Pan Pacific Symposium Conference Proceedings

Evaluation of Molded Underfill Packages Using Acoustic Micro Imaging

Author: Janet E. Semmens
Company: Sonoscan, Inc.
Date Published: 2/14/2012   Conference: Pan Pacific Symposium

Abstract: Acoustic Micro Imaging (AMI) has been used over the past years to successfully evaluate the quality of various microelectronic components. Acoustic micro imaging uses high frequency ultrasound (5 to 500 MHz) to image the internal features of samples. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations and voids). Flip chip devices are evaluated routinely using AMI for defects in the bump bonds and voids or delaminations in the underfill. Flip chip applications typically require high frequency/high resolution ultrasound for inspection. The ability to introduce the ultrasound beam through the exposed silicon chip prior to encapsulation allows for the use of high frequencies. However, more recently a molded underfill process is being used to underfill and encapsulate the devices at the same time eliminating the possibility of accessing the internal features of the device without passing the ultrasound through the acoustically lossy molding compound.

The encapsulation requires lower frequencies than what is ideal for flip chip inspection. The molding compound introduces interference with imaging not only due to absorption (and resulting attenuation) of the high frequency signal but scattering due to a high concentration of filler particles and in many cases the presence of heavy voiding above the interfaces of interest. This presents a challenge to find a suitable frequency for analysis. Spectral analysis of the ultrasound echo at the interface shows the frequency content of the received echoes in response to the encapsulant. In molded underfill packages the downshift in frequency can be significant and certain transducers may not have sufficient frequency content in the range required to reach the interface. The frequency domain analysis provides a useful method of determining the appropriate frequency transducer to evaluate the devices. In addition reconstructing images from the frequency domain content at specified sequential single frequencies illustrates what frequency or range of frequencies will provide the best detection of the internal features or flaws.

This paper provides a brief background describing molded underfill packages and discusses the technical issues involved with acoustic microscopy evaluation of molded underfill packages including Fourier domain frequency analysis and imaging to determine the most suitable transducer and method for evaluation of the devices.

Key Words: 

Acoustic Micro Imaging (AMI), Flip Chip, Molded Underfill (MUF)

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