Effects of Palladium Content on the Properties and High-Humidity Reliability of Silver Alloy Wires

Abstract

This study investigated changes in electrical and mechanical properties, as well as the high humidity reliability of 18 μm-diameter silver alloy wires with palladium contents of 1.1, 2.5, 3.0, and 4.0 wt%, for semiconductor packaging. We also compared the formation of Free Air Ball (FAB) and the high humidity reliability of joints in a reducing atmosphere with 4N (99.99%) gold wire of the same diameter. The wire resistance and hardness values were measured using a 4-point probe and a micro-hardness tester. The FAB formed in atmospheric and reducing atmospheres, as well as the high humidity reliability after bonding, were analyzed using field emission scanning electron microscopy, energy dispersive spectrometry, and a mechanical strength analyzer. As the palladium content increased in the silver alloy wire, the electrical resistance increased, showing a similar level to gold wire up to 2.5 wt%, after which it exhibited a further increase. The hardness values also increased sharply, but the rate of increase slowed down after 2.5 wt%. FAB exhibited excellent sphericity across the entire range of palladium content. After bonding, a uniform distribution of palladium was observed at the joint interface. The high humidity reliability showed improvement with increasing palladium content. Palladium 1.1 wt% exhibited differences compared to 2.5 and 3.0 wt%, and further improvement was observed with an increase to 4.0 wt%. This is attributed to the incorporation of palladium into the vulnerable Ag₃ Al intermetallic compound, resulting in the formation of a corrosion-resistant (AgPd)₃Al phase. As a result, micro-diameter silver alloy wires with a diameter of 18 μm or less demonstrated stable FAB formation, bonding processes, and properties that make them candidates to replace conventional gold wires, particularly under conditions with palladium content exceeding 2.5 wt%.