Anil Kunwar

Modeling the diffusion-driven growth of a pre-existing gas bubble in molten tin

Finite element method is utilized to solve the diffusion equation and model the diffusion driven growth of a pre-existing spherical gas bubble in molten tin at the solder/substrate interface for reflow time of 120 s and temperature of 250 °C. The gibbs free energy change required for determining the equilibrium concentration at liquid solder/gas bubble boundary was calculated using the thermodynamic polynomial coefficients. The rate of change of radius, as function of concentration flux, is calculated using the lagrangian mesh update methodology. With an initial diameter of 20 μm, the bubble growth is calculated as a function of contact angle. When the wetting angle is varied from a value of 30° to 135°, the numerical calculation has yielded the final sizes for the bubble to change from 62.87 μm to 82.8 μm respectively. The effect of wetting transition in the growth of bubble was studied by the in-situ observation of bubble dynamics through synchrotron radiation imaging technique. The scanning electron microscopy images of the morphologies of intermetallic compounds influenced by growing bubble in Sn/Cu solder joint and bubble pictures obtained through synchrotron radiation are utilized to get the experimental size of the bubble. The mean experimental bubble diameter has been obtained as 76.39 μm. The growing bubble inhibits the growth of intermetallic compound at its vicinity and thereby reduces the strength of solder joints.

A numerical model for diffusion driven gas bubble growth in molten Sn-based solder

As soldering process is a high temperature phenomenon accompanied by surface reaction kinetics, the computational model for solder bubble growth can be a strong arena for describing those aspects which are otherwise difficult to be understood through the experimental methods. In this paper, the growth of gaseous bubbles in the molten tin solder has been modeled using finite element method. The advection-diffusion and lagrangian mesh adaptation equations have been utilized to obtain the numerical solution of concentration flux and radius change for a single spherical bubble. Utilizing the axisymmetric coordinate system (2D), the final bubble diameter has been obtained to be 31.06 μm at a working temperature of 350 °C. The experimental images of the voids have been obtained from Synchrotron Radiation Imagaing Technique and Scanning Electron Microscopy. Since the diffusion limited void growth is found to increase with the greater value of working temperature whereas decrease with the higher magnitudes of viscosity and surface tension of the solder alloys, these properties need to be addressed at high temperature applications. Future works in this area include the addition of the roles of the Intermetallic Compounds and bubble interface coalescences.

Modelling the melting of Sn0.7Cu solder using the enthalpy method

The temperature and velocity distribution during the transient melting phase change in Sn-0.7Cu solder alloy has been modelled using finite element method. The simulation of melting front dynamics has been performed by an Eulerian method, commonly known as enthalpy method. CALPHAD technique is utilized for the computation of enthalpy of the solder material. The flow in the liquid regime of the solder is assumed incompressible and natural convection effects are incorporated through the Boussinesq approximation. The asymptotic viscosity is employed for the mushy zone. For a horizontal finite temperature gradient imposed on the square domain, the mushy zone is thinner at the upper half whereas thicker at the lower of the initially liquid region. Consequently, the melting front within the temperature range 500 K - 503 K moves faster towards the solid in the upper portion of the geometry. The velocity at the peripherial zones of the liquid undergoing convection flow reaches its maximum magnitude of 13 mm/s.

Positive feedback on imposed thermal gradient by interfacial bubbles in Cu/liquid Sn-3.5Ag/Cu joints

Synchrotron radiation x-ray imaging technique was applied for in-situ observation of interfacial bubbles in Cu/molten Sn-3.5Ag/Cu joint undergoing thermomigration. The heating plate temperature was maintained at 350 °C and reflow time of 1 h was considered. Interfacial bubbles at hot side, favoured by wetting transition for growth, increase the effective thermal gradient in the solder medium. The interfacial voids lower the resistance of Sn-based solder joints to thermomigration. The numerical model for effective thermal conductivity and heat transfer in the inhomogeneous medium was implemented using finite element method.

