Thermosonic Ball & Wire Bonding Materials
Thermosonic wire bonding is a well-known process which combines heat, ultrasonic energy and force to bond small wires, typically on the order of one or two thousandths of an inch in diameter and on the order of hundreds of thousandths of an inch in lengths. These wires complete an electrical path from a metalized surface on a microchip to another metalized surface on the substrate of the circuit. Bonding occurs through the process of atomic diffusion.
Platinum (Pt) wire has excellent biocompatibility and good stimulation and sensing properties. This makes it a top choice for use in defibrillators and pacemaker electrodes. In order for Pt wire to be the best choice for an application, the application must require the benefits of Pt’s properties including excellent biocompatibility hardness and stiffness. The application must also be able to support an increase in material cost, an increase in bonding temperature, and an increased potential for damage to the microchip (called “cratering”).
Gold (Au) wire is less expensive than Pt wire, due to the desirability and rarity of platinum. Cost is a relatively significant factor in its selection as a material for interconnecting microelectronic circuits. Gold is also an easier material to wire bond because it is a softer material than platinum, and uses only half the amount of heat to thermosonically bond making its application less limited than platinum.
Gold and Aluminum (Al) wire are used in wedge bonding. Al wire is bonded using an ultrasonic bonding process at room temperature, while Au wire wedge bonding uses a thermosonic bonding process with heating up to 150˚C. The Al wire wedge bonding process is the most common due to the lower cost and lower working temperature.