Scanning Acoustic Microscopy (SAM)

Scanning Acoustic Microscopy (SAM) is a failure analysis technique used for detecting disbonds or delaminations between package interfaces, e.g., interfaces between the plastic resin package material, the die, the die paddle, the leadframe, the die attach material, etc.

It basically consists of sending a sound wave through the package, and interpreting the interaction of the sound wave with the package. A typical scanning acoustic microscope (see Figure 1) may employ either pulse echo or through transmission inspection to scan for disbonds or delaminations. Pulse echo inspection consists of interpreting echos sent back by the package while through transmission inspection consists of interpreting the sound wave at the other end of the package, after it has passed through the latter. The ultrasonic wave frequency used ranges from 5 to 150 MHz.

Figure 1. Example of a Scanning Acoustic Microscope

The sound wave may be generated by a piezoelectric crystal, or transducer, that has been cut to provide a specific frequency. It is activated by a high voltage pulse from a transmitter, which is also known as the pulser. The activation would cause the transducer to vibrate at the specified frequency, which transmits an ultrasonic wave through the package.

This wave travels to the specimen through a medium or couplant, which is usually deionized water since sound waves could not travel through air at the frequencies used. The wave travels through the specimen's material at the material's velocity, with a portion of it being reflected back everytime it hits an interface within the material.

In the pulse echo method, the same transducer is used as sender and receiver of the sound waves. Pulses are repeated using repetition rates at which the echoes from one pulse will not interfere with those of another, e.g., 10-20 KHz. The echoes received by the transducer are converted to voltages, amplified, digitized, and presented to the user as an image.

In the through transmission technique, separate transducers are used to send and receive sound waves, both of which are on opposite sides of the specimen. The absence and presence of signals mean bad and good bonding, respectively.

Scanning acoustic microscopy has several modes. The A-scan mode is the real-time oscilloscope waveform of the acoustic signals based on the reflected echoes, or acoustic data collected at a single X-Y portion or point.

The B-scan mode is the cross-sectional display showing the ultrasonic reflection of the various interfaces along the depth of the package, or acoustic data collected along the X-Z plane at depth A. A B-mode scan furnishes a two-dimensional (cross-sectional) description along a test line (Y).

The C-scan mode is the display of the image of reflected echoes at the focused plane of interest, or acoustical data collected along an X-Y plane at depth Z. A C-mode scan furnishes a two-dimensional (area) description at a particular depth (Z) (see Figure 2). Usually the B-scan images are based on the C-scan image for precise determination of the depth of flaws detected.

Figure 2. Examples of C-SAM Photos;

red areas denote full delamination while yellow

areas denote slight delamination

When performing Scanning Acoustic Microscopy, the following must be observed :

1) The units must be placed in the sample holder such that their upper surfaces are parallel to the scanning plane of the acoustic transducer. Air bubbles must be swept away from the unit surfaces and from the bottom of the transducer head.

2) The transducer with the highest center frequency that still provides sufficient signal-to-noise ratio for good imaging must be selected. The transducer must be normal to the plane of the sample and the scan path must be parallel to the plane of the stage.

Failure Mechanisms/Attributes Tested For: Plastic-to-Leadframe Delamination, Plastic-to-Die Delamination, Plastic-to-Die Attach Delamination, Die Attach Voids, Internal Cracks, etc. Internal delaminations generally need to be addressed (especially those that occur on the die surface and bonding fingers) because they can lead to serious reliability issues such as neck breaks, heel breaks, and corrosion.

FA Lab Equipment; Basic FA Flows; Package Failures; Die Failures

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