K. H. Prakash

Interface reaction between copper and molten tin-lead solders

The formation and growth of Cu–Sn intermetallic film at the interface between molten Sn–Pb solders and Cu were studied at different temperature and exposure times. The η-phase (Cu6Sn5) was observed to form at all conditions except at the lowest Sn level of 27 wt% and at the two highest temperatures of 290 and 310°C. The var epsilon-phase (Cu3Sn) was then obtained. At high Sn contents and short times, a cellular film with a rugged interface was obtained which evolved into a compact film with a scalloped interface as the Sn content decreased and exposure time increased. The var epsilon-phase film always formed with a relatively planar interface. The intermetallic grains showed preferred crystallographic orientations. Thickness measurements showed that the net growth rate depends not only on diffusion through the film but also on the film dissolution. Precipitation of the η-phase whiskers was detected during cooling, particularly when the temperature was significantly high.

Isothermal and thermal cycling aging on IMC growth rate in lead-free and lead-based solder interface

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu and Ni/Au) metallization is a major concern for long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer can affect the solder joint reliability. Analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging were conducted. Experimental study of IMC layer growth between Sn3.8Ag0.7Cu and Ni/Au surface finish by isothermal aging versus thermal cycling (TC) aging was investigated to develop a framework for correlating IMC layer growth behavior. An integrated model for IMC growth was derived to describe the Ni-Cu-Sn IMC growth behavior subject to TC aging. Comparison of modeling and test results showed that IMC layer growth rate under TC aging was accelerated. It is noted that IMC layer growth study from various references showed different experimental data and growth kinetic parameters for both liquid-state and solid-state reactions.

Chitosan-mediated crystallization and assembly of hydroxyapatite nanoparticles into hybrid nanostructured films

The synthesis and subsequent assembly of nearly spherical nano-hydroxyapatite (nHA) particles in the presence of trace amounts of the polysaccharide chitosan was carried out employing a wet chemical approach. Chitosan addition during synthesis not only modulated HA crystallization but also aided in the assembly of nHA particles onto itself. Solvent extraction from these suspensions formed iridescent films, of which the bottom few layers were rich in self-assembled nHA particle arrays. The cross-section of these hybrid films revealed compositional and hence structural grading of the two phases and exhibited a unique morphology in which assembled nHA particles gradually gave way to chitosan-rich top layers. Transmission electron microscope and selected area electron diffraction studies suggested that the basal plane of HA had interacted with chitosan, and scanning electron microscope studies of the hybrid films revealed multi-length scale hierarchical architecture composed of HA and chitosan. Phase identification was carried out by X-ray diffraction (XRD) and Rietveld analysis of digitized XRD data showed that the basic apatite structure was preserved, but chitosan inclusion induced subtle changes to the HA unit cell. The refinement of crystallite shape using the Popa method clearly indicated a distinct change in the growth direction of HA crystallites from [001] to [100] with increasing chitosan concentration. The paper also discusses the likelihood of chitosan phosphorylation during synthesis, which we believe to be a pathway, by which chitosan molecules chemically interact with calcium phosphate precursor compounds and orchestrate the crystallization of nHA particles. Additionally, the paper suggests several interesting biomedical applications for graded nHA–chitosan nanostructured films.

Isothermal and thermal cycling aging on IMC growth rate in Pb-free and Pb-based solder interfaces

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu, Ni, Au metallizations) is one of the major concerns in long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer plays a major role on the life-time of the solder joints. As the diffusion and the reaction are thermally activated processes, the growth rate of the IMC layer is sensitive to temperature changes. Critical review and analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging is conducted. Through isothermal annealing the growth rate of the IMC layer at a given temperature and its general dependence on temperature can be determined by plotting the measured IMC layer thickness against time at iso-temperatures. However, real solder joints in electronic assemblies undergo thermal cycling due to environmental temperature changes and/or power on/off cycles. Therefore, data obtained by thermal cycling (TC) aging exposure is more practical in understanding IMC growth behavior in solder joints. However, in TC aging tests, the IMC growth rate is often determined also by plotting the measured IMC thickness against the accumulated time exposure (corresponding to the number of cycles of TC aging). This can lead to incorrect physics-of-failure characterization of IMC growth kinetics for TC aging effects, as IMC growth is temperature and time dependent and is expected to have thermally activated thresholds below which at a certain temperature no IMC growth is expected. Therefore, it is obvious that the whole TC cycle does not contribute to IMC growth and hence using accumulated time corresponding to TC cycle time is not justified. Hence, in this study, comparison between IMC layer thickness data by isothermal aging versus thermal cycling aging are used to develop a framework for correlating IMC layer growth behavior between isothermal and thermal cycling effects.

Synthesis and Characterisation of Hydroxyapatite Nano-Rods/Whiskers

The use of precipitation chemistry to synthesize hydroxyapatite (HA) normally yields nano-crystals with various morphology and sizes depending on the synthesis conditions (e.g., temperature, concentration, level of agitation, pH, etc.). This study involved the synthesis of HA nano-rods/whiskers by modifying an existing chemical route for HA synthesis. The nano-rods were characterised for their structure and morphology and subsequently sintered. The mechanical properties of the sintered compacts were also assessed.

Effect of Super Saturation Level on the size and morphology of Hydroxyapatite precipitate

Hydroxyapatite (HA) is synthesized through acid-base reaction by adding an aqueous solution of orthophosphoric acid to an aqueous solution of calcium hydroxide maintained at 40, 80 and 100°C. X-ray diffraction of the precipitate particles revealed HA as the predominant phase in all the temperatures and that the morphology of the particles changed from needle-shaped at 40°C to spheroid at 100°C. The changes in the morphology with temperature were analyzed taking into account the driving force for the HA precipitation and the super saturation level of Ca2+and PO43-ions with respect to HA. It appears that the effect of temperature on morphology is less pronounced compared to that of super saturation level. The analysis also indicated that the super saturation level of the reactants, especially the concentration of Ca2+ions, played a predominant role on the precipitate morphology for this classical acid-base reaction.

Hydroxyapatite-Chitosan Hybrid Nano-Materials

Self-assembled nano-materials are currently an area of research with high throughput due to the opportunities it provides to fields ranging from semiconductor engineering to gene delivery. There is also considerable interest in nano-particulate systems that attain a lower energy state by self-assembly through favorable and repeated surface interactions as they mimic those commonly found in natural biological systems. This work presents a simple route to first synthesise a highly stable suspension of nano-hydroxyapatite (~40nm) with chitosan and subsequently self-assemble the suspended nano-hydroxyapatite particulates onto a substrate.