Faculty Publications
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Item Effect of Reflow Time on Wetting Behavior, Microstructure Evolution, and Joint Strength of Sn-2.5Ag-0.5Cu Solder on Bare and Nickel-Coated Copper Substrates(Springer New York LLC barbara.b.bertram@gsk.com, 2016) Sona, M.; Prabhu, K.N.The effect of reflow time on wetting behavior of Sn-2.5Ag-0.5Cu lead-free solder on bare and nickel-coated copper substrates has been investigated. The solder alloy was reflowed at 270°C for various reflow times of 10 s, 100 s, 300 s, and 500 s. On bare copper substrate, the intermetallic compound (IMC) thickness increased with increase in reflow time, whereas on Ni-coated Cu substrate, the IMC thickness increased up to 300 s followed by a drop for solder alloy reflowed for 500 s. The spreading behavior of the solder alloy was categorized into capillary, gravity (diffusion), and viscous zones. Gravity zone was obtained from 3.8 ± 0.43 s to 38.97 ± 3.38 s and from 5.99 ± 0.5 s to 77.82 ± 8.84 s for the Sn-2.5Ag-0.5Cu/Cu and Sn-2.5Ag-0.5Cu/Ni/Cu system, respectively. Sn-2.5Ag-0.5Cu solder alloy was also reflowed for the period corresponding to the end of the gravity zone (40 s and 80 s on bare and Ni-coated Cu, respectively). The joint strength was maximum at reflow time of 40 s and 80 s for the Sn-2.5Ag-0.5Cu/Cu and Sn-2.5Ag-0.5Cu/Ni/Cu system, respectively. The dynamic contact angle at the end of the gravity (diffusion) zone (?gz) was found to be a better parameter compared with the stabilized contact angle (?f) to assess the effect of the wettability of the liquid solder on the microstructure and joint strength. The present investigation reveals the significance of the gravity zone in assessment of optimum reflow time for lead-free solder alloys. © 2016, The Minerals, Metals & Materials Society.Item The Effect of Wetting Gravity Regime on Shear Strength of SAC and Sn-Pb Solder Lap Joints(Springer New York LLC barbara.b.bertram@gsk.com, 2017) Sona, M.; Prabhu, K.N.The failure of solder joints due to imposed stresses in an electronic assembly is governed by shear bond strength. In the present study, the effect of wetting gravity regime on single-lap shear strength of Sn-0.3Ag-0.7Cu and Sn-2.5Ag-0.5Cu solder alloys reflowed between bare copper substrates as well as Ni-coated Cu substrates was investigated. Samples were reflowed for 10 s, Tgz (time corresponding to the end of gravity regime) and 100 s individually and tested for single-lap shear strength. The single-lap shear test was also carried out on eutectic Sn-Pb/Cu- and Sn-Pb/Ni-coated Cu specimens to compare the shear strength values obtained with those of lead-free alloys. The eutectic Sn-Pb showed significantly higher ultimate shear strength on bare Cu substrates when compared to Sn-Ag-Cu alloys. However, SAC alloys reflowed on nickel-coated copper substrate exhibited higher shear strength when compared to eutectic Sn-Pb/Ni-coated Cu specimens. All the substrate/solder/substrate lap joint specimens that were reflowed for the time corresponding to the end of gravity regime exhibited maximum ultimate shear strength. © 2017, ASM International.Item Effect of Multiple Reflow Cycles and Al2O3 Nanoparticles Reinforcement on Performance of SAC305 Lead-Free Solder Alloy(Springer New York LLC barbara.b.bertram@gsk.com, 2018) Tikale, S.; Prabhu, K.N.The effect of Al2O3 nanoparticles reinforcement on melting behavior, microstructure evolution at the interface and joint shear strength of 96.5Sn3Ag0.5Cu (SAC305) lead-free solder alloy subjected to multiple reflow cycles was investigated. The reinforced SAC305 solder alloy compositions were prepared by adding Al2O3 nanoparticles in different weight fractions (0.05, 0.1, 0.3 and 0.5 wt.%) through mechanical dispersion. Cu/solder/Cu micro-lap-shear solder joint specimens were used to assess the shear strength of the solder joint. Differential scanning calorimetry was used to investigate the melting behavior of SAC305 solder nanocomposites. The solder joint interfacial microstructure was studied using scanning electron microscopy. The results showed that the increase in melting temperature (TL) and melting temperature range of the SAC305 solder alloy by addition of Al2O3 nanoparticles were not significant. In comparison with unreinforced SAC305 solder alloy, the reinforcement of 0.05-0.5 wt.