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Item Thermal analysis and microstructure of ZA8 alloy solidifying against chills(Springer India sanjiv.goswami@springer.co.in, 2012) Ramesh, G.; Prabhu, K.N.Thermal analysis during solidification of ZA8 alloy against copper, hot die steel and stainless steel chills instrumented with thermocouples was carried out in the present work. The investigation showed that the chill material and coating had a significant effect on the cooling curve of the casting. When casting was solidified against chills, the liquidus and eutectic start temperature of the casting remained nearly the same whereas eutectoid transformation occurred at a higher temperature. Cooling rate curve of the casting solidified against coated chill indicated that formation of solid shell and subsequent re-melting in the case of high thermal conductivity coated chill whereas in lower thermal conductivity coated chill, the re-melting of solid shell was absent. It was found that chilling during solidification causes the morphology of dendrites transform to nearly rounded shape with refinement of lamellar eutectic.Item Wetting behavior of solders(2010) Kumar, G.; Prabhu, K.N.Lead bearing solders have been used extensively in the assembly of modern electronic circuits. However, increasing environmental and health concerns about the toxicity of lead has led to the development of lead-free solders. Wetting of solders on surfaces is a complex and important phenomenon that affects the interfacial microstructure and hence the reliability of a solder joint. The solder material reacts with a small amount of the base metal and wets the metal by intermetallic compound (IMC) formation. The degree and rate of wetting are the two important parameters that characterize the wetting phenomenon. Contact angle is a measure of the degree of wetting or wettability of a surface by a liquid. Spreading kinetics in a given system is strongly affected by the experimental conditions. In reactive systems like soldering, wetting and chemical interfacial reactions are interrelated, and hence for successful modeling, it is essential to assess the effect of interfacial reactions on kinetics of wetting. Solder wetting necessarily involves the metallurgical reactions between the filler metal and the base metal. This interaction at the solder/base metal interface results in the formation of IMCs. During soldering an additional driving force besides the imbalance in interfacial energies originates from the interfacial reactions. The formation of IMC has significant influence on contact angle. The presence of IMCs (thin, continuous, and uniform layer) between solders and substrate metals is an essential requirement for good bonding. Optimum thickness of an IMC layer offers better wettability and an excellent solder joint reliability. However, due to their inherent brittle nature and tendency to generate structural defects, a too thick IMC layer at the interface may degrade the joint. In this paper, the factors affecting the wetting behavior of solders and evolution of interfacial microstructure are reviewed and discussed. Copyright © 2010 by ASTM International.Item Wetting behavior of solders(ASTM International, 2011) Kumar, G.; Prabhu, K.N.Lead bearing solders have been used extensively in the assembly of modern electronic circuits. However, increasing environmental and health concerns about the toxicity of lead has led to the development of lead-free solders. Wetting of solders on surfaces is a complex and important phenomenon that affects the interfacial microstructure and hence the reliability of a solder joint. The solder material reacts with a small amount of the base metal and wets the metal by intermetallic compound (IMC) formation. The degree and rate of wetting are the two important parameters that characterize the wetting phenomenon. Contact angle is a measure of the degree of wetting or wettability of a surface by a liquid. Spreading kinetics in a given system is strongly affected by the experimental conditions. In reactive systems like soldering, wetting and chemical interfacial reactions are interrelated, and hence for successful modeling, it is essential to assess the effect of interfacial reactions on kinetics of wetting. Solder wetting necessarily involves the metallurgical reactions between the filler metal and the base metal. This interaction at the solder/base metal interface results in the formation of IMCs. During soldering an additional driving force besides the imbalance in interfacial energies originates from the interfacial reactions. The formation of IMC has significant influence on contact angle. The presence of IMCs (thin, continuous, and uniform layer) between solders and substrate metals is an essential requirement for good bonding. Optimum thickness of an IMC layer offers better wettability and an excellent solder joint reliability. However, due to their inherent brittle nature and tendency to generate structural defects, a too thick IMC layer at the interface may degrade the joint. In this paper, the factors affecting the wetting behavior of solders and evolution of interfacial microstructure are reviewed and discussed. Copyright © 2010 by ASTM International.Item Effect of cooling rate during solidification of Sn-9Zn lead-free solder alloy on its microstructure, tensile strength and ductile-brittle transition temperature(Elsevier Ltd, 2012) Prabhu, K.N.; Deshapande, P.; Satyanarayan, S.Solidification rate is an important variable during processing of materials, including soldering, involving solidification. The rate of solidification controls the metallurgical microstructure at the solder joint and hence the mechanical properties. A high tensile strength and a lower ductile-brittle transition temperature are necessary for reliability of solder joints in electronic circuits. Hence in the present work, the effect of cooling rate during solidification