Characteristics and Reliability of Al and Al-coated Cu Wires for High Power Applications

We investigated the characteristics and reliability of thick aluminum (Al) and Al-coated Cu wire for high power device applications in terms of resistance to thermal fatigue. Thick bonding wires are those that are in the thickness range of 125-500 um. The most common bonding wire used in packaging power devices is the 99.99% or 4N purity Al with relatively large grains of >25um or more. Results from 4N Al wire were contrasted with that from alternative wires composed of fine-grain 99% or 2N purity Al wire and Al-coated Cu wire, both designed to provide higher performance. The fine-grain 2N Al has grain sizes of 20um or less and the Al-coated Cu wire has 50:50 content ratio of Al and Cu. In terms of breaking load, the Al-coated Cu wire and 2N Al wire showed 3X & 1.67X higher breaking loads than the 4N Al wire, respectively. The Al-coated Cu wire indicated 18.6% higher elongation while the 2N Al wire had 85.8% lower elongation than 4N Al wire. Units were assembled using these 3 wire types (4N Al, 2N Al and Al-coated Cu wire). For both 4N and 2N Al wires, similar horizontal crack signatures were observed after temperature cycling conditions of TC1 (-65°C to 150°C) 1000 cycles and TC2 (-65 °C to 175°C) 750 cycles, respectively. These horizontal cracks followed the crack initiation sites at the heel and toe areas of the wedge bond around 4-10um above the bonding pad area. The Al-coated Cu wire which has higher yield strength and lower CTE showed lower plastic strain in the simulation runs. After temperature cycling at TC1 conditions for 2000 cycles, the wire-bond pad interface of Al-coated Cu wire revealed fine cracks with its propagation path branched along the Al-Cu wire interface or near the wire and the bond pad area. The much finer crack for the Al-coated Cu wire after thermal cycling indicated lower degradation of the wedge bonding than the Al wire and these results correlate well with the predicted performance based on simulation.