Dewen Tian

Effect of solidification on solder bump formation in solder jet process: Simulation and experiment

To investigate the influence of the solidification on the solder bump formation in the solder jet process, the volume of fluid (VOF) models of the solder droplets impinging onto the fluxed and non-fluxed substrates were presented. The high speed camera was used to record the solder impingement and examine the validity of the model. The results show that the complete rebound occurs during the process of the solder droplet impinging onto the fluxed substrate, whereas a cone-shaped solder bump forms during the process of the solder droplet impinging onto the non-fluxed substrate. Moreover, the solder solidification results in the lift-up of the splat periphery and the reduction in the maximum spread factor.

Three-dimensional modelling of solder droplet impact onto a groove

Solder jet technology is widely applied in MEMS packaging. To optimize and control the packaging process, a three-dimensional model was developed to investigate the process of solder droplet impact on a groove with two rectangular pads. In this model, a volume-of-fluid (VOF) method was explored to track the droplet surface and to account for the presence of inertia, viscosity, gravitation, surface tension and wetting effects. The phenomenon of bubble formation was investigated. The numerical model was validated with visualizations of the transient impact processes. The results show that the deformation along the groove is greater than that along the pads. The interactions of the solder gas and solder pad have a significant influence on the droplet shape. The kinetic energy, surface energy and viscous dissipation are the main energy components that influence the impact dynamics, whereas the gravitational potential energy can be neglected. The influences of the impact velocity, droplet size and groove angle on the droplet impingement were quantitatively evaluated.

Effect of thermal aging on microstructure, shear and mechanical shock failures for solder ball bonding joint

The interfacial microstructure evolution between Sn-3.5Ag-0.75Cu solder ball and Au/Ni/Cu pad was investigated under as-bonded and thermal aging conditions. Moreover, the shear and mechanical shock tests were carried out to study the failure mechanisms. Results showed that only one intermetallic compound, fine needle-like AuSn4 was found at the solder/pad interface under as-bonded condition. After aging, Ag3Sn appeared in the solder bulk as a particle-like or elongated structure. Meanwhile, the intermetallic compounds layer became flat and a quaternary intermetallic compound Cu-Sn-Ni-Au appeared between AuSn4 layer and Ni(NiFe) layer. In addition, parts of AuSn4 spalled into the solder bulk with long hour aging. In the shear test, the solder joint exhibited a ductile failure. While in the mechanical shock test, the cracks initiated at the right end of the horizontal pad and propagate through the interfaces between IMCs. Furthermore, the factors dominating the failure mode were investigated which consists of external force and stress distribution, interfacial reaction and strain rate

Modeling of Micropitch Shift of a Magnetoelectrical Sensor During Laser Solder Ball Bonding Process

With the development of optoelectronics and microelectromechanical systems (MEMS) packaging, laser soldering has become an extensively used interconnection technique in electronic manufacturing industry. Postsolder shift in assembling of such components is the most challenging issue to affect the packaging yields. To maintain a high coupling efficiency or accuracy, tight control of postsolder shift is required. In the present work, a 3-D thermal-mechanical coupled finite element model was developed to investigate the time-dependent pitch shift induced by the laser solder ball bonding process. This model accounted for the laser interaction, the heat conduction, the thermal induced deformation and the phase change of the solder and could reflect the actual laser soldering process. The modeling results show different deformation mechanisms in the prebumping and reflow processes. The pitch shift is mainly induced by the thermal shrinkage of solder. The pitch angles obtained from the finite element analysis are in good agreement with those from the measurement using laser goniometer.

Effect of Thermal Aging on the Microstructure Evolution and Solder Joint Reliability in Hard Disk Drive Under Mechanical Shock

The effect of thermal aging on the microstructure evolution and solder joint reliability in hard disk drive (HDD) under mechanical shock was investigated. Significant coarsening of Ag3 Sn particles was found in SnAgCu solder, and AuSn4 intermetallic compound (IMC) changed from needle-type to layer-type during aging. For as-soldered SnAgCu solder joints after mechanical shock, the cracks were initiated in AuSn4 at the corner of the solder joints, and mainly propagated along the thin Ni3 Sn4 IMC layer. After aging at 150 degC for 21 days, the cracks were mainly propagated along the solder, Ni3 Sn4, Au-Sn-Ni-Cu, and Au-Cu-Sn. The significant coarsening of microstructure was found in SnPb solder joints, and only microcracks were found on the surfaces of as-soldered and aged solder joints after mechanical shock.

Prediction of Micro Pitch Motion of a Microsensor Component during Laser Soldering Interconnection Process

Generally, the purpose of soldering is to provide a robust joint. However, for a microsensor assembly, precise position or appropriate pose of the component in operation is also a major concern to ensure monitoring the target effectively. In the present work, a three-dimensional finite element thermal-mechanical indirectly coupled model was developed to investigate the pitch motion induced by laser irradiation during the soldering process. Two kinds of subroutines were designed for modeling the behavior of the laser and the elements above the melting point. The laser soldering includes two independent processes: pre-bumping and reflow. Results on both processes indicate that the pitch angle becomes to be dominant during the solder solidification. Two different pitch motion mechanisms in pre-bumping and reflow were discussed in detail. Pitch angle obtained from the finite element analysis is in good agreement with that from the experiment

Numerical simulation of solder spreading and solidification during solder jet bumping process

A VOF model is developed to simulate the solder spreading and solidification during solder jet bumping process. This model is based on fixed mesh method, and accounts for the surface tension, wetting effects, and heat transfer with solidification. The visualizations of the transient impact processes are employed in order to compare and validate the numerical model presented. Results show the spreading and recoiling process coupled with the solidification leads to a final cone-shaped solder bump. The variation of gravitational potential energy in the impingement is too small to be neglected. The simulated results are in excellent agreement with the photographic images.

A Computational Model of a Molten Solder Droplet Spreading on Orthogonal Pads

A dynamic model based on energy balance method for describing the solder profile evolution on orthogonal pads is proposed. This model combines the fluid mechanics and the surface physics and accounts for the inertial, viscous and gravitational forces, the interfacial tensions and the equilibrium contact angle of the spreading system. A corresponding computational scheme is developed without invoking the spherical cap assumption. In order to verify the model predictions, the high speed videography technique is used to capture the instantaneous solder profile. The results show that the model prediction is in good agreement with the experimental spreading process. The characteristic of this model is that it can predict not only the equilibrium shape of solder joint but also the instantaneous shape during the spreading process.