Echo Separation
An advanced digital signal processing technique that separates ultrasonic reflections from different depths based on phase shifts improves the resolution of scanning acoustic microscopy. Implementing phase inversion depth gating allows the inspection system to isolate the echo from the mold compound interface from the echo of the silicon die surface. This separation allows the detection of sub-micron voids that would otherwise be obscured by strong adjacent reflections.
Signal Processing
The process begins with the transmission of two consecutive ultrasonic pulses with opposite phases into the component package. During phase inversion depth gating, the system sums the received echoes to cancel out the symmetric reflections from superficial layers while reinforcing the asymmetric reflections from deeper interfaces. This selective reinforcement improves the signal-to-noise ratio at the specific depth where delamination is suspected.
By adjusting the time gate, the operator can focus the inspection on a specific internal layer of interest.
Interface Imaging
Automated analysis of the processed signal creates a high-contrast image of the internal structures. Areas of high reflection indicate air-filled voids, whereas uniform areas suggest a secure bond. This imaging method provides a reliable way to qualify the adhesive integrity of plastic ball grid array packages.
