Clean metallic surfaces have high surface energies and
make strong metallic bonds when in contact [13]. However,
direct contact of fresh, clean surfaces is generally screened
by contamination and oxide layer. Hence, contamination
and oxide layer at the contact interface is an important
factor to be considered in determining the effects of the
parameters on bondability. In the ultrasonic bonding
process, the contamination and oxide layer will be broken
down or removed by the ultrasonic induced vibration at
interface. Therefore, for analysis convenience, the ultrasonic
ball bonding process could be arbitrarily divided into
three main stages. The first stage involves the removal of
the contamination and oxide layer covering the interface
between bump and pad. Secondly, as the contamination
and oxide layer is broken down or removed, the underlying
fresh metal surfaces are brought into contact under a
normal force. Thirdly, interatomic forces will come into
operation and atomic bonding will occur. Intermetallic or
interdiffusion processes, if any, will also occur at this stage.
Such understanding and the experimental results described
in the previous section indicate that the slip area instead of
the entire contact area at interface plays a crucial role in
determining bondability