Microelectronics Reliability

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1 Microelectronics Reliability 53 (2013) Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: Ex situ observations of fast intermetallic growth on the surface of interfacial region between eutectic SnBi solder and Cu substrate during solid-state aging process P.J. Shang a,b,, L. Zhang a,, Z.Q. Liu a, J. Tan a, J.K. Shang a,c a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China b Department of Physics, The Chinese University of Hong Kong, Shatin, New Territory, Hong Kong c Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA article info abstract Article history: Received 6 May 2012 Received in revised form 20 February 2013 Accepted 21 February 2013 Available online 1 April 2013 We focused on the surface microstructure and morphology changes of Cu/eutectic SnBi/Cu joint during solid-state aging process through ex situ scanning electron microscopy (SEM) observations and energy dispersive X-ray spectroscopy (EDXS) analysis. Different intermetallic compound (IMC) growth behaviors on the surface from the bulk of solder joints were observed and investigated. The results indicated that at the initial stage of solid-state aging IMCs protruded from the interfacial region with two different microstructures, the small particles on the Cu surface and chunk-type IMC at the solder side. With the increment of solid-state aging time, although the total thickness of IMCs increased very slowly, Cu 3 Sn phases grew fast by the consumption of Cu 6 Sn 5 phase. Growth kinetic analyses for IMC on the surface and in the bulk of solder joints revealed that the IMC growth on the surface was faster than that in the bulk at the initial stage. For the long-term aging, although the total IMC thickness was still thicker than that in the bulk, the rate of the IMC growth on the surface was slower than that in the bulk. The growth of IMC on the surface of interfacial region was divided into two distinct stages, which corresponds to different IMC growth behaviors. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Corresponding authors. Address: Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , China (P.J. Shang). addresses: pjshang@imr.ac.cn (P.J. Shang), lzhang@imr.ac.cn (L. Zhang). For a successful transition to Pb-free manufacturing in electronic assembly, one of the first challenges to the industry is the selection of a replacement solder alloy. So far, many different solder alloys have been proposed as potential Pb-free solder candidates [1,2]. Among these Pb-free alloy families, eutectic SnBi alloy (58Bi 42Sn) has attracted much attention because of its excellent physical and mechanical properties [3 5]. Its low melting point (the melting point of eutectic SnBi solder is 412 K) is benefit for the step-by-step soldering processes and soldering temperature-sensitive components and substrates. Moreover, Sn Bi solder offers a higher strength and superior creep resistance compared to the Sn Pb solder [6]. During the soldering process, the melted solder alloys spread and wet on the metal substrate and then react with the metal substrate to form intermetallic compound(s) (IMCs) at the interface. The formation of IMC(s) plays an important role in controlling the mechanical and electrical properties at the solder joint in microelectronic package [7]. Therefore, a large number of studies have been reported about the microstructure, morphology evolution and growth kinetics of interfacial IMC(s) during reflowing and/or subsequent solid-state aging process [8 15]. The traditional method to characterize the interfacial microstructure and IMC(s) growth in the solder joints in the literatures is to fabricate the cross-sectional samples. And then in order to characterize the interfacial reactions, the cross section of solder joints were carefully polished in order to have a clear appearance for SEM observations. However, the mechanical polishing may seriously destroy the surface state of interface. The embellished microstructure of interface might be different from the original condition. Chen and Chen [16] studied the interfacial reactions between eutectic SnZn solder and bulk or thin-film Cu substrates. In their studies, prominent solder deformation in the bulk type Cu substrate and the extrusion out of interface of chunk-type IMC grains in the thin film type Cu substrate during solid state annealing were observed in the in situ SEM observations, these detailed information might be omitted if the sample was embellished after polishing. In the eutectic SnZn/Cu joint, both of Sn and Zn react with Cu to form IMC. Therefore, the interfacial microstructures of eutectic SnZn/ Cu joint are complicated. As we know, Cu Sn reaction is the most important one in all of the binary metallic systems due to extensive application of Cu and Sn in electronic interconnections [17]. In order to completely understand interfacial reaction and microstructure evolution between Cu and Sn, a Cu/eutectic SnBi solder/ Cu sandwich structure was fabricated and conducted ex situ SEM observation in vacuum ambience. Although the morphology of /$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.

