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Cu joints using a high speed lap-shear test
Microelectronic Engineering 91 (2012) 147–153
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Evaluation of drop reliability of Sn–37Pb solder/Cu joints using a high speed lap-shear test Seong-jae Jeon a,b, Jong-Woong Kim c, Byunghoon Lee b, Hak-Joo Lee a, Seung-Boo Jung b, Seungmin Hyun a,⇑, Hoo-Jeong Lee b,⇑ a b c
Division of Nano-Mechanical Systems Research, Korea Institute of Machinery & Materials, 104 Sinseongno Yuseong-gu, Daejeon, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea Display R&D Division, Korea Electronics Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam, Gyeonggi-do, Republic of Korea
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Article history: Received 23 March 2011 Received in revised form 27 August 2011 Accepted 8 September 2011 Available online 22 September 2011 Keywords: Drop reliability Sn–37Pb Lap-shear test Fracture mode
a b s t r a c t This study provides a framework for evaluating the drop reliability of solder joints using a high-speed lap-shear test. The test specimens employed here were Sn–37Pb/Cu under bump metallization (UBM) solder joints and were aged for 120 h at different temperatures (120, 150, and 170 °C) to examine the effects of aging. We tested them at different loading speeds in the range of 0.01–500 mm/s. A careful analysis of the stress–strain graphs attained from the tests, coupled with characterization of the fracture surfaces, discloses that the fracture mode shifts from ductile to brittle as the loading speed escalates. A map of the fracture mode shows that how readily the mode transition with the loading speed occurs can be directly translated into the drop reliability of the solder joint, disclosing that the drop reliability would deteriorate as the aging temperature increases possibly due to the increased IMC thickness. These results underscore the utility of the experimental methodology adopted in this study to evaluate the drop reliability of solder joints. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In an age hand-held or portable electronic devices (for example, cellular phones) are greatly popular, of new concern are their mechanical reliability against mechanical impacts that such mobile devices would experience when they are accidentally dropped [1–14]. Such a drop could damage solder joints mounted on the printed circuit boards (PCB) of the devices in a different way. For example, mechanical damage the solder joints of stationary electronic devices experience is typically a low-speed mechanical loading such as thermal fatigue or creep caused by heat generated during device operation, the major failure mode of which is often characterized as a ductile fracture. According to recent studies on such drop damage, the dominant failure mode is a brittle fracture developed between the solder alloys and bond pads [1–4]. Thus, there is an urgent need to develop a new test methodology to systematically evaluate the drop reliability. The board level drop test (BLDT), which utilizes a mechanical shock table to provide a specific shock vibration, has been proposed by the joint electron devices engineering council (JEDEC) [15]. While this method offers a reliable test scheme that success⇑ Corresponding authors. Tel.: +82 42 868 7981; fax: +82 42 868 7884 (S. Hyun), tel.: +82 31 290 7365; fax: +82 31 290 7410 (H-.J. Lee). E-mail addresses: [email protected] (S. Hyun), [email protected] (H.-J. Lee). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.09.006
fully simulates drop events, it could be costly and time-consuming, and in addition the data obtained from these tests does not provide much scientific insight into the failure mechanism. Several other test methods, such as the ball impact test (BIT), high-speed ballshear test (HSS), high-speed cold ball-pull test (HSCBP), etc., have been proposed to gain drop reliability data on the chip-scale package (CSP) level [5–14]. In these tests, the solder joints on chips do not experience exactly a dropping event but receive high-speed mechanical loading (for BIT, 1500 mm/s and for HSS, 1300 mm/ s), which is characteristic of the mechanical impacts materials would receive when dropped. Several recent studies based on these test methods have demonstrated that drop reliability could be extracted from characterizing the mechanical data attained from such tests [10–14]. Nevertheless, the stress states that solder joints are subjected to during the tests are quite complicated and hence it is not straightforward to interpret the fracture behavior of solder joints from the perspective of materials mechanics. The lap-shear test offers an effective way of characterizing the mechanical properties of solder joints by imposing a simple shear stress condition on the joints, as illustrated in Fig. 1, and generating a stress–strain graph [16–20]. The stress–strain curves produced from lap-shear tests are similar to those generated from a uni-axial tensile test and thus provide an opportunity to examine the mechanical behavior of solder joints in a systematic way by connecting them with the literature data obtained from traditional
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code, ANSYS 10.0. The Sn–37Pb solder was considered to have elastic–plastic properties, while the other materials were treated like linear elastic materials. Large deformation was activated to simulate the mechanical deflection of the solder joint structure during a high-speed lap shear test. 3. Results and discussion
Fig. 1. (a) A schematic diagram of a test specimen and (b) a photograph of the test apparatus used in the study.
metallurgy. Here, we utilized high-speed lap-shear test (maximum loading speed, 500 mm/s) to evaluate the mechanical behavior of solder joints under high-speed loading and hence the drop reliability. Although this applied stress state may be too simple to closely replicate the complicated mechanical loading states of drop events, the high-speed lap-shear test could provide a glimpse into the fundamental aspects of the mechanical behavior of solder joints under high speed loading by generating stress–strain curves over a wide range of strain rates. In this study, we tested mainly Sn–37Pb solder at various loading speeds ranging from 0.01 to 500 mm/s. We also tested solders after aging treatments at elevated temperatures (120, 150, and 170 °C). We also examined the fractured morphology using scanning electron microscopy to correlate the stress– strain curves obtained from the tests and the fracture mode. 2. Experimental procedures Sn–37Pb (wt.%) solder balls, 300 lm in diameter, were used in this study. The solder balls were attached to Cu under bump metallization (UBM) pads with the size of 500 500 lm on a FR-4 substrate with the dimension of 15 4 1 mm and were reflowed in a reflow machine (infrared 4-zone reflow machine, RF-430-N2, Japan Pulse Laboratory Co. Ltd., Japan). Fig. 1(a) shows a schematic diagram of a test specimen used in this study. The height of the solder joint was maintained at around 120 lm. Some samples underwent a thermal aging process at various temperatures (120, 150 and 170 °C) for 120 h. We characterized the microstructure of the solder joints and the fracture morphology using a scanning electron microscope (SEM, HITACHI S-3000H) equipped with an energy dispersive X-ray spectrometer (EDS). We tested the as-reflowed and aged samples in a micro-tensile machine (Tytron 250, MTS System Co., USA) shown in Fig. 1(b). The tests were done under a displacement control mode and various loading speeds (0.01, 0.1, 1, 10, 100 and 500 mm/s) were used. For each set of the samples, we tested at least 10 specimens. Within each group, more than 80% of them showed a similar curve shape and the range of the ultimate shear strength was very narrow. There were, however, somewhat significant variations in elongation. A gripper was used to hold the samples in place on an X–Y table fixture which is adjustable to off-set the height of the solder joint. To analyze the deformation behavior, we carried out finite element analysis (FEA) using a commercial finite element
