A Spring Model of Oblique Incidence Ultrasound for Evaluation of INterfaces in IC Packaging

摘要

There are many interfaces that exist in IC packaging, such as that between mold compound and silicon chip, and that between silicon die and copper lead-frame. The quality of the interface is becoming a critical issue in the reliability testing of IC packaging and during the manufacturing process as the packages become thinner and desner. Common defects such as delamination and crack can be detected using C-mode scanning acoustic microscope (CSAM) with sufficient confidence. However, weak interface due to weak adhesion and poor cohesion has often gone undetected and many become potential defective area at a later stage. There is a also a strong desire to evaluate the interface quality quantitatively, so the potential defective area can be evaluated and identified early. This paper describes the possible nondestructive evaluation method to characterize the quality of the interfaces that typically existed in IC packagign by using the oblique incidence ultrasound propagation. An interface spring model is used to predict the ultrasonic reflection coefficient relationship with varying interfacial spring constants and incidence angle. The sensitive parameters (incident angles) are identified and are to be used for testing. Numerical results shows that the oblique incidence reflection coefficient depends strongly on the interface quality, especially in the sensitive angle where it is more sensitvie than that for normal incidence. The study demonstrates that the quantification of the interface quality is possible, and oblique reflection coefficient ca be used as a criterion.