Advanced finite element model on Copper wire ball bonding

The mechanism of wirebonding bondability for ultra thin Copper wire is described in this research. Two major analyses are conducted in the present paper. In the first, the characteristic of heat affected zone (HAZ) and free air ball (FAB) on thin Copper (Cu) wire have been carefully experimental measured. Thin film interfacial micro-tribology behavior between Cu FAB and Al pad is examined by Atomic Force Microscopy (AFM). Secondary, the dynamic response on Aluminum (Al) bond pad and beneath the pad during wirebonding process has been successfully predicted by finite element analysis (FEA). Tensile mechanical properties of ultra thin Cu wire before/after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Experimental obtained hardening constant in Hell-Petch equation has significantly influence on the localize stressed area on Al pad. This would result in Al pad squeezing (large plastic deformation) around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB is also carefully investigated by nano indentation instruments. A real-time secondary EFO scheme has been conducted to reduce the strength of Cu wire and increase the bondability. All the measured data serves as material inputs for the finite element model based on explicit software ANSYS/LS-DYNA. A series of comprehensive FEA parametric predictions have been performed in this paper.