Stabilization of bonding force during ultrasonic wire and ribbon bonding

To improve the quality of ultrasonic wire and ribbon bonding, the author propose a methodology for stabilizing the bonding force by compensating the rigidity of ultrasonic transducer (UST) mount in the ultrasonic / thermosonic bonding cycle. The author analyze the construction of ultrasonic technological systems and factors affecting the stability of the bonding process. The bonding force is controlled by an electromagnetic unit based on a coil in the field of a constant magnet, the force being directly proportional to the flowing current in the coil. The rigidity of ultrasonic transducer mount was compensated by the data obtained during the preliminary calibration of the change in the mount force over the entire UST overrun range. The calibration in this case is performed with no current flowing through the coil. The force value can be picked up from a digital force sensor. The force values are simultaneously compared with the digitized signal of the deformation sensor. The obtained data is stored in the memory of the wire bonder. In the bonding cycle, after the moment of contact is determined, the drive unit moves the bonding head vertically by the value of a predetermined distance of approximately one diameter of the bonding wire. This causes the movable part of the UST mount to rise and the force to increase. This increase is compensated by the automatic reduction of the current in the electromagnetic coil, which allows maintaining the preset force at the specified level. The bonding force during bonding is compensated in the same way, with the difference that the vector of force compensation changes – the force should increase with an increase in the deformation of the bonding wire. The implementation of the proposed algorithm made it possible to improve the bonding force stabilization to 20% when bonding thick wire, as well as to improve bonding quality. The proposed solution is also applicable in other technological ultrasound bonding systems, including bonding wire with the diameter of <100 microns.

Wire Bonding: The Ultrasonic Bonding Mechanism

Wire bonding is a welding process. During both ball and wedge bonding, wire and bond pad are massively deformed between the bond tool and the anvil of the bond pad or substrate. The dominant variables affecting deformation are ultrasonic energy, temperature, bond force and bond time. Deformation exposes new surface material that is clean and has not been exposed to atmospheric contamination and oxidation. As the new wire and bond pad surfaces mix, they form diffusion couples that grow and transform into the intermetallic weld nugget. The initial mixing is not at equilibrium in that it does not initially form the compounds described by the equilibrium phase diagram, but temperature and time very quickly allows diffusion to relax the initial mixture into the equilibrium phase diagram compounds. This paper will discuss the mechanisms behind the formation of ball and wedge bonds.