Microstructure, Mechanical, and Electrical Properties and CorrosionAnalysis of Lead-Free Solder CSI Joints onCu Substrate Using Novel Concentrated Solar Energy Soldering (CSES) Method

In this study, “Concentrated Solar Energy (CSE)” is applied for soldering and is named as the “Concentrated Solar Energy Soldering (CSES) technique.” Lead-free solders are used for comparing novel CSES technique solder joints with CSI (Conventional Soldering Iron) solder joints. Tensile strength, bending/flexural strength, microhardness, electrical resistivity, fractography, optical microstructures, SEM microstructures, and EDS analysis were used to compare CSES solder joints with CSI solder joints. -e salt spray test (SST) was used for corrosion analysis of CSES and CSI solder joints. Experimental results reveal that the tensile response, bending strength, and microhardness of the CSES solder joints were almost equal to those of the CSI solder joints. It was observed that after corrosion, the tensile strength, bending strength, and microhardness for CSES and CSI solder joints reduced. CSES solder joints were found to be slightly better than CSI solder joints for electrical resistivity. Electrical resistivity for all the cases studied was found to increase slightly after corrosion. Interfacial integrity and effect of corrosion were clearly observed on CSES and CSI solder joint microstructures.-e intermetallic compounds (IMC) formed in case of CSES solder joints were found to be having slightly less thickness than that of CSI solder joints. -e results demonstrate CSES technique as a green technology and a potential substitute to the CSI method.


Introduction
A proper solder material and soldering method ensures good integration of mechanical and electric connections for electronic devices. SnPb solders were used for the past few decades due to their good mechanical and corrosion resistance properties. Advantages of being economical and good wettability added to their extensive use [1][2][3][4][5][6][7]. In July 2006, the European Reduction of Hazardous Substances (RoHS) banned lead [8,9]. Since then, the development of lead-free solder alloy has gained importance. As a result, the Sn-Ag-Cu (SAC) solder is developed as an alternative solder alloy. is solder alloy with different compositions has found wide acceptance in the industry owing to the amalgamation of good solder properties accompanied by reasonably decent mechanical properties [10][11][12].
As a result of their ease of usage, acceptable creep rate, and good solderability, joint strength, and ductility, SAC solders are currently used widely in industries. However, the main disadvantage is the solder cost. Because of the Ag content, the price of the SAC solder is high. Changing Ag price causes fluctuations in the alloy cost. Sn-Cu-based solders are less expensive alternatives to SAC 305 solders, and a number of options with different additives are available. Even SAC solders with less than 1% Ag content are also good alternatives to SAC 305 [2].
Corrosion of the solder joint is a major concern in soldering applications [13]. Many researchers are working on corrosion analysis of solder joints. A decrease in the life of electronic devices was found to be associated with pitting corrosion in solder joints [14].
Due to scarcity of fossil fuel, numerous researchers are concentrating on an alternative energy source [15][16][17]. Concentrated Solar Energy (CSE) using Fresnel lens was found to be viable n surface hardening of steel (see Figure 1) [18,19].
Concentrated Solar Energy (CSE) is a potent substitute for material processing. It is being used in a variety of material processing applications [20]. Most of the literature available related to CSE for material processing are on different solar collectors. For Fresnel lens for material processing, there is very less work available, though they offer consistent flux on the absorber [21]. It was found in the literature that nobody has worked on soldering using CSE. e motivation for this research is to apply CSE (by using Fresnel lens) to lead-free soldering applications and establish this green technology.

Concentrated Solar Energy Soldering (CSES) Experimental
Setup. In this study, concentrating solar energy is applied for soldering and named as the "Concentrated Solar Energy Soldering (CSES) technique" [22]. e CSES setup has Fresnel lens of 470 mm in diameter for concentrating solar energy, supported by the base frame (see Figure 2).
Soldering fixture was movably fixed on a base frame below the Fresnel lens as shown in Figure 3. Focal length was adjusted to get the desired temperature of 370°C at a focal point to heat the solder wire to enable soldering. Soldering fixture was provided the arrangement to fix and adjust the proper gap between the substrates for soldering to get a spot size of 0.6 mm [22]. Two-axis solar tracking was achieved with sensors, motors, and controller unit. Figure 4 shows the CSES experimental setup. e CSES technique discussed in this work is patented with the patent application no: 201721009049 [23] and can be used in remote areas and even space to solder various components, circuits, etc.