% of Al2O3 nanoparticles improved the solder wettability. The addition of nanoparticles in minor quantity effectively suppressed the Cu6Sn5 IMC growth, improved the solder joint shear strength and ductility under multiple reflow cycles. However, the improvement in solder properties was less pronounced on increasing the nanoparticle content above 0.1 wt.% of the solder alloy. © 2018, ASM International.Item The effect of reflow temperature on time at the end of gravity zone (TGZ) of Sn-3.8Ag-0.7Cu solder alloy(ASTM International, 2020) Panikar, R.S.; Amogha Skanda, V.; Tikale, S.; Prabhu, K.N.The reflow time for solder until the end of the gravity zone (Tgz) is considered to be the optimum reflow time for obtaining high mechanical performance from lead-free solders. In the present work, the effect of reflow time and temperature on Tgz of Sn-3.8Ag-0.7Cu (SAC387) lead-free solder alloy reflowed on the copper substrate has been investigated. The evolution of interfacial microstructure and solder bond shear strength under different reflow temperatures and time was assessed. Solder balls weighing 0.08 ± 0.01 g were reflowed at 260°C, 280°C, and 300°C for reflow times of 30 s, 60 s, 120 s, and 240 s. Times at the end of the gravity zone for SAC387 solder were obtained as 110 ± 5 s, 55 ± 5 s, and 23 ± 3 s for reflow temperatures of 260°C, 280°C and 300°C, respectively. The contact angle for SAC387 solder on the copper substrate at Tgz was found to be 25.5° ± 0.2° for all reflow temperatures. Scanning electron microscopy revealed the formation of a Cu6Sn5 intermetallic compound (IMC) layer at the interface. The IMC layer thickness increased with increase in reflow temperature and time. Maximum solder joint strength was obtained at Tgz reflow times for all reflow temperatures. Microstructures of samples reflowed beyond the gravity zone showed secondary Cu6Sn5 precipitation in the solder bulk. The present investigation reveals a reduction in Tgz reflow time for SAC387 lead-free solder at higher operating reflow temperatures. © © 2020 by ASTM InternationalItem Performance and reliability of Al2O3 nanoparticles doped multicomponent Sn-3.0Ag-0.5Cu-Ni-Ge solder alloy(Elsevier Ltd, 2020) Tikale, S.; Prabhu, K.N.The effect of Al2O3 nanoparticles addition on melting, microhardness, microstructural, and mechanical properties of multicomponent Sn-3Ag-0.5Cu-0.06Ni-0.01Ge (SACNiGe) solder alloy was investigated. The shear strength of the capacitor assemblies under varying high-temperature environments for different nanocomposites was assessed and the reliability of the joint was determined using Weibull analysis. The SACNiGe solder doped with 0.01 and 0.05 wt% Al2O3 nanoparticles to prepare nanocomposites and tested on the solder joints for their performance and reliability under different thermal conditions. Plain copper and Ni[sbnd]P layer coated substrates were used to investigate the effect of different surface finish on the joint reliability. The addition of ceramic nanoparticles in small amounts did not affect the melting parameters of the solder. In comparison with the bare solder alloy, nanocomposites yielded about 20% increase in tin-climb height and 14% higher microhardness. The dispersion of ceramic nanoparticles in the matrix and presence of Ni and Ge elements in the solder resulted in substantial microstructure refinement and about 24% supression in intermetallic compounds (IMCs) growth at the joint interface. In comparison with the bare Cu substrate, the Ni[sbnd]P coating on the substrate provided a strong diffusion barrier, promoted thin and complex (Cu, Ni)6Sn5 IMC layer formation at the interface, and significantly retarded the IMC growth kinetics under elevated temperature conditions. Under varying thermal conditions, nanoparticles doped solder compositions showed about 20% increase in the joint shear strength value. The reliability of joints improved appreciably with the addition of 0.05 wt% Al2O3 nanoparticles in the solder. Samples with SACNiGe+0.05Al2O3 nanocomposite reflowed on Ni[sbnd]P coating showed about 32% higher reliability than that on the uncoated-copper substrate. The SACNiGe solder joint performance and reliability could be significantly improved by minor weight percent addition of Al2O3 nanoparticles in the presence of Ni[sbnd]P coating on the substrate. © 2020 Elsevier