Journal Articles

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    Inference of optimal speed for sound centrifugal casting of tin
    (Metallurgical Society of CIM, 2009) Mukunda, P.G.; Rao, S.; Rao, S.S.
    During centrifugal casting when a mould is rotated at low and very high speeds, defects are found in the final castings. Obtaining the critical speed for sound castings should not be a matter of guessing but based on experience. The defects in the casting are mainly due to the behaviour of the molten metal during the teeming and solidification process. The motion of molten metal at various speeds and its effect during casting are addressed in this paper. Tin is taken as an experiment fluid and its performance during various rotational speeds is discussed. The microstructures and hardness of all the castings are also investigated. © Canadian Institute of Mining, Metallurgy and Petroleum.
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    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.
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    Investigation into creep behaviour of Sn-40%Pb alloy using impression creep method
    (2009) Udaya Prasanna, H.U.; Udupa, K.R.; Prabhu, K.N.
    The creep behaviour of Sn-40%Pb hypereutectic alloys cast in the molds made of different materials was investigated using impression creep technique in the temperature range from zero to 32 °C and under the punching stress of 50 MPa. The creep curves.ie, profiles of indentation depth against time are generated and steady state creep rates (SSCRs) are determined. Activation energy was calculated knowing creep rates at different temperature levels. Standard metallographic technique was used to determine the grain size of alloys which were poured into different molds. It was found that SSCR, at all the temperature levels of testing, is a function of grain size of the material. The activation energy being in the range of 10kJ/mol -12 kJ/mol, suggests that the probable creep mechanism is dislocation glide aided by vacancy diffusion. Results of the experiments are discussed.
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    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.
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    Wetting behaviour and interfacial microstructure of Sn-Ag-Zn solder alloys on nickel coated aluminium substrates
    (2011) Satyanarayan, S.; Prabhu, K.N.
    Wetting behaviours of two lead free solders (Sn-2·625Ag- 2·25Zn and Sn-1·75Ag-4·5Zn) on nickel coated aluminium substrates were investigated. Sn-2·625Ag-2·25Zn exhibited better wettability compared to Sn-1·75Ag-4·5Zn solder. Contact angles of the solders increased with increasing roughness of the substrate. The Young-Dupre equation was used to evaluate the work of adhesion of solder on the substrate. Sn-2·625Ag-2·25Zn solder exhibited higher work of adhesion than Sn-1·75Ag-4·5Zn. A thin continuous layer of Ni 3Sn was detected at the interface between Sn-2·625Ag- 2·25Zn solder and nickel coated Al substrate. Sn-1·75Ag- 4·5Zn solder exhibited scallop intermetallic compounds (IMCs) growing into the solder field as well as a thin continuous IMC in some cases. Ni 3Sn and Ni3Sn4 IMCs were observed at the interface of Sn-1·75Ag-4·5Zn solder and nickel coated Al. © 2011 Institute of Materials, Minerals and Mining.
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    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.
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    Effect of purging gas on wetting behavior of Sn-3.5Ag lead-free solder on nickel-coated aluminum substrate
    (Springer Science and Business Media, LLC, 2013) Prabhu, K.; Varun, M.; Satyanarayan, S.
    The wetting characteristics of Sn-3.5Ag lead-free solder alloy on nickel-coated aluminum substrates in air (ambient), nitrogen, and argon atmospheres were investigated. The contact angles for the solder alloy obtained under air and argon atmospheres were in the range of 36 -38. With nitrogen atmosphere the contact angle was found to be significantly lower at about 26. Solder solidifying in air exhibited needle-shaped tin-rich dendrites surrounded by a eutectic matrix. The amount of tin dendrites decreased in argon atmosphere. However, the morphology of tin dendrites transformed from needle-shaped to nearly non-dendritic shape as the soldering atmosphere was changed from air to nitrogen. The interfacial microstructures revealed the presence of Ni 3Sn and Ni3Sn4 IMCs at the interface. The enhanced wettability observed under nitrogen atmosphere is attributed to the higher thermal conductivity of nitrogen gas and the formation of higher amount of Ni3Sn IMCs at the interface compared to air and argon atmospheres. © 2012 ASM International.
