Wire bonding optimization with fine copper wire for volume production

The adoption of copper wire in high volume wire bonding production has proven successful as seen in the tremendous increase in copper wire consumption in the past year. This increase largely came from the conversion from gold to copper wire in fine pitch devices which previously had been more conservative due to concerns related to copper wire reliability. Fine pitch applications with bond pad openings of 45um and less using 0.8mil and less wire diameter require more stringent control on bonding qualities. Specially designed copper hardware that maintains an oxygen free environment, as well as more elaborate wire bonding processes were required to meet the challenges in copper wire bonding, some with success but often with compromises to production throughput and mean-time-between-assist (MTBA). Palladium-coated copper (Pd-Cu) wire has emerged as an alternative candidate to bare Cu wire especially in 0.8mil and even finer wire diameter applications. Its ability to form uniformly shaped free-air-balls (FABs) with nitrogen instead of forming gas, better bondability on lead surfaces, as well as its resistance to oxidation and corrosion tend to offset the higher cost of Pd-Cu wire compared to bare Cu wire. The advantages of Pd-Cu wire are readily seen in terms of production throughput and MTBA. Both bare Cu and Pd-Cu wires have been widely studied in terms of their hardness and FAB formation. Second bond bondability has also been compared with Pd-Cu wire demonstrating higher stitch strength and a wider process window without the use of elaborate bonding processes. For first bond, both bare Cu and Pd-Cu wires are seeing challenges in pad damage control in the form of peeling and cracking, with Pd-Cu wire showing smaller working process windows in most cases. Much attention has been put in studying the higher hardness of Pd-Cu wire compared to pure Cu wire, though the difference is not too significant. FABs generated from Pd-Cu wire have been studied with respect to vario- - us factors such as electric-flame off (EFO) parameters, types of inert gas, and the gas flow rate used to maintain the oxygen free environment. Correlation between these parameters and the distribution of Pd on the surface of a FAB was used to explain the different degrees of hardness seen. It has been claimed that both EFO parameters and the type of inert gas used affect the mixing of Pd into the FAB and result in different FAB hardness. The control of FAB formation by means of EFO parameters and inert gas used showed a certain influence on the first bond process window. In this paper, experiments were carried out to understand the difference in FAB formation and first bond and second bond process windows using 0.8mil bare Cu and Pd-Cu wires in fine pitch packages. This understanding helps in setting up a robust process with special attention to process parameters that are critical in controlling key bonding performance like ball size, shear, stitch pull strength, pad splash, and pad damage. It is the objective of this paper to demonstrate the feasibility of fine pitch Cu wire processes for high volume production.