Process characterization of Cu & Pd coated Cu wire bonding on overhang die: Challenges and solution

Wire bonding continues to be the predominant interconnect method in IC packaging. Though copper (Cu) wire bonding has been evaluated for more than a decade in replacing gold (Au) wire bonding, it is still a challenging process to adopt on several application fronts. Cost motivation is driving factor, the Cu wire bonding are replacing the Au wires in the assembly packaging production floors. Recent emergence of palladium (Pd) coated Cu wire has created great interest with the expectation to achieve enhanced bond ability and reliability over Cu wire. Cu wire bonding is being evaluated for various packaging configurations and packages. For this paper, Cu wire and Pd coated Cu wire bonding was evaluated on overhang die. Thin overhang die wire bonding imposes many challenges on the bonding process since the die could bend and twist during the wire bond process. Cu wire being harder than Au wire requires higher energy which shall further amplify the bending and twisting during wire bonding. Process characterization was completed with good bonding results for various overhang configurations using special bonding features. The process was developed with consistent ball shape, Al remnant below the bonded ball and looping across the overhang area. With Pd coated Cu wire, higher second bond stitch pull value was achieved. The results also reveal, the wire bonder capability is the key to succeed in thin overhang dice wire bonding using Cu and Pd coated Cu wire. In this article, we briefly review the Cu wire and Pd coated Cu wire performance on overhang dice and discuss key challenges such as 1.first bond formation 2.second bondability, stitch pull strength 3) looping optimization.