Acoustic microscopy

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.

However, due to technological limitations at the time, no such instrument could be constructed, and it was not until 1959 that Dunn and Fry[2] performed the first acoustic microscopy experiments, though not at very high frequencies.

The scientific literature shows very little progress toward an acoustic microscope following the Dunn and Fry experiments up until about 1970 when two groups of activity emerged, one headed by C.F.

The first efforts to develop an operational acoustic microscope concentrated upon high-frequency adaptations of low-frequency ultrasonic visualization methods.

One early system employed Bragg diffraction imaging,[3] which is based upon direct interaction between an acoustic-wave field and a laser light beam.

[4] The original device is based upon a suspension of asymmetric particles in a thin fluid layer which, when acted upon by acoustic energy, produce visual reflectivity changes.

Kessler and Sawyer[6] developed a liquid crystal cell that enabled sound to be detected by hydrodynamic orientation of the fluid.

Advancements of this instrument, a scanning acoustic microscope, have to do with achieving very high resolution, novel modes of imaging, and applications.

In 1970, the Korpel and Kessler group began to pursue a scanning laser detection system for acoustic microscopy.

[9] In 1980, first high-resolution (with a frequency up to 500 MHz) through-transmission mode SAM was built by Roman Maev and his students at his Laboratory of Biophysical Introscopy of the Russian Academy of Sciences.

[10] First commercial SAM ELSAM with the broad frequency range from 100 MHz up to ultra high 1.8 GHz was built at the Ernst Leitz GmbH (Wetzlar, Germany) by the group led by Martin Hoppe and his consultants Abdullah Atalar (Stanford Univ., USA), Roman Maev(Russian Academy of Sciences, Russia).

Using the same transducer to pulse ultrasound and receive the return echoes meant that the acoustic image could easily be constrained to a depth of interest.

Ultrasound that is reflected from an internal feature, or (in some applications) that has traveled through the entire thickness of the sample, is used to make acoustic images.

The return echoes were gated on the depth where the backside mold compound interfaces with the back side of the die paddle.

The samples imaged by acoustic microscopes are typically assemblies of one or more solid materials that have at least one surface that is either flat or regularly curved.

Because of their ability to find visualize features non-destructively, acoustic microscopes are widely used in the production of electronic components and assemblies for quality control, reliability and failure analysis.

In the acoustic image ultrasound was pulsed through the black mold compound (plastic), and reflected from the interface between the overlying mold compound and the top surface of the silicon die, the top surface of the die paddle, delaminations (red) on top of the die paddle, and the outer portion (lead fingers) of the lead frame.
Side-view diagram
In the plastic-encapsulated IC, gating was on a depth that included the silicon die, the die paddle and the lead frame.
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.)
Gating was then changed to include only depth of the die attach material that attaches the silicon die to the die paddle. The die, the die paddle, and other features above and below the die attach depth are ignored. In the resulting acoustic, shown above slightly magnified, the red areas are voids (defects) in the die attach material.