Room-Temperature Bonding with Pd Coated Cu Wire on Al Pads: Ball Bond Optimization with 2-Stage Methodology

Abstract

Wirebonding performed at elevated temperatures is the standard interconnect process for integrated circuits, typically with the use of low-cost copper bonding wire. However, for specific applications it is necessary for wire bonds to be reliably joined at room-temperature. This paper details the development of a room-temperature ball bonding process using a 2-stage optimization method. The first stage optimizes ball geometry by applying a 32 design of experiment to bonding parameters impact force (IF) and electric flame-off (EFO) current. In the second stage bond shear strength is optimized by stepwise increase in ultrasonic amplitude. Target ball bond values were attained at optimized parameters: IF of 1331 mN, EFO current of 59.9 mA, and an ultrasonic amplitude of 26.46 US%. Pad lift during bonding was observed at excessive ultrasonic amplitudes above 40 US%, as determined by optical images at the bond interface. Bonding parameters at room-temperature (23°C) were increased when compared to a high temperature process (175°C) to account for reduced thermal energy. For the same geometry at room-temperature a 7 % increase to impact force was required. EFO current levels remained relatively constant between the two bonding temperatures. For the same shear strength at room-temperature a 18 % increase in ultrasound amplitude was required. The confirmed average shear strength achieved via the room-temperature process was 116 MPa. Higher values are possible.