More uniform Pd distribution in free-air balls of Pd-coated Cu bonding wire using movable flame-off electrode

The currently high price of gold wire has led to the development of bonding wire made from palladium-coated-copper as a lower cost alternative. Increasing the uniformity of the Pd distribution in free-air balls, formed by melting the wire end with an electric spark, is of interest as it can influence the uniformity of process and reliability. To study this Pd distribution, free-air balls are made using four distinct electrical flame-off (spark) processes with short and long spark times from wire bonders with fixed and movable electrodes. Elemental analysis of the free-air ball surfaces reveal a higher Pd concentration on the movable electrode free-air balls than on the fixed electrode free-air balls. Elemental analysis of cross-sections show that the Pd distribution in free-air balls made with a fixed electrode has Pd trails flowing from the neck into the Cu ball centre. Furthermore , micro-and nano-voids are observed to follow the Pd trails. In contrast, free-air balls made with a movable electrode exhibit less severe voiding and retain a uniform, thin Pd layer along the surface up to the tip of the free-air ball (shorter spark time). This can help to increase process consistency and reliability. 1 Introduction The demand for more cost effective bonding wire has increased in the microelectronics packaging industry due to the soaring cost of gold [1, 2]. In terms of thermosonic ball bonding , Cu has emerged as an alternative material due to its low cost. Cu also has better electrical and thermal properties compared to Au. However, Cu bonding wire has limitations such as short floor life, reduced bondability, and harder free-air-balls (FABs) compared to Au wire [3, 4]. To overcome some of these limitations, Uno [5] reported the application of a submi-cron thick layer of Pd onto the bare Cu wire resulting in a Pd-coated Cu (PCC) wire. The main benefits of the Pd coating are extended shelf life, widened second bond window, and increased performance under most reliability tests [5-9]. However , compared to Au wire bonds PCC wire bonds can perform worse under humid conditions due to moisture corrosion induced cracking [10]. To improve the reliability of PCC ball bonds under humid conditions , it is worthwhile to better understand how Pd is distributed in FABs and bonded balls. Previous studies by Stephan et al. [6, 7] showed that the Pd distribution is influenced by the electrical flame-off (EFO) parameters. In particular, a …