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.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.
Earlier wire bonding methods were thermocompression bonding,which used heat and pressure and ultrasonic bonding,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.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.
