Alexander Coucoulas (Father Of Thermosonic Bonding*)
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Ultrasonic welding of aluminum wires to tantalum thin film circuits (1400A) deposited on glass substrates resulted in low strengths that was related to strain hardening of the deforming lead wire which was believed to have caused the nucleation of cracks in the glass substrate. The deformation of the wire was required to obtain a sufficient contact area. In order to eliminate the glass failure mode, the bond region was preheated to temperatures where a minimum amount of strain hardening occurred during the ultrasonic bonding cycle.
By combining thermal and ultrasonic energy the wire deformed with a minimum amount of strain hardening and showed that recrystallization occurred in the deformed wire. This was evidence that the wire was deforming near or at the hot working as opposed to cold working temperature during the bonding cycle. Finally, the bond strength significantly improved with no evidence of glass substrate cracking when thermal and ultrasonic energy was used to form the bonds.
This investigation was directed at developing a reliable bonding process for connecting wires to silicon integrated circuits referred to as the “Chip”. It resulted in a new wire bonding process known worldwide as “Thermosonic Bonding”.
*as stated by Harman, G., Wire Bonding In Microelectronics”, 3rd Edition, McGraw-Hill, 2000