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

Abstract

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 center. 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.