Influence of Titanium Oxide on Structure, Corrosion and Soldering Properties of Sn₈₂Bi₁₅Zn₃ Alloy

Soldering is a low temperature metallurgical joining process. In electronics applications low temperature and reversibility are especially important because of the materials involved and the necessity for reworking and making engineering changes. Solder joining is a wetting process followed by a chemical reaction. Wettability is a function of the materials to be joined, with Cu, Ni, Au, and Pd, as well as alloys rich in one or more of these metals, being particularly amenable to soldering. The chemical reaction following wetting is between the molten solder and the joining metallurgy to form an intermetallic phase region at the interface. Microstructure, wettability and physical properties of Sn_96-xZn₄Bi_x alloys are reported by El-Bediwi et al. [1]. The melting temperature of Sn₉₆Zn₄ alloy decreased after adding bismuth. But contact angle of Sn₉₆Zn₄ alloy varied after adding bismuth. Adding silver caused a significant increase in bismuth-tin-zinc alloy strengthens with a little decreased in melting temperature [2]. El-Bediwi et al. [2] reported that, there is a significant decrease in melting temperature of bismuth-tin-zinc alloy with a very little increase in strengthens after adding indium. Microstructure, thermal parameters, wettability and electrochemical corrosion process of Bi₃₀Sn₅₀Sb₁₀A₁₅Zn₃Cu₂, Bi₂₅Sn₆₁Sb₅Zn₄Al₃Ag₂, and Bi₂₀Sn₆₀Sb₇A₁₅Zn₃Cd₃Cu₂, alloys have been studied [3]. Microstructure, wettability behavior, corrosion parameters, thermal properties of quaternary bismuth- tin based alloy have been investigated using different experimental techniques and the results show that, some properties of Bi60Sn40 alloy improved after adding Sb-Zn or Sb-Ag elements [4]. Tin- zinc eutectic alloy has been considered as a candidate for lead free solder materials because of its low melting point, excellent mechanical properties and low cost [5-7]. Mostly solders are based on Sn-containing binary and ternary alloys. Several elements have been selected as alloying elements such as Zn, Bi, Cu, Ag, Sb and so on [8-10]. Specific researchers [11-15] reported that, tin- zinc eutectic solder alloy is poor wettability, reliability, strength, easy oxidation and microvoid formation. To avoid these disadvantages or improve the properties of it, they added minor amount of Bi, Cu, In, Ag, Al, Ga, Sb, Cr, Ni, Ge elements to develop ternary and even quaternary Pb free alloys. The aim of our work was to study the effect of titanium dioxide on microstructure, soldering properties and corrosion of tin bismuthzinc alloy.

High purity (tin, bismuth and zinc) metal and titanium dioxide (white color) are used to prepare Sn₈₂Bi₁₅Zn₃(Ti₂O)_x (x=0,0.3,0.6,0.9 and 1.2) alloys. These alloys (mixed metals and Ti2O by weight percentage) are melted then normal casted on substrate in air. The samples from alloys are prepared in convenient shape for all tests such as microstructure, thermal parameters, wettability and corrosion behavior. Microstructure of used alloys was performed using Shimadzu X-ray Diffractometer, (Dx-30, Japan) Cu–Ka radiation with l=1.54056 Å at 45kV and 35mA and Ni–filter, in the angular range 2q ranging from 0 to 100° in continuous mode with a scan speed 5deg/min and scanning electron microscope (JEOL JSM-6510LV, Japan). The polarization studies were performed using Gamry Potentiostat/Galvanostat with a Gamry framework system based on ESA 300. Gamry applications include software DC105 for corrosion measurements and Echem Analyst version 5.5 software packages for data fitting. The differential scanning calorimetry (DSC) thermographs were obtained by Universal V4. 5A TA instrument with heating rate 10k/min in the temperature range 0-400 ^oC.

X- ray diffraction analysis

Figure 1: x-ray diffraction patterns of Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloys.

Figure 2: FWHM versus 4tanθ for Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloys. alloys.

Table 1: x-ray diffraction patterns of Sn_82-xBi₁₅Zn₃(Ti₂O)xalloys.

Table 2: Lattice microstrain of Sn_82-xBi₁₅Zn₃(Ti₂O)x alloys.

X-ray diffraction patterns, Figure 1, of Sn₈₂Bi₁₅Zn₃(Ti₂O)x (x=0.3,0.6,0.9 and 1.2) alloys have lines corresponding to tetragonal β-Sn phase, hexagonal Bi phase, Zn phase with undetected Ti2O or intermetallic compounds and solid solution from dissolved atoms changed its matrix structure. The details of x-ray analysis such as the peak intensity (crystallinity), peak broadness (crystal size) and peak position (orientation) of Sn₈₂Bi₁₅Zn₃ alloy changed after adding different ratio of titanium dioxide as listed in Table 1. That is because Ti₂O nanoparticles formed undetected intermetallic compounds with dissolved atoms in matrix alloy. Lattice microstrain, ԑ, which determined from the relation between full width half maximum, FWHM, and 4tan using Williamson and Hall equation [15] is presented in Figure 2. Lattice microstrain of Sn₈₂Bi₁₅Zn₃ alloy varied (increased) after adding titanium dioxide as listed in Table 2.

Scanning electron microscope analysis (SEMA)

Figure 3a: SEM of Sn₈₂Bi₁₅Zn₃ alloy.

Figure 3b: SEM of Sn_81.7Bi₁₅Zn₃(Ti₂O)_0.3 alloy.

Figure 3c: SEM of Sn_81.4Bi₁₅Zn₃(Ti₂O)_0.6 alloy.

