Nanoscale bondability study on copper-aluminum intermetallic compound using molecular dynamics simulation

In this paper, the growth mechanism of intermetallic compound (IMC) layer between Copper (Cu) free air ball (FAB) and Aluminum (Al) bond pad is carefully examined. The test vehicle is pd-coated Cu wirebonds on Al pad in plastic ball grid array (PBGA) package. Palladium (Pd), the anti-oxide material coated on Cu wire will be blended in the Cu FAB when the ball is formed by an electrical flame-off (EFO). Preliminary results demonstrated that IMC cracks from the edge of bonding interface and spreads into the center area. This is the cause of open fail. The IMC between Cu and Al was initially generated in the form of CuAl₂, and gradually increased the content of Cu and turned into CuAl when the working temperature was increased. The final stage of IMC growth is Cu₉Al₄ and the aluminum pad will be vanished as the result of Cu diffusivity. Bondability on nanoscale IMC of CuAl₂, CuAl and Cu₉Al₄ are also cautiously investigated by using molecular dynamics (MD) simulations. Atomic-level tensile stress and tensile strain are predicted to examine the bonding strength of two IMCs along the bonding interface. Interfacial fracture is different in different tensile speed as well as the working temperature. A series of experimental works and MD simulations are conducted in this research.