High speed lap-shear tests were first performed at different loading speeds for the as-reflowed samples. The stress–strain curves obtained from the tests exhibit the typical shape of a stress–strain curve, as shown in Fig. 2. (Note: the stress–strain curves shown in this paper are based on engineering stress and engineering strain.) In all the curves, it appears that the solder joint has experienced a large amount of plastic deformation, 200–300%. The shear modulus estimated from the slope in the initial linear region is around 0.4 GPa. For the sample deformed at the lowest strain rate (i.e., 0.01 mm/s), the yield strength at 0.2% offset is found around 28 MPa while the ultimate shear strength being 41 MPa, which is somewhat lower than the values measured at a similar strain rate using dog-bone shaped specimens [21,22]. A noticeable feature observed in Fig. 2 is the sensitive variation of the shape of the stress–strain curves with the loading speed: the shear strength increases and the elongation decreases as the strain rate increases. In particular, such increase of flow stress with a strain rate is common in plastically-deforming materials (in the present case, solder) and can be the best described as follows [23]:
r ¼ Cðe_ Þm where r, e_ , m, C are the flow stress, strain rate, strain-rate sensitivity, and proportional constant, respectively. As such, plotting the flow stress as a function of the strain rate on a semi-logarithmic scale would produce a linear relationship with the slope being the strain-rate sensitivity, m. Fig. 5 displays such linear relation between the ultimate shear strength and the strain rate for the as-reflowed samples (denoted as the black squares). The linear relationship clearly displayed in the graph indicates that the data follows the power-law relation of the strain rate and the flow stress predicted in the above equation. The strain-rate sensitivity attained from the slope is found approximately 0.04, somewhat lower than the values (0.06–0.09) measured from dog-bone shaped specimens of SnPb solders at low strain rates (106–103 s1) [24–27]. Next, we annealed as-reflowed samples in an oven at various temperatures (120, 150 and 170 °C) for 120 h to investigate the effects of aging. The microstructure changed significantly after the
Fig. 2. Stress–strain curves measured from as-reflowed samples at a wide range of loading speeds.
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Fig. 3. Electron micrographs that show a cross-sectional view of the solder joints of (a) as-reflowed, (b) 120 °C-aged, (c) 150 °C-aged, and (d) 170 °C-aged samples.
annealing: the needle-like features, a typical shape of eutectic phases, inside the solder has coarsened substantially and the thickness of the intermetallic compound (IMC) layer has increased considerably (not shown here). Fig. 3 shows cross-sectional SEM images of the as-reflowed and the annealed samples. The as-reflowed sample shows a thin IMC layer and a composition analysis using EDS confirmed the IMC layer to be Cu6Sn5 (not shown here). The layer grew thicker in the samples annealed at elevated temperatures and for the 170 °C-aged sample a thin layer of Cu3Sn has emerged between the Cu and the Cu6Sn5 layer, which is consistent with the results found in the literature [28–30]. Comparing the SEM images unfolds that with the aging temperature increasing, the IMC thickness has increased from around 2 to 10 lm, as summarized in Table 1. We also carried out lap-shear tests on the samples aged at high temperatures. The stress–strain curves obtained from the tests are shown in Fig. 4. A careful comparison between those graphs forges several interesting observations. First, the overall flow stress has become somewhat lower for samples aged at higher temperatures, and the ductility has diminished substantially as the aging temperature increases, which leads to a significant reduction in toughness. Secondly, the sensitive dependence of the flow stress on the strain rate can still be observed in the samples aged at high temperatures. Fig. 5 shows a plot of the ultimate shear strength and the strain rate in a semi-logarithmic scale for the aged samples. An interesting point shown in the graph is that the curves for samples aged at high temperatures somewhat deviates from linearity at high strain rates. The deviation from linearity might be an indication that the deformation, which mainly occurred in the plastically-deforming element (that is, solder) at low strain rates, is shifting to the IMC at high strain rates, in particular more noticeably in high temperature-aged samples. In addition, the reducing flow stress with the aging appears palpable in the graph, which could be attributed to the coarsening of the microstructural features during the aging. Lastly, it is interesting to note that for the samples aged at higher temperatures, the overall shape of stress–strain curves changes distinctly as the loading speed varies. For the 150 °C-aged samples, for example, samples loaded at lower loading speed (0.01 or 0.1 mm/s) display a substantial amount of elongation and then a gradual drop in flow stress before the stress falls to a complete
fracture, which is the typical shape of stress–strain curves for the as-reflowed and 120 °C-aged samples at all loading speeds. In contrast, samples tested at higher loading speeds (10–500 mm/s) show a somewhat different shape of flow stress curves in that the elongation is smaller but still significant and the stress drops sharply immediately after it reaches the maximum. The samples aged at 170 °C reveals another shape of stress–strain curves at a high loading speed. Here, the stress increases sharply and drops abruptly with almost no plastic deformation. Such differences in the shape of stress–strain graphs hint that there may be several different deformation modes. Thus, we analyzed the fractured surfaces of the samples using SEM to examine the fracture mode active in the samples and to correlate it with the stress–strain curves. Fig. 6 shows three sets of a stress–strain curve and a fractured morphology image taken from the sample after the test, each set representing one of the three fracture modes found in our samples. For the case shown in Fig. 6(a), Mode I, the severely elongated shape of the fractured solder suggests that the solder had experienced a great amount of plastic deformation before it fractured, which is consistent with the profile of the stress–strain curve. The fracture occurred through the solder with the crack running from a lower corner of one side to a higher corner of the other side. This is a typical ductile fracture occurring through solder. The fracture morphology of Mode II also reveals a profile significantly elongated in the shear direction (see the upper side of the fractured solder in Fig. 6(b)), which suggests a substantial amount of plastic deformation. In contrast to Mode I, however, Mode II demonstrates an abrupt failure: the fractured surface is quite flat and sharp with the crack running slightly above the interface between the solder and the IMC layer. Thus we found SnPb from both sides of the fractured surfaces. This behavior is consistent with the shape of the stress–strain curve, which shows a substantial amount of plastic deformation and a subsequent sharp drop in stress. In Mode II, thus, the solder joint deforms plastically to some extent until the fracture occurs abruptly along the interface. The samples aged at 150 °C and deformed at high loading speeds (100 or 500 mm/s) belong to this mode. The mode shown in Fig. 6(c), Mode III, shares similarities with the fracture mode typically observed in dropping events. In Fig. 6(c), the shape of the fractured solder does not show any sign of plastic deformation. A closer examination of the fracture surface
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Fig. 5. A semi-logarithmic plot of ultimate shear strength versus strain rate for samples.
Table 1 The thicknesses of the IMC layers for the samples bonded at various temperatures.
