramic crucible and stirred for 30 min in order to obtain a uniform mixture. Then, the mixture was subjected to a reflowing process and fabricated into a solid ingot. Finally, the as-casted ingot was rolled into a thin sheet with a thickness of 20 mm. The specimen preparation process can be briefly summarized as follows: First, a solder sheet was placed on a clean Cu substrate (10 mm 5 mm 0.5 mm). Then, a custom-made aluminum fixture was used to hold the substrate and the solder sheet, as shown in Fig. 1, which allowed to control the thickness of the solder coating. Finally, the sample was soldered in a reflow oven. The peak temperature during the reflow soldering was 300C and the dwell time at the peak temperature was 150s. After reflow soldering, the specimens were cooled to room temperature (20C) on an aluminum heat sink. Thermal aging and thermal cycling tests were conducted to investigate the effect of the addition of POSS-silanol on the whisker growth. The thermal aging experiments were performed at a temperature of 200C. For the thermal cycling tests, the temperature was varied between 40 and 85C. The dwell time was 10 min at both the maximum and minimum temperature. Fig. 2 shows the real temperature profile recorded by the thermocouple attached to the specimen's surface during the thermal cycling process. The microstructure and grain orientation of the specimens was studied by scanning electron microscopy (SEM, Hitachi S-3400
operated at 20 kV) and electron backscatter diffraction (EBSD), respectively. The specimens for the EBSD investigations were prepared using a precision etching-coating system to remove the strained layer. Energy dispersive X-Ray spectroscopy (EDX) was performed to determine the phase composition of the specimens.
3. Results and discussion 3.1. EBSD analysis of the specimens after reflow soldering Previous studies suggested that the addition of POSS-silanol could alter the microstructure of the solder matrix and influence their performance [16e18]. However, considering the small size of POSS-silanol and the small volume percentage, such changes might occur on a grain-scale level. Consequently, it is difficult to identify the POSS-silanol particles on the SEM images. Therefore, an EBSD analysis was conducted to compare the microstructure of the SAC305 solder material with and without POSS-silanol. The orientation images obtained for the specimens after reflow soldering are shown in Fig. 3. Comparing Fig. 3(a) and (d), the modified SAC305 solder matrix shows a more random distribution of orientations and a finer grain size of the Sn phases. The distribution of orientations could be better characterized using the (001) pole figures. As shown in Fig. 3(c) and (f), the solder modified with POSS-silanol features a more discrete pattern of orientations. Fig. 3(b) and (e) show the orientation images obtained for the Cu6Sn5 phase in the solder joints. Comparing Fig. 3(b) and (e), both the pure SAC305 and the modified SAC305 feature the typical scallop-shaped Cu6Sn5 layers after reflow soldering. However, the
Fig. 1. Schematic illustration of the test specimen and the custom-made aluminum fixture.
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Fig. 3. Orientation images and (001) pole figures: (a), (b) and (c) SAC305; (d), (e) and (f) SAC305 modified with POSS-silanol.
morphology of the Cu6Sn5 layer was thicker and sharper in the pure SAC305 than in the SAC305 modified with POSS-silanol. In addition, numerous Cu6Sn5 IMC phases were found dispersed in the SAC305 solder matrix, but only a low amount of Cu6Sn5 was observed in the modified solder. These results demonstrate that the addition of POSS-silanol results in a finer microstructure with random Sn grain orientations, which inhibits the growth of interfacial Cu6Sn5 as well as the migration of Cu6Sn5 into the solder matrix during the solidification process. These microstructure characteristics are beneficial to reducing the stress level and mitigating whisker growth. Firstly, the growth of Cu6Sn5 was identified as the driving force for whisker formation because of the volume expansion effect [11,12]. Therefore, inhibiting the growth of the interfacial Cu6Sn5 phase is expected to reduce the driving force for whisker formation. Secondly, the migrated Cu6Sn5 usually precipitates at the Sn grain boundaries [19]. Although its presence at the grain boundaries can enhance the mechanical performance, it would negatively affect the stress relief along the grain boundaries and induce a build-up of stress around those IMCs. This indicates that preventing the migration of Cu6Sn5 also results in an inhibition of whisker growth. The inhibitory effect of the added POSS-silanol on the formation of Cu6Sn5 and its contribution to a finer and stable microstructure have not been well explained in literature. A possible explanation for the effect might be as follows: The active eOH groups of the POSS-silanol strongly bonded with the metallic matrix via the formation of SieOeSn bonds within and at the grain boundary regions of the solder alloy. At the same time, the inert groups on the POSS-silanol prevented an agglomeration of the POSS-silanol particles [17]. Therefore, POSS-silanol might act as a barrier, effectively
blocking the grain boundary diffusion pathways of the atoms and inhibiting the solid state reaction between Cu and Sn which drives the formation of IMCs at the grain boundaries. 3.2. Evolution of the microstructure under isothermal aging conditions Fig. 4 shows the evolution of the microstructure at the interface under isothermal aging conditions. As demonstrated in Fig. 4(a) and (e), the solder matrix of the modified SAC305 exhibits a finer microstructure and a thinner interfacial IMCs layer after reflow soldering than the pure SAC305 specimen. This result is consistent with the results of the EBSD analysis mentioned above. After aging at 200C for 16 h, the scallop-shaped interfacial IMC layer was transformed and featured a planar morphology after aging, as shown in Fig. 4(b) and (f). This interfacial IMC layer continued to coarsen with increasing aging time. In addition, a continuous Cu3Sn layer appeared in-between the Cu6Sn5 and the Cu substrate. However, it is worth noting that the total IMC layer (counting both Cu3Sn and Cu6Sn5) in the pure SAC305 solder was much thicker than the IMC layer in the solder modified with POSS-silanol. In contrast, the Cu3Sn layer in the modified solder grew much faster than in the pure SAC305 solder. Fig. 5 shows the thickness of the combined IMC layer and the Cu3Sn layer as a function of the square root of the aging time. The thickness of the IMC layer was found to increase proportional to the square root of the aging time. These results are consistent with the results published by Li et al., but the growth rate was higher than the rate reported by Li et al. because the aging temperature used in this study was higher [20]. Fig. 5 shows that the total IMC layer
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Fig. 4. Evolution of the microstructure under different aging conditions: (a)e(d) pure SAC305, and (d)e(h) SAC305 modified with POSS-silanol.
grew faster in the pure SAC305 solder. However, the growth rate of the Cu3Sn layer was higher in the modified solder. These results suggest that the added POSS-silanol inhibited the growth of the Cu6Sn5 layer but promoted the growth of the Cu3Sn layer. This phenomenon can be explained as follows: The formation and growth of Cu6Sn5 and Cu3Sn depend on the interdiffusion of Sn and Cu, as illustrated in Fig. 6(a). In the SAC305 modified with POSS-silanol, the modifier is considered to be bonded to the Sn atoms. Therefore, the Sn atoms bonded to the POSS-silanol diffuse slower, which inhibits the growth of the Cu6Sn5 layer, as shown in Fig. 6(b). However, the Cu atoms from the substrate continue to diffuse into the already formed Cu6Sn5 layer. Therefore, the Cu3Sn layer grows at the expense of Cu6Sn5. Although this reaction also occurs in SAC305, the solder matrix can supply sufficient Sn atoms to react with the Cu atoms from the substrate to preferentially form
Cu6Sn5. In summary, the modifier POSS-silanol reduces the Cu6Sn5 formation rate by inhibiting the interdiffusion of Sn and Cu in the solder/interfacial IMC boundary region. However, in the Cu6Sn5 region, due to the continued supply of Cu atoms, Cu6Sn5 can easily be converted into Cu3Sn, resulting in a more rapid growth of the Cu3Sn layer, as shown in Fig. 6(c). This special IMC growth behavior facilitated the inhibition of the volume expansion effect. For the formation of Cu6Sn5, Cu atoms must first diffuse into the Sn lattice and occupy the sites of the Sn atoms. The reaction then generates a molar volume expansion of 36.56 cm3/mol (DV ¼ VCu6Sn5 e 5 VSn, molar volume of the different phases is listed in Table 1). Concerning the formation of Cu3Sn5, the reaction between Cu and Cu6Sn5 generates a molar volume expansion of 18.49 cm3/mol (DV ¼ 5 VCu3Sn 5 VCu6Sn5) [11,12]. On the one hand, the added POSS-silanol inhibited the
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L. Ma et al. / Journal of Alloys and Compounds 657 (2016) 400e407 Table 1 Comparison of selected properties of the materials involved in this study. Materials
3
Density (g/cm ) Molar volume (cm3/mol) CTE (ppm/ C) Young's Modulus (GPa)
Fig. 5. Thickness of the IMC layer as a function of the square root of the aging time.
growth of Cu6Sn5. On the other hand, it promoted the transition of Cu6Sn5 into Cu3Sn. These two aspects both helped to account for the volume expansion, thereby reducing the compressive stress. It must be pointed out that no whisker growth was observed even in the unmodified SAC305 specimens during and after thermal aging. This may be attributed to the high aging temperature. A high annealing temperature and a post-annealing treatment at 150C for 1 h are effective ways to reduce whisker formation, as previously reported in literature [21,22]. Hence, the stress concentration due to the growth of the IMCs may be offset by the relaxation effect at elevated temperatures. Although no whisker growth was observed during the thermal aging experiments in this study, there is a possibility of whisker formation at high temperature. However, whisker growth was observed in Sn-based solders even at temperatures higher than 150C in another study [23]. Therefore, the current study might still increase understanding of the whisker growth mechanism and might help to find means to inhibit their formation. 3.3. Evolution of the microstructure under thermal cycling conditions Fig. 7 compares the whiskers growth at the surface of the pure SAC305 and the modified SAC305 solder after thermal cycling. For the pure SAC305 solder, whiskers were observed after 625 cycles, and most of them emerged along the crack, as shown in Fig. 7(b). The roughness of the microstructure of the pure SAC305 increased
Cu
Sn
Cu6Sn5
Cu3Sn
8.95 7.10 17.30 117.00
7.286 16.29 22.00 46.46
8.26 118.01 16.30 85.56
11.33 27.30 19.00 108.30
with the number of thermal cycles. In Fig. 7(c), the whiskers are larger, and some new whiskers with a smaller size appeared close to other microcracks. No further changes were observed when the total number of thermal cycles was increased to 3910 (Fig. 7(d)), which suggests that the internal stress had reached an equilibrium state. For the modified solder, a few tiny whiskers grew along a crack after 625 cycles, as shown in Fig. 7(f). No new whiskers formed when the number of thermal cycles was increased, but the crack became wider (Fig. 7(g) and (h)). Fig. 8 compares the interface region between the solder matrix and the Cu substrate for two different specimens during thermal cycling. As shown in Fig. 8(a) and (d), the microstructures were similar after reflow soldering. However, a serious deformation of the solder could be observed along the interface in the pure SAC305 solder, as shown in Fig. 8(b). With increasing number of thermal cycles, a crack appeared along the interface. For the modified solder, there was no obvious deformation or no cracks occurred at the interface region. The results shown in Figs. 7 and 8 demonstrate that the addition of POSS-silanol could effectively inhibit whisker formation and relieve the damage to the interfacial regions during thermal cycling. The reason for this effect is analyzed and discussed below. The whisker and damage formation mechanism in the pure SAC305 and the SAC305 modified with POSS-silanol is illustrated in Fig. 9. A mismatch in the coefficient of thermal expansion (CTE) between the different phases is considered the main cause for whisker growth and interfacial damage [24]. The CTE values of the different materials are compared in Table 1. There are obvious differences in the CTE between the Sn matrix and the CuSn IMCs or the Cu substrate. Therefore, an alternating stress is generated at the interface regions during thermal cycling, as illustrated in Fig. 9(a). It was noting that the CTE of Sn listed in Table 1 was in average. Sn features a body-centered tetragonal (BCT) structure, and the CTE of the c-axis is almost twice the CTE of the a-axis [25,26]. Therefore, in addition to the mismatch between the Sn matrix and the CuSn IMCs or the Cu substrate, a CTE mismatch between Sn grains with different orientations exists as well. Hence, in the pure SAC305 solder joint, cracks are very likely to occur among the Sn grains with the maximum divergence in orientation. Subsequently, with the crack representing a weak point, the growth of Sn whiskers was promoted at this site due to
Fig. 6. Evolution of the microstructure at the interface of SAC305 due to the addition of POSS-silanol during thermal aging: (a) early stage, (b) intermediate stage, and (c) advanced stage of diffusion.
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Fig. 7. Whisker growth at the surface of the solder during thermal cycling: (a)e(d) pure SAC305; (e)e(h) SAC305 modified with POSS-silanol.
the compressive stress originating from the bottom of the crack. The compressive stress was caused either by the misorientation of the Sn grains or the mismatch between the Sn matrix and the CuSn IMCs or the Cu substrate at the bottom of the crack, as shown in Fig. 9(b). However, when POSS-silanol was introduced, the SieOeSn bonds within and at the grain boundary regions reinforced the solder matrix. Previous studies [17,18] reported that the shear strength and micro hardness of SAC305 solders improved by about 42.86% and 11.18%, respectively, when 3 wt% POSS-silanol was
added to the SAC305 solder. Therefore, the movement and distortion of metal atoms caused by the CTE mismatch was limited by the pinning effect of the POSS-silanol. Furthermore, the POSS-silanol inhibited the growth of interfacial IMCs. This means that the compressive stress at the bottom was reduced and was not sufficient to drive the whisker out of the surface even when the crack appeared, as shown in Fig. 9(c). The combination of these two effects significantly inhibited whisker formation and interfacial deformation by stiffening the solder matrix and decreasing the driving force for whisker formation.
