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Tin whisker growth from micro-alloyed SAC solders in corrosive climate
Journal of Alloys and Compounds 616 (2014) 116–121
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tin whisker growth from micro-alloyed SAC solders in corrosive climate Balázs Illés a,⇑, Barbara Horváth b a b
Department of Electronics Technology, Budapest University of Technology and Economics, Egry József St. 18, Budapest 1111, Hungary National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 22 April 2014 Accepted 13 July 2014 Available online 21 July 2014 Keywords: Tin whisker Corrosion SAC Metals and alloys
a b s t r a c t In this paper the tin whisker growth properties of lead-free SAC (SnAgCu) solder alloys were investigated. There different alloys were studied: two micro-alloyed SAC (SnAgCu + Bi + Sb) and the widely applied SAC305 as reference. FR4 test board was developed for 0805 chip resistor components. Solder joints were fabricated on imm-Sn surface finish. The samples were aged in elevated temperature and different humidity levels to induce the tin whisker formation: at 85 °C/85RH% and 85 °C/20RH%. The test duration was 3000 h. Whisker growth was checked after every 500 h by scanning electron microscope. The developed whiskers were etched by focused ion beam and the cross-sections were observed by scanning ion microscope and transmission electron microscope. It was shown that the lead-free SAC solder alloys can also develop tin whiskers and the whiskering ability depends on the corrosion behavior of the SAC solders. Copper and bismuth precipitations were found at the whisker roots and within the whiskers which could effect on the number, length and type of the developed whiskers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Tin whiskers are 1–10 lm thick and 10–500 lm long crystal filaments which grow out from the tin layer and can cause short circuits in the electronics appliances. Tin whisker growth is induced by compressive mechanical stresses [1] which can originate from: residual stresses in the tin electroplatings; mechanical loads; growth of intermetallics and oxide layer; and temperature change. Transition to lead-free electronics resulted in the common use of SnAgCu (SAC) solder alloys in the electronics industry. Previously tin–lead alloys (63Sn37Pb and 60Sn40Pb) were mainly used. The lead content in these alloys was found to be highly effective in restraining tin whisker growth. Nowadays, a new tendency in the development of SAC solder alloys is to reduce the expensive silver content (and therefore the price of the SAC solders) with micro-alloying of further metals (antimony, bismuth, etc.). These new SAC alloys are the so called ‘‘low silver content micro-alloyed SAC solders’’ (mSAC). In the mSAC alloys the tin content is higher (over 98 wt%) but the silver contentment is much lower (under 1 wt%) than in a simple SAC alloys (Sn 96 wt%, Ag 3 wt%). Further aim of the novel mSAC alloys is to improve the quality of the solder joints. In the simple SAC alloys the higher silver content can cause large amount of Ag3Sn intermetallics (IMC) when applied to fast ⇑ Corresponding author. E-mail address: [email protected] (B. Illés). http://dx.doi.org/10.1016/j.jallcom.2014.07.103 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
cooling. These very brittle parts can be the starting-point of cracks inside the solder joints [2]. It is also found that the mechanical properties of mSAC alloys can be improved by micro-alloying. Perevezentsev et al. have proven that germanium micro-alloying increases the mechanical strength of SAC solder joints by 18% [3]. Pandher et al. have observed similar results by micro-alloying with bismuth [4]. Nadia et al. have shown that adding copper nanoparticles can improve the wetting-ability of SAC solders [5]. However, apart from the previously discussed advantages of mSAC alloys, the alloying of further elements into the SAC alloys and the high tin content can result in reliability problems. Hua et al. have demonstrated that bismuth alloyed tin has high risk to grow bismuth-enriched tin extrusions at high temperature environment [6]. Chuang et al. have reported that alloying 0.5 wt% cerium into the Sn–3Ag–0.5Cu improves the solderingability, however also results in tin whisker growth at an extremely high rate [7]. Li et al. have found that neodymium alloyed tin can also produce tin whiskers with different morphologies and the humidity plays an important role in whisker growth. The selective oxidation of NdSn3 compound is serious in water vapor, which results in chrysanthemum-like, rod-like whiskers and tin extrusions [8]. Horváth et al. have also found that high tin content SnCu solder alloys (Cu1–5 wt%) can grow whiskers under corrosive climate [9]. The corrosive environment is still only occasionally used during the tin whisker researches. Tin whisker studies under corrosive climate started in 2006. Oberndorff et al. have found that in high
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B. Illés, B. Horváth / Journal of Alloys and Compounds 616 (2014) 116–121 Table 1 Alloy composition of the investigated solders. Solder alloy
Sn
Ag
Cu
Bi
Sb
SAC305 mSAC1 mSAC2
96.5 98.4 98.9
3.0 0.8 0.3
0.5 0.7 0.7
– 0.1 0.1
– – 0.01
Fig. 3. Hundreds of whiskers on mSAC2 solder joint, 85 °C/85RH% test after 3000 h.
SAC305
Average whisker density [pcs./0.04 mm2 ]
Fig. 1. First whiskers on an mSAC2 solder joint, 85 °C/85RH% tests after 1000 h.
SAC305 mSAC1 mSAC2
Average whisker length [µm]
mSAC1 mSAC2
Time [h] Fig. 4. Average whisker length, 85 °C/85RH% tests.
of corrosive climate was investigated on tin whisker growth from mSAC (and simple SAC) solder alloys.
