Flip Chip Underfill Void - Application Note 2271
Sample & Method
Flip chip imaged on C-SAM at 300 MHz frequency. Ultrasound was gated on the chip-to-bump interface, a depth that also includes the cured underfill material.
The large dark feature is a trapped bubble, or void, in the underfill. Since gating at this depth also includes the bonds of the solder bumps to the chip face, it is possible to see that all bumps have the same appearance – namely, the dark center and gray outline expected of a good bond in this type of flip chip.
The curved dark bands inside the void are not traces but probably ridges of underfill material.