Is copper wire bonding ready for automotive applications?

Abstract

Wire bonding has been the predominant interconnect technology for more than 40 years and even today is holding a market share of more than 85%. The standard wire material has been gold albeit copper and aluminum wires have been used for 20 or so years in power applications. In the past few years, economics, i.e. the explosive increase in gold commodity pricing, have lead to a rapid conversion of fine diameter gold wire packages to copper wire packages. The conversion was lead by consumer products which were most price sensitive and perceived to be least demanding in reliability due to the nature of the application and the relatively short life. For these products, a good track record has been established which then lead to an extension of the applications to computer and communication applications. Now, automotive and networking applications are also implementing copper wires as interconnects without sacrificing their stringent reliability and long life requirements. All of the recent conversion activity has spurned many research and application publications about copper wire bonding Discussion 1 Section Header to report on copper wire itself, the bonding process and intermetallic compound formation, effect of mold compounds, reliability performance, etc. Wire suppliers have developed and introduced new copper based wires, mold compound suppliers have purified mold compounds to suppress corrosion and equipment suppliers have developed copper wire specific bonders. Here, the successful implementation and `long term' copper wire bonding experience for a wide range of applications and range of wafer nodes will be reported. A strict methodology for assessing bondability for any new device will be discussed. This methodology was found to be necessary due to the subtle differences in pad design even when devices are built in the same fab. Clearly, advanced wafer nodes based on LK and ELK dielectrics have rather small process windows and do require rigorous tuning of bonders control of assembly processes and manufacturing floor as well as specific training of operators. Such a methodology plus the proper choice of materials are essential for the success and the long term reliability of copper wire bonds as will be demonstrated in this paper. It will be shown that JEDEC tests can be extended to exceed the typical test durations by more than five or six times. The high reliability of copper wire bonds is based on the very slow copper aluminum intermetallic growth. Such reliability performance as well as near gold assembly yields lead to the level of confidence to start automotive application qualifications. The first set of results of successful automotive application qualification per ACE-Q100 requirements will be presented to demonstrate readiness for manufacturing.