Where x-ray imaging fails - delamination, crack, and micro-pore detection using ultrasonic reflection tomography in a scanning acoustic microscope

In recent years x-ray imaging techniques have dramatically expanded their range of applications. However, there are still specific problems where x-ray techniques fail due to fundamental physical reasons. These “no-go” applications can be divided into three main categories, (i) applications in which the x-ray absorption is too large, so that no significant transmission signal can be detected (e.g. pores in high density materials like tungsten carbide), (ii) applications in which the die gap of a planar delamination or a micro-crack is so much smaller than the transmitted thickness that no significant change in X-ray transmission can be observed (e.g. micro-cracks with nanometer die gaps), (iii) applications in which the absorption contrast between adjacent materials is too small to be detected (e.g. silicon chip/mold compound combinations). Most of the above mentioned applications can be successfully treated by using high-frequency scanning acoustic microscopy (SAM). These ultrasonic microscopes are typically working in the frequency range between 10 and 400 MHz and are using differences in acoustic impedance for imaging purposes.