Fig. 4. Stress–strain curves for samples aged at high temperatures, (a) 120 °C, (b) 150 °C, and (c) 170 °C. The loading speed was varied from 0.01 to 500 mm/s.
indicates that the crack propagated through the IMC layer — a brittle fracture through IMC. It should be noted that this fracture mode, in particular, is observed only in samples deformed under high loading speeds; for instance, the samples aged at 170 °C and tested at high loading speeds (100 or 500 mm/s) correspond to this mode. Such traits suggest that this mode corresponds to the fracture typically observed in dropping events [1–4]. The schematic diagrams shown in Fig. 7 summarize the three fracture modes, a ductile fracture through solder (Mode I), an abrupt fracture after
Temperature
Thickness (lm)
As-reflowed 120 °C 150 °C 170 °C
2.04 2.77 3.87 8.53
some plastic deformation (Mode II), and a brittle fracture through IMC (Mode III). Disclosing the three fracture modes prevalent in our samples critically helped us fully understand the deformation behavior of the solder joints captured in the stress–strain curves of Figs. 2 and 4. By revisiting the stress–strain curves and identifying the fracture mode for each curve, we were able to generate a fracture mode map that demonstrates how actively the dominant fracture mode shifts as the strain rate varies, as shown in Fig. 8. The samples aged at 170 °C exhibit the most dynamic shift in the fracture mode as the strain rate escalates: the fracture mode swings from Mode I at low loading speeds (0.01 or 0.1 mm/s) to Mode II at moderate loading speeds to Mode III at high loading speeds (100 or 500 mm/s). For the 150 °C-aged samples, the fracture mode switches from Mode I for the samples tested at low loading speeds (0.01 or 0.1 mm/s) to Mode II for the samples tested at high loading speeds (100 or 500 mm/s), but Mode III was not observed. For the as-reflowed and the 120 °C-aged samples, Mode I is dominant at all loading speeds. These results underline how differently a materials joint responds — and fails — to a mechanical loading applied at different speeds. The general trend observed in these results is that the main fracture path shifts from the interior of the solder to the interface to the IMC layer as the strain rate increases, leading to a ductile failure through solder (Mode I) at low loading speeds and a brittle failure through IMC (Mode III) at high loading speeds. This implies that the deformation has taken place mostly in the interior of the solder at low loading speeds, but with the loading speed escalating fast loading has prompted deformation concentration near the interface or in the IMC layer, for the plastically-deforming solder would not be able to respond quickly to high-speed loading. Such shift in the fracture mode with the loading speed has also been observed in studies using other high-speed test methods such as ball impact tests and board level drop tests [31–33].
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Fig. 6. Electron micrographs of solder joints after fracture and the stress–strain graphs of the tested sample, each set of which represents one of the three failure modes found in this study, (a) Mode I, (b) Mode II and (c) Mode III. For the electron micrographs, the images of the top and bottom sides of the fractured solder joints were taken separately and joined together. The insets of (b) and (c) show magnified images taken around the crack paths.
Another interesting point is that the mode transition occurs more swiftly in the samples aged at higher temperatures. The SEM analysis on the aged samples, presented earlier (see Fig. 3), indicated that the aging had brought about two noticeable consequences on the microstructure of the solder joints: the coarsening
of the microstructural features and the increase of the IMC thickness. Some works using such test methods as ball impact tests reported a similar mode transition and attributed it to microstructural coarsening [33]; however, the coarsening does not account for the swifter mode transition observed in our study, for solder with
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Fig. 7. Schematic diagrams that summarize the three fracture modes, (a) a ductile fracture through solder (Mode I), (b) an abrupt fracture after some plastic deformation (Mode II) and (c) a brittle fracture through IMC (Mode III).
Fig. 8. A map of the fracture mode that illustrates the systematic change of the dominant fracture mode with the loading speed.
larger grains would have a lower strength and hence the deformation is more likely to concentrate in the solder, which is opposite to the results found in our study. For the data set generated in our study, such mode transition behavior is better explained by the increase of the IMC thickness. We carried out finite element analysis for solder joints with different IMC thicknesses to check if the IMC thickness influences the deformation distribution across the solder and the IMC during loading. The distribution of plastic strain attained from the FEM analysis, shown in Fig. 9, discloses that a sample with a thin IMC layer exhibits no clear sign of deformation concentration while one with a thick IMC layer displays deformation concentration along the region located just above the interface, which confirms the influence of the IMC thickness on the deformation concentra-
tion. This simulation result implies that at a high loading speed, such deformation concentration would occur more readily for a sample with a thick IMC than one with a thin IMC, facilitating the fracture mode transition. Thus we suggest that the increased IMC thickness of the samples aged at higher temperatures has instigated the swifter mode transition. With full understanding of the deformation behavior of solder joints over a wide range of loading speeds, we are now able to use this information to evaluate the drop reliability. First, it should be noted that the fracture behavior of Mode III is characteristic of the fracture mode usually observed in drop failures [1–4]. For the samples aged at the highest temperature, as the loading speed increases, the fracture mode promptly shifts to Mode III. This means that this material is likely to fail in a brittle manner upon mechan-
Fig. 9. Distribution of equivalent plastic strain in Sn–37Pb/Cu solder joints with IMC layers with different thicknesses: (a) 2 and (b) 10 lm.
