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2 Journal of Alloys and Compounds 550 (2013) Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: The effects of temperature and humidity on the growth of tin whisker and hillock from Sn5Nd alloy Cai-Fu Li, Zhi-Quan Liu, Jian-Ku Shang Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China article info abstract Article history: Received 11 September 2012 Received in revised form 24 September 2012 Accepted 24 September 2012 Available online 29 September 2012 Keywords: Tin whisker Hillock NdSn 3 Oxidation Growth mechanism The effects of exposure time, temperature and humidity on the growth of tin whisker and hillock from Sn5Nd alloy were investigated via scanning electron microscopy. It was found that tin whiskers grew from NdSn 3 compound, while hillocks grew from the tin matrix around the NdSn 3 compound, which was induced by the oxidation of NdSn 3 compound by oxygen and water vapor in the ambient. More tin whiskers and/or hillocks were extruded from the substrate with longer exposure time, higher temperature and higher humidity. This resulted in the formation of various morphologies of tin extrusions at different storage conditions, including thread-like, spiral, flute-like, claw-like, sprout-like, chrysanthemum-like and rod-like whiskers, as well as hillocks. Tin whisker was extruded from the crack of the surface Nd(OH) 3 layer which serves as the mold of tin whisker growth. And the proposed growth models of tin whisker and hillock on Sn Nd alloy can explain the diversity of the whisker morphology. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Tin whisker s investigation has a history of more than 60 years [1 3]. Tin whisker can short the nearby circuits, and induce great disasters [4]. Lead was often alloyed during tin plating to restrain tin whisker growth in the electronic industry [5,6], until restrictions by European Union and Japan were published to ban the use of hazardous lead [7]. So the ever versatile SnPb soldering and coating had to give way to lead free soldering and pure tin coating. In order to improve the properties of the lead free solders, rare earth (RE) elements were added into the solders. And the results do show improved wettability, creep strength as well as tensile strength and reduced interfacial reaction rate as respected [8 12]. But tin whiskers were also found to grow from the RE-doped lead free solders [13 18]. The influence of temperature and humidity on tin whisker growth from tin coating has been investigated since the early years [1,19]. The results showed that low temperature and low relative humidity could reduce whisker growth on tin coating, while higher temperature and humidity accelerate tin whisker growth, which was attributed to the more atom mobility and in-depth oxidation of tin at higher temperature and/or higher humidity according to their opinions [19,20]. Spontaneous tin whisker growth from REdoped lead free solders was resulted from the oxidation of RE Sn Corresponding author. Tel./fax: address: zqliu@imr.ac.cn (Z.Q. Liu). intermetallic compounds [15,21,22]. And thread-type whiskers were found to growth at room temperature, while hillock-type whisker grew at 150 C [15,23]. Besides, it was reported that the trace water vapor in room ambience (humidity) had significant influence on the oxidation of RE-Sn intermetallic compound [14,16,24]. But, the investigation on the effects of temperature and humidity on tin whisker growth is still limited for RE Sn alloys. So a systematic study is deserved to explore the effects of temperature and humidity on tin whisker growth to evaluate the application of RE-doped lead free solders in different applicable conditions. Generally, tin whisker has a thread-like or filament morphology, while Levy and Kammerer [25] found spiral polygon whiskers grown from compressed tin plate. Ribbon-like whiskers [26] as well as flute-like whiskers [2] were also reported in previous studies. Osenbach [27] reviewed the morphology of tin whiskers grown from tin plate, showing flower, helix