Impacts to fine pitch copper wire bonding quality by external airflow

Abstract

Smaller diameter Cu wire has been incorporated in actual volume production and migrated into fine pitch IC packages in the past few years. Copper is easily oxidized by forming copper oxide that will hinder the welding process between copper to Aluminum bond pad surface. Forming gas must be used to protect the copper from reacting with oxygen for this reason. Intensive studies that focused on EFO parameter, inert gas flow rate and wire types were mainly conducted to achieve robust FAB (free air ball) process in the past. The key for a reliable finer copper ball process is not only the formation of defect-free FAB during EFO sparks stage but slight oxidation during search stage and bonding stage shall not be neglected. This paper discusses the issues and challenges encountered during the development of ultra fine copper ball. It also covers the impacts to the quality of ultra fine copper balls by possible oxidation during search stage, bonding stage and external air flow or air turbulence. Performance and resistance to disturbance by external air flow using different nozzles design from ASM are also compared in this paper.