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ical impact during drop events a low drop reliability. For the samples aged at low temperatures, in contrast, the ductile fracture mode remains persistent not switching to the brittle mode even at high loading speeds, meaning that this solder joint would be able to absorb a high speed impact by allowing some plastic deformation a high drop reliability. As such, the thorough characterization of the stress–strain behavior of these solder joints over a wide range of strain rates produces the conclusion that the drop reliability of SnPb/Cu UBM solder joints would deteriorate as the aging temperature increases. In addition, it should be noted that in the literature, the results from investigations on the effects of aging are somewhat scattering. For example, a study on SnAgCu/ electroless-Ni/immersion-Au UBM using a high speed ball shear test reported the deterioration of the drop reliability due to reduced interface strength after aging at high temperatures whereas Xu et al., in a study on SnPb/electroless-Ni/immersion-Au UBM using a high speed ball impact test, reported an increase in impact toughness after aging, attributing it to grain coarsening in the solder [32,33]. Such incoherency in the reports in the literature highlights the complex nature of drop failures and the difficulties of evaluating drop reliability accurately, which emphasizes that the data captured in this study should be carefully used in junction with those obtained from other techniques to fully understand drop failures. The results of our study demonstrate that our experimental methodology, imposing a simple shear strain at wide range speeds and sifting through the data extracted from the stress–strain curves, could critically helps us gain key information directly tied to the drop reliability. As the strain rate escalates, the fracture mode shifts in the sequence of Modes I–III. How readily this characteristic swing of the fracture mode with the strain rate occurs can be translated directly to the materials’ ability to withstand a high speed loading, which is closely linked to the drop reliability. In addition, one should remember that the stress states the materials would experience in real applications are likely to be more complicated than a pure shear stress condition adopted in this study. Thus, the data obtained from this material should be carefully interpreted in conjunction with the results obtained from board level drop tests. 4. Summary This report presents a unique way of utilizing the test method of straining specimens at various speeds, once a popular experimental methodology for traditional metallurgists, to evaluate the drop reliability of solder joints for microelectronics. We chose SnPb/Cu UBM solder joints aged at different temperatures as a test sample. A careful analysis of stress–strain curves obtained from the lap-shear tests, combined with the SEM analysis, elucidated three fracture modes (Modes I–III) active in the solder joints; and illuminating the fracture mode for each sample generated a map of the fracture mode, which displayed how the fracture mode of each set of samples shifts from ductile to brittle with the loading speed increasing. Noting that a prompt shift in the fracture mode from ductile to brittle represents a low drop reliability, we used this map to evaluate the drop reliability of the solder joints and found that the drop reliability would deteriorate as the aging temperature increases possibly due to the increased IMC thickness. The methodology adopted in this study offers a unique path to evaluate the drop reliability of solder joints, providing a cheap and quick
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means of evaluating the drop reliability when used in conjunction with the board level test. More importantly, it could help us examine the fundamental aspect of the mechanical behavior of solder joints during drop failure. Acknowledgments The authors appreciate a financial support from the Grant (SC0800) ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea. This study was also conducted through the Practical Application Project of Advanced Microsystems Packaging Program of Seoul Technopark, funded by the Ministry of Knowledge Economy. In addition, this research was, in part, financially supported by the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement of Technology (KIAT) through the Workforce Development Program in Strategic Technology. References [1] J.-W. Jang, A.P. De Silva, J.E. Drye, S.L. Post, N.L. Owens, J.-K. Lin, D.R. Frear, IEEE Trans. Electron. Packag. Manufact. 30 (2007) 49–53. [2] T.T. Mattila, J.K. Kivilahti, J. Electron. Mater. 34 (2005) 969–976. [3] Y.H. Xia, C.Y. Lu, X.M. Xie, J. Electron. Mater. 36 (2007) 1129–1136. [4] W. Peng, M.E. Marques, J. Electron. Mater. 36 (2007) 1679–1690. [5] J.W. Kim, J.K. Jang, S.O. Ha, S.S. Ha, D.G. Kim, S.B. Jung, Microelectr. Relia. 48 (2008) 1882–1889. [6] Y.-S. Lai, H.-C. Chang, C.-L. Ye, Microelectr. Relia. 47 (2007) 2179–2187. [7] J.W. Kim, Y.C. Lee, S.S. Ha, S.B. Jung, J. Mater. Sci.: Mater. Electron. 20 (2009) 17–24. [8] K.T. Tsai, F.L. Liu, E.H. Wong, R. Rajoo, Solder Surf. Mount. Technol. 16 (2006) 12–17. [9] E.H. Wong, C.S. Selvanayagam, S.K.W. Seah, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, D.R. Frear, M. Leoni, Y.-S. Lai, C.-L. Yeh, J. Electron. Mater. 37 (2008) 829–836. [10] E.H. Wong, R. Rajoo, S.K.W. Seah, C.S. Selvanayagam, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, M. Leoni, P.L. Eu, Y.-S. Lai, C.-L. Yeh, Microelectr. Relia. 48 (2008) 1069–1078. [11] F.B. Song, S.W.R. Lee, K. Newman, B. Sykes, S. Clark, In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 364–372. [12] F.B. Song, S.W.R. Lee, K. Newman, B. Sykes, S. Clark, In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 1504–1513. [13] M.E. Johnson, H.Y. Lu, D. Lawhead, T. Tessier, D. Scott, A. Curtis. In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 536–542. [14] K. Newman, In: IEEE 55th Electronic Components and Technology Conference, 2005, pp. 1194–1201. [15] Solid State Devices, Board level drop test method of components for handheld electronic products. In: JESD22-B111, 2003. [16] S.-C. Her, Compos. Struct. 47 (1999) 673–678. [17] Y.-L. Shen, N. Chawla, E.S. Ege, X. Deng, Acta Mater. 53 (2005) 2633–2642. [18] M.A. Matin, J.G.A. Theeven, W.P. Vellinga, M.G.D. Geers, Microelectr. Relia. 47 (2007) 1262–1272. [19] J.H.L. Pang, B.S. Xiong, IEEE Trans. Comp. Packag. Technol. 28 (2005) 830–840. [20] N. Chawla, R.S. Sidhu, J. Mater. Sci.: Mater. Electron. 18 (2007) 175–189. [21] W.J. Plumbridge, C.R. Gagg, J. Mater. Sci.: Mater. Electron. 10 (1999) 461–468. [22] T.K. Hwang, S.B. Lee, Key Eng. Mater. 297–300 (2005) 825–830. [23] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill, New York, 1988. [24] E.H. Wong, C.S. Selvanayagam, S.K.W. Seah, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, D.R. Frear, M. Leoni, Y.-S. Lai, C.-L. Yeh, Mater. Lett. 62 (2008) 3031–3034. [25] F.Q. Lang, H. Tanaka, O. Munegata, T. Taguchi, T. Narita, Mater. Character. 54 (2005) 223–229. [26] F.L. Zhu, H.H. Zhang, R.F. Guan, S. Liu, Microelectr. Eng. 84 (2007) 144–150. [27] I. Shohji, T. Yoshida, T. Takahashi, S. Hioki, Mater. Sci. Eng. A 336 (2004) 50–55. [28] C.E. Ho, S.C. Yang, C.R. Kao, J. Mater. Sci.: Mater. Electron. 18 (2007) 155–174. [29] H.K. Kim, H.K. Liou, K.N. Tu, J. Mater. Res. 10 (1995) 497–504. [30] T.Y. Lee, W.J. Choi, K.N. Tu, J.W. Jang, S.M. Kuo, J.K. Lin, D.R. Frear, K. Zeng, J.K. Kivilahti, J. Mater. Res. 17 (2002) 291–301. [31] J. Varghese, A. Dasgupta, Microelectr. Relia. 47 (2007) 1095–1102. [32] D.S. Liu, C.Y. Kuo, C.L. Hsu, G.S. Shen, Y.R. Chen, K.C. Lo, Mater. Sci. Eng. A 494 (2008) 196–202. [33] Y. Xu, S. Ou, K.N. Tu, K. Zeng, R. Dunne, J. Mater. Res. 23 (2008) 1482–1487.