on microstructure, impact and tensile properties of Sn-9Zn lead-free solder alloy was investigated. Four different cooling media (copper and stainless steel moulds, air and furnace cooling) were used for solidification to achieve different cooling rates. Solder alloy solidified in copper mould exhibited higher cooling rate as compared to other cooling media. The microstructure is refined as the cooling rate was increased from 0.03 to 25 °C/s. With increase in cooling rate it was observed that the size of Zn flakes became finer and distributed uniformly throughout the matrix. Ductile-to-brittle transition temperature (DBTT) of the solder alloy increased with increase in cooling rate. Fractured surfaces of impact test specimens showed cleavage like appearance and river like pattern at very low temperatures and dimple like appearance at higher temperatures. The tensile strength of the solder alloy solidified in Cu and stainless moulds were higher as compared to air and furnace cooled samples. It is therefore suggested that the cooling rate during solidification of the solder alloy should be optimum to maximize the strength and minimize the DBTT. © 2011 Elsevier B.V.Item Effect of Mn on cooling behaviour and microstructure of chill cast Zn-Al (ZA8) alloy(2012) Ramesh, G.; Vishwanatha, H.M.; Prabhu, K.N.In the present work, the effect of manganese addition to ZA8 alloy on thermal analysis parameters, heat transfer and microstructure was investigated. The thermal analysis parameters were found to be significantly affected by chemical modification of ZA8 alloy. Cooling curve and differential scanning calorimetry analyses of modified alloy showed nucleation of new phase other than b dendrites. Chilling of modified alloy resulted in decreased liquidus temperature and enhanced eutectoid transformation. Further, chilling avoids the formation of intermetallic compounds in modified alloy. The heat flux transients were estimated using inverse modelling during solidification of unmodified and modified alloys against different chills. The peak heat flux decreased on addition of Mn to ZA8 alloy. Differential scanning calorimetry analysis indicated that the addition of Mn to ZA8 alloy decreases the heat of solidification. The addition of Mn to ZA8 alloy increased the contact angle, indicating decreased wettability of the modified alloy on the chill surface. The microstructure of ZA8 with Mn showed an increased amount of b phase and a decreased amount of eutectic. X-ray diffraction analysis confirmed the formation of MnAl6 intermetallics in Mn added ZA8 alloy. Chilling with chemical modification resulted in enhanced decomposition of b phase. © 2012 Institute of Materials, Minerals and Mining.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 simultaneous refinement and modification by cerium on microstructure and mechanical properties of Al-8% Si alloy(Taylor and Francis Ltd. maney@maney.co.uk, 2016) Vijayan, V.; Prabhu, K.N.The effect of cerium melt treatment on microstructure and mechanical properties of gravity die cast Al-8% Si alloy was studied. The addition of Ce transformed the large columnar ?-Al grains into fine equiaxed grains and modified the acicular eutectic Si into fine fibrous form. The addition of Ce resulted in the formation of needle-shaped complex Ce intermetallic along the grain boundaries. Electrical conductivity measurements are used for the first time to reveal the effect of Ce addition on modification of the eutectic Si. The electrical conductivity of the alloy increased with Ce melt treatment. Scanning Electron Microscopy results suggests that the Ce particles did not heterogeneously nucleate the ?-Al grains, instead the fine equiaxed grains were formed through an invariant reaction between the liquid melt and Ce phase. The ultimate tensile strength, % elongation and hardness of the alloy significantly improved due to simultaneous modification and refinement. © 2016 Informa UK Limited, trading as Taylor & Francis Group.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 Bond shear strength of Al2O3 nanoparticles reinforced 2220-capacitor/SAC305 solder interconnects reflowed on bare and Ni-coated copper substrate(Springer, 2021) Tikale, S.; Prabhu, K.N.The influence of Al2O3 nanoparticles addition in trace amounts and electroless Ni–P substrate coating on the microstructure development and bond shear strength of Sn-3.0Ag–0.5Cu (SAC305) solder joint were investigated. The performance and reliability of the 2220-capacitor joints with Al2O3 nanoparticle reinforced nanocomposites reflowed on Cu and Ni–P coated substrate were analyzed under varying high-temperature environments. The addition of nanoparticles enhanced the wettability and microhardness of the solder and considerably refined the joint microstructure. The dispersion and adsorption of Al2O3 nanoparticles resulted in the suppression of intermetallic (IMC) growth at the interface and refinement of the ?-Sn grains as well as IMC precipitates into the matrix. The Ni–P coating on the substrate significantly retarded the IMC growth kinetics resulting in the formation of a thin and uniform IMC layer at the joint interface. The thermal stability and performance of the joint under high-temperature environments were enhanced due to the Ni–P coating on the substrate. Compared to the unreinforced SAC305 solder joint with bare Cu substrate, joints with SAC305 + 0.05Al2O3 composite showed about 17% higher shear strength with bare Cu substrate and about 27% higher strength with Ni–P coated substrate. The Weibull distribution analysis indicates a significant improvement in joint reliability of the 2220-capacitor/SAC305 solder assembly using SAC305 + 0.05Al2O3 nanocomposite and Ni-coated substrate. The ANOVA study suggests that the solder joint performance majorly depends on the operating environment, solder composition, and the substrate finish. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature.