Investigation of the oxidation process at the coppersolder interface with atomic force microscopy Attila Bonyár, Tamás Hurtony Department of Electronics Technology Budapest University of Technology and Economics Budapest, Hungary bonyar@ett.bme.hu Szabolcs Dávid Preclinical Imaging and Biomarker Center Gedeon Richter Plc. Budapest, Hungary davidsz@richter.hu Abstract In this work the investigation of copper-solder interface with Atomic Force Microscope (AFM) is presented. Multiple contact and tapping-mode AFM images were made on presoldered cross-sectional SAC (Sn-Ag-Cu) solder joint samples. The formation of oxide layer was observed and the structural properties of oxide were monitored in time. Our aim was to determine the maximum idle time between the sample preparation and the further analysis, based on only the monitored corrosion profile of the copper-solder interface. Keywords intermetallic compounds, Atomic Force Microscope (AFM), corrosion I. INTRODUCTION The mechanical properties of the lead free solder alloys are mainly depending on the microstructure and the shape of the formed joint, so during metallographic observations the above mentioned properties are aimed to be highlighted. Nowadays the most common method for the microstructure investigation of solder joints is the cross-sectioning combined with different microscopy techniques [1]. The quality of the cross-section sample preparation defines the possible information that can be obtained by the observation. The resistance of materials used in lead-free solders against corrosion in aqueous solutions has already been investigated [2], and it has been exposed that the corrosion of tin actually determines the behavior of corrosion of the solder itself. Hence several publications mention tin to be corrosion resistant. However, even if the sample was prepared with the gentlest care and precision under laboratory environment a thin film of native oxide usually forms on the surface of the tin sample. In spite of the fact that the polycrystalline tin oxide is an n-type semiconductor it is still transparent in the visible light region (widely used as transparent electrodes) [3], thus the presence of the native oxide is generally cannot be observed by optical microscope. However, former studies have shown that under natural conditions tarnishing of tin can sometimes be observed in indoor atmospheric conditions [4 6]. Sometimes tin may also suffer from corrosion by mineral acids and organic acids in the presence of air [5]. For revealing the fine microstructure of intermetallic compounds in lead free solder alloys, we have recently developed a sample preparation method which is based on selective electrochemical etching [7, 8]. During this special preparation technique we use the polished surface of the sample as a working electrode, so it has to be electronically conductive. Since the electrical conductivity of the tin oxide is far behind the conductivity of the tin, this oxide layer could insulate the samples from the electrolyte and it could mask the surface during the etching process as can be seen in the images made with scanning electron microscope (SEM) in Fig. 1. In this study the formation of oxide layer at the copper-solder interface was investigated with atomic force microscope (AFM) and the structural properties of oxide were monitored in time. The aims of our investigation were: 1) to determine whether the formation of the fine tin oxide layer which is invisible to the optical microscope could be examined with AFM or not and 2) to determine whether it is possible to monitor the overall corrosion state of the specimen (including the tin surface) based on only the oxidation state of the copper surface, which could be much more easily monitored.
Fig. 1. Scanning electron microscope images of intermetallic solder joint microstructures. Left: after the complete and selective removal of the Sn phase; right: the oxide layer masks the selective etching. II. MATERIALS AND METHODS For the soldering experiments SENJU Sn96.5Ag3.0Cu0.5 solder paste was printed onto a single Cu sheet and was soldered with hot air soldering. For the metallographic inspections the samples were embedded in an acryl based (Technovit 4006) resin. First they were ground by SiC grinding papers (grit size: 80, 320, 500, 1200) and polished by 9 m, 3 m and 1 m and finally OP-S (0.04 m) diamond and SiC suspensions. The optical microscopic images were taken by an Olympus BX51 upright microscope. Atomic force microscope measurements were done with a Veeco (lately Bruker) diinnova type microscope in full contact- and tapping-mode with 1024x1024 resolution. The PID values were optimized according to the user manual. Budget Sensors SiNi soft contact tips were used for contact-mode imaging, while Bruker RTESPA-CP chips were used for tapping mode. Scanning electron microscope images were made with a FEI Inspect S50 microscope with a Bruker Quantax EDX (energy dispersive X-ray analysis) system. Selective electrochemical etching of the solder joints were done in 1 % H 2SO 4 with an amperometric method as described in our previous publications [7, 8]. For the controlled oxidation of the samples they were put into an ESPEC SH-241 HAST (highly accelerated stress test) chamber with the following conditions: relative humidity (RH) 90%, temperature 25 o C. Two time intervals (90 min and 180 min) were examined. AFM measurements were done at three different areas of the samples copper surface, copper-solder interface, solder surface as can be seen in Fig 2. Fig. 2. Optical microscope image of the specimen with the three measurement areas highlighted. A. Area 1 Copper surface III. RESULTS AND DISCUSSIONS In this chapter we investigate the effect of oxidation on the copper surface. Fig. 3 left shows a contact-mode AFM image from the Cu part of the sample (area 1) directly after the sample preparation. No sign of oxidation can be observed.