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Evaluation of drop reliability of Sn–37Pb solder/Cu joints using a high speed lap-shear test Seong-jae Jeon a,b, Jong-Woong Kim c, Byunghoon Lee b, Hak-Joo Lee a, Seung-Boo Jung b, Seungmin Hyun a,⇑, Hoo-Jeong Lee b,⇑ a b c
Division of Nano-Mechanical Systems Research, Korea Institute of Machinery & Materials, 104 Sinseongno Yuseong-gu, Daejeon, Republic of Korea School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Republic of Korea Display R&D Division, Korea Electronics Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam, Gyeonggi-do, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 March 2011 Received in revised form 27 August 2011 Accepted 8 September 2011 Available online 22 September 2011 Keywords: Drop reliability Sn–37Pb Lap-shear test Fracture mode
a b s t r a c t This study provides a framework for evaluating the drop reliability of solder joints using a high-speed lap-shear test. The test specimens employed here were Sn–37Pb/Cu under bump metallization (UBM) solder joints and were aged for 120 h at different temperatures (120, 150, and 170 °C) to examine the effects of aging. We tested them at different loading speeds in the range of 0.01–500 mm/s. A careful analysis of the stress–strain graphs attained from the tests, coupled with characterization of the fracture surfaces, discloses that the fracture mode shifts from ductile to brittle as the loading speed escalates. A map of the fracture mode shows that how readily the mode transition with the loading speed occurs can be directly translated into the drop reliability of the solder joint, disclosing that the drop reliability would deteriorate as the aging temperature increases possibly due to the increased IMC thickness. These results underscore the utility of the experimental methodology adopted in this study to evaluate the drop reliability of solder joints. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction In an age hand-held or portable electronic devices (for example, cellular phones) are greatly popular, of new concern are their mechanical reliability against mechanical impacts that such mobile devices would experience when they are accidentally dropped [1–14]. Such a drop could damage solder joints mounted on the printed circuit boards (PCB) of the devices in a different way. For example, mechanical damage the solder joints of stationary electronic devices experience is typically a low-speed mechanical loading such as thermal fatigue or creep caused by heat generated during device operation, the major failure mode of which is often characterized as a ductile fracture. According to recent studies on such drop damage, the dominant failure mode is a brittle fracture developed between the solder alloys and bond pads [1–4]. Thus, there is an urgent need to develop a new test methodology to systematically evaluate the drop reliability. The board level drop test (BLDT), which utilizes a mechanical shock table to provide a specific shock vibration, has been proposed by the joint electron devices engineering council (JEDEC) [15]. While this method offers a reliable test scheme that success⇑ Corresponding authors. Tel.: +82 42 868 7981; fax: +82 42 868 7884 (S. Hyun), tel.: +82 31 290 7365; fax: +82 31 290 7410 (H-.J. Lee). E-mail addresses: [email protected] (S. Hyun), [email protected] (H.-J. Lee). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.09.006
fully simulates drop events, it could be costly and time-consuming, and in addition the data obtained from these tests does not provide much scientific insight into the failure mechanism. Several other test methods, such as the ball impact test (BIT), high-speed ballshear test (HSS), high-speed cold ball-pull test (HSCBP), etc., have been proposed to gain drop reliability data on the chip-scale package (CSP) level [5–14]. In these tests, the solder joints on chips do not experience exactly a dropping event but receive high-speed mechanical loading (for BIT, 1500 mm/s and for HSS, 1300 mm/ s), which is characteristic of the mechanical impacts materials would receive when dropped. Several recent studies based on these test methods have demonstrated that drop reliability could be extracted from characterizing the mechanical data attained from such tests [10–14]. Nevertheless, the stress states that solder joints are subjected to during the tests are quite complicated and hence it is not straightforward to interpret the fracture behavior of solder joints from the perspective of materials mechanics. The lap-shear test offers an effective way of characterizing the mechanical properties of solder joints by imposing a simple shear stress condition on the joints, as illustrated in Fig. 1, and generating a stress–strain graph [16–20]. The stress–strain curves produced from lap-shear tests are similar to those generated from a uni-axial tensile test and thus provide an opportunity to examine the mechanical behavior of solder joints in a systematic way by connecting them with the literature data obtained from traditional
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code, ANSYS 10.0. The Sn–37Pb solder was considered to have elastic–plastic properties, while the other materials were treated like linear elastic materials. Large deformation was activated to simulate the mechanical deflection of the solder joint structure during a high-speed lap shear test. 3. Results and discussion
Fig. 1. (a) A schematic diagram of a test specimen and (b) a photograph of the test apparatus used in the study.
metallurgy. Here, we utilized high-speed lap-shear test (maximum loading speed, 500 mm/s) to evaluate the mechanical behavior of solder joints under high-speed loading and hence the drop reliability. Although this applied stress state may be too simple to closely replicate the complicated mechanical loading states of drop events, the high-speed lap-shear test could provide a glimpse into the fundamental aspects of the mechanical behavior of solder joints under high speed loading by generating stress–strain curves over a wide range of strain rates. In this study, we tested mainly Sn–37Pb solder at various loading speeds ranging from 0.01 to 500 mm/s. We also tested solders after aging treatments at elevated temperatures (120, 150, and 170 °C). We also examined the fractured morphology using scanning electron microscopy to correlate the stress– strain curves obtained from the tests and the fracture mode. 2. Experimental procedures Sn–37Pb (wt.%) solder balls, 300 lm in diameter, were used in this study. The solder balls were attached to Cu under bump metallization (UBM) pads with the size of 500 500 lm on a FR-4 substrate with the dimension of 15 4 1 mm and were reflowed in a reflow machine (infrared 4-zone reflow machine, RF-430-N2, Japan Pulse Laboratory Co. Ltd., Japan). Fig. 1(a) shows a schematic diagram of a test specimen used in this study. The height of the solder joint was maintained at around 120 lm. Some samples underwent a thermal aging process at various temperatures (120, 150 and 170 °C) for 120 h. We characterized the microstructure of the solder joints and the fracture morphology using a scanning electron microscope (SEM, HITACHI S-3000H) equipped with an energy dispersive X-ray spectrometer (EDS). We tested the as-reflowed and aged samples in a micro-tensile machine (Tytron 250, MTS System Co., USA) shown in Fig. 1(b). The tests were done under a displacement control mode and various loading speeds (0.01, 0.1, 1, 10, 100 and 500 mm/s) were used. For each set of the samples, we tested at least 10 specimens. Within each group, more than 80% of them showed a similar curve shape and the range of the ultimate shear strength was very narrow. There were, however, somewhat significant variations in elongation. A gripper was used to hold the samples in place on an X–Y table fixture which is adjustable to off-set the height of the solder joint. To analyze the deformation behavior, we carried out finite element analysis (FEA) using a commercial finite element
High speed lap-shear tests were first performed at different loading speeds for the as-reflowed samples. The stress–strain curves obtained from the tests exhibit the typical shape of a stress–strain curve, as shown in Fig. 2. (Note: the stress–strain curves shown in this paper are based on engineering stress and engineering strain.) In all the curves, it appears that the solder joint has experienced a large amount of plastic deformation, 200–300%. The shear modulus estimated from the slope in the initial linear region is around 0.4 GPa. For the sample deformed at the lowest strain rate (i.e., 0.01 mm/s), the yield strength at 0.2% offset is found around 28 MPa while the ultimate shear strength being 41 MPa, which is somewhat lower than the values measured at a similar strain rate using dog-bone shaped specimens [21,22]. A noticeable feature observed in Fig. 2 is the sensitive variation of the shape of the stress–strain curves with the loading speed: the shear strength increases and the elongation decreases as the strain rate increases. In particular, such increase of flow stress with a strain rate is common in plastically-deforming materials (in the present case, solder) and can be the best described as follows [23]:
r ¼ Cðe_ Þm where r, e_ , m, C are the flow stress, strain rate, strain-rate sensitivity, and proportional constant, respectively. As such, plotting the flow stress as a function of the strain rate on a semi-logarithmic scale would produce a linear relationship with the slope being the strain-rate sensitivity, m. Fig. 5 displays such linear relation between the ultimate shear strength and the strain rate for the as-reflowed samples (denoted as the black squares). The linear relationship clearly displayed in the graph indicates that the data follows the power-law relation of the strain rate and the flow stress predicted in the above equation. The strain-rate sensitivity attained from the slope is found approximately 0.04, somewhat lower than the values (0.06–0.09) measured from dog-bone shaped specimens of SnPb solders at low strain rates (106–103 s1) [24–27]. Next, we annealed as-reflowed samples in an oven at various temperatures (120, 150 and 170 °C) for 120 h to investigate the effects of aging. The microstructure changed significantly after the
Fig. 2. Stress–strain curves measured from as-reflowed samples at a wide range of loading speeds.
