The Method

Before determination of the proposed signal processing algorithm, it is necessary to analyse what happens when the ultrasonic signals meets a delamination in the IC chip. The IC chip can be considered as a multilayered structure which is differentiated by the ultrasonic signal. Each of the layers has different physical properties, including acoustic impedance. As it is known, the reflection coefficient strongly depends on the difference in acoustic impedance. The reflection occurs on each boundary between the layers. Transmitted and reflected signals are reflected on the other boundaries. This leads to signals containing irregular numbers of reflections and are different for different IC chips. In all cases the reflection from the delamination is much stronger due to the big difference in the acoustic material of the chip and air. This feature is exploited by the ordinary maximum amplitude algorithm for the defect detection. Another feature of the signal in the case of the delamination in chip is that the multiple reflections occur between delamination and chip boundaries.

The developed algorithm consists of the following steps:

• The surface reflection time is determined tsurf
• The cross-correlation of the signal part below the surface is calculated yc(t)
• The maximal reflection is found
• The delay time between maximal reflection and surface is calculated Dt1
• The position of next reflections estimated (2Dt1, 3Dt1...)
• The cross-correlation functions amplitudes is calculated at these positions
• The deviation from the decay low is calculated
• All deviations are integrated and the resulting values are used to create a C-Scan image

Fig.1 'A' Scan showing key data used by the algorithm

Fig.2(a) Processed Image. Red indicates delamination

Fig.2(b) Processed Image using bigger step to improve processing speed