Conventional Soldering Iron (CSI) Soldering Experimental
Setup. SARA 71, a proportional integral derivative (PID) controlled soldering station by SARA Mechatronics, was used for "Conventional Soldering Iron (CSI)." It has the temperature range of 150°C to 480°C. Special fixture was made for CSI soldering to hold the Cu substrates properly and ensure the gap between them as 0.8 mm. e temperature of soldering iron can be adjusted in the soldering station.

Solder and Substrate Materials.
In this study, Alpha Assembly Solutions' commercial lead-free solders SAC305, SACX0307, and 99C and one lead -containing solder SnPb   were used for experimentation. All the solders contained 2.2% flux. Copper substrates used in this study had a purity level of 99.9% Cu. Details for solder material contents are given in Table 1.
For this study, the dimensions of the Cu substrate used were length � 50 mm; width � 5 mm; thickness � 0.5 mm. As electronic industries use soldering for microjoining [25], in this study, it was proposed to study a microbutt joint of length � 0.8 mm; width � 5 mm; thickness � 0.5 mm.
A butt joint was successfully obtained with CSES technique by using SAC305 and SACX0307 lead-free solders. e initial results achieved with CSES technique were promising [22].

Sample Preparation for
Testing. For different mechanical tests (tensile test, bending test, and microhardness test) and electrical resistivity test, the Cu substrate of length � 50 mm; width � 5 mm; thickness � 0.5 mm was prepared. SAC305, SACX0307, 99C, and SnPb solders were used to obtain a butt joint between two Cu substrates with the CSES technique and CSI method. A microbutt joint of length � 0.8 mm; width � 5 mm; thickness � 0.5 mm was obtained. In the air, the solder joint was allowed to solidify. Excess solder on Cu plates was removed by grounding and polishing after solidification. Prior to different mechanical testing, 1200 grit SiCp paper [26,27] was used to further ground the CSES and CSI solder joints to take out extra solder at the edges.

Tensile Test.
e tensile tests were conducted at room temperature with a crosshead speed of 2 mm/min. Results were averaged for five CSES and CSI solder joints. e ASTM test standard E8M-09 was used to determine the tensile strength of the CSES and CSI solder joints [26,28,29].

Fractography.
e mode of failure exhibited by CSES and CSI solder joints was determined by fractography investigation. An optical microscope was used to take fractographs on tensile fracture surfaces.

Bending/Flexural
Test. At room temperature, the bending/flexural tests were conducted with a crosshead speed of 2 mm/min. e results were averaged after testing five CSES and CSI solder joints. e ASTM D 790-03 standard was used for finding the bending/flexural properties of the CSES and CSI solder joints.

Microhardness Test.
e microhardness tests on CSES and CSI solder joints were conducted at room temperature.
e Vickers pyramidal diamond indenter with a facing angle of 136°was used for the test. A digital microhardness tester was used to measure Vickers microhardness. For the testing, a 10 gf indenting load and dwell time of 15 seconds were used. e results of five CSES and CSI solder joints were averaged to get microhardness of the CSES and CSI solder    joints. Polished CSES and CSI solder joints were used for measuring microhardness according to the ASTM standard E384-99 [30].

Electrical Resistivity Test.
In this study, electrical resistivity tests were conducted at room temperature and with a probe spacing of 1.5875 mm. A four-point probe method [26,31] was used to check the electrical resistivity of CSES and CSI soldered joints, as shown in Figure 6.
At least five CSES and CSI solder joints were tested, and the results were averaged. e electrical resistivity properties of the CSES and CSI solder joints were determined as per the ASTM standard ASTM B193-16 [26].

Microstructural Characterization.
Microstructural characterization investigation was performed on metallographically polished extruded CSES and CSI solder joints to examine the morphological features of grains and second phases. Etching was done to observe the grain boundaries. e etching solution was prepared by mixing 5 vol.% HCl in ethanol [32]. Initially, an optical microscope was used to take the microstructure at 400×. en, to reveal the microstructure at the interface of the solder and the substrate, it was taken at a higher magnification using SEM. e composition of the elements was determined with the help of EDS attached with SEM machine.

Salt Spray Test. Salt spray test (SST) was used for corrosion analysis of CSES and CSI joints.
e SST is a widely used technique for estimating corrosion resistance of soldered joints [13,[33][34][35]. In this study, in an enclosed chamber at 35°C, CSES and CSI solder joints were exposed to a fog created from a 5 wt.% NaCl solution for 96 hours. e salt spray test was conducted as per ASTM B-117 [13,33,34]. Tensile strength, fractography, bending/flexural strength, microhardness, electrical resistivity, microstructural characterization, and EDS analysis were done for these corroded samples.
e properties before and after corrosion were analysed for CSES and CSI solder joints.

