C-SAM and Q-BAM
PBGA Virtual Cross-Section - Application Note 2602
Sample & Method
The square feature is the critical interface, and its red color indicates that the molding compound is entirely delaminated from the die face - except for a small black-yellow questionable area. The cross-sectional Q-BAM image was made along the horizontal line drawn through the questionable area. The cross-sectional view in the top half of the Q-BAM image shows that the "questionable" feature is actually a crack in the molding compound. The crack extends upward diagonally from the interface, but does not reach the surface. Since the crack is outside of the gated depth, it loses signal amplitude and appears dark in the c-scan image.
A plastic BGA package. At left above: planar C-SAM image. At right: Q-BAM image based on the C-SAM image. The planar image is gated and focused on the interface between the top surface of the die and the molding compound above the die.