and other morphologies of tin whiskers. As to RE doped lead free solders, thread-like and hillocklike whiskers were observed [15,23], but other morphologies were less investigated and summarized. Since Nd doped solders has better solderability, higher mechanical property as well as improved interfacial reaction [9], in this work Sn5Nd alloy was selected as substrate to study the growth behavior of whisker and hillock at different temperatures and humidity. Various morphologies of whiskers were observed at different growth conditions. The growth mechanisms of whisker and hillock were proposed to explain the whisker and hillock growing phenomenon observed in RE-doped lead free solders /$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

3 232 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) Experimental procedures Pure tin (99.97%) rolled into sheet and bulk Neodymium (Nd) were weighted basing upon the weight ratio of Sn5Nd (Sn:Nd = 95:5) alloy. Then Nd was enwrapped within the tin sheet immediately to protect Nd from oxidation. The alloy was heated within a graphite crucible in electric arc furnace and cooled with water. The whole melting process was operated in vacuum atmosphere. After the melting, Sn5Nd alloy was cut into small pieces (about mm) with linear cutting machine. The pieces were milled, polished and rinsed with alcohol to get smooth and clean surfaces. After the samples were prepared, they were stored at six different conditions: (a) 15 C, dry condition; (b) 15 C with ice around; (c) room ambience (about C, 25 50% relative humidity); (d) room temperature with water around (about C, 70 80% relative humidity); (e) 60 C, dry condition (in dry cabinet, about 15% relative humidity); and (f) 60 C with water around (about 90% relative humidity). The samples were observed by scanning electron microscope (SEM) after some time storage at the above conditions for different periods. Part of the stored samples were embedded in resin to prepare the cross sectional samples of the alloy. These samples were also milled, polished and rinsed with alcohol to get smooth and clean surfaces. X-ray diffraction (XRD) was done on a D/max 2400 diffractometer using Cu Ka (k = nm) radiation, and SEM observation was carried out with LEO Supra 35 SEM operated at 20 kv. 3. Results The microstructure of Sn5Nd alloy was characterized by XRD and SEM. The lower line in Fig. 1a is the XRD result from the fresh sample surface. It can be seen clearly that the alloy is composed of pure b-sn and small amount of NdSn 3 compound. The SEM image is shown in Fig. 1b. We can see that NdSn 3 compound is imbedded in the pure tin matrix as indicated by the arrows. It is noteworthy that this figure was taken after storage at 15 C for some time, and NdSn 3 compound was slightly oxidized. So we can differentiate NdSn 3 compounds from the surrounding tin matrix clearly. After storage at different temperatures and humidities, the sample endured selective oxidation. The oxidation products were examined with the XRD, and the results were depicted with the upper line in Fig. 1a. The new phase indicated with black solid stars was identified as Nd(OH) 3 rather than Nd 2 O 3, which is same to the selective oxidation of NdSn 3 compound at room ambience [14,16]. It means that the selective oxidation of NdSn 3 compound in the alloy also results in pure tin and Nd(OH) 3. We found that various tin whiskers and hillocks grew from the samples when they were kept at different atmosphere. In this section, we will discuss the growth phenomena concerning the effects of time, temperature and humidity on tin whisker and hillock growth, as well as summarize the morphologies of tin whiskers and hillocks The effect of time on whisker and hillock growth Tin whiskers grew from the samples after storage for some time, and more tin whiskers and hillocks grew with longer time as shown in Fig. 2, which depicts the whisker growth phenomenon of Sn5Nd alloy after storage at room ambient condition for 27, 63 and 227 days, respectively. It can be seen from the figures that the