Ultrasonic Bonding on Unstable Pin

In power electronics modules, ultrasonic wire bonding is a common method to make electronic connections between the connector pins and the IGBTs. In these modules the connector pins are often residing on top of the plastic frame. Due to the pins being in positions which are hard to reach, clamping of these pins is either suboptimal or not used. This poor or absent clamping combined with the plastic frame's elasticity (softness) means that the pin has more freedom to move compared to the bonding on a metal substrate or IC. In our experiments we measured the pin and the plastic frame displacement with a laser Doppler vibrometer during the ultrasonic heavy wire (400 um in diameter Al wire) bonding process. We measured that the press fitted pin can move laterally along the ultrasonic excitation axis (2.0 ± 0.2) um whereas the frame under the pin moved (0.3 ± 0.1) um. This indicates that the pin slips over the frame while bonding. The slipping of the pin is also visible on the ultrasonic frequency waveforms of the transducer. While molded pins in general are thought to be more stable compared to the press fitted pins, similar behavior is seen in heavy wire bonding where high ultrasonic power is needed. We measured molded frame displacement (0.6 ± 0.2) um while bonding on the pin. In this paper we show how to use process traces and visual inspection to detect unstable pins and how to improve bondability on unstable pins by selecting process parameters that are optimized for the unstable pins rather than stable surfaces.