ABSTRACT

The Uniform Droplet Spray (UDS) process, developed in the Laboratory for Manufacturing and Productivity at MIT, offers many advantages over such conventional metal droplet production processes as gas atomization. The process exploits the acoustically-amplified capillary instability phenomenon of laminar liquid jets to produce uniform liquid droplets, which allows for greater control of the thermal state and mass flux of the droplet spray. 

The present work investigates the varios basic phenomena associated with teh UDS process. The thesis comprises three primary parts. The first part investigates the break-up behavior of molten metal jets and compares the results to theories by Lord Raleigh and others. This was achieved by spraying pure tin into a chamber containing less than 5 ppm of oxtgen. The second part investigaes the viability of the UDS process for producing large solder balls that can be used for Ball-Grid Array (BGA) Integrated Circuit packaging. it also examines whether the balls so produced are comparable to or better than those produced by conventional processes. The final part investigates the role of surface oxidation in the break-up behavior of liquid metal jets. Again, pure tin was used in this part of the investigation to study the effect of surface oxidation on jet break-up. 

From the experimental results and theoretical considerations it is shown that: (i) the break-up behavior of a molten metal jet can be explained by Rayleigh's theory, provided that there is sufficiently low oxygen environment, (ii) large solder calls can be produced cost effectively by the UDS process for BGA packaging, and they are comparable to or better than those that are produced by conventional processes in terms of dimensional tolerance and microstructure, and (iii) the jet of molten tin does not break into a train of uniform droplets if the spray chamber contains more than 1500 ppm of oxygen. Implications of these results to the desin and control of the UDS process are discussed and research topics for further investigation are suggested.

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