2 900 P.J. Shang et al. / Microelectronics Reliability 53 (2013) eutectic SnBi ball on Cu during reflowing has been investigated by Kim et al. in their previous work [18], due to the lack of liquid solder and rosin flux during solid-state aging process, the driving force of IMC growth might be converted. Therefore, the key point of this work focused on the evolution of interfacial microstructure during solid-state aging process. The different evolution behaviors of interfacial microstructure and fast IMC growth on the surface of interfacial region were investigated and discussed. 2. Experimental procedure As mentioned above, in order to evaluate the IMC growth at the interfacial region during solid-state aging, the Cu/solder/Cu sandwich sample was prepared. The metal substrate used in this experiment was oxygen-free high purity copper (OFHC) plates with a size of 10 mm 7mm 2 mm. The surfaces of copper plates were ground and carefully polished by 0.5 lm diamond paste, and then rinsed in-order in acetone, alcohol and distilled water in an ultrasonic bath. Two copper plates coated with SnBi solder paste were placed together face to face and heated on a heating plate at 443 K for about 20 s. The reflowed sample was cut cross-sectionally and ground with SiC papers following by carefully polishing with 0.5 lm diamond paste. The interfacial microstructure of the reflowed sample was characterized by SEM before it was sealed in vacuum quartz tube with argon shield to prevent the polished cross-sectional surface from oxidation further. During solid-state aging, the quartz tube was immerged into silicone oil bath at 393 K and aged for different times. In order to compare the different IMC growth behaviors of surface with that in the bulk of solder joints, the other group of sample was prepared by following the same process as the above one. However, the sample was polished before SEM observation, which represents the microstructures in the bulk of solder joint. Focused ion beam (FIB) dual-beam system (FEI Nova 2000 Nanolab system) equipped with energy-dispersive X-ray Spectroscopy (EDXS) was employed to characterize the interfacial microstructures and identify the IMCs formed between the solder and Cu substrate during aging process. In order to evaluate the characteristics of interfacial region, an identical view-field with bone like eutectic pattern was chosen as an observed region (see Fig. 1). The thickness measurement of IMCs was done by the following steps: (i) the commercial image analysis software Adobe Photoshop (2003 Adobe Systems Incorporated) was used to enhance the image contrast that received from the SEM images of pixels, and stood out the IMC layers; (ii) create a binary image from the enhanced image and save it to BMP format; (iii) measure the data bar and the IMC layers in the one image and obtain their pixels number respectively; Here, the third step was done by the software written by ourselves. In this approach, we can quantify the mean thickness of the intermetallic layers, even if the layer is not uniform. 3. Results and discussion The backscattered electron image shown in Fig. 1a illustrated the typical microstructure of eutectic SnBi/Cu solder joint after reflowing for a short time. The microstructure of eutectic SnBi is composed of two phases. The white contrast in Fig. 1 represents Fig. 1. SEM images of the interfacial microstructures of eutectic SnBi/Cu in different process conditions: (a) as-reflowed sample. (b) Aging for 1 day at 393 K, the inset image is the higher magnification one of the interfacial IMC. (c) Aging for 2 days. (d) Aging for 3 days.