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    Spreading behavior and evolution of IMCs during reactive wetting of SAC solders on smooth and rough copper substrates
    (2013) Satyanarayan, S.; Prabhu, K.
    The effect of surface roughness of copper substrate on the reactive wetting of Sn-Ag-Cu solder alloys and morphology of intermetallic compounds (IMCs) was investigated. The spreading behavior of solder alloys on smooth and rough Cu substrates was categorized into capillary, diffusion/reaction, and contact angle stabilization zones. The increase in substrate surface roughness improved the wetting of solder alloys, being attributed to the presence of thick Cu 3Sn IMC at the interface. The morphology of IMCs transformed from long needle shaped to short protruded type with an increase in the substrate surface roughness for the Sn-0.3Ag-0.7Cu and Sn-3Ag-0.5Cu solder alloys. However, for the Sn-2.5Ag-0.5Cu solder alloy the needle-shaped IMCs transformed to the completely scallop type with increase in the substrate surface roughness. The effect of Ag content on wetting behavior was not significant. © 2013 TMS.
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    Effect of Sn doping on structural, optical, electrical and wettability properties of oriented ZnO nanorod arrays
    (2013) Santhosh Kumar, A.; Nagaraja, K.K.; Nagaraja, H.S.
    Herein we present a modified sol gel route for the one step fabrication of oriented ZnO nanorod arrays. The method is seed layer free, and nanorods directly attach to a substrate. We also present the effect of tin (Sn) content on the crystallinity, microstructural, optical and electrical properties of the ZnO nanorod arrays. Thermo gravimetric (TG) curves of gel precursors showed that most of the organic groups and other volatiles were removed at about 450 C. X-ray diffraction patterns confirmed that the films were polycrystalline in nature with (002) preferred orientation. The texture coefficient, grain size, dislocation density and lattice parameters of the ZnO arrays were determined. The SEM micrographs revealed that the undoped and 1 at.%Sn doped films were composed of nanorods and the concentration of 2 at.%Sn doping hindered the rod like structure growth and modulated into granular nature. UV-visible transmission spectroscopy indicated that the transparency of the films increased with Sn content. On Sn doping, the films also exhibited a red shift and slight shrinkage of band gap. The electrical studies revealed that 1 at.% of Sn doping enhanced electrical conduction in ZnO films and beyond that the distortion caused in the lattice reduced the conductivity. The contact angle of the ZnO nanostructures varied between 91 and 115 depending upon the Sn content. Therefore, 1 at.%Sn doping into ZnO nanorods improves the crystallinity, electrical conductivity and water contact angle. © 2013 Springer Science+Business Media New York.
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    Solder joint reliability of Sn-0·7Cu and Sn-0·3Ag- 0·7Cu lead-free solder alloys solidified on copper substrates with different surface roughnesses
    (2013) Satyanarayan, S.; Prabhu, K.
    The bond shear test was used to assess the integrity of Sn-0·7Cu and Sn-0·3Ag-0·7Cu lead-free solder alloy drops solidified on copper substrates with smooth and rough surface finishes. Solder alloys solidified on smooth substrates required higher shear force compared to that on rough substrates. Sn-0·3Ag-0·7Cu alloy required higher shear energy than Sn-0·7Cu alloy. Solder alloys solidified on smooth substrate surfaces exhibited complete ductile failure. On rough copper surfaces, solder alloys showed a transition ridge characterized by sheared intermetallic compounds (IMCs) and the presence of dimples. The peak shear strength decreased with increase in contact area of the solder bond on the substrate. Smooth surface and the presence of minor amount of Ag in the solder alloy enhance the integrity of the solder joint. © 2013 Institute of Materials, Minerals and Mining.