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Fig. 3. Electron micrographs that show a cross-sectional view of the solder joints of (a) as-reflowed, (b) 120 °C-aged, (c) 150 °C-aged, and (d) 170 °C-aged samples.
annealing: the needle-like features, a typical shape of eutectic phases, inside the solder has coarsened substantially and the thickness of the intermetallic compound (IMC) layer has increased considerably (not shown here). Fig. 3 shows cross-sectional SEM images of the as-reflowed and the annealed samples. The as-reflowed sample shows a thin IMC layer and a composition analysis using EDS confirmed the IMC layer to be Cu6Sn5 (not shown here). The layer grew thicker in the samples annealed at elevated temperatures and for the 170 °C-aged sample a thin layer of Cu3Sn has emerged between the Cu and the Cu6Sn5 layer, which is consistent with the results found in the literature [28–30]. Comparing the SEM images unfolds that with the aging temperature increasing, the IMC thickness has increased from around 2 to 10 lm, as summarized in Table 1. We also carried out lap-shear tests on the samples aged at high temperatures. The stress–strain curves obtained from the tests are shown in Fig. 4. A careful comparison between those graphs forges several interesting observations. First, the overall flow stress has become somewhat lower for samples aged at higher temperatures, and the ductility has diminished substantially as the aging temperature increases, which leads to a significant reduction in toughness. Secondly, the sensitive dependence of the flow stress on the strain rate can still be observed in the samples aged at high temperatures. Fig. 5 shows a plot of the ultimate shear strength and the strain rate in a semi-logarithmic scale for the aged samples. An interesting point shown in the graph is that the curves for samples aged at high temperatures somewhat deviates from linearity at high strain rates. The deviation from linearity might be an indication that the deformation, which mainly occurred in the plastically-deforming element (that is, solder) at low strain rates, is shifting to the IMC at high strain rates, in particular more noticeably in high temperature-aged samples. In addition, the reducing flow stress with the aging appears palpable in the graph, which could be attributed to the coarsening of the microstructural features during the aging. Lastly, it is interesting to note that for the samples aged at higher temperatures, the overall shape of stress–strain curves changes distinctly as the loading speed varies. For the 150 °C-aged samples, for example, samples loaded at lower loading speed (0.01 or 0.1 mm/s) display a substantial amount of elongation and then a gradual drop in flow stress before the stress falls to a complete
fracture, which is the typical shape of stress–strain curves for the as-reflowed and 120 °C-aged samples at all loading speeds. In contrast, samples tested at higher loading speeds (10–500 mm/s) show a somewhat different shape of flow stress curves in that the elongation is smaller but still significant and the stress drops sharply immediately after it reaches the maximum. The samples aged at 170 °C reveals another shape of stress–strain curves at a high loading speed. Here, the stress increases sharply and drops abruptly with almost no plastic deformation. Such differences in the shape of stress–strain graphs hint that there may be several different deformation modes. Thus, we analyzed the fractured surfaces of the samples using SEM to examine the fracture mode active in the samples and to correlate it with the stress–strain curves. Fig. 6 shows three sets of a stress–strain curve and a fractured morphology image taken from the sample after the test, each set representing one of the three fracture modes found in our samples. For the case shown in Fig. 6(a), Mode I, the severely elongated shape of the fractured solder suggests that the solder had experienced a great amount of plastic deformation before it fractured, which is consistent with the profile of the stress–strain curve. The fracture occurred through the solder with the crack running from a lower corner of one side to a higher corner of the other side. This is a typical ductile fracture occurring through solder. The fracture morphology of Mode II also reveals a profile significantly elongated in the shear direction (see the upper side of the fractured solder in Fig. 6(b)), which suggests a substantial amount of plastic deformation. In contrast to Mode I, however, Mode II demonstrates an abrupt failure: the fractured surface is quite flat and sharp with the crack running slightly above the interface between the solder and the IMC layer. Thus we found SnPb from both sides of the fractured surfaces. This behavior is consistent with the shape of the stress–strain curve, which shows a substantial amount of plastic deformation and a subsequent sharp drop in stress. In Mode II, thus, the solder joint deforms plastically to some extent until the fracture occurs abruptly along the interface. The samples aged at 150 °C and deformed at high loading speeds (100 or 500 mm/s) belong to this mode. The mode shown in Fig. 6(c), Mode III, shares similarities with the fracture mode typically observed in dropping events. In Fig. 6(c), the shape of the fractured solder does not show any sign of plastic deformation. A closer examination of the fracture surface
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Fig. 5. A semi-logarithmic plot of ultimate shear strength versus strain rate for samples.
Table 1 The thicknesses of the IMC layers for the samples bonded at various temperatures.
