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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 bu...
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... It may be inferred that the formation of bubbles and interfacial voids at the interface can affect the growth of the IMC. As studied by Kunwar et al., the formation of bubbles at the interface could hinder the diffusion of Cu at the area of the bubbles and the resulting poor strength of the solder joints [17]. As per Figure 4e,f, most primary Cu 6 Sn 5 formed are the "in-plane" branched type, and some of the primary crystals that start nucleates near the interface area are formed in the faceted hollow rods. ...
The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid–liquid–solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.
... numerical simulations in MOOSE software. Two different cases of initial conditions are chosen for performing the numerical simulation, and the results are presented in Fig. 4. The images (a1)-(b1) inside the left box of the figure, correspond to the study of wetting transition in a single bubble at liquid-solid interface, that has been reported in Kunwar et al. Kunwar et al. (2015). The initial condition for wettability parameter (α s ) is set as − 0.15 (corresponds to initial contact angle of 76.5 • ), and the bubble attains an initial morphology of image (a1). Then the simulation is allowed to proceed at α s =-0.677 (set at material property section of the simulation) till the bubble attains a more spherical sha ...
Designing means and methods to detect the presence of interfacial bubbles and intermetallic compounds (IMCs) during hot dipping solder coating of Cu ribbon, can help in the production of defect-free PV ribbons. A mechanistic study of Cu6Sn5 IMC grain growth and bubble morphology evolution at the solder-substrate interface is performed with phase field simulation. A machine learning model is utilized to identify the occurrence of bubble(s) and IMC at the material interface of liquid solder and solid Cu. The datasets for the microstructural images consisting of bubble(s), IMC and planar solder/Cu interface are generated using in situ synchrotron radiation (SR) imaging experiment techniques. The integration of in situ SR radiography based non-destructive testing experiments with convolutional neural network model to intelligently detect the interfacial microstructures paves the path for potential industrial application of this technique in the smart manufacturing of defect free and reliable PV ribbon material.
... For Cu 6 Sn 5 IMC material, µ and E are taken as 0.31 and 112.3 GPa respectively [18,19,20,21]During the growth of IMC during interfacial reaction, there is the development of stress and so volumetric body stress of 306.5 MPa per unit volume is exerted on the RVE. The solution of Eq. 9 obtained by using finite element method in Elmer multiphysics software [22], gives the von Mises stress distribution as shown in Fig. 10. The magnitude of simulation results are in agreement with those of [23]. ...
In order to investigate the effect of \(\text {TiO}_{2}\) nanoparticles on growth behavior of interfacial \(\text {Cu}_6\text {Sn}_5\) intermetallics compounds (IMCs) in Pb-free Sn/Cu system, the solder joints are fabricated by using flux doped with different content (0.0–2.0 wt% of flux) and particle diameter (5 nm and 50 nm) of \(\text {TiO}_{2}\). In context of isothermal reflow soldering at 250 °C and subsequent air cooling, the increase in reflow duration from 10 to 120 s was characterized with an increment in IMC layer thickness and grain size, due to the enhancement of Cu flux contribution for Ostwald ripening during constant temperature reflow and precipitation kinetics during cooling. The increased proportion of \(\text {TiO}_{2}\) nanoparticles in flux was found to reduce the growth of IMC layer and grain size. The suppression effect on IMC was more pronounced for 5 nm particles as compared to the 50 nm \(\text {TiO}_{2}\). The \(\text {TiO}_{2}\) nanoparticles, adsorbed on IMC plane can retard the growth of the latter. Presence of sufficient amount of a given sized \(\text {TiO}_{2}\) nanoparticles among IMCs, by increasing the effective stress at the localized interfaces, and causing the breaking of brittle \(\text {Cu}_6\text {Sn}_5\) during growth stage; can help in the inhibition of IMC whisker formation. Particle diameter and mass proportion of \(\text {TiO}_{2}\) nanoparticles are important for soldering materials design.
... The environmental and human health related benefits of lead-free Sn-based solders are the motivating factors to encourage the researchers and technologist to study about these alloys. Some of the key reliability issues for solder joints, such as Sn electrodeposits and whiskers [1,2], voids and bubbles [3], thermomigration [4] and electromigration [5], introduced by the reaction between Sn based alloys solder and Cu substrate, which are related about the formation of intermetallic compounds (IMCs) [6,7]. The topic of IMC is considered in the scope of this present study, and particularly, Cu 6 Sn 5 intermetallic compound was generalized as IMC owing to its rapid occurrence during reflow soldering of solder joints. ...
... With the initial condition and BCs defined for the system, Eq. 3 was solved along with Navier-Stokes equation using finite element method (FEM) in Elmer multiphysics software [3,42]. Since the co-ordinate of the geometry was scaled by 1.0 × 10 −3 in the finite element analysis (FEA), the time-step size was accordingly scaled to 0.001 s. ...
The growth behaviour of Cu6Sn5 intermetallic compound (IMC) at the solder height controlled post-spread Sn/Cu interface is investigated for different initial solder ball volumes, reflow temperatures and cooling rates. Because of the limited solder thickness at the periphery, the IMC retains a preferential scalloped morphology, even after cooling. For solder balls of initial diameters of 500, 1000 and 1700 μm, with the maximum solder height at the peripheral regime not exceeding 150 μm, reflowed at 250 °C and undergoing air cooling, it has been revealed that IMC characterized with larger layer thickness and grain diameter, correspond to the sample of smaller ball size. The increase in reflow temperature for a solder of initial size of 500 μm, is characterized by the increase in IMC thickness, developments of few but quite large faceted planes over the original scalloped morphology and non-uniformity in the grain diameter. In contrary the to air cooling (cooling rate ≥ 4.0 K/s), the IMCs obtained at the thin film zone, for furnace cooling (cooling rate = 0.037 K/s), are very larger, both in grain size and layer thickness. Moreover, the scalloped Cu6Sn5 surface in furnace cooled specimens, bear many but tiny facets. Finite element analysis is utilized to numerically estimate the diffusion of Cu into the geometrical volumes of solder. Ostwald ripening and film height are the important parameters defining the growth behavior of the compound in miniaturized solder joints.
