Scanning Acoustic Microscopy (SAM) is an inspection that uses focused sound to investigate, measure, or obtain an internal image of an object. It is commonly used in failure analysis and non-destructive evaluation. The semiconductor industry has found the SAM, particularly the C-mode (C-SAM), useful in detecting voids, cracks and delamination within microelectronic packages.
FIGURE: C-SAM analysis on ADC128S. A-SCAN analysis.
C-SAM creates images by generating a pulse of ultrasound. The pulse reflects off of the material interfaces and, once detected by the transducer, allows to create the image. Imaging resolution and depth penetration are crucial for obtaining valid results. The amount of ultrasound reflected at the interface is based on the differences in the materials at the interface. The more different the materials the more ultrasound reflected.
The objective of the C-SAM test depends on the type of parts. As an example of its application, this technique is used for ultrasonic inspection of die attach in accordance with MIL-STD-883, method 2030.2. The purpose of this examination is to non-destructively detect unbonded regions, delaminations and/or voids in the die attach material and at interfaces within devices.
For certain devices or die attach materials, it is difficult to draw a clear distinction between well-bonded and poorly bonded conditions. While applying the above method this aspect is covered according to the device design. The ultrasonic inspection instrument is selected and adjusted as necessary to obtain satisfactory images and achieve maximum image details within the sensitivity requirements for the device. In the case of reflection mode or transmission mode images, care should be taken to ensure that the ultrasound penetrates and is sensitive to the entire die attach interface or bulk material area of interest.
FIGURE: Delaminations observed during C-SAM analysis on DAC121S
Ultrasonic images are then inspected to determine that each device conforms to the standard and defective devices are rejected. The image is viewed under the appropriate magnification so as to determine acceptance or rejection. Automated percentage void or bond area calculations can be utilized instead of visual analysis upon confirming that the automated method is at least equal to the accuracy of the visual method.
C-SAM has been demonstrated to be an effective methodology for identification of potential problems in different type of products where voids or delamination can jeopardize the long term reliability. Although most acceptance criteria are based on relative values (an increased defect size after exposing the devices to reliability tests such as thermal cycling or burn-in) absolute values (percentage of affected area) can provide valuable information about potential cause of failures during screening and/or acceptance testing.
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