Fig. 4. Stress–strain curves for samples aged at high temperatures, (a) 120 °C, (b) 150 °C, and (c) 170 °C. The loading speed was varied from 0.01 to 500 mm/s.
indicates that the crack propagated through the IMC layer — a brittle fracture through IMC. It should be noted that this fracture mode, in particular, is observed only in samples deformed under high loading speeds; for instance, the samples aged at 170 °C and tested at high loading speeds (100 or 500 mm/s) correspond to this mode. Such traits suggest that this mode corresponds to the fracture typically observed in dropping events [1–4]. The schematic diagrams shown in Fig. 7 summarize the three fracture modes, a ductile fracture through solder (Mode I), an abrupt fracture after
Temperature
Thickness (lm)
As-reflowed 120 °C 150 °C 170 °C
2.04 2.77 3.87 8.53
some plastic deformation (Mode II), and a brittle fracture through IMC (Mode III). Disclosing the three fracture modes prevalent in our samples critically helped us fully understand the deformation behavior of the solder joints captured in the stress–strain curves of Figs. 2 and 4. By revisiting the stress–strain curves and identifying the fracture mode for each curve, we were able to generate a fracture mode map that demonstrates how actively the dominant fracture mode shifts as the strain rate varies, as shown in Fig. 8. The samples aged at 170 °C exhibit the most dynamic shift in the fracture mode as the strain rate escalates: the fracture mode swings from Mode I at low loading speeds (0.01 or 0.1 mm/s) to Mode II at moderate loading speeds to Mode III at high loading speeds (100 or 500 mm/s). For the 150 °C-aged samples, the fracture mode switches from Mode I for the samples tested at low loading speeds (0.01 or 0.1 mm/s) to Mode II for the samples tested at high loading speeds (100 or 500 mm/s), but Mode III was not observed. For the as-reflowed and the 120 °C-aged samples, Mode I is dominant at all loading speeds. These results underline how differently a materials joint responds — and fails — to a mechanical loading applied at different speeds. The general trend observed in these results is that the main fracture path shifts from the interior of the solder to the interface to the IMC layer as the strain rate increases, leading to a ductile failure through solder (Mode I) at low loading speeds and a brittle failure through IMC (Mode III) at high loading speeds. This implies that the deformation has taken place mostly in the interior of the solder at low loading speeds, but with the loading speed escalating fast loading has prompted deformation concentration near the interface or in the IMC layer, for the plastically-deforming solder would not be able to respond quickly to high-speed loading. Such shift in the fracture mode with the loading speed has also been observed in studies using other high-speed test methods such as ball impact tests and board level drop tests [31–33].
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Fig. 6. Electron micrographs of solder joints after fracture and the stress–strain graphs of the tested sample, each set of which represents one of the three failure modes found in this study, (a) Mode I, (b) Mode II and (c) Mode III. For the electron micrographs, the images of the top and bottom sides of the fractured solder joints were taken separately and joined together. The insets of (b) and (c) show magnified images taken around the crack paths.
Another interesting point is that the mode transition occurs more swiftly in the samples aged at higher temperatures. The SEM analysis on the aged samples, presented earlier (see Fig. 3), indicated that the aging had brought about two noticeable consequences on the microstructure of the solder joints: the coarsening
of the microstructural features and the increase of the IMC thickness. Some works using such test methods as ball impact tests reported a similar mode transition and attributed it to microstructural coarsening [33]; however, the coarsening does not account for the swifter mode transition observed in our study, for solder with
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Fig. 7. Schematic diagrams that summarize the three fracture modes, (a) a ductile fracture through solder (Mode I), (b) an abrupt fracture after some plastic deformation (Mode II) and (c) a brittle fracture through IMC (Mode III).
Fig. 8. A map of the fracture mode that illustrates the systematic change of the dominant fracture mode with the loading speed.
larger grains would have a lower strength and hence the deformation is more likely to concentrate in the solder, which is opposite to the results found in our study. For the data set generated in our study, such mode transition behavior is better explained by the increase of the IMC thickness. We carried out finite element analysis for solder joints with different IMC thicknesses to check if the IMC thickness influences the deformation distribution across the solder and the IMC during loading. The distribution of plastic strain attained from the FEM analysis, shown in Fig. 9, discloses that a sample with a thin IMC layer exhibits no clear sign of deformation concentration while one with a thick IMC layer displays deformation concentration along the region located just above the interface, which confirms the influence of the IMC thickness on the deformation concentra-
tion. This simulation result implies that at a high loading speed, such deformation concentration would occur more readily for a sample with a thick IMC than one with a thin IMC, facilitating the fracture mode transition. Thus we suggest that the increased IMC thickness of the samples aged at higher temperatures has instigated the swifter mode transition. With full understanding of the deformation behavior of solder joints over a wide range of loading speeds, we are now able to use this information to evaluate the drop reliability. First, it should be noted that the fracture behavior of Mode III is characteristic of the fracture mode usually observed in drop failures [1–4]. For the samples aged at the highest temperature, as the loading speed increases, the fracture mode promptly shifts to Mode III. This means that this material is likely to fail in a brittle manner upon mechan-
Fig. 9. Distribution of equivalent plastic strain in Sn–37Pb/Cu solder joints with IMC layers with different thicknesses: (a) 2 and (b) 10 lm.
