Role of intermetallics on the mechanical fatigue behavior of Cu-Al ball bond interfaces

摘要

The mechanical fatigue behavior of Cu-Al interfaces occurring in thermosonic ball bonds -typically used in microelectronic packages for automotive applications - is investigated by means of a specially designed fatigue test technique. Fully reversed cyclic shear stresses are induced at the bond interface, leading to subsequent fatigue lift off failure and revealing the weakest site of the bond. A special focus is set on the role of interfacial intermetallic compounds (IMC) on the fatigue performance of such interfaces. Therefore fatigue life curves were obtained for three representative microstructural states: The as-bonded state is compared to two annealed states at 200 degrees C for 200 h and at 200 degrees C for 2000 h respectively. In the moderately annealed state two IMC layers (Al2Cu, Al4Cu9) could be identified, whereas in the highly aged state the original pad metallization was almost entirely consumed and AlCu is formed as a third IMC. Finally, the crack path is traced back as a function of interfacial microstructure by means of electron microscopy techniques.