Faculty Publications
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Item Severity of quenching and kinetics of wetting of nanofluids and vegetable oils(2009) Jagannath, V.; Prabhu, K.N.In the present work, the suitability of vegetable oil blends with mineral oil and alumina based nanofluids as quench media for industrial heat treatment was investigated. Sunflower oil, palm oil, and mineral oil were used for preparing the blends. Alumina based nanofluids of varying concentrations ranging from 0.01-4 % were used. The size of alumina particles was about 50 nm. The severity of quenching and heat transfer coefficients were estimated during quenching of copper probes. Heat transfer coefficients were estimated using a lumped heat capacitance model. The static contact angle was measured on copper substrates having a surface texture similar to the probes used for estimation of heat transfer coefficients. A dynamic contact angle analyzer was used for this purpose. The measured contact angles of nanofluids on copper were high compared to oils, indicating poor wetting by quench media that are polar in nature. Wetting characteristics had a significant effect on heat transfer coefficients estimated during quenching. Copyright © 2009 by ASTM International.Item Thermal contact at solder/substrate interfaces during solidification(2009) Nyamannavar, S.; Prabhu, K.N.Heat flux transients at the solder/substrate interface during the solidification of Sn-37Pb and Sn-3.5Ag solder alloys against metallic substrates were estimated by the lumped heat capacitance model and the contact condition was assessed by scanning electronic microscopy (SEM). Copper substrates yielded maximum contact heat flux followed by brass and aluminium substrates. The SEM study in the solder/substrate interfacial region revealed the existence of a clear gap with the aluminium substrate. A conforming contact was obtained with copper and brass substrates. © 2009 Institute of Materials, Minerals and Mining.Item Wetting behaviour and evolution of microstructure of Sn-Ag-Zn solders on copper substrates with different surface textures(2010) Satyanarayan, S.; Prabhu, K.N.The effect of surface roughness on wetting behaviour and evolution of microstructure of two lead-free solders (Sn-2.625Ag-2.25Zn and Sn-1.75Ag-4.5Zn) on copper substrate was investigated. Both solders exhibited good wettability on copper substrates having rough surface and lower wettabilty on smooth surfaces. The contact angles of solders decreased linearly with increase in surface roughness of the substrate. The exponential power law, ?=exp(-K ?n), was used to model the relaxation behaviour of solders. A high intermetallic growth was observed at the interface particularly on copper substrates with rough surface texture. A thin continuous interface showing scallop intermetallic compounds (IMC) was obtained on smooth surfaces. With an increase in surface roughness, the IMC morphology changed from scallop shaped to needle type at the Sn-2.625Ag-2.25Zn solder/substrate interface and nodular to plate like IMCs for Sn-1.75Ag-4.5Zn solder matrix. Copyright © 2010 by ASTM International.Item Wetting behaviour and evolution of microstructure of Sn-Ag-Zn solders on copper substrates with different surface textures(ASTM International, 2011) Satyanarayan, S.; Prabhu, K.N.The effect of surface roughness on wetting behaviour and evolution of microstructure of two lead-free solders (Sn-2.625Ag-2.25Zn and Sn-1.75Ag-4.5Zn) on copper substrate was investigated. Both solders exhibited good wettability on copper substrates having rough surface and lower wettabilty on smooth surfaces. The contact angles of solders decreased linearly with increase in surface roughness of the substrate. The exponential power law, ?=exp(-KT -1), was used to model the relaxation behaviour of solders. A high intermetallic growth was observed at the interface particularly on copper substrates with rough surface texture. A thin continuous interface showing scallop intermetallic compounds (IMC) was obtained on smooth surfaces. With an increase in surface roughness, the IMC morphology changed from scallop shaped to needle type at the Sn-2.625Ag-2.25Zn solder/ substrate interface and nodular to plate like IMCs for Sn-1.75Ag-4.5Zn solder matrix. Copyright © 2010 by ASTM International.Item Effect of temperature and substrate surface texture on wettability and morphology of IMCs between Sn-0.7Cu solder alloy and copper substrate(2012) Satyanarayan, S.; Prabhu, K.N.In the present work, the effect of soldering temperature (270 and 298 °C) and substrate surface texture (0.02 and 1.12 ?m) on wetting characteristics and morphology of intermetallic compounds (IMCs) between Sn-0.7Cu lead-free solder on copper substrates was investigated. It was found that increase in temperature and substrate surface roughness improved the wettability of solder alloy. However, the effect of surface roughness on wettability was significant as compared to that of temperature. The spreading of solder alloy was uniform on smooth substrate, whereas spreading of the alloy on rough substrate resulted in an oval shape. The morphology of IMCs transformed from long needle shaped to short and thick protrusions of IMCs with increase in surface roughness of the substrate. Needle shaped and thick protruded intermetallics formed at the solder/Cu interface were identified as Cu 6Sn 5 compounds. The formation of Cu 3Sn IMC was observed only for the spreading of solder alloy at 298 °C which contributed to improvement in the wettability of solder alloy on both smooth and rough substrate surfaces. © Springer Science+Business Media, LLC 2012.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 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 The effect of nanocoatings on critical heat flux (CHF) under pool boiling conditions(ASTM International, 2021) Reddy, S.; Prabhu, K.N.; Nayak, U.V.The effect of nanomaterial coating on heat transfer characteristics of a copper substrate during pool boiling with distilled water (>99.9 %) was studied. The peak on the pool boiling curve between nucleate and transition boiling is termed the critical heat flux. Multi-walled carbon nanotubes and graphene were used to obtain coatings on the copper substrate. Experiments were carried out at atmospheric pressure. The results showed enhanced critical heat flux for the coated substrates. Graphene-coated copper showed the highest critical heat transfer characteristics compared with that on bare and multi-walled carbon nanotube-coated substrates. The study has significant implications for the enhancement of heat transfer during quench heat treatment. © © 2021 by ASTM International.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