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ARTICLE:
Ira M. Cohen
Department of Mechanical Engineering and Applied Mechanics, School of Engineering and Applied Sciences, University of Pennsylvania, Philadelphia, PA 19104-6315 Portonovo S. Ayyaswamy
School of Engineering and Applied Science, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, 297 Towne Building, 220 S. 33rd Street, Philadelphia, PA 19104-6315
ABSTRACT Low energy plasmas are those for which the bulk temperature may be only slightly elevated from the ambient although the electron temperature may be tens of thousands of degrees Kelvin. This situation occurs when a gas is only weakly ionized (1CH or less) but large electric fields may be present to impart energy to electrons. This kind of plasma has found an interesting application in melting the tips of ultrafine wires as described below. The properties and description of low energy plasmas are quite different from those of welding plasmas, for example, and have heretofore received much less attention in the heat transfer literature.
In the assembly of microelectronic chips, the most reliable means to connect the circuitry in the chip to terminals that connect with the outside world is by a process called wirebonding. This is done by automated machinery in which an electric discharge (called an electronic flame off or EFO) is used to heat and melt the end of a very fine (25 μm diameter) wire. The liquid metal surface tension causes the molten metal to roll up into a ball. A special capillary tool then squashes and welds this ball onto a bond pad on the chip. A loop is formed to a second bond made on a mounting frame with leads to connect to the outside world. We will review here the processes from the initial breakdown of the gap between the wire end and an auxiliary electrode called a wand, to the steady state arc discharge, the heating and melting of the wire, the subsequent roll up and ball formation, and final cooling. We will follow by numerical simulation the formation of the plasma, treat the heat transfer from the plasma to the wire, track the phase change fronts in the nascent ball as it rolls up, and back as die ball cools and solidifies. Accounts of both enlarged scale and real scale experiments to validate the computational simulations will be given.  download article
27-78 pages
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