Tensile Strength.
e experimental results of tensile testing are presented in Table 2 and represented graphically in Figure 7. e results show that the tensile response of the CSES solder joints is almost equal to that of the CSI solder joints. e results obtained show that SAC305 CSES (before and after corrosion) solder joints exhibited higher tensile strength than SACX0307 and 99C CSES (before and after corrosion) solder joints. SAC305 CSES (before and after corrosion) solder joints also exhibited marginally higher average tensile strength than SnPb CSES (before and after corrosion) solder joints. ese results are also found to be in a good agreement with earlier research [36]. It was observed that, after corrosion, the tensile strength for CSES and CSI solder joints reduced. is is attributed to interdendritic or pitting corrosion effect on the CSES and CSI solder joints.

Fractography.
e representative fractographs of the solder joints made of different solders before and after corrosion are shown in Figure 8. Fractography images show that the fracture was relatively ductile in nature and was always found at the solder portion of solder joints and not near the substrates, which were joined.  Table 3 shows the experimental results of the bending/flexural test. e results revealed that the bending/flexural strength of the CSES solder joints is comparable with that of CSI solder joints. It was observed that SAC305 CSES (before corrosion) solder joints had more bending/flexural strength than SACX0307, 99C, and SnPb CSES (before and after corrosion) solder joints. Also, SAC305 CSI (before corrosion) solder joints were found to have more bending/flexural strength than SACX0307, 99C, and SnPb CSI (before and after corrosion) solder joints. With corrosion, bending/flexural strength for CSES and CSI solder joints was found to be reducing (see Figure 9).

Microhardness.
e microhardness test results of CSES and CSI solder joints are given in Table 4. e results of CSES solder joints revealed that the microhardness of the CSES and CSI solder joints is almost similar. It was observed that SAC305 CSES (before corrosion) solder joints had higher average microhardness than SACX0307, 99C, and SnPb CSES (before and after corrosion) solder joints. Also, SAC305 CSI (before corrosion) solder joints exhibited higher average microhardness than SACX0307, 99C, and SnPb CSI (before and after corrosion) solder joints. e reduction in microhardness for CSES and CSI solder joints was observed after corrosion (see Figure 10). 3.5. Electrical Resistivity. Table 5 presents the experimental results of electrical resistivity of CSES and CSI solder joints which were compared with the bare copper substrate. e results plotted, as shown in Figure 11, showed that the electrical resistivities of the CSES (before and after corrosion) solder joints were slightly better than those of CSI (before and after corrosion) solder joints. Electrical resistivity of SAC305 CSES (before and after corrosion) solder joints was observed to be better than that ofvSACX0307, 99C, and SnPb CSI (before and after corrosion) solder joints. e corrosion effect on electrical resistivity can be clearly seen from the results obtained. After corrosion, electrical resistivity for all the cases studied was found to be slightly increased and this was consistent with the observation of other researchers [37]. Figures 12 and 13 show optical and SEM micrographs, respectively, for different lead-free and SnPb solder joints achieved with the CSES technique and CSI method (before and after corrosion). Optical and SEM micrographs at solder portion of different lead-free and SnPb solder joints achieved with the CSES technique and CSI method (before and after corrosion) are shown in Figures 14 and 15, respectively. e microstructure of Cu/SAC305/Cu CSES (before corrosion) solder joint shows coarse dendritic grains of the tin-rich solid solution in a eutectic matrix (see Figures 12(a), 13(a), 14(a), and 15(a)). As in the case of CSES solder joints, continuous heat was provided to join two substrates, which melts the solder faster but solidifies slower compared to the CSI solder joint. e coarse microstructure was thus obtained.