Scanning electron micrographs (SEM) of Sn₈₂Bi₁₅Zn₃(Ti2O) x (x=0,0.3,0.6,0.9 and 1.2) are shown in Figure 3a-3e. SEM of Sn₈₂Bi₁₅Zn₃ alloy has lamellar structure (tin phase is a gray color, bismuth phase is a black color) contained white color (which is zinc or bismuth-tin or cluster from dissolved atoms) with different shape and size. Sn_81.7Bi₁₅Zn₃(Ti₂O)_0.3 alloy has homogenous structure from tin phase (gray color), slabs and spherical shape from bismuth phase (black color) and little spherical grain or around slab from undetected phases (white\or gray) such as zinc or Ti2O or intermetallic phases. Sn_81.4Bi₁₅Zn₃(Ti₂O)_0.6 alloy has heterogeneous structure formed from tin phase as a gray color contained bismuth phase as black color and little spherical with slab at grain boundary of undetected phases (zinc or Ti₂O or intermetallic phases) as gray/ or white color. Sn_81.1Bi₁₅Zn₃(Ti₂O)0.9 alloy has heterogeneous matrix structure (slabs and spherical with different size\ orientation) as gray color contained lamellar structure bismuth phase as black color and undetected phases (spherical/ around grains) as gray or white color. Sn_80.8Bi₁₅Zn_{3(Ti2O)1.2 alloy has heterogeneous matrix structure of tin phase (gray color) contained spherical bismuth phase (black color) surround by white\or gray color as zinc or Ti2O or intermetallic phases. From SEMA, adding Ti2O to Sn82Bi15Zn3 alloy caused a change in its matrix microstructure (quantity, size and orientation of formed phases).}

Figure 3d: SEM of Sn_81.1Bi₁₅Zn₃(Ti₂O)_0.9 alloy.

Figure 3e: SEM of Sn_80.8Bi₁₅Zn₃(Ti₂O)_1.2 alloy.

Solder properties

Figure 4a: DSC graph of Sn₈₂Bi₁₅Zn₃ alloy.

Solder alloys are characterized by the melting temperature being a strong function of composition. Differenitional scanning calometric graphs of Sn₈₂Bi₁₅Zn₃(Ti₂O)x (x=0,0.3,0.6,0.9 and 1.2) alloys which have more peaks (related more phases) are shown in Figure 4a-4e. Adding titanium dioxide caused a change in DSC shape, intensity and broadness of Sn₈₂Bi₁₅Zn₃ alloy like x-ray diffraction patterns. That is because Ti₂2O changed matrix alloy microstructure such as formed phases and atoms dissolved formed a cluster of atoms or solid solution. Melting temperature of Sn₈₂Bi₁₅Zn₃ alloy varied after adding Ti₂O as listed in Table 3. Sn_81.4Bi₁₅Zn₃(Ti₂O)0.6 alloy has low melting temperature.

Figure 4b: DSC graph of Sn_81.7Bi₁₅Zn₃(Ti₂O)_0.3 alloy.

Figure 4c: DSC graph of Sn_81.4Bi₁₅Zn₃(Ti₂O)_0.6 alloy.

Figure 4d: DSC graph of Sn_81.1Bi₁₅Zn₃(Ti₂O)_0.9 alloy.

Figure 4e: DSC graph of Sn_80.8Bi₁₅Zn₃(Ti₂O)_1.2 alloy.

Table 3: Lattice microstrain of Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloys.

The contact angle Sn₈₂Bi₁₅Zn₃ alloy decreased after adding different ratio from titanium dioxide as presented in Table 4. That is meant, the spreading of molten Sn₈₂Bi₁₅Zn₃ alloy increased in copper surface substrate.

Table 4: Contact angles of Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloys

Corrosion behavior

Figure 5: Electrochemical polarization curves of Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloy.

Electrochemical polarization curves of Sn₈₂Bi₁₅Zn₃(Ti₂O)_x (x=0.3,0.6,0.9 and 1.2) alloys in 0.25M HCl are shown in Figure 5. They show, the corrosion potential of used alloys exhibited a negative potential. Also the cathodic and the anodic polarization curves showed similar corrosion trends. The corrosion potential (E_Corr), corrosion current (I_Corr) and corrosion rate (Corr_Rate) of Sn₈₂Bi₁₅Zn₃(Ti₂O)_x alloys are presented in Table 5. Corrosion rate and corrosion current of Sn₈₂Bi₁₅Zn₃ alloy decreased (varied) after adding Ti2O. That is because adding different ratio of Ti₂O caused formed new phases and cluster of atoms or solid solution which effects on atoms segregation and their reactivity with HCl. We recommended to study the effect of Ti₂O from 0.1 to 0.9 with 0.1 step.

Table 5: Corrosion parameters of Sn_82-xBi₁₅Zn₃(Ti₂O)_x alloys.

1. Adding Ti₂O to Sn₈₂Bi₁₅Zn₃ alloy caused a change in its matrix microstructure (crystallinity, size and orientation of formed phases in matrix).

2. Lattice microstrain of Sn₈₂Bi₁₅Zn₃ alloy varied (increased) after adding titanium dioxide.

3. Melting temperature of Sn₈₂Bi₁₅Zn₃ alloy varied (decreased) after adding Ti₂O.

4. The contact angle Sn₈₂Bi₁₅Zn₃ alloy decreased after adding different ratio from titanium dioxide.

5. Corrosion rate of Sn₈₂Bi₁₅Zn₃ alloy decreased (varied) after adding Ti₂O.

6. Soldering properties such as melting temperature and spreading (contact angle) of Sn₈₂Bi₁₅Zn₃ alloy improved and very closed to lead- tin commercial solder alloy (M.P=183 ^o C and contact angle ^o 19) after adding Ti₂O.

7. Corrosion resistance of Sn₈₂Bi₁₅Zn₃ alloy also is improved by adding Ti₂O.

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