Airborne ultrasonic inspection at 2 MHz; its application to SMT

Airborne ultrasonics offers certain advantages in inspection systems. Compared with vision systems, the signal acquisition time can be much shorter, signal processing is much simpler, and range information is obtained directly, rather than by complex triangulation methods. This ability to measure distance to an object, or the difference between two distances, makes ultrasonic techniques the obvious choice for many inspection situations. The advantage of airborne ultrasonics over conventional NDT ultrasonics is, of course, that no water bath or other liquid couplant is required. In the work described here, continuous-transmission frequency modulation (CTFM) is used with 2 MHz transducers to achieve a distance-measuring resolution in air of less than 25 μm (0.001 inch)