2. Experimental
Time [h] Fig. 2. Average whisker density, 85 °C/85RH% tests.
humidity, tin whiskers growth is induced by oxidation and corrosion of the tin layer [10]. Nakadaira et al. have reported that corrosion can spread inside the tin layer during high humidity aging and induces compressive stress, which causes whiskering inside and around the corroded area [11]. Kim et al. have conducted a whisker research with application of corrosive environment in 2007 [12], however they have recognized the role of tin oxide formation only two year later, when they have stated that the volume increase of SnO2 by the oxidation produces compressive stress on the tin layer and it can cause the whisker growth [13]. Consequently, the tin whiskering resistance of mSAC solder alloys is still questionable. Therefore, in this research the effect
In our experiment, three different solder alloys were investigated from the aspect of whiskering: two mSAC and the widely applied SAC305 as reference. The alloy composition can be seen in Table 1. FR4 test boards were developed with copper wiring and imm-Sn surface finish for 18 pieces of 0805 size chip resistors. The size of the test boards were 40 45 mm. The solder pastes were deposited onto the test boards by a DEK248 stencil printer. The used stencil was 150 lm thick, laser cut from stainless steel. The resistors were placed by a DIMA semi-automatic pick&place machine. The soldering was carried out by a DIMA infrared reflow oven according to the suggested thermal profiles of the different solder alloys. Two test boards were produced for each solder alloy, which means 72 solder joints from each alloys. The samples were aged in elevated temperature and humidity levels to induce tin whisker growth. Two different accelerated life-time tests were carried out for solder joints [14]: 85 °C/85RH% to study the effect of a corrosive climate and 85 °C/20RH% as a reference dry test. The test duration was 3000 h. Whisker growth was checked after every 500 h by a FEI Inspect S50 Scanning Electron Microscope (SEM) (Acc. Voltage 20 kV). The cross-sections of the whiskers were developed using a JEM-9320FIB Focused Ion Beam (FIB) (with a Ga Ion Source and Acc. Voltage of 30 kV) and observed with a FIB Scanning Ion Microscope Image (FIB-SIM). The samples were coated with approx. 50 nm carbon layer for surface protection before the FIB examination. The cross-sectional images were observed with a 45° tilt angle. The cross-sections of the whisker and the layer underneath
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Table 2 Maximum tin whisker lengths. Time (h)
1000 1500 2000 2500 3000
Max. length [lm] 85 °C/85RH% SAC305
mSAC1
mSAC2
0 12 30 50 50
0 22 35 37 59
19 30 51 55 65
Fig. 5. Typical tin whiskers in the 85 °C/85RH% test environment. were also observed by a JEM-2100F-2 200 kV Field Emission Gun Transmission Electron Microscope (TEM) and by Energy-dispersive X-ray spectroscopy (EDX) analysis in order to identify the layer elements. The electron diffraction (crystal orientation study) were also performed by the Scanning Area Electron Diffraction (SAED) unit of the JEM-2100F-2 TEM. The axial length of a whisker was determined according to the JESD201 standard (measuring the distance between the tip of the whisker and the surface). The measurement accuracy was about ±5 lm. The whisker densities were calculated at 1600 magnification. The applied unit of the whisker density was pieces (pcs.)/0.04 mm2. The average whisker lengths and densities were calculated from 25 measurements.
3. Results First tin whiskers were detected on the mSAC2 joints at 85 °C/ 85RH% test environment at 1000 h (Fig. 1). The cracked surface on the SEM micrograph is flux residue remaining from the solder paste. After 1000 h, the 85 °C/85RH% test has produced whiskers on all of the samples. Fig. 2 shows the average whisker densities measured on samples stored in 85 °C/85RH% environments. No significant difference has been observed between the alloys in the 1500–2500 h time period, the number of developed tin whiskers has increased linear in time (from 3–8 to 25–28 pcs./ 0.04 mm2). The relatively high deviations are common in the whisker researches. After 2500 h, the number of developed whiskers
has shown saturation for SAC305 and mSAC1 alloys while the mSAC2 has increased linearly further and reached a 38 pcs./ 0.04 mm2 value at the end of the test. Fig. 3 shows an mSAC2 solder joint after testing in 85 °C/85RH% for 3000 h. The average whisker lengths measured on samples stored in 85 °C/85RH% environments have shown higher differences (Fig. 4) than the average densities (Fig. 2). The mSAC2 alloy has produced longer tin whiskers than others. The length statistics were almost similar in the case of the mSAC1 and SAC305 alloys. The increase of length was linear in time during the whole test. The average lengths have reached 32, 24 and 22 lm at 3000 h in the case of mSAC2, mSAC1 and SAC respectively. Tin whisker appearance was not detected in the tests conducted in the 85 °C/20RH% environment. The maximum whisker lengths are also important in aspect of microelectronics reliability due to the possible short circuit development. Table 2 contains the maximum whisker lengths detected during the study. Nowadays, the applied minimum raster distance in electronics industry is 200 lm. After harsh and long environment tests, the maximum detected whisker length on SAC solder joints was 65 lm (mSAC2, 85 °C/85RH%, 3000 h). So it can be concluded that the probability of ‘‘SAC solder’’ whisker caused short circuits is low, however it cannot be neglected. In the case of the 85 °C/85RH% test most of the whiskers was highly twisted nodule type (Fig. 5) with highly varying diameters (between 0.5 and 10 lm), however some hillocks and filaments were also detected. 4. Discussion According to the results of the different test environments, corrosion plays an important role in the tin whisker growth from solder alloys since only the corrosive environments have produced whiskers. We have not observed tin whisker appearance on the samples stored in 85 °C/20RH%. In the case of the 85 °C/85RH% test, the whiskering is significant from the aspects of microelectronics reliability. The mSAC1 and SAC305 alloys have shown similar whisker resistance ability and it was better than the mSAC2 alloy. The root cause of the different behavior can come from the followings: different corrosion resistance of the alloys and different recrystallization speed of the alloys [9]. In this case the mechanical stress induced by the Sn–Cu intermatallics layer growth at the copper pads can be excluded since the thick solder joint (compared to a surface finishing) can easily relax this kind of stress. Recrystallization (grain growth) reduces the total area of the grain boundary in the joints which reduces the internal energy (stress). The recrystallization starts above a given temperature which is typically at 0.4–0.7 of the liquidus temperature of the alloy. In our case, the investigated alloys have almost the same liquidus temperatures (SAC305: 492 K; mSAC1: 498 K; mSAC2: 501 K). Therefore the recrystallization speed has to be also similar, so this factor can be excluded from the further analyses. According to the JEDEC guidelines, the condensed moisture at higher temperature can induce tin whisker growth by the
Fig. 6. FIB cross-sections of an SAC305 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
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Fig. 7. FIB cross-sections of mSAC1 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
Fig. 8. FIB cross-sections of mSAC2 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
Fig. 9. TEM analysis of the SAC305 samples aged at aged 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) EDS element maps.
