Surface Wave Acoustic Microscopy for Rapid Non-destructive Evaluation of Silicon Nitride Balls

A rapid nondestructive evaluation (NDE) technique based on surface wave acoustic microscopy (SWAM) has been developed for inspection of surface/subsurface defects in silicon nitride (Si3N4) balls. This technique exploits advantages of convergent surface acoustic waves generated by a sharply focused ultrasonic transducer by placing the transducer's focal point below the ball surface, or defocusing. The use of surface waves to interrogate an area, rather than a point as in conventional acoustic microscopy, at a time on the ball surface can greatly improve the efficiency of defect detection. To ensure the inspection of surface defects with SWAM, we first simulated surface wave generation and propagation on a Si3N4 ball using finite element method (FEM). Then we tested different ultrasonic transducers for effective surface wave generation and detection. Transducers with varying frequency, f-number, defocusing distance, etc. for surface defect detection were experimentally compared. C-shape surface cracks on Si3N4 balls of different sizes were successfully detected. As a result, a SWAM prototype for rapid detection of surface defects in silicon nitride balls was successfully demonstrated.