Department of Electrical Engineering, Mahabad Branch, Islamic Azad University, Mahabad, Iran
Received: 13/07/2014; Revised: 12/08/2014; Accepted: 16/08/2014
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The structure, electrical resistivity, oxidation and corrosion behavior of the Sn-Pb eutectic alloy has been determined in this paper. The results can be summarized in the following notes: The alloy exhibited two phase mixtures, cubic lead as a solid solution and tetragonal tin. The lattice parameter, a of Pb was calculated and found to be 4.935 Å, which is a smaller value compared with that of pure Pb. The alloy exhibited Ohmic behavior and has electrical resistivity 24.6 μΩ.cm at room temperature. The resistivity increased linearly with temperature and the temperature coefficient of resistivity was calculated and found to be 11x 10-3 C-1. Oxidation behavior of the Sn-Pb eutectic alloy was described through the weight gain with time during the TGA test. Rate of the weight gain was found to be decreased with increasing aging time after the first 20 minutes due to the formation of oxide layer which may act as a protective layer and prevent further oxidation. The alloy exhibited good corrosion resistance for long run.
Sn-Pb eutectic alloy, oxidation and corrosion behavior, electrical resistivity.
Soldering process is one of the most important methods used for joining electronic components in electronic devices. The advances in transistors, diodes and specially integrated circuits have been revolutionized in electronic manufacturing throughout the world. These components are of very little values as individual components and to be use, they must be electrically connected to each other and mechanically to the printed circuit board by the so-called solder alloys. It based on the tin-lead alloys because of their unique combination of material properties and low cost [1-5]. The popularity of these alloys is due to their relatively low melting point, aggressive bonding characteristics, good wicking tendencies and good electrical continuity. The present paper aims to study the structure, electrical properties, corrosion and oxidation behaviors of the eutectic tin-lead solder alloy, which is still in use until today although, there are many regulations and legislations tend to prevent the use of lead due to adverse health effects.
The provided alloy has long wires form of average diameter 1 mm. X-ray diffraction technique with Cu-Kα radiation was used to study the structure of this alloy. Electrical resistivity was also calculated from the I-V curve using a simple circuit of Ohm’s law. Furthermore, the circuit was connected to a heater plate to determine the temperature coefficient of resistivity. Thermal gravimetric analysis (TGA) in the presence of Oxygen atmosphere at 120 ºC for 2.5 h. Corrosion behavior of the alloy sample in a chemical solution of 2 Mol HCl versus exposure time in the range of 0 to 50 minutes at steps of 10 min were done using the weight lost method .
Fig.(3.1) shows the XRD pattern of the Sn-Pb eutectic alloy. It shows that the pattern contains peaks due to solid solution of Sn in face center cubic Pb and a solid solution of little Pb in the body center tetragonal Sn. The binary equilibrium phase diagram of Sn-Pb  indicates that, the solubility limit of Sn in Pb is 19.1wt.% and the solubility limit of Pb in Sn is 2.5wt.%. Details of these phases are presented in table (3.1). It was found that, the lattice parameter, a of Pb solid solution is equal to 4.935 Å, which is a lower value than that of pure Pb (4.949 Å). This reduction could be attributed to the solubility of Sn of lower atomic radius than that of Pb matrix phase in Pb solid phase, which may cause a contraction of the unit cell of Pb phase.
Fig.(3.2) shows the variation of weight against aging time of the Sn-Pb eutectic alloy kept at a fixed temperature of 120 ºC. It shows a rapid increase of the weight in the first 20 minutes, followed by a nearly constant weight with time. This increase can be attributed to the formation of some oxide layers on the surface of the alloy sample. From this figure, the rate of the weight gain of the Sn-Pb eutectic alloy was calculated and the result is illustrated in Fig.(3.3). It demonstrated that, the rate of the weight gain increases rapidly with aging time in the first 20 minutes, followed by a steeper range of decreasing the rate. This decrease could be attributed to the precipitation of some oxides layer that may act as protective layers and prevent further oxidation.
Fig.(3.4) shows the current-voltage relation of the Sn-Pb eutectic alloy. It shows a linear relation indicating that, the alloy is Ohmic material. The electrical resistivity of the alloy sample was first calculated many times and the average value was taken into account. It was found that, the electrical resistivity is equal to 24.6 ± 1 μΩ.cm, which is a high value when compared with a wide range of lead-free solders [8-11]. After that, the circuit was connected to a heater plate and recorded the value of the voltage versus temperature in the range of the room temperature to 91 ºC then calculating the electrical resistance with temperatures as illustrated in Fig.(3.5). In addition, the electrical resistivity was also calculated in this range of temperatures and the result is illustrated in Fig.(3.6). It shows a linear increase of resistivity with temperature. This increase could be attributed to the disturbances in the lattice order due to the thermal vibration which may cause scattering of conduction electrons. From the slope of the straight line, the temperature coefficient of resistivity (α) was calculated and found to be 0.011 C-1.
Fig.(3.7) shows the variation of the weight loss (mg/cm2) of the Sn-Pb eutectic alloy sample in a solution of 2 Mol HCl versus exposure time in the range of 0 to 50 minutes. It shows that, the weight loss increases rapidly through the first 10 minutes followed by a nearly constant loss with increasing the exposure time. From this figure, corrosion rate was calculated and the results are illustrated in Fig.(3.8). It shows a rapid increase of the rate in the first 10 minutes followed by continuous decrease. The stability of this alloy after the first 10 minutes may be due to the formation of a protective layer of mixed oxides of Pb and Sn on the surface of the alloy which may prevent the continuous attack of corrosive ions. This reveals the stability of the alloy against the medium for long run. By surface’s examination, it was noticed that it hasn’t any type of localized corrosion which means only a general corrosion occurred.
The structure, electrical resistivity, oxidation and corrosion behaviors of the eutectic Sn-Pb alloy have been studied and analyzed using X-ray, Ohm’s law circuit connected to a heater, thermal gravimetric analysis and weight loss method, respectively. The results showed that, alloy exhibited two phase α-Pb and β-Sn. The room temperature electrical resistivity of the alloy was calculated to be 24.6 ± 1 μΩ.cm. Also, the resistivity varies linearly with temperature in the range of the room temperature to 91 ºC. The temperature coefficient of resistivity was also calculated and found to be 11 x 10-3 C-1. The alloy exhibited good oxidation and corrosion resistance for long run.