| As the electronic packaging industries are initiating their shift towards lead free soldering and miniaturization of solder bumps, the reliability of the solder joints has been a topic of significant concern among the researchers. For a liquid tin-based solder attached to a copper substrate or sandwiched between the substrates, the interface of the solder-substrate system becomes the site for planar microbubbles and intermetallic compounds (IMCs). The solder bubbles or voids reduce the effective area of the solder joint surface or section and thus decrease the net strength of the material. Moreover, these voids act as the site of stress concentration and are a risk factor for the failure of the joints and connections. IMCs, more prevalently, Cu6Sn5 are brittle in nature and their thickness is considered a factor affecting the overall reliability of the solder-substrate system. Both bubbles and IMCs, occurring at the interface of the material, characterize the vulnerability of interfacial zone of the solder-substrate system. The study of voids and IMCs in lead free solder-substrate system provides a unified approach for the overall interfacial reliability in the material. In microelectronic devices, the solder joints are subjected to voltage and thermal gradients. Thus, the study of IMCs growth under these field gradients can be a tool for solder materials design. As the effect of thermal and electric field gradients on IMCs are more pronounced in molten solders, the present study deems it more applicable from the accuracy view-point to study the case of reflow soldering.The use of accurate experimental and numerical model in the assessment of interfacial voids and IMCs can enable in the deduction of useful data and inferences.In the present work, an ap-plication software DANPHE based on existing Multiphysics Object-Oriented Simulation Envi-ronment (MOOSE) framework has been developed to perform mesoscale finite element analysis of IMC under field gradients. Additionally, numerical models have been developed on (ⅰ) Elmer-a Finite Elment Method (FEM/FEA) software to depict the bubble growth process and (ⅱ) FiPy-a Finite Volume Method (FVM) software to calculate IMC size under electromigration.The following list enumerates the major contents of the present study:(1) In-situ visualization of the growth of a pre-existing bubble of 20μm on the interface of a liquid Sn/Cu joint was performed at 250°using synchrotron radiation(SR) imaging tech-nique. A FEM model was developed in Elmer software to perform the simulation analysis of this process. It was evidently noticed through the experiment that wetting angle of the bubble over the dynamic IMC changed from an initial obtuse to the right angle (point of bubble’s hemisphericity) and progressively to an acute angle till it assumed a completely spherical shape. In conformance to the experimental observation, the numerical model has predicted that the bubble’s size is greater for bigger contact angle. This finding from the FEA leads to the inference that the bubble growth rate is faster at the initial growth stage and is retarded with the progress.(2) On the basis of impact made by the growing bubble on the IMCs, the latter can be cat-egorized as underdeveloped, perturbed and fully developed IMCs. The underdeveloped IMCs are just beneath the bubble and they cannot grow at all due to the restriction caused by the bubble in Sn supply to the Cu substrate. The perturbed region is the vicinity area of the bubble, where the IMC growth is partially hindered by the void. Finally, the fully developed IMCs are the Cu6Sn5 compounds that are far from the bubble and are virtually unaffected by it. The border zone between the perturbed and fully developed IMC region can be utilized in the prediction of the final diameter of the bubble.(3) The presence of bubbles in the interface of the solder and substrate renders the medium inhomogeneous. The material properties of the solder medium are altered by such interfa-cial bubbles. An Asymptotic Expansion Homogenization (AEH) approach based on FEM has been utilized for estimation of effective material properties-Cu diffusivity and com-posite solder thermal conductivity. These effective properties could be used for assessing the effect of bubble on IMC growth in fields and field gradients.(4) In order to study of IMC growth at the cold end of Cu/molten Sn/Cu joint subjected to thermal gradient, DANPHE application was developed within the MOOSE framework and then FEA was performed for the thermomigration test. The experimentally measured thickness values of the intermetallic compounds in the cold end of solders were obtained through in-situ visualization via synchrotron radiation imaging technique at heating plate temperatures of 250 and 350℃. In the FEA within DANPHE application, the coeffi-cients of the thermomigration term, namely diffusivity(D) and heat of transport(Q*) as well as the parameters in the boundary condition, namely dissolution rate constant (kd) and saturated concentration (Cs) have been modelled as temperature dependent materials properties.Thus, the simulation results of the numerical model,which are in agreement to the experimental observation, could be utilized to explain the pronounced acceleration in thermal gradient enhanced IMC growth in cold end at higher temperatures.In addition to this, it is observed experimentally that, at T=350℃, the thickness of cold end IMC in Sn is larger than that of Sn-3.5Ag, thereby implying that the formation of Ag3Sn compounds in the SnAg solders suppresses the growth of IMC.(5) In the similar way, for conducting electromigration tests at 250℃, pure Sn solder sand-wiched between copper substrates maintained at distances of 450μm and 1.243 mm, were subjected to current densities of 0.56×103 and 3.0×103A/cm2 respectively for 1 hour. The real-time tracking of the anode end Cu6Sn5 IMC thickness was done using CCD camera along with SR x-ray imaging technique. Additionally, the numerical anal-ysis of the joule heating and electrotransport of Cu species was conducted by utilizing DANPHE interface of MOOSE framework.The temperature increase in the liquid solder medium under smaller current density was negligible whereas this increment was around 4℃ for the bigger magnitude of volume current density. The thickness curves exhibited a linear kinetics of IMC growth during electrodeposition. Moreover, larger current den-sity triggers the formation of bigger sized Cu6Sn5 IMC. While the bigger current density is favorable for larger IMC size, the joule heating effect associated with it might reduce the effective charge number in connection to the transport of Cu species and thereby, cause a slight retardation effect in the compound’s growth. For bigger samples, the ini-tial presence of local concentration gradients induces diffusion process that substantially hinders the Cu species transport to the anode whereas the concentration gradients are re-duced at the later stage of the experiment.In such samples, IMC growth rate is relatively faster in the later phase of the electromigraion test. Apart from FEM, a finite volume method(FVM) based numerical model consisting of coupled set of transient-diffusion-source and level-set solvers was developed using FiPy software for calculating the anode end IMC thickness increase. |
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