LtdItem Wettability and Interfacial Heat Transfer During Solidification of Al–Si Alloy (A413) Melt Droplets on Metallic Substrates(Springer Science and Business Media Deutschland GmbH, 2024) Shamil, K.M.; Kamala Nathan, D.; Prabhu, K.N.Wetting and heat transfer during solidification were studied with Al–Si alloy (A413) melt droplets on copper and stainless steel cylindrical substrates. As the molten metal cools down and solidifies, the interface changes from the initial liquid/solid contact to solid/solid contact, leading to variations in the rate of interfacial heat transfer. The effect of substrate roughness and melt superheat on the droplet contact angle and heat flux was investigated. A smooth substrate surface resulted in higher casting surface roughness, and the contact angle of solidified droplets increased with an increase in substrate roughness. Surface profile analysis indicated the presence of an air gap between the substrate and solidified droplet. The gap width was used to determine the variation of heat transfer coefficient (HTC) along the radial direction of the substrate surface at the end of solidification. Deformation of the solidified droplet was inverse for copper versus stainless steel resulting in an increase in HTC for copper and a decrease for stainless steel. © 2023, American Foundry Society.Item Effects of Phosphorus Treatment on Cooling Behavior, Heat Transfer, Microstructure, and Mechanical Properties of Hypereutectic Al-23%Si Alloy(Springer, 2025) Vijayan, V.; Prabhu, K.N.The influence of phosphorus (P) treatment on the microstructure, cooling behavior, interfacial heat flux, and mechanical properties of hypereutectic Al-23 Si alloy is investigated in the present work. Computer-aided cooling curve analysis revealed that higher cooling rates suppressed silicon cluster agglomeration and promoted nucleation of primary silicon at lower undercooling. The addition of P increased the nucleation temperature of primary silicon and resulted in finer silicon crystals. Eutectic silicon nucleation was facilitated by P treatment, with refined primary silicon acting as nucleation sites. Interfacial heat flux analysis demonstrated that P addition decreased the heat flux, attributed to the presence of less conductive primary silicon, and unmodified eutectic silicon microstructure. Microstructural analysis revealed the refinement of primary silicon and transformation of its morphology to polyhedral shape with P treatment. Heat treatment improved tensile properties, with refined primary silicon and copper precipitation contributing to enhanced strength. The morphology and composition of copper intermetallic varied with P and Cu content, influencing mechanical properties. These findings provide insights into optimizing alloy compositions and processing conditions for hypereutectic Al-Si alloys in various industrial applications. © ASM International 2023.Item Effect of Ni nanoparticles reinforcement on wettability, microstructure and mechanical properties of SAC387 lead-free solder alloy(Elsevier Ltd, 2025) Muhammed, H.J.; Prabhu, K.N.The study investigates the influence of nickel (Ni) nanoparticles on the wettability, microstructure, and mechanical properties of Sn-3.8Ag-0.7Cu (SAC387) lead-free solder alloy. Nanocomposite solders containing 0.3 wt% and 0.5 wt% Ni were prepared and reflowed at temperatures of 230 °C, 250 °C, and 270 °C to evaluate their performance on copper substrates with a surface roughness (Ra) of 0.01 ± 0.002 ?m. Wettability improved with increasing reflow temperature; however, the addition of Ni nanoparticles had minimal direct impact on spreading behavior. Microstructural analysis revealed enhanced formation of interfacial intermetallic compounds (IMCs), particularly (Cu,Ni)6Sn5 which contributed to improved joint stability. The optimal mechanical performance was observed at 250 °C with 0.3 wt% Ni addition, yielding a 51.14 % increase in shear strength compared to the unreinforced solder. Microhardness also improved significantly by 43.7 % at the IMC layer and 18.3 % in the solder bulk. Weibull analysis further confirmed higher joint performance with Ni nanoparticle incorporation. These findings highlight the potential of addition of Ni nanoparticles in improving the performance of SAC387 solder joints in electronic packaging. © 2025 Elsevier Ltd