Evaluation of quality of silicon/copper interfaces in IC packaging

The silicon die and copper lead-frame 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 ultrasonic microscopy method with sufficient confidence. However, weak interface due to weak adhesion and poor cohesion has often gone undetected and may become potential defective area at a later stage. There is a desire to study the interface quality quantitatively, so the potential defective area can be evaluated and identified early. This paper describes the work in evaluating the quality of the interfaces that typically exist in IC packages by using longitudinal ultrasonic wave propagation with contact transducers. An interface spring model is used to predict the ultrasonic reflection coefficient relationship with varying interfacial spring constants. Experimental results of normal incidence reflection coefficients are obtained from the two-layered specimen bonded with die attach adhesive under varying conditioning process that simulates the degrading of the interface, and from samples under shear loading. Good qualitative agreement between the measurement and the prediction is obtained, and shows the reflection coefficient depends strongly on the interface quality. The study demonstrates that the quantification of the interface quality is possible, and reflection coefficient can be used as a criterion.