Michael G. Oravecz

In situ elastic property characterization of flip-chip underfills

The elastic properties of processing related inhomogeneities, such as filler settling and voids, are characterized using the acoustic microscope. A procedure to calculate the acoustic impedance of materials, and hence the elastic properties, at internal interfaces is proposed. The acoustic impedance of the silica filled polymer material under the die is measured at different locations corresponding to areas of different brightness in the acoustic image. Correlating the acoustic impedance measurements with the HS-model indicated that a) darker areas are regions where the underfill has a homogeneous distribution of filler; b) lighter areas are regions characterized by filler settling. Destructive cross sectioning and microscopy confirmed the above predictions. Further, the elastic properties of the different areas adjacent to the die are estimated using the model. The relatively quick, nondestructive technique presented in this paper could be useful in advanced process control, rapid yield management and in providing input into package reliability studies (such as finite element analysis).

Resolution and signal loss in acoustic microscopy of encapsulated IC packages

This paper examines the degradation of resolution and signal strength in pulsed acoustic microscopy of plastic encapsulated ICs. The effects of attenuation of ultrasound in water and plastic encapsulants, and transducer characteristics such as bandwidth, focal length and center frequency, are investigated. Ignoring the consequences of attenuation or spectral characteristics of the transducer could lead to overestimation of the resolution by as much as a factor of two in high frequency inspection. The novel procedure presented in this paper can not only help predict a realistic resolution, but can also be applied in selecting the optimum combination of transducer characteristics for specific materials.

Acoustical Imaging/NDE of Complex Geometries Using Refraction and Reflection Transmission Techniques in Acoustic Microscopy

The application of scanning laser acoustic microscopy (SLAM) for flaw detection, metallurgical characterization and evaluation of processing variable influence has been limited to components (ceramic turbine blades, plastic IC packages, metal cylinders, etc.) which allow access to two surfaces. Such specimens were tested based only on amplitude variations of transmitted 10 to 500 MHz sonic energy. This paper presents practical SLAM applications where parts are tested based on amplitude variations of refracted and/or reflected 10 to 100 MHz sonic energy. This distinction allows testing of components that restrict access to only one surface and of components where the area of interest is masked by intervening material. For example, miniature welded or brazed corner and T joints, thin wall tubing and the bonding of small electronic parts on thick heat sinks can be nondestructively tested using refraction and reflection SLAM imaging.