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Abstract. Soldering alloys Sn-3.5Ag were oxidized at 170 °C with atmospheric air flow in the horizontal tube furnace. The oxide growth after exposing to high temperature was observed with optical microscope and scanning electron microscope (SEM). The occurrence of tin oxide (SnO) formed on reacted sample has been proven by using X-ray diffraction (XRD) and supported by energy dispersive X-ray analysis (EDAX) and X-ray photon electron (XPS). Increasing the exposure time of alloys samples to high temperature increased the weight loss indicating the spallation of oxides scale was occurred. Spallation of metal oxide scale was presumably due to the stress growth and also the differences of thermal expansions between alloy and metal oxide scale during cooling. High temperature corrosion behaviors of soldering alloy Sn-3.5Ag will be discussed based on the experimental results from the kinetic of oxidation, analysis of the surface morphology and the cross-section observations.
Soldering is well-known metallurgical joining method that uses for the interconnection and packaging of virtually all electronic devices and circuits. The Sn-Ag binary eutectic solder is one possible lead free solder replacement for the Sn-Pb. This alloy system is very important because it is generally recognized as the first choice for free-lead solder. Binary eutectic alloy Sn-Ag has been considered in this research due to its low melting temperature, 221 °C and this alloy provides better solderability and better mechanical properties (Tomlineson & Fullylove, 1992). According to El-Bahay et al, this alloy also possesses good ductility and better creep and thermal resistance than Sn-Pb solder (El-Bahay et al. 2004). Research on Sn-Ag alloy system has been conducted in some area such as fracture behavior, microstructure evolution and cooling rate effects on microstructure and mechanical behavior (Ding et al. 2005; Ochoa et al, 2003; Yang et al., 1994).
However, the behavior of this lead-free alloy in high temperature corrosion has not widely reported, although it is important in many applications of electronic components in high technology equipments such as high-powered automobile and nuclear power generator. Therefore, studies of oxidation behaviors of soldering alloy Sn-Ag are essential in order to predict its lifetime and alloy soldering development. In the present work, the eutectic lead-free solder Sn-3.5Ag was prepared. The objective of this paper is to study the corrosion behavior of Solder Sn-3.5Ag alloy on the high temperature oxidation at 170 oC in flowing the atmosphere gas. This research was undertaken in order to obtain a better understanding of the oxidation behavior and microstructure morphology at elevated temperature of soldering Sn-3.5Ag alloy.
The materials investigated was prepared by melting lead-free solder alloy of Sn-3.5Ag (in %wt) at 300 oC and poured into a casting cyclinder carbon steel mould with diameter of 14 mm. Carbon fibre was used to stir the molten alloy to ensure the alloy was mixed evenly and did not conglomeration. After that, samples were allowed to cool slowly down to room temperature for solidification. The sample then removed from the casting of carbon steel mould and cut into a disk shape about 2.5 mm and 14 mm with a diamond cutting machine. The alloys were cut into coupons which were ground to a 1200 grit finish and cleaned ultrasonically in ethanol immediately prior use.
Alloy coupons were reacted at 170 Â°C with atmospheric gas flow at a total pressure of approximately 1 atm. The experiment test was performed in a horizontal tube furnace for a period of time. The oxidized coupons were weighted initially and reweighted after certain reaction time. The weight loss was calculated by measuring the weight difference. Samples characterizations were conducted using X-ray diffraction (XRD), scanning electron micrography (SEM), X-ray photo spectroscopy (XPS) and energy dispersive analysis of X-rays (EDAX) to identify oxide phases, optical metallography, product morphologies, and also microanalysis respectively.
Results and Discussions
The kinetics of oxidation of solder Sn-3.5Ag alloy samples was shown in Figure 1. The weight loss was observed during exposure time. The phenomenon is believed due to spallation of metal oxide on the Sn-3.5Ag alloy. The spallation seems to be increased proportionally with exposure time. On the other hand, the growth rate of the oxides that stick in metal substrate is slow compare to the spallation rate. Spallation of metal oxide scale could be caused by differences in thermal expansion between the alloy and the oxide formed during cooling (Li et al. 2001). Besides that, the scale factor in the mechanical defect will cause failure of the chemical reaction occurs when the exposed alloy surface sites for cracks and / or oxide scale excluded during cooling process (Othman et al. 2010).
After exposure for 20 hours, there was little change in sample weight. This is due to the greatly occurred of grain oxide formation.Â While no weight changes seen on the alloy samples for 45 hours of exposure time up to 70 hours. It shows that the formation of grain oxides and the spallation occur with almost similar rate. However, the weight loss of the solder Sn-3.5Ag alloy starts to dramatically increase at 100 hours. The increase of the weight loss occurs because the greater spallation rate. This also caused the increased in thickness of the grain oxide on the sample that motivated the growth of stress occurs. Oxide scale fragmentation leads to this growth stress.
