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thermosonic bonding
Thermosonic bonding is the most widely used wire bonding method to electrically connect silicon integrated circuits. It was introduced by Alexander Coucoulas in the 1960s.〔Harman, G., Wire Bonding In Microelectronics], McGraw-Hill, Chapt. 2, pg.36〕〔Coucoulas, A., Trans. Metallurgical Society Of AIME, “Ultrasonic Welding of Aluminum Leads to Tantalum Thin Films”, 1966, pp. 587–589. abstract https://sites.google.com/site/coucoulasthermosonicbondalta〕〔Coucoulas, A., “Hot Work Ultrasonic Bonding – A Method Of Facilitating Metal Flow By Restoration Processes”, Proc. 20th IEEE Electronic Components Conf. Washington, D.C., May 1970, pp. 549–556.https://sites.google.com/site/hotworkultrasonicbonding〕 Owing to the reliability of thermosonic bonds, they are used extensively to connect central processing units (CPUs), which are encapsulated integrated circuits that serve as the "brains" of a computer.
Thermosonic bonding is a akin to friction welding, since it creates a scrubbing action at the contact points between the pre-heated deforming lead-wire and the metallized pads of the silicon integrated circuit. In addition, it produces a temporary ultrasonic or acoustic softening in the lead wire, which facilitates its deformation to form the desirable contact area at relatively low temperatures and forces. As a result of the frictional action and acoustic softening induced in the lead wire during the bonding cycle, thermosonic bonding can be used to reliably bond high melting point lead wires, such as those made of gold and lower cost aluminum and copper, with relatively low bonding parameters. This ensures that the fragile and costly silicon integrated circuit chips are not exposed to damaging high temperatures or mechanical stresses during the bonding process.
==History==
Earlier wire bonding methods were thermocompression bonding, which used heat and pressure and ultrasonic bonding,〔Carlin, B. (1960) ''Ultrasonics''. McGraw-Hill Book Co.〕 which used vibratory energy and pressure. Thermosonic bonding improved upon the reliability of the earlier processes by preheating the lead wire and metallized chip prior to introducing the ultrasonic cycle.〔Coucoulas, A., “Hot Work Ultrasonic Bonding – A Method Of Facilitating Metal Flow By Restoration Processes”, Proc. 20th IEEE Electronic Components Conf. Washington, D.C., May 1970, pp. 549–556.https://sites.google.com/site/hotworkultrasonicbonding〕 It resulted in eliminating the incidences of cracking in the fragile and costly silicon chip. The improvement occurred because pre-heating the lead-wire softened it which facilitated its deformation in forming the required contact area during the ultrasonic bonding cycle. Under these conditions, the onset of recrystallization (metallurgy) or hot working of the deforming wire tends to occur while it is forming the required contact area. Owing to hot working during the bonding cycle, the final deformed-bonded wire would be in a relatively soft and stable state. If the wire was ultrasonically deformed at room temperature, it would tend to strain hardened (cold working) and therefore tend to transmit damaging mechanical stresses to the silicon chip. Thermosonic bonding, initially referred to as hot work ultrasonic bonding, was found to work on bonding aluminum and copper wires to tantalum and palladium thin films on aluminum oxide and glass substrates which simulated the metallized chip.
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