Fig. 3. Contact-mode AFM images of the Cu part (area 1) of the specimen. Left: directly after the sample preparation; right after 90 min in the HAST chamber. Scan areas: 50x50 m 2. After 90 min in the HAST chamber small dots appear on the surface as can be seen in Fig. 3 right and Fig. 4 in higher magnifications. The size of the oxide grains is in the 50-100 nm range and their distribution is not homogeneous. After 90 min in the HAST chamber the oxidation of the Cu surface started at specific spots, but there is no continuous oxide formed on the surface yet. Fig. 4. Contact-mode AFM images of the Cu part (area 1) of the specimen. Left: after 90 min in the HAST chamber, scan area: 30x30 m 2 ; right the highlighted area in 10x10 m 2. Following the 180 min in the HAST chamber the surface of the copper seems more homogenous at smaller magnifications (e.g. 30x30 m 2 ), as can be seen on Fig. 5. At first sight the surface looks more like a freshly prepared sample (Fig. 3 left) than an oxidized one. However, in higher magnifications we can see that a continuous oxide layer consisting of scallopy grains covers the Cu surface. Based on the 2x2 m 2 image in Fig. 6 right, the estimated size of the oxide grains is in the 10-50 nm range, which is significantly smaller than the particle size observed after 90 min in the HAST chamber. Fig. 5. Contact-mode AFM images of the Cu part (area 1) of the specimen. Left: after 180 min in the HAST chamber, scan area: 30x30 m 2 ; right the highlighted area in 10x10 m 2.
Fig. 6. Contact-mode AFM images of the Cu part (area 1) of the specimen. Left: after 120 min in the HAST chamber, scan area: 5x5 m 2, highlighted area from Fig. 5; right the highlighted area in 2x2 m 2. Conclusively based on these images we can say that the oxidation of the Cu surface can be monitored well with AFM. The oxidation of the surface starts as oxide-spots which connect with each other and form a continuous oxide given enough time. B. Area 2 Copper-solder interface Fig. 7 presents contact-mode AFM images from the copper-solder interface of the sample. The OP-S suspension at the late stages of the sample preparation (polishing) process slightly etches the Sn phase so the fine intermetallic microstructures are revealed in the bulk solder. Various IMCs can be identified on the AFM images: scallopy type Cu 6Sn 5 intermetallic layer (IML), Cu 6Sn 5 hexagonal tube and other particles, Ag 3Sn fibers which form cells with diameters in the 10 m range. Fig. 7. Contact-mode AFM images of the Cu-solder interface part (area 2) of the freshly prepared specimen. Left: scan area: 50x50 m 2 ; right 16x16 m 2. Fig. 8 presents images which illustrate the intermediary phase of the continuous oxide layer formation on the Cu part of the interface. These samples were stored in the laboratory for 30 and 150 min subsequently at approximately 25 o C temperature and 87 % RH. Fig. 8. Contact-mode AFM images of the Cu-solder interface part (area 2) of the specimen. The images were recorded after storing the sample in approx 25 o C in 87 % RH for 30 min (left) and 150 min (right). Scan areas: 30x30 m 2. Higher magnification images from the interface after 180 min in the HAST chamber are presented in Fig. 9. It can clearly be seen that the continuous oxide formation stops at the borderline of the Cu 6Sn 5 intermetallic layer. The surface of the scallopy IML seems to be completely smooth as it was directly after the polishing. Fig. 9. Contact-mode AFM images of the Cu-solder interface part (area 2) of the specimen after 180 min in the HAST chamber. Left: 10x10 m 2 ; right 5x5 m 2 from the highlighted area. C. Area 3 solder surface Fig. 10 present contact mode AFM images from the freshly polished solder surface in two magnifications. In smaller magnifications the various intermetallic compounds (Cu 6Sn 5, Ag 3Sn) can clearly be identified in the bulk solder. However, the fine microstructure of the Sn phase is blurred by the polishing (Fig. 9 right).
