Dynamic Behavior And Shape Of Molten Droplet Impact On Planar And Cornaer Structures

The reliability of the electrical joints are closely related to their joint shapes. With the increasing complexity of the packaging structure and the emerging interconnection techniques in electronic manufacturing, traditional shape prediction methods cannot meet the needs of the practical applications. A two-phase flow model based on VOF methods of fluid dynamics has been developed to predict the joint shape in electronic packaging. In the model, the fluid is treated as the mixture of gas and liquid, and only one set of fluid and heat transfer equations should be solved. The free surface is reconstructed from the value of VOF using piecewise linear method. The phase transition is handled using enthalpy-porosity methods. The wetting effect between the solder and the pads is considered by adding a boundary condition of wall adhesion. The governing equations of the moment, the continuity, VOF, and the energy are solved by Fluent software, which is a universal solver of computational fluid dynamics based on finite volume method.The predictive ability of VOF model on the joint shape after the reflow process has been investigated from the aspects of wettablity, the pad structure, the solder mask, bridging, and the complex surface. Results show that the proposed model can predict the case that the solder flow out of the pad under large solder volume condition and the droplet shape evolution during the formation process compared with the tradiational Surface Evolver model. In addition, it is convenient for the VOF model to simulate the solder bridging process and other more complex joint shape as it can simulate the process of solder merging and splitting. The comparison between the results from the VOF model and the experimental or those from the Surface Evolver model indicates VOF model has a high accuracy for predicting the joint shape after reflow process.The dynamic process of solder droplet impact onto the corner pads has been simulated by VOF model. Results show that the droplet deforms more serious along the pad interface direction than other directions. The maximum deformation along the pad interface direction can be used as the criterion to determine if the solder bridging has occurred. During the period from the solder impacting to the maximum solder spread is reached, the kinetic energy decreases, the viscous dissipation increases, the surface energy decreases first and then increases, and the gravitational potential energy has a small value and can be neglected. The maximum shear stress occurs nearby the contact line, while the maximum vorticity and strain rate occur where the droplet shape has a sudden change. To investigate the bubble formation mechanism, the process before the solder contacting with the pads has been simulated using a finer mesh. It is found that the gas under the droplet bottom becomes concave under the action of compressed gas with an increasing pressure, and finally the droplet will capture some gas to form a bubble inside. However, no bubble is observed during the impingement with the impact point offset from the pads interface because of the gas expulsion when the solder spreads from one side pad to the other side. In addition, the effects of the solder droplet size, the impact velocity, and the included angle between the pads are quantitatively investigated. The understanding of these effects is helpful for the controlling and optimization of the solder jetting processThe comparison experiment of the millimeter-sized solder droplet impact on the pre-fluxed and non-fluxed substrates has been designed to investigate the solder bump formation. It is found that the droplet will rebound completely from the pre-fluxed substrate, and a cone-shaped bump forms on the non-fluxed substrate. The predicted results are in good agreement with the results from the validation experiment using a high speed videographer. For the micron-sized solder bump formation, ripples will form on bump surface under the coupling of droplet oscillations and solidification, while a smooth bump will be obtained if there is no droplet oscillation. Solidificaction plays an important role on the solder bump formation. On the one hand, solidification increases the oscillation damping, which leads to a reduction in the oscillation number; and on the other hand, solidification decreases the kinetic energy, which results in the decrease of the maximum spread of droplet. Moreover, solidification weakens the interaction between the solder and the ambient gas or the substrate. The reciprocal transformation among all the energy components during the solder bump formation accords with energy conservation, which can be described as follows: the kinetic energy oscillates damply; the surface energy oscillates first and then decreases; the viscous dissipation increases monotonously, while the gravitational potential energy is very small and can be neglected.Orthogonal experimental design method is applied to analyze the influences of the process factors on the solder bump formation. It is found that impact velocity, droplet size, and initial temperature are significant factors to the bump height, while impact velocity and droplet size are the significant factors to the bump base diameter. The single factor analysis is also performed to distiguish the difference between bump shapes in detail with the variation of one of the following factors: impact velocity, solder droplet size, droplet temperature, substrate temperature, and thermal contact resistance. In this way, the results of single factor analysis can be regarded as the validation of orthogonal experiment, which illustrates the achieved conclusions here are universe and representative. Furthermore, the effects of different process factors on solidification time and surface ripple formation can be explored.