density of tin extrusions on the sample surface increased with the exposure time. The selective oxidation of NdSn 3 in the alloy results in pure tin and Nd(OH) 3, and Fig. 2d f shows the surface microstructure of the oxidation products. Tin is in the form of tin whiskers, hillocks as well as sprouts, while there is a compact layer of Nd(OH) 3 over the original NdSn 3 compound. The selective oxidation will introduce great volume expansion according to the oxidation formula proposed in Ref. [14]. Because NdSn 3 compound is surrounded by the tin matrix and covered by the surface Nd(OH) 3, the volume expansion was restricted by the surrounding tin matrix as well as the compact surface Nd(OH) 3 layer. Therefore, compressive stress forms and provides the driving force for tin whisker and hillock growth. With the prolonging oxidation, there are more released tin atoms and expansion to sustain the driving force, so more whiskers and hillocks grow from the sample as time goes on. Thus we can see that the whole tin matrix surface became uneven after 227 days as depicted in Fig. 2c. Moreover, it is very interesting that many nano-size tin whiskers grew out of the surface Nd(OH) 3 layer after 227 days storage as indicated by the arrowhead in Fig. 2f, and some of them are very straight. The diameter of these nano-size whiskers is between 20 nm and 200 nm, which gives us a clue to fabricate tin nanowires. Since we got the conclusion that the oxidation of NdSn 3 compound is a diffusion-controlled process [14], the oxidation becomes very slow after 200 days. So tin atoms released from oxidation become less, and these accumulated tin atoms are extruded from the very small cracks of surface Nd(OH) 3 layer to form tin nanowires. Therefore, these whiskers are not very long, but usually have small diameters. In all, more whiskers and hillocks grew from the sample to relieve the compressive stress resulted from the continuous oxidation of NdSn 3 compound over time The effect of temperature on whisker growth Fig. 1. (a) The XRD results of Sn5Nd alloy before (the lower) and after (the upper) oxidation (stored at room ambient condition for 25 months) and (b) shows the microstructure of the almost fresh alloy. Fig. 3 shows the surface morphologies of the samples at different temperatures in high relative humidity conditions. Strangely, there were very few thread-like whiskers, while most were needle-like, rod-like, sprout-like and chrysanthemum-like whiskers. It can be seen from Fig. 3a that the sample surface was very smooth with few surface relief, except for the slight oxidation of the imbedded NdSn 3 compound after 27 days storage at 15 C with ice around the sample holder. Fig. 3d shows the details of NdSn 3 compound microstructure. There were only very small tin whiskers with diameters no more than 1 lm and length less than 10 lm, and nano-size tin sprouts grown on NdSn 3 compound.

4 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) Fig. 2. Whisker growth phenomenon for samples stored at room temperature, dry condition for (a) 27 days, (b) 63 days, and (c) 227 days. (d) (f) are the high magnification images of (a) (c) of the corresponding areas indicated by the dashed rectangles, respectively. Fig. 3. Whisker grew from the samples stored for 27 days at humidity conditions: (a) 15 C, (b) room temperature, and (c) 60 C, and (d), (e) and (f) are the high magnification images of (a), (b) and (c), respectively. The growth phenomenon of whiskers and hillocks at room temperature was shown in Fig. 3b and e. We can see specifically that there were more tin whiskers and hillocks after storage at room temperature than at 15 C in high humidity condition. Besides, the surface Nd(OH) 3 layer is broken with tin whisker growth, and whisker grew out of the surface Nd(OH) 3 layer through the crack as indicated by the arrowhead in Fig. 3e. When the temperature elevated to 60 C, the sample experienced great changes. The growth of tin whiskers and hillocks can not release the great compressive stress induced by the severe oxidation effectively in time.