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ical impact during drop events a low drop reliability. For the samples aged at low temperatures, in contrast, the ductile fracture mode remains persistent not switching to the brittle mode even at high loading speeds, meaning that this solder joint would be able to absorb a high speed impact by allowing some plastic deformation a high drop reliability. As such, the thorough characterization of the stress–strain behavior of these solder joints over a wide range of strain rates produces the conclusion that the drop reliability of SnPb/Cu UBM solder joints would deteriorate as the aging temperature increases. In addition, it should be noted that in the literature, the results from investigations on the effects of aging are somewhat scattering. For example, a study on SnAgCu/ electroless-Ni/immersion-Au UBM using a high speed ball shear test reported the deterioration of the drop reliability due to reduced interface strength after aging at high temperatures whereas Xu et al., in a study on SnPb/electroless-Ni/immersion-Au UBM using a high speed ball impact test, reported an increase in impact toughness after aging, attributing it to grain coarsening in the solder [32,33]. Such incoherency in the reports in the literature highlights the complex nature of drop failures and the difficulties of evaluating drop reliability accurately, which emphasizes that the data captured in this study should be carefully used in junction with those obtained from other techniques to fully understand drop failures. The results of our study demonstrate that our experimental methodology, imposing a simple shear strain at wide range speeds and sifting through the data extracted from the stress–strain curves, could critically helps us gain key information directly tied to the drop reliability. As the strain rate escalates, the fracture mode shifts in the sequence of Modes I–III. How readily this characteristic swing of the fracture mode with the strain rate occurs can be translated directly to the materials’ ability to withstand a high speed loading, which is closely linked to the drop reliability. In addition, one should remember that the stress states the materials would experience in real applications are likely to be more complicated than a pure shear stress condition adopted in this study. Thus, the data obtained from this material should be carefully interpreted in conjunction with the results obtained from board level drop tests. 4. Summary This report presents a unique way of utilizing the test method of straining specimens at various speeds, once a popular experimental methodology for traditional metallurgists, to evaluate the drop reliability of solder joints for microelectronics. We chose SnPb/Cu UBM solder joints aged at different temperatures as a test sample. A careful analysis of stress–strain curves obtained from the lap-shear tests, combined with the SEM analysis, elucidated three fracture modes (Modes I–III) active in the solder joints; and illuminating the fracture mode for each sample generated a map of the fracture mode, which displayed how the fracture mode of each set of samples shifts from ductile to brittle with the loading speed increasing. Noting that a prompt shift in the fracture mode from ductile to brittle represents a low drop reliability, we used this map to evaluate the drop reliability of the solder joints and found that the drop reliability would deteriorate as the aging temperature increases possibly due to the increased IMC thickness. The methodology adopted in this study offers a unique path to evaluate the drop reliability of solder joints, providing a cheap and quick
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means of evaluating the drop reliability when used in conjunction with the board level test. More importantly, it could help us examine the fundamental aspect of the mechanical behavior of solder joints during drop failure. Acknowledgments The authors appreciate a financial support from the Grant (SC0800) ISTK (Korea Research Council for Industrial Science and Technology) of Republic of Korea. This study was also conducted through the Practical Application Project of Advanced Microsystems Packaging Program of Seoul Technopark, funded by the Ministry of Knowledge Economy. In addition, this research was, in part, financially supported by the Ministry of Knowledge Economy (MKE) and Korea Institute for Advancement of Technology (KIAT) through the Workforce Development Program in Strategic Technology. References [1] J.-W. Jang, A.P. De Silva, J.E. Drye, S.L. Post, N.L. Owens, J.-K. Lin, D.R. Frear, IEEE Trans. Electron. Packag. Manufact. 30 (2007) 49–53. [2] T.T. Mattila, J.K. Kivilahti, J. Electron. Mater. 34 (2005) 969–976. [3] Y.H. Xia, C.Y. Lu, X.M. Xie, J. Electron. Mater. 36 (2007) 1129–1136. [4] W. Peng, M.E. Marques, J. Electron. Mater. 36 (2007) 1679–1690. [5] J.W. Kim, J.K. Jang, S.O. Ha, S.S. Ha, D.G. Kim, S.B. Jung, Microelectr. Relia. 48 (2008) 1882–1889. [6] Y.-S. Lai, H.-C. Chang, C.-L. Ye, Microelectr. Relia. 47 (2007) 2179–2187. [7] J.W. Kim, Y.C. Lee, S.S. Ha, S.B. Jung, J. Mater. Sci.: Mater. Electron. 20 (2009) 17–24. [8] K.T. Tsai, F.L. Liu, E.H. Wong, R. Rajoo, Solder Surf. Mount. Technol. 16 (2006) 12–17. [9] E.H. Wong, C.S. Selvanayagam, S.K.W. Seah, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, D.R. Frear, M. Leoni, Y.-S. Lai, C.-L. Yeh, J. Electron. Mater. 37 (2008) 829–836. [10] E.H. Wong, R. Rajoo, S.K.W. Seah, C.S. Selvanayagam, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, M. Leoni, P.L. Eu, Y.-S. Lai, C.-L. Yeh, Microelectr. Relia. 48 (2008) 1069–1078. [11] F.B. Song, S.W.R. Lee, K. Newman, B. Sykes, S. Clark, In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 364–372. [12] F.B. Song, S.W.R. Lee, K. Newman, B. Sykes, S. Clark, In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 1504–1513. [13] M.E. Johnson, H.Y. Lu, D. Lawhead, T. Tessier, D. Scott, A. Curtis. In: IEEE 57th Electronic Components and Technology Conference 2007, pp. 536–542. [14] K. Newman, In: IEEE 55th Electronic Components and Technology Conference, 2005, pp. 1194–1201. [15] Solid State Devices, Board level drop test method of components for handheld electronic products. In: JESD22-B111, 2003. [16] S.-C. Her, Compos. Struct. 47 (1999) 673–678. [17] Y.-L. Shen, N. Chawla, E.S. Ege, X. Deng, Acta Mater. 53 (2005) 2633–2642. [18] M.A. Matin, J.G.A. Theeven, W.P. Vellinga, M.G.D. Geers, Microelectr. Relia. 47 (2007) 1262–1272. [19] J.H.L. Pang, B.S. Xiong, IEEE Trans. Comp. Packag. Technol. 28 (2005) 830–840. [20] N. Chawla, R.S. Sidhu, J. Mater. Sci.: Mater. Electron. 18 (2007) 175–189. [21] W.J. Plumbridge, C.R. Gagg, J. Mater. Sci.: Mater. Electron. 10 (1999) 461–468. [22] T.K. Hwang, S.B. Lee, Key Eng. Mater. 297–300 (2005) 825–830. [23] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill, New York, 1988. [24] E.H. Wong, C.S. Selvanayagam, S.K.W. Seah, W.D. van Driel, J.F.J.M. Caers, X.J. Zhao, N. Owens, L.C. Tan, D.R. Frear, M. Leoni, Y.-S. Lai, C.-L. Yeh, Mater. Lett. 62 (2008) 3031–3034. [25] F.Q. Lang, H. Tanaka, O. Munegata, T. Taguchi, T. Narita, Mater. Character. 54 (2005) 223–229. [26] F.L. Zhu, H.H. Zhang, R.F. Guan, S. Liu, Microelectr. Eng. 84 (2007) 144–150. [27] I. Shohji, T. Yoshida, T. Takahashi, S. Hioki, Mater. Sci. Eng. A 336 (2004) 50–55. [28] C.E. Ho, S.C. Yang, C.R. Kao, J. Mater. Sci.: Mater. Electron. 18 (2007) 155–174. [29] H.K. Kim, H.K. Liou, K.N. Tu, J. Mater. Res. 10 (1995) 497–504. [30] T.Y. Lee, W.J. Choi, K.N. Tu, J.W. Jang, S.M. Kuo, J.K. Lin, D.R. Frear, K. Zeng, J.K. Kivilahti, J. Mater. Res. 17 (2002) 291–301. [31] J. Varghese, A. Dasgupta, Microelectr. Relia. 47 (2007) 1095–1102. [32] D.S. Liu, C.Y. Kuo, C.L. Hsu, G.S. Shen, Y.R. Chen, K.C. Lo, Mater. Sci. Eng. A 494 (2008) 196–202. [33] Y. Xu, S. Ou, K.N. Tu, K. Zeng, R. Dunne, J. Mater. Res. 23 (2008) 1482–1487.