Failure Mechanism of Fly Die in Eutectic Bonding

Eutectic materials are widely used in high-temperature and high-reliability applications due to their excellent thermal and mechanical properties compared with other bonding materials. However, the high bonding temperature and high yield strength of eutectic materials can also cause some reliability issues during the die bonding process, such as die cracking and fly die. The fly die issue, which refers to the phenomenon of silicon die flying away from the bond pad after the collet moves away, is thoroughly studied in this paper. This paper emphasizes the following two aspects: 1) evaluation of the bonding strength when fly die happens and 2) finding out the driving forces for this phenomenon. It was found out that the expansion of trapped air in the bondline cavity is the key driving force. As the eutectic die bonding temperature is usually very high, the trapped air will expand and generate high pressure. Meanwhile, the eutectic alloy is still in liquid state when the collet moves away, which results in a weak bonding strength. When the potential energy stored in the trapped air breaks the weak bonding of the liquid eutectic, the die will fly away. Fly die most possibly takes place when the wetting area is less than 18% of the overall die bottom surface according to thermodynamic calculation. This theory was supported by both experimental observations and computational fluid dynamic simulations. Optimization methods were finally proposed to reduce die flying risk based on the knowledge obtained.