Table 3 Element composition at the measurement points in Fig. 9.
Table 4 Element composition at the measurement points in Fig. 10.
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
M1 M2 M3 M4 M5 M6 M7 M8
94.51 94.24 94.37 60.22 66.01 66.14 51.16 88.03
0.98 1.12 0.64 0.49 0.84 0.59 0.14 2.58
0.30 0.44 0.49 1.98 0.31 2.12 13.33 0.98
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
4.21 4.19 4.51 37.31 32.83 31.16 35.37 8.41
M1 M2 M3 M4 M5 M6
95.96 95.41 39.14 29.89 35.70 69.39
2.35 1.73 1.42 14.80 60.60 1.20
0.31 0.44 55.15 44.37 0.84 0.90
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
1.38 2.42 4.29 10.94 2.86 28.51
corrosion of the tin layer [15]. Osenbach et al. have proved that the layer where the corroded area intersects the non-corroded area is coincident with a no apparent height difference. This indicates that the surface of the layer is pinned through during the corrosion process. In this way, the corrosion-induced excess tin atoms are pushed within the original volume of the tin layer and create
localized stress and excess strain energy [16]. Corrosion can spread not only towards the top free surface but it can also spread towards the inside of the layer causing mechanical stress on the neighboring tin grains. Nakadaira et al. assume that during the increase of corroded area it is getting in contact with the whisker base grains and affecting the growth of the whisker [11]. The Pilling-Bedworth Ratio (PBR) gives the volume expansion during a metal oxide
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B. Illés, B. Horváth / Journal of Alloys and Compounds 616 (2014) 116–121 Table 5 Element composition at the measurement points in Fig. 11.
Fig. 10. TEM analysis of the mSAC1 samples aged at 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) TEM-SAED analyzes results.
formation. PBR for SnO2 and SnO on pure tin is 1.31 and 1.26, respectively [17]. This means, that the formation of SnOx results in considerable mechanical stress in the solder joints. The difference of the volume between the two oxides is due to the different crystal structure and the place of the oxygen atom between the tin atoms. According to the literature the SAC305 and SAC405 solder alloys have the highest corrosion resistances from the lead-free solder alloys and bismuth content solder alloys have usually shown worse results [18]. Cross-sections of the developed whiskers and the layer underneath were prepared by FIB in order to evaluate the level of corrosion in the solder joints. When comparing the cross-sections of the samples with SIM, it was observed that the volume of the corroded area is much smaller under the SAC305 and mSAC1 whiskers (Figs. 6 and 7) than under the whiskers from mSAC2 (Fig. 8). In the upper 10 lm of the mSAC2 solder joints – which is the relevant region from the aspect of whisker growth – the corroded area ratio is
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10
90.88 92.57 93.42 53.34 67.96 25.92 67.65 61.03 92.47 57.26
1.04 0.88 1.64 0.10 1.13 0.58 0.90 1.12 0.66 0.28
1.04 1.01 0.68 5.04 0.18 50.33 1.41 1.78 0.68 39.47
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
7.04 5.54 4.26 41.51 30.73 23.17 30.04 36.07 6.19 2.99
usually over 80%. In the case of the mSAC1 and SAC305, this ratio was much lower between 10% and 50%. In order to analyse the cross-sections more precisely, TEM lamellas were fabricated from the samples. The red rectangles in the figures mark the area of the TEM observations. The TEM analysis results of the SAC305 sample are presented in Fig. 9 and Table 3. The TEM analysis has approved the results of the FIB-SIM analyses. Localized corrosion has been found under the SAC305 whisker. The atomic present of the oxygen in the corrosion spots is over 30%. Corroded copper precipitations are also observable at the whisker root (see M7 and in Table. 4). The PBR for Cu2O and CuO on pure copper is 1.67 and 1.72, respectively [17]. Hence, in case of the oxidation of the copper grains in the alloy, the volume can expand up to +72% depending on the ratio of the developed copper-oxide type. This causes further mechanical stress (beside the tin corrosion products) inside the layer. Therefore, the larger the copper precipitations at the whisker root, the more stress will develop in the layer. The TEM analysis results of the mSAC1 sample are presented in Fig. 10 and Table 4. In this case, the TEM lamella was prepared along a nodule type tin whisker in order to study the grain structure of the whisker. The solder joint was also corroded under the mSAC1 whisker. However the atomic present of the oxygen at the localized corrosion is slightly lower than at SAC305. Copper and also some silver precipitations have also been found close to the whisker root (M3, M4, M5, only M4 was corroded). According to the diffraction analyses of the grains, it was found that the studied whisker is not mono-crystalline and apart from tin other elements such as copper and bismuth were also detected in the whisker in traces (in the G5 grain). The G1 base grain of the whisker is fragmented. The G1 base grain has [1 1 1] orientation, while the grains G2, G3 and G4 have [0 0 1] orientation with minor tilt differences. According to the measurements, this means a 53°, 52° and 48° tilt compared to the G1 base grain. The grains G6, G7 and
Fig. 11. TEM analysis of the mSAC2 samples aged at aged 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) EDS element maps.