5 234 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) Thus tin matrix would experience creep under the compressive stress. Series of mountains ranges went up along the NdSn 3 compound after 27 days storage (Fig. 3c). Interestingly, no common long whiskers grew with the giant surface relief. There were only chrysanthemum-like whiskers on the sample as can be seen in Fig. 3c and f. Tin whisker growth would experience two thermodynamic activation processes, the selective oxidation of NdSn 3 compound and the fast diffusion of released tin to the whisker root. At 15 C, the selective oxidation of NdSn 3 compound by water vapor and oxygen was very gentle because of the low temperature, which results in slight compressive stress. And the diffusion or flow of the released tin atoms was also very slow. These all lead to less and smaller tin whiskers and tin sprouts on NdSn 3 surface, as well as fewer hillocks on tin matrix as shown in Figs. 3a and d compared with room temperature and 60 C. At 60 C, the selective oxidation of NdSn 3 compound by water vapor and oxygen was severe, which results in large amount of tin atoms as well as great compressive stress in the alloy. And the diffusion or flow of the released tin atoms was fast at 60 C. But tin atoms released from oxidation still cannot go fast enough to release the compressive stress in a very short time. Under such conditions, the compressive stress increased gradually with time. This surface Nd(OH) 3 layer would break down when it could not endure the accumulated stress anymore, and tin atoms erupted, which would lead to a large quantity of chrysanthemum-like tin whiskers and tin sprouts on NdSn 3 surface. Because of the long-term large compressive stress in tin matrix, low melting tin would deform by creep under compressive stress at 60 C. And series of tin mountain ranges went up from tin matrix at this condition in Fig. 3c The effect of humidity on whisker growth Fig. 4 illustrates the whiskers and hillocks which grew under different relative humidities at 60 C. Clearly, there were many thread-like whiskers on the sample keeping in dry cabinet as indicated by the arrowheads in Fig. 4a. Many of the whiskers were very long, more than 100 lm. Besides, tin matrix which was very smooth and flat in the beginning had many hillocks after 27 days. While there were large quantities of chrysanthemums-like whiskers grown from the sample in high relative humidity condition, even without any long thread-like whiskers. Few short rod-like whiskers could be found on this sample as depicted in Fig. 4d. Compared with the surface relief in dry condition, there were larger mountain ranges along the NdSn 3 compound in high relative humidity condition. The oxygen source of NdSn 3 oxidation is the water vapor and oxygen in the air, and the trace amount of water vapor in ambient condition could have much effect on the oxidation of the compound [14,24]. The oxidation was very severe when the sample was in high relative humidity condition and resulted in great compressive stress in very short time. So fresh tin atoms released from selective oxidation erupted from almost everywhere of the Nd(OH) 3 layer under sudden pressure. Therefore, more tin extrusions were observed on the samples in high relative humidity condition as can be seen in Figs. 4b and d. Moreover, because the long-term compressive stress was much higher than in dry condition, tin matrix crept in a much higher rate and the surface relief was great under this humid condition. So the water in the air plays an important role in whisker and hillock growth through the oxidation of NdSn 3 in Sn5Nd alloy Morphologies of whiskers and hillocks under different conditions As shown in Fig. 5, there were many whiskers with different morphologies grown from the samples. Thread-like whiskers are mostly found on the samples stored in ambient conditions at room temperature and 60 C, and these whiskers have diameter between 50 nm and 10 lm and a length usually longer than 5 lm. As indicated by the arrowhead shown in Fig. 5a, the thread-like whisker has a length of 190 lm and the diameter is about 1 lm. Moreover, many spiral whiskers were discovered on the samples in ambient conditions at room temperature and 60 C, and one of them is shown in Fig. 5b. As can be seen clearly, the whisker also had striations along and across the growth direction as indicated by the arrowheads. Besides, there is another special whisker indicated in Fig. 5c. It looks like a flute. The flute-like whisker has a diameter of 500 nm at the tip (part I), then distorted through an abrupt kink and enlarged its diameter to about 1 lm (part II) at the same time. Then the whisker distorted and gradually enlarged its diameter to Fig. 4. Whiskers growth phenomenon from the sample after storage at 60 C for 63 days under different humidity conditions, (a) and (c) dry, (b) and (d) humidity condition.