Microstructural Characterization.
As shown in Figures 12(b), 13(b), 14(b), and 15(b), the microstructure of the Cu/SAC305/Cu CSI (before corrosion) solder joint shows fine dendritic grains of the tin-rich solid solution in a eutectic matrix with microporosity. Microporosity observed has reduced the tensile and bending strength of the solder joint when compared with that of the Cu/SAC305/Cu CSES (before corrosion) solder joint. As there was intermittent heat provided to join the two substrates, in case of CSI joints, it takes more time for solder to melt and faster solidification is realized compared with the CSES solder joint, leading to a refined microstructure. e microstructure of the Cu/SAC305/Cu CSES (after corrosion) solder joint shows coarse dendritic grains of the   (c)). e pitting corrosion was observed in the solder joint leading to reduced tensile and bending strength of the solder joint when compared with that of the Cu/SAC305/Cu CSES (before corrosion) solder joint (see Tables 2 and 3). Fine dendritic grain microstructure of the tin-rich solid solution in a eutectic matrix with microporosity was observed for the Cu/SAC305/Cu CSI (after corrosion) solder joint (see Figures 12(d), 13(d), 14(d), and 15(d)). Tensile and bending strength of the solder joint compared with the Cu/ SAC305/Cu CSI (before corrosion) solder joint were found to be reduced because of microporosity and pitting corrosion which were observed to have occurred in the solder joint (see Tables 2 and 3). e microstructure of the Cu/SACX0307/Cu CSES (before corrosion) solder joint shows coarse dendritic grains of the tin-rich solid solution in a eutectic matrix (see Figures 12(e), 13(e), 14(e), and 15(e)). Figures 12(f), 13(f), 14(f), and 15(f), the microstructure of the Cu/SACX0307/Cu CSI (before corrosion) solder joint shows fine dendritic grains of the tin-rich solid solution in a eutectic matrix with microporosity. Microporosity observed has reduced the tensile and bending strength of the solder joint when compared with that of the Cu/SACX307/Cu CSES (before corrosion) solder joint (see Tables 2 and 3). e microstructure of the Cu/SACX0307/Cu CSES (after corrosion) solder joint shows coarse dendritic grains of the tin-rich solid solution in a eutectic matrix (see Figures 12(g), 13(g), 14(g) and 15(g)). e pitting corrosion was observed in the solder joint, which reduced the tensile and bending strength of the solder joint compared with that of the Cu/ SACX307/Cu CSES (before corrosion) solder joint (see Tables 2 and 3).

As shown in
Fine dendritic grains of the tin-rich solid solution in a eutectic matrix with microporosity was observed for the Cu/ SACX0307/Cu CSI (after corrosion) solder joint (see Figures 12(h), 13(h), 14(h), and 15(h)). Tensile and bending CSES before corrosion CSES a er corrosion CSI before corrosion CSI a er corrosion strength of the solder joint compared with the Cu/SACX307/ Cu (before corrosion) CSI solder joint were found to reduce because of microporosity and pitting corrosion which was observed to have occurred in the solder joint (see Tables 2  and 3). e microstructure of the Cu/99C/Cu CSES (before corrosion) solder joint shows coarse dendritic grains of the tin-rich solid solution in a eutectic matrix (see Figures 12(i), 13(i), 14(i), and 15(i)). As shown in Figures 12(j), 13(j), 14(j), and 15(j), the microstructure of the Cu/99C/Cu CSI (before corrosion) solder joint shows the fine dendritic grains of the tin-rich solid solution in a eutectic matrix with microporosity. Microporosity observed has reduced the tensile and bending strength of the solder joint compared with that of the Cu/99C/Cu CSES (before corrosion) solder joint (see Tables 2 and 3). e microstructure of the Cu/99C/Cu CSES (after corrosion) solder joint shows coarse dendritic grains of the tin-rich solid solution in a eutectic matrix (see Figures 12(k), 13(k), 14(k), and 15(k)). Severe interdendritic corrosion was observed in the solder joint, which reduced the tensile and bending strength of the solder joint compared with that of the Cu/99C/Cu CSES (before corrosion) solder joint (see Tables 2 and 3).
Fine dendritic grains of the tin-rich solid solution in a eutectic matrix with microporosity was observed for the Cu/ 99C/Cu CSI (after corrosion) solder joint (see Figures 12(l), 13, 14(l), and 15(l)). Tensile and bending strength of the     CSI (before corrosion) CSI (a er corrosion) Figure 11: Plot of electrical resistivity of pure Cu and different lead-free and SnPb solder joints created using CSES and CSI methods (before and after corrosion).
solder joint compared with the Cu/99C/Cu CSI (before corrosion) solder joint were found to be reduced because of microporosity and severe interdendritic corrosion which were observed to have occurred in the solder joint (see Tables 2 and 3). e microstructure of the Cu/SnPb/Cu CSES (before corrosion) solder joint shows globules and slightly dendritic structure in a eutectic matrix (see Figures 12(m), 13, 14(m) and 15(m)).
As shown in Figures 12(n), 13(n), 14(n), and 15(n), the microstructure of the Cu/SnPb/Cu CSI (before corrosion) solder joint shows globules and dendrites (lead-rich solid solution) in a eutectic matrix with microporosity. Microporosity observed led to reduction in the tensile and bending strength of the solder joint when compared with that of the Cu/SnPb/Cu CSES (before corrosion) solder joint (see Tables 2 and 3). e microstructure of the Cu/SnPb/Cu CSES solder joint (after corrosion) shows globules and slightly dendritic of the lead-rich solid solution in a eutectic matrix (see Figures 12(o), 13(o), 14(o), and 15(o)). Interdendritic corrosion was observed in the solder joint, which reduced the tensile and bending strength of the solder joint compared with that of the Cu/SnPb/Cu CSES (before corrosion) solder joint (see Tables 2 and 3).
Globules and dendrites of the lead-rich solid solution in a eutectic matrix with microporosity were observed for the Cu/ SnPb/Cu CSI (after corrosion) solder joint (see Figures 12(p), 13(p), 14(p), and 15(p)). Tensile and bending strength of the solder joint compared with that of the Cu/SnPb/Cu CSI SnPb CSES before corrosion CSES a er corrosion CSI before corrosion CSI a er corrosion Figure 12: Representative optical micrographs showing microstructure evolution at solder joints of different lead-free and SnPb solder joints created using CSES and CSI methods (before and after corrosion).  Figure 13: Continued.