B. Illés, B. Horváth / Journal of Alloys and Compounds 616 (2014) 116–121
G8 have the same orientation as G1 [1 1 1], therefore these are possible subgrains of G1 with a tilt of 7°, 1.5° and 1° compared to G1. The grain G5 has [0 1 0] orientation which results another 41° tilt compared to the G1 base grain. The same effect of copper in the tin whiskers was observed by Horváth [19], who states that even only a few diffusing copper atoms can fragment the whisker and create subgrains within the large abnormal base grain. This results in twisted nodule type polycrystalline whiskers instead of filament whiskers, as it was found in our case. This effect can also reduce the maximum length of the developed whiskers. Therefore alloying copper into pure tin coatings may have a positive effect on the reliability of the electronics appliances. The TEM analysis results of the mSAC2 sample are presented in Fig. 11 and Table 5. Under the mSAC2 whisker the solder joint is highly corroded. The localized corrosion form a continuous corroded layer within the atomic present of the oxygen is usually over 30%. Copper precipitations are observable at the whisker root which are also corroded (see M4 and M6 in Table 3). Here, not only some diffusing copper atoms (as at mSAC1) but continuous copper precipitations have been found inside the whisker. This proves the slightly detectable presence of copper and bismuth at mSAC1. 5. Conclusions The tin whiskering ability of micro-alloyed SAC (SnAgCu + Bi + Sb) and simple SAC (SnAgCu) solder alloys was investigated by corrosive and dry environmental tests. The main conclusions are the followings: Corrosion plays an important role for tin whisker growth in the SAC solder alloys since only the corrosive environments have produced whiskers. The whiskering was significant in the aspects of microelectronics reliability at the 85 °C/85RH% test. The longest detected whisker was 65 lm and the highest density reached 55 pcs./ 0.04 mm2 after 3000 h. The mSAC1 and SAC305 alloys have shown similar and better whisker resistance ability than the mSAC2 alloy. The root cause of the different behavior probably comes from the different corrosion resistance of the alloys. In the upper 10 lm of the mSAC2 solder joints the corroded area ratio is usually over 80%. This ratio is much lower for mSAC1 and SAC305, between 10% and 50%. Copper precipitations have been found at the whisker roots. During the oxidation of copper, the volume can expand up to +72% which cause further mechanical stress in the layer. The larger the copper precipitation at the whisker root, the more stress will develop in the layer. Copper and bismuth traces have also been found within the whisker which can fragment the whisker grain. This may result in twisted nodule type polycrystalline whiskers instead of
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filament whiskers. This effect can also reduce the maximum length of the developed whiskers. Therefore alloying copper into pure tin coatings may have a positive effect on the reliability of the electronics appliances.
Acknowledgement The research leading to these results has received funding from the ProProgressio foundation. References [1] B.-Z. Lee, D.N. Lee, Spontaneous growth mechanism of tin whiskers, Acta Mater. 46 (1998) 3701–3714. [2] O. Krammer, T. Garami, Reliability investigation of low silver content microalloyed SAC solders, in: Proceedings of 35th International Spring Seminar on Electronics Technology (ISSE), Bad Aussee, 2012, pp. 149–154. [3] B.N. Perevezentsev, M.N. Kurmaev, Microalloying of alloys of the Sn–Ag–Cu system, Met. Sci. Heat Treat. 51 (2009) 540–543. [4] R.S. Pandher, B.G. Lewis, R. Vangaveti, B. Singh, Drop shock reliability of leadfree alloys – effect of micro-additives, in: Proceedings of 57th Electronic Components and Technology Conference (ECTC), Reno, 2007, pp. 669–676. [5] A. Nadia, A.S.M.A. Haseeb, Understanding the effects of addition of copper nanoparticles to Sn–3.5Ag solder, Solder. Surf. Mt. Tech. 23 (2011) 68–74. [6] C.-C. Hua, Y.-D. Tsai, C.-C. Lin, G.-L. Lee, S.-W. Chen, T.-C. Lee, T.-C. Wen, Anomalous growth of whisker-like bismuth–tin extrusions from tin-enriched tin–Bi deposits, J. Alloys Comp. 472 (2009) 121–126. [7] T.-H. Chuang, C.-C. Chi, Effect of adding Ge on rapid whisker growth of Sn– 3Ag–0.5Cu–0.5Ce alloy, J. Alloys Comp. 480 (2009) 974–980. [8] C.-F. Li, Z.-Q. Liu, J.-K. Shang, The effects of temperature and humidity on the growth of tin whisker and hillock from Sn5Nd alloy, J. Alloys Comp. 550 (2013) 231–238. [9] B. Horváth, T. Shinohara, B. Illés, Corrosion properties of tin-copper alloy coatings in aspect of tin whisker growth, J. Alloys Comp. 577 (2013) 439–444. [10] P. Oberndorff, M. Dittes, P. Crema, P. Su, E. Yu, Humidity effects on Sn whisker formation, IEEE Trans. Electron Pack. Manuf. 29 (2006) 239–245. [11] Y. Nakadaira, S. Jeong, J. Shim, J. Seo, S. Min, T. Cho, S. Kang, S. Oh, Growth of tin whiskers for lead-free plated leadframe packages in high humid environments and during thermal cycling, Microelectron. Reliab. 48 (2008) 83–104. [12] K.-S. Kim, J.-H. Kim, S.-W. Han, The effect of postbake treatment on whisker growth under high temperature and humidity conditions on tin-plated Cu substrates, Mater. Lett. 62 (2008) 1867–1870. [13] K.-S. Kim, J.-M. Yang, J.-P. Ahn, The effect of electric current and surface oxidization on the growth of Sn whiskers, Appl. Surf. Sci. 256 (2010) 7166– 7174. [14] Zs. Illyefalvi-Vitéz, P. Németh, O. Krammer, J. Pinkola, Comparison of accelerated life-time test methods of Pb-free solder joints, in: Proceedings of 16th European Microelectronics and Packaging Conference, Oulu, 2007, pp. 419–424. [15] JEDEC JP002, Current tin whiskers theory and mitigation practices guideline, March 2006. [16] J.W. Osenbach, J.M. DeLucca, B.D. Potteiger, A. Amin, R.L. Shook, F.A. Baiocchi, Sn corrosion and its influence on whisker growth, IEEE Trans. Electron Pack. Manuf. 30 (2007) 23–35. [17] S.D. Cramer, B.S. Covino, ASM Handbook, Corrosion: Fundamentals, Testing, and Protection vol. 13A, Ohio, 2003. [18] B.Y. Wu, Y.C. Chan, M.O. Alam, Electrochemical corrosion study of Pb-free solders, J. Mater. Res. 21 (2006) 62–70. [19] B. Horváth, Influence of copper diffusion on the shape of whiskers grown on bright tin layers, Microelectron. Reliab. 53 (2013) 1009–1020.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Tin whisker growth from micro-alloyed SAC solders in corrosive climate Balázs Illés a,⇑, Barbara Horváth b a b