3 P.J. Shang et al. / Microelectronics Reliability 53 (2013) the Bi-rich phase and the gray one is Sn-rich phase. Only a thin and wavy-like IMC layer was observed besides the parental materials at the solder/cu joint. The EDXS analysis reveals that the IMC formed at the interface contains at.% Cu and at.% Sn, which corresponds to Cu 6 Sn 5 without any Bi dissolved. However, based on the Cu Sn binary phase diagram and our previous works, although the Cu 6 Sn 5 phase was the main products at the interface after reflowing for a short time, Cu 3 Sn can nucleate and grow to about 100 nm between Cu and Cu 6 Sn 5 by transmission electron microscopy observations [19]. Here, the Cu 3 Sn phase is too thin to be detected by the limited resolution of FIB in the present study. Fig. 1b shows the cross sectional image of eutectic SnBi/Cu solder joints after solid-state aging for 1 day at the temperature of 393 K in vacuum. Compared to the interfacial microstructures prior to solid-state aging, there were obvious changes occurred at the interface. It could be seen that a very thick IMC layer, which was approximately 6.01 lm, protruded from the interfacial region between eutectic SnBi and Cu substrate after aging for 1 day at 393 K (Fig. 1b), it was noted that the interfacial morphology is quite different from the sample embellished after polishing. The details of interfacial IMC microstructure were shown in a higher magnification in Fig. 1b. The IMCs protruded from the interfacial region were composed of many small particles beside the Cu substrate, and the structure of which is loose and porous. On the solder side, some chunk-like IMC particles extruded out of the interface between solder and Cu substrate. In the subsequent solid-state aging process, with the increase of the total thickness of IMC, the small particles always formed firstly prior to IMC layers on the Cu substrate, as shown in Fig. 1c and d, and more and more chunk-like IMC particles extruded out. Moreover, if we use the white contrast Bi phase at the solder/imc interface as marker, which was pointed out by arrowheads in Fig. 1a d, it is clear to see that the IMC layer grows mainly toward Cu side during solidstate aging. Fig. 2 shows the cross section secondary electron images of eutectic SnBi/Cu interface after solid-state aged for 1 day at 393 K and corresponding X-ray elemental mapping analysis. The IMC layer in X-ray elemental mappings of Cu, Sn and Bi was sketched out by dashed lines in Fig. 2b d. The distributions of Cu and Sn in the IMC layer are uniform, therefore, although the thickness of IMC increases to about 6 lm, it is reasonable to presume that only one kind of IMC phase exists at the interface during this initial solidstate aging process. The results of the EDXS quantification indicate the IMC layer should be identified as Cu 6 Sn 5. The atomic ratio between Cu and Sn is 55% and 45% and correctly quantifies as Cu 6- Sn 5. The Cu 3 Sn phase was still not able to be detected. Furthermore, almost no Bi was observed in the IMC layer, as shown in Fig. 3d. However, based on the electron probe microanalysis (EPMA) results of Liu and Zhang [20], the solubility of Bi in Cu 6 Sn 5 was about 1.80 ± wt.%. Here, the absence of Bi in Cu 6 Sn 5 layer is limited by the resolution of EDXS. This result is also verified by the following EDXS line-scan analysis across the interface. The EDXS line-scan technique was employed to confirm the general interfacial morphology and the stoichiometry of IMCs. Fig. 3b shows the EDXS line-scan results across the interface of the as-reflowed sample. Only a very thin Cu 6 Sn 5 layer could be detected in the bulk of the interface, and it is consistent with the morphological observations shown in Fig. 1a. After the sample was solid-state aged for 1 day, a flat step can be seen in the EDXS line-scan result in Fig. 3d, which corresponds to the Cu 6 Sn 5 phase in Fig. 1b. Significant variations were found in Fig. 3f after the sample was solid-state aged for 2 days at 393 K in vacuum. According to Fig. 3f, two distinct steps appeared at the compositional line scan result, which means that two kinds of IMCs have formed at the interface. The EDXS quantification analysis confirmed that the two kinds of IMC at the solder and Cu side should be Cu 6 Sn 5 and Cu 3 Sn, respectively. Moreover, it can be seen that the Cu 3 Sn phase grew fast and occupied up to half of the total IMCs layer. Fig. 2. Morphology of eutectic SnBi/Cu joint aged for 1 day: (a) secondary electron image; (b) X-ray mapping of Cu; X-ray mapping of Sn; (d) X-ray mapping of Bi.