CSES
(before corrosion) solder joint were found to be reduced because of microporosity and interdendritic corrosion which were observed to have occurred in the solder joint (see Tables 2 and 3).
Overall results of microstructural characterization reveal that in the case of CSES joint, continuous heat was provided to join two substrates, which melts solder material faster but solidification requires more time compared to the CSI solder joint, resulting in the coarse microstructure. e coarsening was due to higher heat input. In CSES solders joints, heat input and flow were considered to be better when compared with that of CSI solder joints in view of fusion quality and improved mechanical properties.
As there was intermittent heat provided to join two substrates, in the case of CSI joints, which takes more time to melt and achieve faster solidification of the solder when compared to CSES solder joint, resulted in comparatively refined microstructure. e observations indicate that in the case of CSI joints, the finer microstructure was evolved due to less heat input and faster cooling rate in comparison to the coarser grained CSES joints.
Because of more homogenized heat input, the alloying element distribution and microstructure seem to be better in case of CSES joints. is is seen as the main reason why less corrosion is observed in case of CSES solder joints than CSI solder joints. Since corrosion resistance is related to solder chemical composition, its effect was observed in both CSES and CSI solder joints. But, CSES solder joints fared marginally better in corrosion resistance than CSI solder joints. However, both are susceptible to corrosion under similar conditions. As corrosion is the property of composition, improvement in chemical composition of solder material can improve corrosion resistance of solder joints.
Further, the results of microstructural examination using SEM on extruded CSES and CSI solder joints revealed no cracks or debonding at the interface which means the interfacial integrity was good (see Figures 13 and 15 bonding between the substrate and solder mostly depends on the integrity of the intermetallic compound (IMC) layer. e long-term reliability of solder joints depends on how, at the interface, IMCs are formed as reaction products between the solder and the substrate during soldering [38][39][40][41][42]. In this study, IMCs were clearly observed in Cu/ SAC305/Cu solder joints (see 13(k), 13(m), and 13(o)) had slightly less thickness than CSI (before and after corrosion) solder joints (see Figures 13(b), 13(d), 13(f ), 13(h), 13(j), 13(l), 13(n), and 13(p), which reduced the brittleness in CSES joints thus making CSES solder joints marginally better in strength than CSI solder joints. is can be attributed to the melting and solidification of solder characteristics and better fusion quality in case of CSES solder joints when compared to CSI solder joints. EDS analysis results showed different elements in a solder joint as well as at IMCs for Cu/SAC305/ Cu, Cu/SACX0307/Cu, and Cu/99C/Cu and Cu/SnPb/Cu (before corrosion and after corrosion) solder joints. Elements' wt.% and at.% of intermetallic compounds (IMCs) and at the solder portion of the solder joint for different solder joints are shown in Figures 13 and 15 Figure 15: Continued.

Conclusions
e following conclusion can be drawn from the present study: (1) e results showed that the tensile response of CSES solder joints is almost equal to that of CSI solder joints. (2) Bending/flexural strength of CSES solder joints was found to be comparable with CSI solder joints. (3) Microhardness measurement of CSES and CSI solder joints was practically similar. (4) Electrical resistivities of CSES solder joints were slightly better than that exhibited by CSI solder joints. (5) For CSES and CSI solder joints, interfacial integrity was found to be good, as there were no cracks or debonding observed at the interface.
(6) e intermetallic compounds (IMCs) formed in case of CSES solder joints exhibited slightly less thickness than CSI solder joints, which reduced the brittleness in CSES solder joints. (7) It was observed that after corrosion, the tensile strength, bending strength, and microhardness for CSES and CSI solder joints reduced. With corrosion, electrical resistivity for all the cases studied was found to increase slightly. e effect of corrosion on CSES and CSI solder joints was clearly seen in the microstructures.