Next Generation Nickel-Based Bond Pads Enable Copper Wire Bonding

Abstract

Copper wire bonding has huge cost advantages over gold wire bonding. As a result, low pin count, heavy wire applications have already been converted to copper wire and many companies are in high volume production. Recently, with the price of gold skyrocketing, conversion of high pin count (>250 I/O), high performance applications to Cu has dramatically accelerated. These high performance devices are increasingly relying on low-k materials under the bond pads. Unfortunately, the 33% greater hardness of Cu compared to gold places even greater stress on these inherently fragile materials. This can result in difficulties with pad damage and cratering of the underlying structures. Advancements have been made to copper wire bonders, tools, and wire that have resolved many of these issues and made fine pitch copper wire bonding feasible. Ni-based bond pads have emerged to solve the pad damage problem. Nickel is about 50% harder than copper and four times harder than aluminum so that it provides greater protection against the higher stress resulting from Cu ball bonding, as well as damage during probe. This is especially beneficial for devices with low-k active circuitry under the bond pad. NiPd, NiPdAu, and/or NiAu have demonstrated their great robustness to receive the Cu wire bonding with a huge wire bonding window without any splash and with excellent reliability.