What is a Phase Inversion image?
Ultrasonic echoes occur when there is a discontinuity in the path. The discontinuity can be a normal transition from one material to another (e.g. mold compound to die) or because of a defect (e.g. disbond or delamination). The most commonly used indicator is the strength (amplitude) of the return signal. One of the properties of any material is the velocity of sound. When the ultrasonic impulse crosses a boundary between two materials, the phase of the echo signal depends on the relative speeds of sound in the two materials. If the impulse is traveling from a slower material to a faster material, the phase is unchanged. When it travels from a faster material to a slower material, the phase is inverted. When the phase of the echo is different from what is expected, it may be indicative of a defect. Specifically, when the normal path is from slower to faster, but the signal is phase-inverted, this indicates an abnormal condition. For example, plastic molding compound is slower than sili
