The Influence of Contamination on Aluminum-Gold Intermetallics

The uncontrolled formation of aluminum-gold intermetallics such as "purple plague" (AuA12) has not been considered a major semiconductor and/or hybrid reliability problem for several years, due to the advent of dependable lower temperature assembly processes which include: thermosonic gold ball bonding, ultrasonic aluminum wedge bonding, and epoxy attachment of both die and substrates. Today, however, intermetallics are beginning to appear again even in the lower temperature production environment. Isolated instances of ball bond lifting after long burn-ins at 125°C are being noted in high reliability programs. Rapid intermetallic consumption of second or tail bonds has been reported in hybrids with aluminum substrate metallization and gold thermosonic bonding. These processes are definitely accelerated by the 125°C thermal exposure, but have occurred so rapidly that contamination appears to be the principal driving source. A case study is presented in which a switch in substrate attach epoxy caused a catastrophic acceleration of intermetallic formation and widespread bond failures after the hybrids were exposed to a thermal testing environment. Details of the failure investigation process are presented including: impurity searches using energy dispersive X-ray, scanning Auger microprobe, and secondary ion mass spectrometry; epoxy cure cycle validations using residual gas analysis methods; the empirical use of the wire bond pull as the process validation method; and the analysis of sealed package ambient both pre and post burn-in.