Acoustic softening characterization to improve copper wire bonding FEM simulation

Wire bonding remains the dominant technique for connecting integrated circuit chips to metal lead frames or organic substrates in semiconductor packaging. This semiconductor packaging process consists forming a ball at the end of a gold, copper or silver wire, crushing and welding it onto a pad using ultrasonic energy. Typically the bond pad material is made of aluminum (A1) top metal layer of some micron thickness, vias, other metals and dielectrics layers that may be very sensitive to deformation and cracks. The wire and Al-pad bonding is achieved through the application of ultrasonic vibration, compression force, and heat forming new intermetallic phases which secures the required adhesion. Hard wires development, specifically Cu wires, has driven extensive research on characterizing the ball-pad interface and enhancing bond pad robustness. Harsh bonding experiments on different pad structures have been performed to identify optimal metal-oxide stack-ups and layouts to prevent cracks. These studies compare the crack response of different metal structures highlighting the importance of underlying Al-based metal structures and their density to pad strength. Bond pad integrity is evaluated through cratering tests, where the ball and pad metal are etched away, followed by an optical inspection of the underlying oxide for damage [1]. Extensive Design of Experiment (DoE) on bonding settings must be performed to improve copper wire bondability on different pad structures and to investigate the bonding settings on the crack formations under the bond pads [2].