In this study, surface morphology of each sample was observed using a scanning electron microscope (SEM). The microstructure of the solder Sn-3.5Ag alloy samples are shown in Figures 2 (a) and 2 (b). These images show that formation of metal oxides is abundant and consistent at the exposure time to 100 h compared to 20 h. This because of the mechanism of oxidation occurs is directly proportional to the exposure time. In addition, the images observed that the fragments of metal oxide scale exist in the alloy samples exposed for 20 h and 100 h.
Figures 3 (a) and 3 (b) show spectrum graph contents of elements found on alloy samples surface exposed for 20 h and 100 h exposure, respectively. These images captured by using energy dispersive analysis of X-rays (EDAX) to study microanalysis. It shows that both the alloy surface containing oxygen, silver and stanum. We observed that silver content in the samples of 100 h is more than 20 h sample. This might cause of non-homogeneous distribution elements of silver in the alloy samples. While the oxygen content in the samples 20 h is higher than the sample of 100 h. This is because EDAX is carried out in a small area on the sample and the oxygen content in the samples of 100 h is slightly less in this area than oxygen in 20 h samples.
The XRD analysis has been done to study the oxide layer on the sample. Figure 4 shows the comparative diffraction pattern of three samples. There are seven peaks obtained from unexposed samples, which is a phase of Sn and Ag3Sn. That phase is in agreement with the phase reported by Ding et al, 2005. The samples exposed for 20 h, the peak observed are ß-phase of SnO, Sn and Ag3Sn. The graph shows that the intensity peak in the sample exposed for 100 h was lower compared with other samples. Peak is found to lower the peak indicates the presence of Sn phase. This shows the composition of the Sn phase in the samples tested for 100 h were lower. This condition may occur due to the presence of SnO. Peak intensity describes the amount of X-ray scattering from each layer in the crystal structure and phase depending on the spread of the atoms in the structure. Therefore, the peak intensity shows the structure and composition of phases formed on the sample.
In this study, X-ray photoelectron spectroscopy (XPS) was used to obtain the chemical composition of materials as well as the oxidation state of their constituent elements. The XPS spectrum for the samples Ag3Sn as a function of time are presented in Figure 5. In this figure, the Sn core level principal peaks can be observed in Figure 5 (a). From the XPS spectrum, we noticed that the shift of peak of Sn and SnO occur when exposed to different temperatures. Also, the SnO peaks notice an increase in the presence as a function of the time exposure.
The same samples were analyzed by scanning electron microscope under magnification 5000X to get a clearer picture and a fine of oxide layers formed on the sample. For a cross section of alloy microstructure analysis by SEM has shown that the stipple oxide formed on the alloy exposed for 20 h is less dense (Figure 6 (a)). As for 100 h exposed time, the oxide is formed more compact (Figure 6 (b)).
Lastly, the EDAX analysis were done for a cross-sectional area as shown in Figure 7. From Table 1 and 2, can be observed that both cross-sectional sample of the alloy has a high element of oxygen to the oxide layer formed. Figures 8 (a) and 8 (b) shows a graph of the spectrum contents of the elements contained in a small area of the cross section of alloy samples exposed for 20 h (Figure 8 (a)) and the cross-section sample of 100 h (Figure 8 (b)).
Table 1 and Figure 8a shows the carbon and copper are the elements that exist in the area to be tested on samples exposed for 20 h. The presence of carbon on the sample may be caused by air gas which contained carbon dioxide (CO2). Alloy samples exposed in a horizontal tube furnace has reacted with carbon dioxide may be formed of tin carbide. In addition, there are some elements of the copper oxide layer on the alloy may caused by the diffusion of copper into the oxide later occurs. Dengar
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According to Choi et al. (Choi et al, 2000), such as pinhole defects that exist in the sample will facilitate the permeation of copper and lead to the formation of metal compounds.
The oxidation behaviour of solder alloy Sn-3.5Ag at 170 ° C in atmospheric air flow has been studied. From analysis of the sample weight change, found that the occurrence of a significant sample weight after 70 hours. This phenomenon is caused by fragments of metal oxide scale due to thermal expansion differences between the alloy of the metal oxide and also due to the growth of metal oxide stress. The thick metal oxides during the oxidation reaction causes the growth of oxide stress becomes higher and the oxide layer cracks and breaks will occur. The formation of nodules / stipple oxide increased with the addition of the SnO exposure. The formation of nodules / stipple SnO can be proved by using X-ray diffraction analysis (XRD) and spectroscopic energy X-ray scattering (EDAX).
The work was carried out at Materials Science and Advance Surface Analysis (MAP) Laboratories, UKM. The author would like to express their sincere thanks for the Grant UKM-GGPM-NBT-089-2010 for the financial support and also Centre for Research and Instrumentation Management (CRIM).