Analysis of the friction processes in ultrasonic wedge/wedge-bonding

Research on wire bonding, the most common chip interconnection technology, has been done for over 40 years by now. The most common description is a model of three phases: friction, approach and interdiffusion (Lang 1988; Osterwald 1999). To quantify this model, with the main aspect at the duration of the friction phase, ultrasonic oscillation was filmed with a high speed camera. A pulsed LED light-source allowed very short exposure times of 2 μs. For the first time it was possible to make the movement of the wire visible in a reflected light exposure. After the friction phase, the end of friction (EOF) occurs. Laser vibration measurements of the tool and the pad amplitude turned out that the EOF correlates with a second, characteristic plateau of the pad amplitude. At this time, the tool amplitude has already shown its characteristic maximum and was attenuated to approximately 80% of the maximum value. After the EOF, when the wedge is bonded to the pad, a bending of the wire towards the wedge in the heel region was observed. This is a possible reason for a weakening of the heel by the US-power. While a change of the wire texture could be proved in recent investigations (Geißler et al. 2006), the exposure of the heel to reverse bending cycle was never verified before.