4 902 P.J. Shang et al. / Microelectronics Reliability 53 (2013) Fig. 3. SEM image and corresponding line-scan analysis of the interface between eutectic SnBi solder and Cu substrate after reflow (a) and (b); aging for 1 days (c) and (d); aging for 2 days (e) and (f) and aging for 3 days (g) and (h).

5 P.J. Shang et al. / Microelectronics Reliability 53 (2013) Fig. 4. SEM images of interfacial microstructure of the samples were aged for 3 and 10 days (a) no surface polishing and (b) after surface polishing, respectively. When the sample was solid-state aged up to 3 days, a distinct Bi peak appeared in-between Cu 3 Sn phase and Cu, as shown in Fig. 3h. Furthermore, the different IMC growth behaviors on the surface and in the bulk were also investigated and compared. Fig. 4 shows the interfacial microstructures of eutectic SnBi/Cu solder joints after the samples were solid-state aged for 3 and 10 days at 393 K. The interfaces of IMC/Cu and solder/imc were marked out by black and white dashed lines, respectively, as shown in Fig. 4. Obviously, the thickness of IMC layer on the surface region of solder joint aged for 3 days was thicker than that in the bulk of solder joint although the sample was solid-state aged for 10 days. Moreover, it seems that the thickness of IMC in the bulk of solder joint in Fig. 4b only corresponded to the thickness of the chunk-like IMC layer, which was pointed out by white arrows in Fig. 4a. Therefore, almost the entire protrudent IMC layer formed on the Cu surface. In order to compare the IMC growth characteristics on the surface with that in the bulk of solder joints, the total thickness of IMCs layer at the interface was calculated by measuring three SEM images for each solid-state aging time. It was noted that IMC protruded from the interfacial region during solid-state aging process, as shown in Fig. 1, thus the IMC grew not only along the planar direction but also along the direction perpendicular to the cross section. It is difficult to correctly measure the IMC thickness on different directions. However, considering of that compared to lateral diffusion of Sn and Cu (surface or short-circuit diffusion) in the present study, the magnitude of species diffusion along perpendicular to the cross section (bulk diffusion) is smaller, so only the IMC thickness along the planar direction was measured and considered. The growth kinetics of IMC was expressed by plotting the square of measured thickness as a function of the aging time, as shown in Fig. 5. It can be seen that the IMC thickness on the surface was much thicker than that in the bulk of solder joints at the same aging time. Especially at the initial solid-state aging stage, the IMC thickness on the surface region increased up to 6 lm, as shown in Fig. 1b. Obviously, the IMC growth on the surface was divided into two different stages, which was presented in Fig. 5 with blue and red lines, respectively. At the initial solid-state aging stage, since the slope of blue line in Fig. 5 is large, it reflects that the IMC on the surface region grows very fast at this stage. Combined with the above SEM observations, it is easy to determine that the growth of Cu 6 Sn 5 phase dominates the increase of total IMCs layer at this stage. In the following solid-state aging process, the total thickness of IMCs layer increased slowly, however, the thickness of Cu 3 Sn phase increased up to half of the total IMCs layer. Therefore, the main event happened in the IMCs layer was Cu 3 Sn phase grew by the consumption of Cu 6 Sn 5 phase at this stage. According to the above results, the IMC formed at the interface exhibits two types of microstructures, the microstructures of IMC Fig. 5. The square of the thickness of total IMC layer as a function of aging time at 393 K on the surface and in the bulk of solder joints. protruded from the Cu surface was loose and small, and the IMC on the solder side was chunk-type and extruded out of the interface. The protrusion growth of IMC from the Cu surface is thought to be attributed to the accumulation and then reaction between Sn and Cu atoms