Department of Electronics Technology, Budapest University of Technology and Economics, Egry József St. 18, Budapest 1111, Hungary National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
a r t i c l e
i n f o
Article history: Received 22 April 2014 Accepted 13 July 2014 Available online 21 July 2014 Keywords: Tin whisker Corrosion SAC Metals and alloys
a b s t r a c t In this paper the tin whisker growth properties of lead-free SAC (SnAgCu) solder alloys were investigated. There different alloys were studied: two micro-alloyed SAC (SnAgCu + Bi + Sb) and the widely applied SAC305 as reference. FR4 test board was developed for 0805 chip resistor components. Solder joints were fabricated on imm-Sn surface finish. The samples were aged in elevated temperature and different humidity levels to induce the tin whisker formation: at 85 °C/85RH% and 85 °C/20RH%. The test duration was 3000 h. Whisker growth was checked after every 500 h by scanning electron microscope. The developed whiskers were etched by focused ion beam and the cross-sections were observed by scanning ion microscope and transmission electron microscope. It was shown that the lead-free SAC solder alloys can also develop tin whiskers and the whiskering ability depends on the corrosion behavior of the SAC solders. Copper and bismuth precipitations were found at the whisker roots and within the whiskers which could effect on the number, length and type of the developed whiskers. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Tin whiskers are 1–10 lm thick and 10–500 lm long crystal filaments which grow out from the tin layer and can cause short circuits in the electronics appliances. Tin whisker growth is induced by compressive mechanical stresses [1] which can originate from: residual stresses in the tin electroplatings; mechanical loads; growth of intermetallics and oxide layer; and temperature change. Transition to lead-free electronics resulted in the common use of SnAgCu (SAC) solder alloys in the electronics industry. Previously tin–lead alloys (63Sn37Pb and 60Sn40Pb) were mainly used. The lead content in these alloys was found to be highly effective in restraining tin whisker growth. Nowadays, a new tendency in the development of SAC solder alloys is to reduce the expensive silver content (and therefore the price of the SAC solders) with micro-alloying of further metals (antimony, bismuth, etc.). These new SAC alloys are the so called ‘‘low silver content micro-alloyed SAC solders’’ (mSAC). In the mSAC alloys the tin content is higher (over 98 wt%) but the silver contentment is much lower (under 1 wt%) than in a simple SAC alloys (Sn 96 wt%, Ag 3 wt%). Further aim of the novel mSAC alloys is to improve the quality of the solder joints. In the simple SAC alloys the higher silver content can cause large amount of Ag3Sn intermetallics (IMC) when applied to fast ⇑ Corresponding author. E-mail address: [email protected] (B. Illés). http://dx.doi.org/10.1016/j.jallcom.2014.07.103 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
cooling. These very brittle parts can be the starting-point of cracks inside the solder joints [2]. It is also found that the mechanical properties of mSAC alloys can be improved by micro-alloying. Perevezentsev et al. have proven that germanium micro-alloying increases the mechanical strength of SAC solder joints by 18% [3]. Pandher et al. have observed similar results by micro-alloying with bismuth [4]. Nadia et al. have shown that adding copper nanoparticles can improve the wetting-ability of SAC solders [5]. However, apart from the previously discussed advantages of mSAC alloys, the alloying of further elements into the SAC alloys and the high tin content can result in reliability problems. Hua et al. have demonstrated that bismuth alloyed tin has high risk to grow bismuth-enriched tin extrusions at high temperature environment [6]. Chuang et al. have reported that alloying 0.5 wt% cerium into the Sn–3Ag–0.5Cu improves the solderingability, however also results in tin whisker growth at an extremely high rate [7]. Li et al. have found that neodymium alloyed tin can also produce tin whiskers with different morphologies and the humidity plays an important role in whisker growth. The selective oxidation of NdSn3 compound is serious in water vapor, which results in chrysanthemum-like, rod-like whiskers and tin extrusions [8]. Horváth et al. have also found that high tin content SnCu solder alloys (Cu1–5 wt%) can grow whiskers under corrosive climate [9]. The corrosive environment is still only occasionally used during the tin whisker researches. Tin whisker studies under corrosive climate started in 2006. Oberndorff et al. have found that in high
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B. Illés, B. Horváth / Journal of Alloys and Compounds 616 (2014) 116–121 Table 1 Alloy composition of the investigated solders. Solder alloy
Sn
Ag
Cu
Bi
Sb
SAC305 mSAC1 mSAC2
96.5 98.4 98.9
3.0 0.8 0.3
0.5 0.7 0.7
– 0.1 0.1
– – 0.01
Fig. 3. Hundreds of whiskers on mSAC2 solder joint, 85 °C/85RH% test after 3000 h.