6 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) Fig. 5. Whiskers with different morphologies. (a) A thread-like whisker and (b) A spiral whisker grew from NdSn 3 compound after storage at 60 C, dry condition for 27 days. (c) A whisker with kinks and flutes grew from NdSn 3 compound after storage at 60 C, dry condition for 62 days. (d) A whisker grew from NdSn 3 compound after storage at room ambience for 62 days. (e) Nanowhiskers grew from NdSn 3 compound after storage at 60 C, dry condition for 227 days. (f) Hillock with striations along and across the growth direction grew from tin matrix after storage at 60 C, dry condition for 227 days. (g) Tin sprouts grew from NdSn 3 compound after storage at 15 C, humidity condition for 227 days. (h) Chrysanthemum-like whisker grew from NdSn 3 compound after storage at room temperature, humidity condition for 62 days. (i) Rod-like whiskers grew from NdSn 3 compound after storage at room temperature, humidity condition for 27 days. 1.5 lm (part III) until another kink. After that the diameter of the whisker changed from 1.5 lm to about 5 lm after three kinks. It can be seen clearly that the whisker had striations and flutes along the enlarged parts. It was found that for this flute-like whisker part III is parallel to part V, and part IV is parallel to part VI. And the whisker is supposed to be single crystal. That is to say, the whisker does not change its direction from part III to part V through double kinks, and so does part IV and part VI. Claw-like whisker was observed as shown in Fig. 5d. The whisker enlarged its diameter from the tip to the base, and had striations along the growth direction. Furthermore, the cross section of the whisker root is similar to the shape of the crack from which the whisker grew, which was also reported in Ref. [24] (see Fig. 9 therein). The whisker was extruded from the crack, and it seems that the crack served as the mold for the extrusion and its shape determines the cross section of the whisker. Nanowhiskers were also observed to grow from the sample stored in ambient conditions at room temperature and 60 C for more than 128 days. Fig. 5e shows many Nanowhiskers grown from the sample stored at 60 C, dry condition for 227 days. The diameter of these nano-size whiskers is between 20 nm and 200 nm. Traditionally, thread-like whiskers, spiral whiskers, flute-like whiskers, claw-like whiskers as well as nanowhiskers grow from the samples stored in ambient conditions at room temperature and 60 C. Besides tin whiskers, there exists other surface relief phenomenon. Many hillocks grew from the tin matrix of the samples stored at room temperature (ambience and high humidity) and 60 C in dry condition. As shown in Fig. 5f, the hillock grew from tin matrix after storage at 60 C in dry condition for 227 days. It has striations along and across the growth direction as indicated by the arrowheads. The hillock was supposed to grow from one tin grain in the matrix, and the striation was induced by the grain boundary. Hillocks can grow much higher when they have enough tin atom supply from the oxidation of the nearby NdSn 3 compound. Tin sprouts were observed to grow from the NdSn 3 compound as shown in Fig. 5g, and they could not grow into long whiskers because of the shortage of supplying tin atoms. These tin sprouts grew from the sample stored at 15 C (dry and humidity conditions).

7 236 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) Chrysanthemum-like whisker was observed in this work as indicated in Fig. 5h, which is composed of a bundle of small tin whiskers. This kind of whisker formed when the sample was stored at 60 C, high relative humidity condition. Moreover, rod-like whiskers (see Fig. 5f) were also observed, which grew from the sample stored in high humidity condition at 15 C and room temperature. Although tin whiskers were observed having the morphological diversities, most of them are supposed to be single crystal except the chrysanthemum-like whisker. And we do not found any effect of the orientation of the NdSn 3 compounds on the whisker formation up to now. It was found that various whiskers grew directly from the original NdSn 3 compound through surface cracks with their roots buried in the surface hydroxide, while hillocks grew from the matrix nearby the NdSn 3 compound Cross-sectional microstructure after oxidation In order to explore the microstructure of the alloys after oxidation, cross sectional samples were prepared after storage for some time. Fig. 6 shows the microstructure of the oxidized alloy after storage in room ambience for 40 months. Compared with Fig. 1b, the original NdSn 3 compound