X-ray images quickly reveal thick or deep defects that offer high contrast. Acoustic, or ultrasound, images show very thin gap-type defects. To be sure I looked at only die-attach defects, I used preliminary acoustic imaging to eliminate samples containing defects in either the die or the substrate. The thickness of a void determines its contrast in an x-ray image. Trapped bubbles that extend from the substrate to the die show the highest contrast with surrounding materials.
Smaller voids appear as lower-contrast spots.
In an acoustic image, part of the ultrasound reflects when it encounters bonded interfaces, but it reflects completely at internal gaps. So, voids and delaminations show up clearly. The acoustic image in Figure 3 shows several small defects and one large defect, which is typical of voids and delaminations. An acoustic-microscopy image of a IC shows voids and delaminations. These types of defects reflect almost all the microscopy system's ultrasound back to the ultrasonic transducer.