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Fig. 8. Evolution of the microstructure at the interface during thermal cycling: (a)e(c) pure SAC305, and (d)e(f) SAC305 modified with POSS-silanol.
Fig. 9. Schematic illustration of the whisker formation process: (a) stress analysis, (b) whisker formation in pure SAC305, and (c) whisker formation in SAC305 modified with POSSsilanol.
4. Conclusions This study aimed to investigate the whisker growth in SAC305 and POSS-silanol-modified SAC305 under either high isothermal temperature aging or thermal cycling conditions and to determine the effect of the addition of POSS-silanol on the whisker growth. In summary, the following conclusions can be drawn: 1. During the thermal aging process, the added POSS-silanol inhibited the interdiffusion of Sn and Cu atoms at the interface region, thereby preventing the formation of Cu6Sn5 but promoting the conversion of Cu6Sn5 into Cu3Sn. 2. During the thermal cycling process, the added POSS-silanol significantly inhibited whisker formation and the deformation of the interface by stiffening the solder matrix and decreasing the driving force for whisker formation. 3. The added POSS-silanol not only reinforced the solder joint, but also reduced the risk of whisker formation under both the thermal aging and the thermal cycling conditions. POSS-silanol can therefore be considered a promising modifier material which enhances the reliability of solders. Acknowledgments This work was supported by the National Natural Science
Foundation of China (Grant No. 51301007), the Beijing Municipal Natural Science Foundation (Grant No. 2144044) and the Joint Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20131103120030). References [1] K.N. Tu, J.C.M. Li, Mater. Sci. Eng. A 409 (2005) 131e139. [2] M.A. Ashworth, G.D. Wilcox, R.L. Higginson, R.J. Heath, C. Liu, R.J. Mortimer, Microelectron. Reliab. 55 (2015) 180e191. [3] P. Eckold, R. Niewa, W. Hugel, Microelectron. Reliab. 54 (2014) 2578e2585. [4] P. Eckold, M.S. Sellers, R. Niewa, W. Hügel, Microelectronics Reliab. (2015), http://dx.doi.org/10.1016/j.microrel.2015.08.018 (in press). [5] B. Horvath, B. Illes, T. Shinohara, Thin Solid Films 556 (2014) 345e353. [6] C.-F. Li, Z.-Q. Liu, J.-K. Shang, J. Alloys Compd. 550 (2013) 231e238. [7] K.N. Tu, C. Chen, A. Wu, J. Mater. Sci. Mater. Electron. 18 (2007) 269e281. [8] Y. Wang, J.E. Blendell, C.A. Handwerker, J. Mater. Sci. 49 (2014) 1099e1113. [9] B. Illes, B. Horvath, J. Alloys Compd. 616 (2014) 116e121. [10] F. Pei, C.L. Briant, H. Kesari, A.F. Bower, E. Chason, Scr. Mater. 93 (2014) 16e19. [11] G.T. Galyon, L. Palmer, Electron. Packag. Manuf. IEEE Trans. 28 (2005) 17e30. [12] J.Y. Song, J. Yu, T.Y. Lee, Scr. Mater. 51 (2004) 167e170. [13] J.W. Cahn, Y. Mishin, A. Suzuki, Acta Mater 54 (2006) 4953e4975. [14] A. Baated, K. Hamasaki, S. Kim, K.-S. Kim, K. Suganuma, J. Electron. Mater. 40 (2011) 2278e2289. [15] K.S. Kim, C.H. Yu, J.M. Yang, Microelectron. Reliab. 46 (2006) 1080e1086. [16] R. Zhang, G. Xu, X. Wang, F. Guo, A. Lee, K.N. Subramanian, J. Electron. Mater 39 (2010) 2513e2521. [17] A. Lee, K.N. Subramanian, J. Electron. Mater 34 (2005) 1399e1407. [18] J. Shen, C. Peng, H. Yin, J. Chen, J. Mater. Sci. Mater. Electron. 23 (2012) 1640e1646.
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