CHANGE OF ULTRASONIC REFLECTIVITY WITH SMALL AIR GAP IN SOLID-SOLID BOUNDARY

Ultrasonic testing is useful for detecting plastic-metal debonding in an IC package. A positive phase echo relative to an input pulse is received from the bond region, whereas a negative phase echo is received from the debonding region because of the difference of acoustic impedance between the plastic and metal in the IC package. The purpose of this study is to examine the change from the positive to the negative echo with a small air gap in this solid-solid boundary. In these experiments, we used a gage block which is made of steel and has a flat surface for micrometer calibration, and a convex lens which is for a Newton ring experiment. as a test specimen. This specimen has a small air gap changing from actual contact with zero space to a gap space of order of microns.Ultrasonic immersion testing was carried out using 5MHz and 25MHz probes. The change of echo height and echo phase with gap space was examined. And then, we could simulate the change of waveform during the transition by mixing tw...