Acoustic Microscopy - Scanning Process

Scanning Process

The ultrasonic transducer raster-scans the top surface of the sample. Several thousand pulses enter the sample each second. Each pulse may be scattered or absorbed in passing through homogeneous parts of the sample. At material interfaces, a portion of the pulse is reflected back to the transducer, where it is received and its amplitude recorded.

In the acoustic image and side-view diagram of the plastic-encapsulated IC to the right, ultrasound was pulsed through the black mold compound (plastic), and reflected from:

  1. the interface between the overlying mold compound and the top surface of the silicon die.
  2. the interface between the overlying mold compound and the top surface of the die paddle.
  3. the interface between the overlying mold compound and delaminations (red) on top of the die paddle.
  4. the interface between the overlying mold compound and the outer portion (lead fingers) of the lead frame.

The portion of the pulse that is reflected is determined by the “Acoustic Impedance”, Z, of the each material that meets at the interface. The acoustic impedance of a given material is the material’s density multiplied by the speed of ultrasound in that material. When a pulse of ultrasound encounters an interface between two materials, the degree of ultrasonic reflection from that interface is governed by this formula:

R = \frac{\left(z_2-z_1\right)}{\left(z_2+z_1\right)}

where R is the fraction of reflection, and z1 and z2 are the acoustic impedances of the two materials.

If both materials are typical solids, the degree of reflection will be moderate, and a significant portion of the pulse will travel deeper into the sample, where it may be in part reflected by deeper material interfaces. If one of the materials is a gas such as air – as in the case with delaminations, cracks and voids – the degree of reflection at the solid-to-gas interface is near 100%, the amplitude of the reflected pulse is very high, and practically none of the pulse travels deeper into the sample.

Read more about this topic:  Acoustic Microscopy

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