Ball bonding is a key interconnect process in microelectronics assembly and there have been a number of detailed research studies on the reliability of both gold (Au) and copper (Cu).

Despite decades of proven performance and reliability with gold, an early study by Onuki ¹ shows that failure in Cu and Au ball bonds can occur in the intermetallic phases and correlates ball lifts with a critical intermetallic thickness.  Slower intermetallic growth in Cu ball bonds on Al metallization is perceived to result in longer times needed to reach critical intermetallic thickness and higher reliability. 

However, more recent studies with copper wires ≤25µm and bond pads ≤1µm reveal problems with reliability in temperature cycling (TCL) ^{2, 3} and stress-corrosion of the Cu-Al phase during pressure cooker testing (PCT) ⁴. ‘Squeeze out’, which is thinning of the bond pad at the regions of highest stress may result in chip damage and reduce TCL and PCT reliability. 

Recent comparisons of encapsulated 25µm diameter Cu and Au wire with metal-coated wires ^{2, 3} show poor copper wires performance in PCT and TCL relative to Au and Pd-plated Cu wire.  Hang et al ⁵ find cracking in Cu-Al intermetallics during high temperature storage testing (HTST) at 250°C.

To establish the likely extent of these observations in-service, there is a need to obtain higher quality data on different material wires, supported by state of the art analytical methods. The Sure Connect initiative is currently sponsoring such work at a leading microelectronics research centre. Recently published results from this project indicate that under moist conditions and in the presence of chlorine, copper ball bonds appear more prone to corrosion than gold ball bonds. ⁶

References

1. J. Onuki, M. Koizumi, I. Araki.  IEEE Trans. On Comp. Hybrids & Manfg. Tech. 12 (1987) 550.
2. S. Kaimori, T. Tonaka, A. Mizoguchi. IEEE Trans. Adv. Packgg. 29 (2006) 227.
3. T. Uno, S. Terashima, T. Yamada.  IEEE Proc. ECTC (2009) 1486.
4. C. W. Tan, A. R.  Daud, M. A. Yarmo.  Appl. Surf. Sci. 191 (2002) 67.
5. C. J. Hang, C. Q. Wang, M. Mayer, Y. H. Tian, Y. Zhou, H. H. Wang.  Microelectronics Reliability 48 (2008) 416.
6. C. D. Breach and Teck Kheng Lee, ICEPT Conference, Shanghai, August 2011