SAC305
Average whisker density [pcs./0.04 mm2 ]
Fig. 1. First whiskers on an mSAC2 solder joint, 85 °C/85RH% tests after 1000 h.
SAC305 mSAC1 mSAC2
Average whisker length [µm]
mSAC1 mSAC2
Time [h] Fig. 4. Average whisker length, 85 °C/85RH% tests.
of corrosive climate was investigated on tin whisker growth from mSAC (and simple SAC) solder alloys.
2. Experimental
Time [h] Fig. 2. Average whisker density, 85 °C/85RH% tests.
humidity, tin whiskers growth is induced by oxidation and corrosion of the tin layer [10]. Nakadaira et al. have reported that corrosion can spread inside the tin layer during high humidity aging and induces compressive stress, which causes whiskering inside and around the corroded area [11]. Kim et al. have conducted a whisker research with application of corrosive environment in 2007 [12], however they have recognized the role of tin oxide formation only two year later, when they have stated that the volume increase of SnO2 by the oxidation produces compressive stress on the tin layer and it can cause the whisker growth [13]. Consequently, the tin whiskering resistance of mSAC solder alloys is still questionable. Therefore, in this research the effect
In our experiment, three different solder alloys were investigated from the aspect of whiskering: two mSAC and the widely applied SAC305 as reference. The alloy composition can be seen in Table 1. FR4 test boards were developed with copper wiring and imm-Sn surface finish for 18 pieces of 0805 size chip resistors. The size of the test boards were 40 45 mm. The solder pastes were deposited onto the test boards by a DEK248 stencil printer. The used stencil was 150 lm thick, laser cut from stainless steel. The resistors were placed by a DIMA semi-automatic pick&place machine. The soldering was carried out by a DIMA infrared reflow oven according to the suggested thermal profiles of the different solder alloys. Two test boards were produced for each solder alloy, which means 72 solder joints from each alloys. The samples were aged in elevated temperature and humidity levels to induce tin whisker growth. Two different accelerated life-time tests were carried out for solder joints [14]: 85 °C/85RH% to study the effect of a corrosive climate and 85 °C/20RH% as a reference dry test. The test duration was 3000 h. Whisker growth was checked after every 500 h by a FEI Inspect S50 Scanning Electron Microscope (SEM) (Acc. Voltage 20 kV). The cross-sections of the whiskers were developed using a JEM-9320FIB Focused Ion Beam (FIB) (with a Ga Ion Source and Acc. Voltage of 30 kV) and observed with a FIB Scanning Ion Microscope Image (FIB-SIM). The samples were coated with approx. 50 nm carbon layer for surface protection before the FIB examination. The cross-sectional images were observed with a 45° tilt angle. The cross-sections of the whisker and the layer underneath
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Table 2 Maximum tin whisker lengths. Time (h)
1000 1500 2000 2500 3000
Max. length [lm] 85 °C/85RH% SAC305
mSAC1
mSAC2
0 12 30 50 50
0 22 35 37 59
19 30 51 55 65
Fig. 5. Typical tin whiskers in the 85 °C/85RH% test environment. were also observed by a JEM-2100F-2 200 kV Field Emission Gun Transmission Electron Microscope (TEM) and by Energy-dispersive X-ray spectroscopy (EDX) analysis in order to identify the layer elements. The electron diffraction (crystal orientation study) were also performed by the Scanning Area Electron Diffraction (SAED) unit of the JEM-2100F-2 TEM. The axial length of a whisker was determined according to the JESD201 standard (measuring the distance between the tip of the whisker and the surface). The measurement accuracy was about ±5 lm. The whisker densities were calculated at 1600 magnification. The applied unit of the whisker density was pieces (pcs.)/0.04 mm2. The average whisker lengths and densities were calculated from 25 measurements.