endured oxidation, and resulted in pure tin and Nd(OH) 3 (the black contrast grains) in Fig. 6a. Fig. 6b shows the detailed microstructure of the oxidation products. It can be seen clearly that Nd(OH) 3 and tin were interlaced, and pure tin was in the form of network connecting the released tin with the tin matrix. It is noteworthy that there was a layer of compact Nd(OH) 3 on the top surface. This layer played an important role in tin whisker growth. Besides, there is a volume expansion about 43% calculated according to the reaction formula [14] during the selective oxidation compared with the volume of the original NdSn 3 compound. Because the NdSn 3 compounds were imbedded in pure tin matrix Fig. 6. Cross sectional microstructure of Sn5Nd alloy after storage at room ambience for about 40 months (the dashed lines indicate the original polished surfaces). in this study, and the compact layer of Nd(OH) 3 was right on the top surface of the original NdSn 3 compound according to the SEM investigation in Figs. 2 and 6b, the volume expansion would be restricted by the tin matrix and the surface layer of Nd(OH) 3, which induces compressive stress in the oxidation products and tin matrix. And the compressive stress stimulates the growth of tin hillocks as indicated in Fig Discussion In previous study, we found by experimental observations that NdSn 3 compound endured selective oxidation when keeping at room ambient condition, and the oxidation products were nanocrystalline Nd(OH) 3 and pure tin [14]. So, the volume expansion during the selective oxidation was calculated to be about 43% according to the proposed reaction formula. While in this study, the great volume expansion would be restricted by the tin matrix and the surface layer of Nd(OH) 3, which induces compressive stress in the oxidation products and tin matrix. And we consider that this compressive stress provides driving force for tin whisker and hillock growth. Fig. 7 demonstrates the growth mechanisms of tin whiskers and hillocks. A compact layer of Nd(OH) 3 forms over the oxidized NdSn 3 compound during the initial oxidation. The afterward oxidation leads to great volume expansion inside the surface layer. And the released tin atoms accumulated to form tin clusters. Then the tin clusters grew bigger and the compressive stress increased with exposure time. As the surface Nd(OH) 3 layer and the tin matrix will restrain the volume expansion, the surface layer is subjected to a tensile stress, and the under released tin layer is in compressive stress state. When the surface Nd(OH) 3 layer cannot endure anymore and the inner compressive stress could not relax from other effective methods in time, the weak point of Nd(OH) 3 layer would crack and tin whiskers would be extruded through the crack under the compressive stress as shown in Fig. 7a. On the other hand, if the Nd(OH) 3 layer was strong enough and the tin grain accompanying the oxidized NdSn 3 compound was at an orientation easy to grow, tin atoms would add to the nearby tin matrix crystal to stimulate the growth of hillocks as shown in Fig. 7b. Choi et al. [28] also considered that the special grain in the coating texture may act as the seed of a whisker. Once tin whisker or hillocks were extruded, the compressive stress at the root relaxed to some extent. Therefore, compressive stress gradient formed, and tin atoms would come from surrounding areas to feed the whisker and hillock under the compressive gradient. As shown in the cross-sectional image of the oxidized NdSn 3 compound (Fig. 6), the tin layers are highly interconnected with Nd(OH) 3 between them, which meant that there were large quantities of grain boundaries and interfaces in the oxidation products. It agrees well with Dudek and Chawla s results using 3-dimension reconstruction [22]. So, tin atoms released from selective oxidation would diffuse and/or flow to the root of tin whiskers and/or hillocks through the grain boundaries and interfaces between released tin and Nd(OH) 3 under the compressive stress gradient in great quantity within short time. This mechanism could explain the morphological diversity of tin whiskers. It seemed that the crack served as a mold during the extrusion according to Fig. 7a. The whisker grew out of the crack whose shape determined the cross section of tin whisker in the beginning. Then a layer of stable and productive tin oxide formed around tin whisker body immediately after the whisker grew out of the crack. Tu [29] assumed that this tin oxide layer worked in restricting whisker from radial growth. And we think that the surface Nd(OH) 3 layer also played an important role in preventing the extruded tin from lateral direction growth. So, the

