The silicon die (chip) and the copper leadframe in IC packaging are bonded by a die attach adhesive, and the quality of the interface is a critical issue in the reliability of IC packaging as well as during the manufacturing process. The common defects such as cracks and delamination can be detected using the C-mode scanning acoustic microscopy. However, a weak interface due to poor adhesion often goes undetected and may become a potential defective area at a later stage. This paper describes the work done to evaluate the quality of the weak interface between a die attach adhesive and a copper leadframe. An interface spring model is used to predict the ultrasonic reflection coefficients. Normal incidence reflection coefficients are measured from a two-layer specimen bonded with a die attach adhesive. The quality of the interface in the samples and its degradation are affected by copper oxidation, and by applying shear stress loading. It is shown that the reflection coefficient depends strongly on both the interface quality and stress loading, indicating that a nondestructive characterization of the interface is possible and the reflection coefficient can be used as a criterion.