3. Results First tin whiskers were detected on the mSAC2 joints at 85 °C/ 85RH% test environment at 1000 h (Fig. 1). The cracked surface on the SEM micrograph is flux residue remaining from the solder paste. After 1000 h, the 85 °C/85RH% test has produced whiskers on all of the samples. Fig. 2 shows the average whisker densities measured on samples stored in 85 °C/85RH% environments. No significant difference has been observed between the alloys in the 1500–2500 h time period, the number of developed tin whiskers has increased linear in time (from 3–8 to 25–28 pcs./ 0.04 mm2). The relatively high deviations are common in the whisker researches. After 2500 h, the number of developed whiskers
has shown saturation for SAC305 and mSAC1 alloys while the mSAC2 has increased linearly further and reached a 38 pcs./ 0.04 mm2 value at the end of the test. Fig. 3 shows an mSAC2 solder joint after testing in 85 °C/85RH% for 3000 h. The average whisker lengths measured on samples stored in 85 °C/85RH% environments have shown higher differences (Fig. 4) than the average densities (Fig. 2). The mSAC2 alloy has produced longer tin whiskers than others. The length statistics were almost similar in the case of the mSAC1 and SAC305 alloys. The increase of length was linear in time during the whole test. The average lengths have reached 32, 24 and 22 lm at 3000 h in the case of mSAC2, mSAC1 and SAC respectively. Tin whisker appearance was not detected in the tests conducted in the 85 °C/20RH% environment. The maximum whisker lengths are also important in aspect of microelectronics reliability due to the possible short circuit development. Table 2 contains the maximum whisker lengths detected during the study. Nowadays, the applied minimum raster distance in electronics industry is 200 lm. After harsh and long environment tests, the maximum detected whisker length on SAC solder joints was 65 lm (mSAC2, 85 °C/85RH%, 3000 h). So it can be concluded that the probability of ‘‘SAC solder’’ whisker caused short circuits is low, however it cannot be neglected. In the case of the 85 °C/85RH% test most of the whiskers was highly twisted nodule type (Fig. 5) with highly varying diameters (between 0.5 and 10 lm), however some hillocks and filaments were also detected. 4. Discussion According to the results of the different test environments, corrosion plays an important role in the tin whisker growth from solder alloys since only the corrosive environments have produced whiskers. We have not observed tin whisker appearance on the samples stored in 85 °C/20RH%. In the case of the 85 °C/85RH% test, the whiskering is significant from the aspects of microelectronics reliability. The mSAC1 and SAC305 alloys have shown similar whisker resistance ability and it was better than the mSAC2 alloy. The root cause of the different behavior can come from the followings: different corrosion resistance of the alloys and different recrystallization speed of the alloys [9]. In this case the mechanical stress induced by the Sn–Cu intermatallics layer growth at the copper pads can be excluded since the thick solder joint (compared to a surface finishing) can easily relax this kind of stress. Recrystallization (grain growth) reduces the total area of the grain boundary in the joints which reduces the internal energy (stress). The recrystallization starts above a given temperature which is typically at 0.4–0.7 of the liquidus temperature of the alloy. In our case, the investigated alloys have almost the same liquidus temperatures (SAC305: 492 K; mSAC1: 498 K; mSAC2: 501 K). Therefore the recrystallization speed has to be also similar, so this factor can be excluded from the further analyses. According to the JEDEC guidelines, the condensed moisture at higher temperature can induce tin whisker growth by the
Fig. 6. FIB cross-sections of an SAC305 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
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Fig. 7. FIB cross-sections of mSAC1 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
Fig. 8. FIB cross-sections of mSAC2 whiskers and the surrounding areas, aged at 85 °C/85RH% for 3000 h.
Fig. 9. TEM analysis of the SAC305 samples aged at aged 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) EDS element maps.
Table 3 Element composition at the measurement points in Fig. 9.
Table 4 Element composition at the measurement points in Fig. 10.
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
M1 M2 M3 M4 M5 M6 M7 M8
94.51 94.24 94.37 60.22 66.01 66.14 51.16 88.03
0.98 1.12 0.64 0.49 0.84 0.59 0.14 2.58
0.30 0.44 0.49 1.98 0.31 2.12 13.33 0.98
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
4.21 4.19 4.51 37.31 32.83 31.16 35.37 8.41
M1 M2 M3 M4 M5 M6
95.96 95.41 39.14 29.89 35.70 69.39
2.35 1.73 1.42 14.80 60.60 1.20
0.31 0.44 55.15 44.37 0.84 0.90
0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0
1.38 2.42 4.29 10.94 2.86 28.51
corrosion of the tin layer [15]. Osenbach et al. have proved that the layer where the corroded area intersects the non-corroded area is coincident with a no apparent height difference. This indicates that the surface of the layer is pinned through during the corrosion process. In this way, the corrosion-induced excess tin atoms are pushed within the original volume of the tin layer and create
localized stress and excess strain energy [16]. Corrosion can spread not only towards the top free surface but it can also spread towards the inside of the layer causing mechanical stress on the neighboring tin grains. Nakadaira et al. assume that during the increase of corroded area it is getting in contact with the whisker base grains and affecting the growth of the whisker [11]. The Pilling-Bedworth Ratio (PBR) gives the volume expansion during a metal oxide
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B. Illés, B. Horváth / Journal of Alloys and Compounds 616 (2014) 116–121 Table 5 Element composition at the measurement points in Fig. 11.
