page_title: Acoustic microscopy
section_title: Gating of the return echoes
hierarchical_section_title: Acoustic microscopy / Scanning process / Gating of the return echoes
caption_attribution_description: English: C-Sam Generated Image
context_page_description: Acoustic microscopy is microscopy that employs very high or ultra high frequency ultrasound. Acoustic microscopes operate non-destructively and penetrate most solid materials to make visible images of internal features, including defects such as cracks, delaminations and voids.
context_section_description: A pulse of ultrasound from the transducer travel nanoseconds or microseconds to reach an internal interface and are reflected back to the transducer. If there are several internal interfaces at different depths, the echoes arrive at the transducer at different times. Planar acoustic images do not often use all return echoes from all depths to make the visible acoustic image. Instead, a time window is created that accepts only those return echoes from the depth of interest. This process is known as "gating" the return echoes.
  Finally, the plastic-encapsulated IC was flipped over and imaged from the back side. The return echoes were gated on the depth where the backside mold compound interfaces with the back side of the die paddle. The small black dots in the acoustic image above are small voids (trapped bubbles) in the mold compound.
caption: Still scanning the top of the sample, the gating of the return echoes was then changed to include only the plastic encapsulant (mold compound) above the die. The resulting acoustic image is shown above. It shows the structure of the particle-filled plastic mold compound, as well as the circular mold marks at the top surface of the component. The small white features are voids (trapped bubbles) in the mold compound. (These voids are also visible in the previous image as dark acoustic shadows.)
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