Experimental Study of Thermosonic Gold Bump Flip-Chip Bonding With a Smooth End Tool

Thermosonic flip-chip (TSFC) bonding is a flexible and rapidly developing package technology, in which ultrasonic energy is used to reduce bonding temperature, time, and force. In this paper, a laboratory TSFC bonder with a smooth end tool and without vacuum or a die collet was developed to bond a gold bump to a silver pad in order to understand the effect of ultrasonic energy on flip-chip bonding strength. With a laser Doppler vibrometer, the ultrasonic vibration of the tool and flip chip were measured during the bonding process. In addition, the energy loss between the tool and chip was observed. The relationship between energy loss, ultrasonic propagation, bump deformation, and bonding parameters was discussed. The effects of bonding parameters on bonding strength and vibration transferred from the tool to chip were experimentally investigated. Results show that energy losses prevent over-bonding phenomenon caused by an excess of ultrasonic power. Good bonding strength can be achieved by optimizing bonding parameters using a tool without a vacuum or collet. Compared to vacuum-based or groove tools, smooth-end tools are able to automatically control the ultrasonic energy applied at the bonding interface.