The study of interficial reaction during rapidly solidified lead-free solder Sn3.5Ag0.7Cu/Cu laser soldering

Interfacial intermetallic compound (IMC) is a necessary condition for the reliability of solder connection. In this study, the fiber lasers were used to solder rapidly-solidified lead-free solder Sn3.5Ag0.7Cu and Cu substrate, investigating the influence of laser soldering process parameters on the growth of IMC at the solid/liquid interface and finding the optimum parameters of laser soldering process. To simulate the IMC growth under actual service conditions, the solder joints under the condition p=50w, v=140mm/min were chosen to age at 150°C. Scanning electron microscopy (SEM) and EDS were used to observe IMCs morphology and analyze the composition of IMCs. The results showed that when the laser power was 50w, the thickness of IMCs formed at the interface decreased with the increase of scanning speed. And the morphology of IMCs also changed with the scanning speed. In the aging process, the thickness of the IMCs increased with the aging time, and the morphology became relatively flat. In addition, the thickness of IMCs at the rapidly-solidified Sn3.5Ag0.7Cu/Cu, which was thicker before the aging process, was thinner than that at the as-cast Sn3.5Ag0.7Cu/Cu in the subsequent aging process. The distribution of Ag3Sn particles formed in the rapidly-solidified lead-free solders was more uniform, which suppressed the growth of Cu6Sn5 in the aging process better.

Effects of soldering temperature and cooling rate on the as-soldered microstructures of intermetallic compounds in Sn-0.7Cu/Cu joint

In this study, Sn-0.7Cu solder alloy, being selected as the research object, was allowed to react with polycrystalline Cu substrate at 250°C, 275°C and 300°C. After soldering reaction of 10 minutes, it underwent cooling in the three types of cooling medium: water, air and heating furnace, along with the simultaneous application of high pressure air for blowing away the liquid solder on the top of the intermetallic compounds (IMCs) of the specimens. Scalloped, faceted, prismatic and hexagonal shape Cu6Sn5 grains were observed at the solder/Cu interface, respectively. Scalloped shape grains were found at 250°C. At higher soldering temperatures such as 275°C and 300°C, faceted grains were dominant and the other two were mainly discovered under smaller cooling rates. With these observations, in-depth analyses of the morphology and size of Cu6Sn5 IMC as well as effect of brazing temperature and cooling rate on the microstructure of the joint were made. Moreover, synchrotron radiation real-time imaging technology, was utilized in observing the dynamic growth behavior of IMC during brazing process, thereby, providing direct evidence to the hypothesis regarding the IMC evolution pattern.

Quantitative polynomial free energy based phase field model for void motion and evolution in Sn under thermal gradient

As the electronic packaging and fabrication industries for solar PV cells assembly or panels are making endeavours in using Pb-free solder joints, the reliability of these joints can be taken as an important research topic. Among several aspects, the control of void formation, growth and evolution in lead-free Sn-based solders can be achieved if the underlying mechanism for such phenomena can be modeled. This study employs a quantitative polynomial free energy based phase field method to model the motion and evolution of void in Sn material under thermal gradient. The effects of imposed temperature gradient to the overall migration rate and profile of the void has been assessed in the finite element model.

Study of electrochemical migration based transport kinetics of metal ions in Sn-9Zn alloy

Abstract In microelectronic circuit exposed to humid environment, the growth phenomenon of dendritic deposits due to the electrochemical migration (ECM) of metallic ions, generates a serious reliability issue. ECM of Sn and Zn ions in deionized water with Sn-9Zn electrodes, has been in-situ studied at gap potentials of 3 and 5 V. At 3 V, tin ions (with 15.61 wt%) dominate the composition of metallic dendrites whereas Zn ions (with 20.99 wt%) show greater presence in the experiment with 5 V. The Nernst-Planck transport equation has been solved using finite element method in order to describe the kinetics of ECM of Sn2+ and Zn2+ ions. The composition of resultant dendrites is governed by the competition between advection rate, transport time scale and anodic surface concentration of metallic species.

Effect of the \(\text {TiO}_2\) Nanoparticles on the Growth Behavior of Intermetallics in Sn/Cu Solder Joints

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