Thermosonic Bonding

Thermosonic Bonding is widely used to wire bond silicon integrated circuits into computers. Alexander Coucoulas was named "Father Of Thermosonic Bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. Owing to the well proven reliability of thermosonic bonds, it is extensively used to connect the central processing units (CPUs), which are encapsulated silicon integrated circuits that serve as the "brains" of today's computers.

A Thermosonic bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies. Figure 1 shows a diagram of a Thermosonic Bonding machine which includes a magnetostrictive or piezoelectric-type transducer which is used to convert electrical energy into vibratory motion which is known as Piezoelectricity. The vibratory motion travels along the coupler system, a portion which is tapered to serve as the velocity transformer. The velocity transformer amplifies the oscilliatory motion and delivers it to a heated bonding tip. It is akin to a friction bond, since the introduction of ultrasonic energy (via a bonding tool vertically attached to an ultrasonic transformer or horn) simultaneously delivers a force and vibratory or scrubbing motion to the interfacial contact points between a pre-heated deforming lead-wire and the metallized pads of a silicon integrated circuit (Figure 2). In addition to the delivery of thermal energy, the transmission of ultrasonic vibratory energy creates an ultrasonic softening effect by interacting at the atomic lattice level of the preheated lead wire. These two softening effects dramatically facilitates the lead wire deformation by forming the desirable contact area using relatively low temperatures and forces. As a result of the frictional action and ultrasonic softening induced in the preheated lead wire during the bonding cycle, thermosonic bonding can be used to reliably bond high melting point lead wires (such as gold and lower cost aluminum and copper) using relatively low bonding parameters. This ensures that the fragile and costly silicon integrated circuit chip is not exposed to potentially damaging conditions by having to use higher bonding parameters (ultrasonic energy, temperatures or mechanical forces) to deform the lead wire in forming the required contact area during the bonding process.

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