from eutectic SnBi solder and Cu surface, respectively. Since the surface energy of interfacial region is higher than that on the Cu or solder surface, the Sn atoms from solder side and Cu atoms from Cu surface will diffuse to the interfacial region by the driving force of reducing the surface energy during the initial stage of solid-state aging. The microstructural observations of solder joint close to the interfacial region indicated that the surface of polished sample should be flat, as shown in Fig. 1a, however, it seems like that the Bi-rich phase was embossed out of surface. As we know that the Bi does not react with the Cu. It means that the Sn atoms in the Sn-rich phase of eutectic SnBi solder close to the interface diffuse to the interface and then reacts with the Cu, but the Bi-rich phase in the eutectic SnBi solder was left. The Sn atoms from the Sn-rich phase of eutectic SnBi solder close to the interface react with Cu atoms to form small Cu 6 Sn 5 particles on the Cu surface. As mentioned above that the structure of Cu 6 Sn 5 layer protrudent from the Cu surface is loose and porous, thus the Cu and Sn atoms can diffuse fast through the boundaries of Cu 6 Sn 5 particles. It is clear that the surface diffusion of Sn is faster than that of Cu. Therefore, the IMC layer mainly grew to Cu side, as shown in Fig. 4a. However, the diffusion of Cu to solder side drove the small Cu 6 Sn 5 particles to grow bigger. The specific interfacial IMCs are influenced by the mass supply of the components that participate in the interfacial reaction, the significant effects on

6 904 P.J. Shang et al. / Microelectronics Reliability 53 (2013) Fig. 6. Morphology of eutectic SnBi/Cu joint aged for 3 day after the surface of sample was polished: (a) secondary electron image; (b) X-ray mapping of Cu; X-ray mapping of Sn; (d) X-ray mapping of Bi. the phase evolution has been demonstrated [21 23], i.e., if the Sn supply is limited, the Cu 6 Sn 5 phase tends to transform into the Cu-rich Cu 3 Sn phase. On the contrary, if the Cu supply is limited as in the thin-film type, the Cu 3 Sn phase is converted to the Cu 6 Sn 5 phase upon the complete consumption of Cu. At the initial stage of solid-state aging, the total thickness of IMCs was thin, and the lateral diffusing distance of Sn atoms was short. Moreover, the Sn atoms can diffuse through grain boundary of small Cu 6 Sn 5 particles to the Cu side. Therefore, at the Cu side of the interface, there is enough supply of Sn which can continue to react with Cu to form Cu 6 Sn 5, and drive the IMC thickness to increase. However, with the increase of thickness of IMC layer, the diffusing distance increase for the diffusion of Sn atoms in solder to Cu side, on the other hand, the segregated Bi formed a discontinuous layer at the Cu/Cu 3 Sn interface, as shown in Fig. 6, which will also block the diffusion of Sn and decrease the IMC growth rate. The Cu-rich ambience inside the Cu 6 Sn 5 phase at the Cu side is conductive to the transformation from Cu 6 Sn 5 to Cu 3 Sn, as shown in Fig. 3. The reason why the chunk-type Cu 6 Sn 5 grains at the solder side are extruded out of the interface during solid-state aging process is unclear. The grain extrusion obviously indicates that the presence of a compressive stress at the interface. The possible reasons resulting from the compressive stress are considered to the mismatch of coefficient of thermal expansion (CTE) of eutectic SnBi solder and Cu substrate or morphology change of Cu 6 Sn 5 in the bulk of solder joint from scallop-type to layer-type. However, it was noted that the coefficients of thermal expansion (CTE) of eutectic SnBi and Cu are m/ C and m/ C, respectively, both of which are close to each other. Therefore, it is clear that the compressive stress induced from the mismatch of coefficient of thermal expansion (CTE) of eutectic SnBi solder and Cu substrate might not be the main factor to extrude the chunk-type Cu 6 Sn 5 grains out of the interface. In addition, the formation of Cu 6 Sn 5 at the solder/cu interface is mainly controlled by the following