Authored By:
Janet E. Semmens
Sonoscan, Inc.
Elk Grove Village, IL, USA
Summary
Earlier studies concerning evaluation of stacked die packages using Acoustic Micro Imaging (AMI) demonstrated the feasibility of using AMI to analyze 3D devices. The construction of the devices evaluated in the studies were typically stacks of silicon chips bonded with an adhesive and using wire bonding for the interconnections. More recently 3D processes include stacked flip chip, silicon interposer, and TSV (Through Silicon Via). The various methods to achieve 3D integration provide challenges to the inspection of the devices using AMI. These challenges include multiple layers, thin silicon layers, different layer thicknesses, varying material properties, small feature sizes and in some cases the devices require analysis post encapsulation.
AMI (Acoustic Micro Imaging) is a non-destructive test method that utilizes high frequency ultrasound in the range of 5 MHz to 500 MHz. Ultrasound is sensitive to variations in the elastic properties of materials and is particularly sensitive to locating air gaps (delaminations, cracks and voids). There is a direct relationship between frequency and resolution in AMI. Higher frequencies have shorter wavelengths and therefore provide higher resolution. Lower frequencies, which have longer wavelengths, provide better penetration of the ultrasound energy through attenuating materials, thicker materials or multiple layer assemblies.
Generally a compromise is found between sufficient resolution and maintaining satisfactory penetration and working distance for a given application. In order to accommodate the changing designs in 3D devices AMI technology is evolving. This includes different frequency transducers, and enhanced software tools and imaging techniques to aid in the detectability of features and assist in locating the sequential levels in the devices.
This paper will demonstrate how recent developments in AMI analysis methods can facilitate evaluation of various types of 3D devices.
Conclusions
During the development of a 3D device construction and materials are changed or modified. Design and construction varies significantly between types of 3D devices and can vary between manufacturers for the same type of device. Fortunately many of the methods that are currently in use to evaluate standard device types can be used or modified to analyze 3D packages. The diversity of 3D device types also necessitates continuing experimentation and development of acoustic methods and transducers to accommodate evolving device technology.
Initially Published in the SMTA Proceedings
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