As to defects of non-fusion, almost 100% ultrasonic reflect; as to very tiny defect, the echo amplitude showed changing characteristics dependent on frequency [16]. From micrograph of the corona bond in Fig. 4(d), we can see the interface fit very tightly and the space is only 1–2 lm, belonging to typical weak combination. In this area, high frequency echo amplitude increases with the decrease of the space, and so the range between the two lowest points in Fig. 10(a) represents the corona bond.