A fundamental understanding of the fluid flow and heat transfer that occurs during the spreading and solidification of a molten droplet on a substrate is crucial because of its potential applications in many industrial areas....[ Read more ]

A fundamental understanding of the fluid flow and heat transfer that occurs during the spreading and solidification of a molten droplet on a substrate is crucial because of its potential applications in many industrial areas.

In this research, thermal contact heat transfer in a rapid contact solidification process was studied. An interfacial temperature sensor, which has a junction thickness of only 1-μm, was developed by utilizing micro-fabrication technique. A sudden falling experiment was conducted as a simulation of a rapid contact solidification process by employing the special sensor to record the rapid temperature changes at the substrate surface. A concise procedure was proposed to determine the interfacial thermal contact conductance from the temperature measurement. The thermal contact conductance during the rapid contact process of the molten metal (Indalloy-158) with the copper substrate was determined. The influence of the initial state of the molten metal on the thermal contact conductance was investigated. A simple correlation for the thermal contact conductance during a rapid contact solidification process was developed.

By introducing the simple correlation into the numerical simulation, for the first time, a non-constant thermal contact conductance, which varies with time and position, was used to simulate the spreading and solidification of a molten droplet impinging on a substrate. Experiments were performed to study the final state of a droplet deposited on a substrate. Qualitative agreement between the numerical and experimental results demonstrated the validity of the present method to use a variable thermal contact conductance. As the thermal contact conductance is self-adjustable, the present method has general utility under different operation conditions.

Numerical simulations were finally applied to examine the behaviour of the micro-droplet soldering process. The effects of the droplet impact velocity, the droplet diameter, and the substrate material on the solder bump shape were evaluated. These simulations could be used to find the optimum process conditions that yield the desired bump shapes, which should aid in the further development of this novel micro manufacturing approach.