... ∇.σ∇φ = 0 (5) wherein, σ is the electrical conductivity of DI. In order to get solution values of C k , the coupled set of Eqs. 4 & 5 is solved using finite element method (FEM) in Elmer Software [24], [25]. The representative element for the finite element domain is taken as a square of length 300 µm with triangular mesh. ...
... In context of the top curvature surface boundary of the solder, no flux BC is applied for the concentration variable. Having initial condition and BCs defined for the system, Eq. (1) was solved for a simulation time of 60 s using finite element method (FEM) in Elmer multiphysics software [34,35]. Since the co-ordinate of the geometry was scaled by 1.0 × 10 −3 in the finite element analysis (FEA), the time step size was accordingly scaled to 0.001 s. ...
... In order to apply differentiated heat transfer boundary conditions at the interface of solder and substrate, the mesh at the bottom of the solder is partitioned within Salome software into two surfaces -central circular face of radius 1 mm (Region I) and peripheral circular band with width of 600 µm (Region II) as shown in Fig. 3(a). Elmer multiphysics software [20,21,22] is utilized for performing numerical solutions using finite element method (FEM) whereas postprocessing and visualization tasks are implemented in Paraview 5.0.1 [23] software. ...
Pure Sn solder balls of initial diameter of 2 mm, having been reflow soldered on Cu substrate at 573.15 K for 1, 2
and 5 min, were subjected to furnace cooling. Upon the start of reflow, the initially spherical solder balls un-
derwent a rapid radial spreading and wetting on Cu substrate and thus attained non-spherical geometry with a
maximum height at the center whereas a minimum one at the periphery. The wetting or interfacial reaction, that
would produce an even scalloped intermetallic compounds (IMCs) during isothermal reflow, resulted in the
growth of dense prismatic IMC rods at the central region during cooling whereas the peripheral scalloped IMCs
were not overriden by the faceted or prismatic rods during the temperature reduction procedure. Reduced
interfacial heat transfer at the central interface, existence of radial isotherms and lowered supply of Cu pre-
cipitates at the peripheral interface owing to the compromised solder height have been discussed as the factors
responsible for the segregated IMC morphology. The abundant precipitation of Cu and the presence of thermal
gradient, sustain a screw dislocation based growth of hexagonal IMC whiskers. The increase in reflow duration is
associated with the enhanced tubularity of the rods. Finite element method has been utilized to accomplish
numerical simulations for heat and mass transfer in solder phase as well as for anisotropic thermal conduction at
the dislocation in evolving IMC.
... In such context, the use of in-situ method such as synchrotron radiation technique has a great merit of correctly characterizing the experimental results. Several research works have been done hither to regarding the use of in-situ synchrotron radiation X-ray monochromatic beam for studying intermetallic compounds and bubbles at the interface of liquid solder and solid copper substrate [7][8][9][10][11][12][13]. However, most of the works stress on the images obtained during the reflow heating or they explicitly highlight the pictures during heating stage. ...
The intermetallic compound growth in Sn/Cu and Sn–3.5Ag/Cu solder joints undergoing cooling has been in-situ observed using synchrotron radiation X-ray imaging technique. The overall thickness of intermetallic compound attained during cooling condition is dependent on the rates of Cu precipitation or deposition from the bulk solder and Cu diffusion from grain boundary at interface. Although the net increase in IMC thickness contributed predominantly by deposition kinetics is greater for air cooling than in furnace cooling from the start temperature of 300∘C for the first 20 min, the former solidifies before 30 min and the latter stays in liquid state for 1 h due to slower cooling rate and attains a bigger IMC of size about 14.5 μm. In context of Sn–3.5Ag solders subjected to air cooling from 275 ∘C, the presence of Ag contributes to the increment in overall IMC thickness during the cooling period. For the improvement in solder joints reliability, faster cooling rate and limiting the Ag content can be employed as the materials design and processing parameters.(http://dx.doi.org/10.1007/s00339-016-0543-4)
The liquid/solid (L/S) interface of dissimilar metals is critical to the microstructure, mechanical strength, and structural integrity of interconnects in many important applications such as electronics, automotive, aeronautics, and astronautics, and therefore has drawn increasing research interests. To design preferential microstructure and optimize mechanical properties of the interconnects, it is crucial to understand the formation and growth mechanisms of diversified structures at the L/S interface during interconnecting. In situ synchrotron radiation or tube-generated X-ray radiography and tomography technologies make it possible to observe the evolution of the L/S interface directly and therefore have greatly propelled the research in this field. Here, we review the recent progress in understanding the L/S interface behaviors using advanced in situ X-ray imaging techniques with a particular focus on the following two issues: (1) interface behaviors in the solder joints for microelectronic packaging including the intermetallic compounds (IMCs) during reflow, Sn dendrites, and IMCs during solidification and reflow porosities and (2) growth characteristics and morphological transition of IMCs in the interconnect of dissimilar metals at high temperature. Furthermore, the main achievements and future research perspectives in terms of metallurgical bonding mechanisms under complex conditions with improved X-ray sources and detectors are remarked and discussed.