8 C.F. Li et al. / Journal of Alloys and Compounds 550 (2013) (a) Tin oxide Whisker (b) Crack Nd(OH) 3 Tin oxide Hillock Tin NdSn 3 Tin matrix Nd(OH) 3 Tin NdSn 3 Tin matrix Fig. 7. Sketch map of growth mechanisms for (a) tin whisker and (b) hillock grown from Sn5Nd alloy (the hatched area is Nd(OH) 3 ). thread-like whiskers usually have unchanged cross sections for most of the time as can be seen in Fig. 2, Figs. 4a and 5a. If the growth of whisker could not release the compressive stress effectively, the inner compressive stress would accumulate, which would result in higher tensile stress in the surface Nd(OH) 3 layer, the crack would spread under the stress. The cross section of the whisker becomes larger with the mold, which is the case in Figs. 5c and d, where the whisker enlarged the diameter during its growth. It is the same that if the crack grows at one side, tin whisker could grow freely from this side, while the other side of the crack still restricts the growth of tin whisker. In such a case, tin whisker would experience partial cross-sectional growth. The free side of the whisker grows faster than the restricted side if the released tin atoms were sufficient to feed the whisker growth. Then the whisker would turn to another common growth orientation, and the partial growth would cease. As a result, a kink will be formed during whisker growth, which is shown in Fig. 5c. If the whisker experiences a long time partial cross-sectional growth, we could observe spiral whisker as shown in Fig. 5b. According to this mechanism, the cross section of tin whisker is determined by the shape of the crack through which tin whiskers are extruded out from the underneath. So the propagation of the crack through which whisker grow influences the afterward morphologies of the whiskers. It is necessary to point out that when the samples were stored at 15 C, b-tin transformed into a-tin after some time, which can also be found in the literature [30]. At the time samples were taken out after 227 days, we found that part of the polished surface became gray. The samples was examined with XRD, and the results showed strong a-tin peaks besides the peaks from b-tin, NdSn 3, and Nd(OH) 3. It indicates that b-tin in the alloy endured b to a phase transition when kept at 15 C. Therefore, we believe that if without any improvement, the RE (Nd) doped lead free solders can neither be used in room temperature condition because of tin whisker growth, nor be used in low temperature condition due to the b to a phase transition. 5. Conclusions In this work, the growth behavior of tin whiskers and hillocks on Sn5Nd alloy was studied at different temperature ( 15 C, room temperature and 60 C) and humidity (ambient air and high relative humidity) conditions. Whiskers grow from the original NdSn 3 compound through surface Nd(OH) 3 layer cracks, while hillocks grow directly from the matrix around NdSn 3 compound. (1) At 15 C, the oxidation of NdSn 3 compound was very gentle resulting in less compressive stress, which leads to very small tin whiskers and tin sprouts growth, fewer hillock formations. The oxidation of NdSn 3 compound at 60 C results in more tin extrusions from original NdSn 3 compound and hillocks from tin matrix, and the whole tin matrix would experience creep under the compressive stress. (2) Many thread-like whiskers grew from the samples at ambient air conditions when the samples were stored at room temperature and 60 C. While in high humidity conditions, the selective oxidation of NdSn 3 compound by water vapor and oxygen was severe, which resulted in chrysanthemum-like, rod-like whiskers and tin extrusions. So the humidity plays an important role in oxidation during whisker growth. (3) The NdSn 3 in the matrix endured oxidation during the storage at different conditions. The oxidation produces interlaced pure tin and Nd(OH) 3, which would induce volume expansion. The resulted pure tin provides the tin source for tin whisker and hillock growth, and the driving force comes from the restriction of the volume expansion by the tin matrix and the surface Nd(OH) 3. Tin whiskers grows through the crack of the surface Nd(OH) 3 layer. Tin atoms released from selective oxidation would diffuse and/or flow to add to the root of tin whiskers and/or hillocks through the grain boundaries and interfaces between released tin and Nd(OH) 3 under the compressive stress. Different morphologies of whisker were observed in this study, and these growth phenomena of tin whisker are clearly explained with the proposed growth model. 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