Overview
The Uniform Droplet Spray (UDS) process, invented and developed at the DBM group at MIT, has many advantages over the conventional gas atomization and spray forming processes. The UDS process exploits the capillary instability phenomenon of liquid jets for producing uniform liquid metal droplets, which allows for a greater control of the thermal state and mass flux of the droplets. The microstructure of the deposit can be readily tailored by controlling the heat and mass fluxes of the droplet spray. Many possible applications of the UDS process exist. These include rapid prototyping, spray forming, spray coating, and uniform metal powder production.
The actual process of generating metal droplets requires specialized equipment due to the very high temperatures involved. Solid stock metal is placed within specially made crucibles. The design and material of the specific crucible is dependent upon the metal that is melted. Most lower melting temp metals, like Tin, are melted in stainless steel crucibles, while higher melt temperature metals like Aluminum are melted in carbon crucibles. At the bottom of the crucible is an orifice that will produce a characterisitic droplet size based on the orifice's diameter. These crucibles are then heated by running current through resistors in contact with the crucible surface or internally mounted within the crucible. A thermocouple is attached for closed loop temperature control.
After the steady state melt temperature is reached and the metal is fully liquified, the chamber in which the spraying will occur is either evacuated or flooded with an inert gas. This aids in the uniform break up and surface oxidation properties of the droplets. To force the droplets out of the crucible a piezo stack actuates a shaft that extends into the crucible. A function generator supplies a signal to a strobe light and to an amplifier that provides the amplified signal to the piezo stack. The frequency at which the metal is agitated affects the mass flow and droplet size. When the stream is initiated, the crucible is slightly pressurized to force out the droplets. After exiting the orifice, the stream of droplets is charged.
To charge the droplets, a voltage is applied to a charging ring immediately below the orifice. This charge induces an opposite charge on the stream, and as the droplets are ejected they retain the induced charge. The induced charge further aids droplet break up and dispersion. To obtain a steady stream of droplets, a visual inspection of the stream with the aid of the strobe light is conducted. When the stream appears to freeze into individual droplets that are stationary, the stream is in stedy state.
This is the starting place for most of the research in the DBM lab. The other projects springboard off this general technique.
The DBM lab is currently working on projects that use the UDS technology.
The first project involves the application of UDS for multi-orificial flows. Future applications will call for more than a single droplet stream. This research involves complex modeling to predict spray distributions of charged droplets from several streams. The droplets not only experience drag and gravinometric forces, but the repelling coulombic forces that result from similar charge. Questions regarding this research should be directed to Jeanie Cherng.
The second project is an control system that controls the production of BGA solder balls. Questions regarding this research should be directed to Wayne Hsiao.