Fig. 10. Contact-mode AFM images of the solder part (area 3) of the freshly polished specimen. Left: scan area: 30x30 m 2 ; right 2x2 m 2. Unlike to the copper surface, after 180 min in the HAST chamber no significant change can be observed on the solder surface (Fig 11). Fig. 11. Contact-mode AFM images of the solder part (area 3) of the specimen after 180 min in the HAST chamber. Left: scan area: 30x30 m 2 ; right 2x2 m 2. Selective electrochemical etching were performed on the sample as a control measurement after 180 min in the HAST chamber and we found that the etching was not blocked by the tin oxide which formed during this time interval. This means that it is not possible to monitor the corrosion state of the tin surface based on the oxidation state of the copper surface, because the copper reaches the full oxide covered stage much faster than the current blocking oxide builds on the tin surface. D. Tapping-mode imaging Tapping-mode AFM images are often used to distinguish materials with different hardness properties, because the phase information of the signal is very sensitive regarding these parameters. Fig. 12 illustrates a case when due to inadequate polishing of the sample, contaminations remained on the surface. Based on only the height map (Fig. 12 right) it could be hard to distinguish the contamination from the Ag 3Sn intermetallic particles present in the solder. In this case the phase information (Fig. 12 left) helps to reveal the contamination with high contrast between the different types of materials. Hence our aim was to determine whether we could gain any more information regarding the oxidation process with the help of the tapping-mode imaging. Fig. 12. Tapping-mode AFM images of the solder part (area 3) of the specimen. Left: phase map; right height map. Scan area: 10x10 m 2. Arrows mark the contamination on the surface after an unsuccessful polishing treatment Figures 14 and 15 present tapping-mode images from the Cu-solder interface after 180 min in the HAST chamber.
Fig. 13. Tapping-mode AFM images of the Cu-solder interface part (area 2) of the specimen after 180 min in the HAST chamber. Left: phase map; right height map. Scan area: 10x10 m 2. It can clearly be seen, that the phase map shows very good contrast compared to the contact mode images regarding the oxide covered area. It can also be seen, that unlike to the Cu surface the Cu 6Sn 5 intermetallic layer is not covered with oxide. However, no significant change can be observed regarding the tin surface before and after the 180 min treatment in the HAST chamber (data not shown). In order to be able to monitor the oxidation process on the tin surface, in the future gentler sample preparation approach (to avoid the blurring of the fine tin microstructures) and longer oxidation times should be applied. Fig. 14. Tapping-mode AFM images of the Cu-solder interface part (area 2) of the specimen after 180 min in the HAST chamber. Left: phase map; right height map. Scan area: 3x3 m 2. IV. CONCLUSIONS The oxidation of a copper-solder interface was investigated with AFM. We found that the oxidation process of the copper surface could be monitored well, however no significant changes could be observed on the tin surface after 180 min in a HAST chamber with 25 o C and 90 % RH. This means that it is not possible to monitor the overall oxidation state of the solder based on only the oxidation state of the copper. We also found that the Cu 6Sn 5 intermetallic layer is also free of any visible oxide traces after this treatment. ACKNOWLEDGMENT The work reported in the paper has been developed in the framework of the project Talent care and cultivation in the scientific workshops of BME project. This project is supported by the grant TÁMOP e 4.2.2.B-10/1 2010-0009. REFERENCES [1] Lin F, Bi W, Ju G, Wang W, Wei X. Evolution of Ag3Sn at Sn 3.0Ag 0.3Cu 0.05Cr/Cu joint interfaces during thermal aging. J Alloys Compd 2011; 509:6666 72. [2] Masato Mori, Kazuma Miura, Takeshi Sasaki, Corrosion Science Volume 44, Issue 4, April 2002, pp. 887 898. [3] Jong-Heun Lee and Soon-Ja Park, Temperature Dependence of Electrical Conductivity in Polycrystalline Tin Oxide, Am Ceram. SOC 73. (9), 1990, pp. 2771-74 [4] T. Stambolov, The Corrosion and Conservation of Metallic Antiquities and Works of Art, Central Research Laboratory for Objects of Art and Science, Amsterdam, 1985. [5] S.C. Britton, The Corrosion Resistance of Tin and Tin alloys, Tin Research Institute, 1952. [6] H. Leidheiser, The Corrosion of Copper, Tin and their Alloys,Wiley, NewYork, 1971 [7] Tamás Hurtony, Attila Bonyár, Péter Gordon, Gábor Harsányi, Investigation of intermetallic compounds (IMCs) in electrochemically stripped solder joints with SEM, Microelectronics Reliability 52, 2012, pp. 1138-1142. [8] Attila Bonyár, Tamás Hurtony, Gábor Harsányi, Selective electrochemical etching for the investigation of solder joint microstructures, Proc. of the 35th International Spring Seminar on Electronics Technology. Salzburg, Austria, 2012.05.09-2012.05.13. IEEE, pp. 89-94