Optimizing Ag wire bonding for memory devices

Abstract

Au wire used to be the most common wire used for wire bonding. Due to semiconductor industry's continuous drive for cost reduction, more and more devices are bonded with alternative wires such as Cu wire, Pd coated Cu wire and Ag alloy wire. This paper will first review the bonding wire trend and pros and cons for each wire type. We will then focus on Ag wire bonding challenges and solutions, especially for memory devices. Cu wire is the lowest cost bonding wire, it has been used in many applications from the low end discrete packages to the high end mobile processors. For fine pitch memory devices, multi-die stack configuration is very common. Cu wire faces bigger challenges since memory devices often require low loops, die to die bonding and bonding on thin over-hang die. For memory devices, Ag wire is an attractive solution that is easier to optimize than Cu wire and can provide significant cost reduction compared to Au wire. In this paper, Ag wire Free Air Ball formation process is studied and DOEs are performed to optimize the ball formation parameters for 18um Ag wire. To characterize the looping performance, we studied various loop height and wire length and optimize Ag wire looping formation for low loop applications. We compare Au, Ag and Cu wire looping performance and show the advantages for using Ag wire for low loop and longer wire length applications. Recommendation for selection of different loop profiles including compressed and uncompressed loops are given to achieve the best looping performance and machine throughput.