Fig. 10. TEM analysis of the mSAC1 samples aged at 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) TEM-SAED analyzes results.
formation. PBR for SnO2 and SnO on pure tin is 1.31 and 1.26, respectively [17]. This means, that the formation of SnOx results in considerable mechanical stress in the solder joints. The difference of the volume between the two oxides is due to the different crystal structure and the place of the oxygen atom between the tin atoms. According to the literature the SAC305 and SAC405 solder alloys have the highest corrosion resistances from the lead-free solder alloys and bismuth content solder alloys have usually shown worse results [18]. Cross-sections of the developed whiskers and the layer underneath were prepared by FIB in order to evaluate the level of corrosion in the solder joints. When comparing the cross-sections of the samples with SIM, it was observed that the volume of the corroded area is much smaller under the SAC305 and mSAC1 whiskers (Figs. 6 and 7) than under the whiskers from mSAC2 (Fig. 8). In the upper 10 lm of the mSAC2 solder joints – which is the relevant region from the aspect of whisker growth – the corroded area ratio is
Component (at.%)
Sn
Ag
Cu
Bi
Sb
O
M1 M2 M3 M4 M5 M6 M7 M8 M9 M10
90.88 92.57 93.42 53.34 67.96 25.92 67.65 61.03 92.47 57.26
1.04 0.88 1.64 0.10 1.13 0.58 0.90 1.12 0.66 0.28
1.04 1.01 0.68 5.04 0.18 50.33 1.41 1.78 0.68 39.47
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
7.04 5.54 4.26 41.51 30.73 23.17 30.04 36.07 6.19 2.99
usually over 80%. In the case of the mSAC1 and SAC305, this ratio was much lower between 10% and 50%. In order to analyse the cross-sections more precisely, TEM lamellas were fabricated from the samples. The red rectangles in the figures mark the area of the TEM observations. The TEM analysis results of the SAC305 sample are presented in Fig. 9 and Table 3. The TEM analysis has approved the results of the FIB-SIM analyses. Localized corrosion has been found under the SAC305 whisker. The atomic present of the oxygen in the corrosion spots is over 30%. Corroded copper precipitations are also observable at the whisker root (see M7 and in Table. 4). The PBR for Cu2O and CuO on pure copper is 1.67 and 1.72, respectively [17]. Hence, in case of the oxidation of the copper grains in the alloy, the volume can expand up to +72% depending on the ratio of the developed copper-oxide type. This causes further mechanical stress (beside the tin corrosion products) inside the layer. Therefore, the larger the copper precipitations at the whisker root, the more stress will develop in the layer. The TEM analysis results of the mSAC1 sample are presented in Fig. 10 and Table 4. In this case, the TEM lamella was prepared along a nodule type tin whisker in order to study the grain structure of the whisker. The solder joint was also corroded under the mSAC1 whisker. However the atomic present of the oxygen at the localized corrosion is slightly lower than at SAC305. Copper and also some silver precipitations have also been found close to the whisker root (M3, M4, M5, only M4 was corroded). According to the diffraction analyses of the grains, it was found that the studied whisker is not mono-crystalline and apart from tin other elements such as copper and bismuth were also detected in the whisker in traces (in the G5 grain). The G1 base grain of the whisker is fragmented. The G1 base grain has [1 1 1] orientation, while the grains G2, G3 and G4 have [0 0 1] orientation with minor tilt differences. According to the measurements, this means a 53°, 52° and 48° tilt compared to the G1 base grain. The grains G6, G7 and
Fig. 11. TEM analysis of the mSAC2 samples aged at aged 85 °C/85RH% for 3000 h: (a) TEM dark field (DF) image and (b) EDS element maps.
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G8 have the same orientation as G1 [1 1 1], therefore these are possible subgrains of G1 with a tilt of 7°, 1.5° and 1° compared to G1. The grain G5 has [0 1 0] orientation which results another 41° tilt compared to the G1 base grain. The same effect of copper in the tin whiskers was observed by Horváth [19], who states that even only a few diffusing copper atoms can fragment the whisker and create subgrains within the large abnormal base grain. This results in twisted nodule type polycrystalline whiskers instead of filament whiskers, as it was found in our case. This effect can also reduce the maximum length of the developed whiskers. Therefore alloying copper into pure tin coatings may have a positive effect on the reliability of the electronics appliances. The TEM analysis results of the mSAC2 sample are presented in Fig. 11 and Table 5. Under the mSAC2 whisker the solder joint is highly corroded. The localized corrosion form a continuous corroded layer within the atomic present of the oxygen is usually over 30%. Copper precipitations are observable at the whisker root which are also corroded (see M4 and M6 in Table 3). Here, not only some diffusing copper atoms (as at mSAC1) but continuous copper precipitations have been found inside the whisker. This proves the slightly detectable presence of copper and bismuth at mSAC1. 5. Conclusions The tin whiskering ability of micro-alloyed SAC (SnAgCu + Bi + Sb) and simple SAC (SnAgCu) solder alloys was investigated by corrosive and dry environmental tests. The main conclusions are the followings: Corrosion plays an important role for tin whisker growth in the SAC solder alloys since only the corrosive environments have produced whiskers. The whiskering was significant in the aspects of microelectronics reliability at the 85 °C/85RH% test. The longest detected whisker was 65 lm and the highest density reached 55 pcs./ 0.04 mm2 after 3000 h. The mSAC1 and SAC305 alloys have shown similar and better whisker resistance ability than the mSAC2 alloy. The root cause of the different behavior probably comes from the different corrosion resistance of the alloys. In the upper 10 lm of the mSAC2 solder joints the corroded area ratio is usually over 80%. This ratio is much lower for mSAC1 and SAC305, between 10% and 50%. Copper precipitations have been found at the whisker roots. During the oxidation of copper, the volume can expand up to +72% which cause further mechanical stress in the layer. The larger the copper precipitation at the whisker root, the more stress will develop in the layer. Copper and bismuth traces have also been found within the whisker which can fragment the whisker grain. This may result in twisted nodule type polycrystalline whiskers instead of
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filament whiskers. This effect can also reduce the maximum length of the developed whiskers. Therefore alloying copper into pure tin coatings may have a positive effect on the reliability of the electronics appliances.
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