interfacial reaction: 6Cu þ 5Sn! Cu 6 Sn 5 ð1þ By this reaction, if we consider the formation of a layer of Cu 6- Sn 5 at the interface between Sn and Cu, Boettinger et al. calculated that the volumetric change should be In other words, the IMC takes up 5.6% less volume than a rule of mixture combination of Sn and Cu reactants from which the IMC forms [26]. Therefore, the volumetric change from the formation of Cu 6 Sn 5 phase also should not be the reason. Based on the flux-driven ripening (FDR) theory of Suh et al. [24], during wetting reaction between molten solder and copper the scallop-type Cu 6 Sn 5 formed at the interface. It was found that both growth and ripening of the scallop-type Cu 6 Sn 5 take place at the solder/metal interface at the same time. Hence the ripening is a non-conservative one which will result in the total volume of scallop-type Cu 6 Sn 5 to increase with reaction time. During solid-state aging the morphology of Cu 6 Sn 5 changes from scallop type to layer type [25]. Although the velocity of species diffusion during solidstate aging is slower than that in wetting reaction, Cu and Sn are still supplied to drive the transformation of IMC morphology from scallop type to layer type, so the morphology change from scallop type to layer type is also considered to be non-conservative process. It is reasonable to presume that this process will also result in the total volume increase with solid-state aging time. Thus the volume increase of Cu 6 Sn 5 grains will generate a compressive stress to neighboring Cu 6 Sn 5 grains, and result in the extrusion of chunk-type Cu 6 Sn 5 grains out of the interface, as shown in Figs. 2 and 5. However, more evidences are needed to verify it. 4. Conclusions The evolution of interfacial IMC at the surface of eutectic SnBi and Cu solder joint after reflow and solid state aging have been studied by ex situ SEM observations using a natural bone-like eutectic SnBi structure as a marker. Aging for 1 day at 393 K caused the appearance of a very thick IMC layer protrubent from the interfacial region between eutectic SnBi and Cu substrate. The X-ray elemental mapping analysis confirmed the IMC layer to be Cu 6 Sn 5.

7 P.J. Shang et al. / Microelectronics Reliability 53 (2013) At the initial stage of solid-state aging, the protrubent Cu 6 Sn 5 layer exhibits two different microstructures, small particles on the Cu surface and chunk-type IMC at the solder side. The protrusion of IMC growth on the Cu surface is thought to be attributed to the accumulation and then reaction between Sn and Cu atoms from eutectic SnBi solder and Cu surface. And the extrusion of chunktype Cu 6 Sn 5 phase resulted from the compressive stress induced from the ripening growth and the morphology change of Cu 6 Sn 5 phase from scallop-type to layer-type with solid-state aging time. The growth kinetic analysis revealed that the growth of protrudent IMC on the surface has two different stages. In the initial stage of soid-state aging, the IMC layer on the Cu surface increased very fast, and the growth of Cu 6 Sn 5 phase dominated the increase of the total IMC growth. Compared to that in the bulk of solder joint, the IMC on the Cu surface grew faster and thicker than that in the bulk of solder joint at the initial stage of solid-state aging. With the extension of solid-state aging time, the IMC growth rate on the Cu surface was very slow, even it is slower than that in the bulk of solder joint. The main event happened at this stage was Cu 3 Sn phase grew by the consumption of Cu 6 Sn 5 phase. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Foundation of China (Grant No ), Basic Research Program of China (Grant No. 2010CB631006) and the Hundred Talents Program of the Chinese Academy of Sciences. References [1] Vianco PT, Rejent JA, Hlava PF. Solid-state intermetallic compound layer growth between copper and 95.5Sn 3.9Ag 0.6Cu solder. J Electron Mater 2004;33: [2] Kim KS, Huh SH, Suganuma K. Effects of intermetallic compounds on properties of Sn Ag Cu lead-free soldered joints. 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