Ultrasonic characterization of the interface between die attach and copper leadframe in IC packaging

Silicon die and copper leadframe in IC packaging are bonded by die attach adhesive, and the quality of the interface is a critical issue in the reliability testing of IC packaging and during the manufacturing process. Common defects such as crack and delamination can be detected using C-mode scanning acoustic microscopy. However, weak interface due to poor adhesion has often gone undetected and may become potential defective area at a later stage. This paper describes the work in evaluating the quality of the weak interface between die attach and copper leadframe. An interface spring model is used to predict the ultrasonic reflection coefficients. Normal incidence reflection coefficients are measured from the two-layered specimen bonded with die attach adhesive. Varying copper oxidation is used to simulate the degrading of the interface, and effect of shear stress loading is discussed. It is shown that the reflection coefficient depends strongly on the interface quality and stress loading, indicating that the nondestructive characterization of the interface is possible and reflection coefficients can be used as a criterion.