Effects of molding compound properties on acoustic evaluation of encapsulated microelectronic devices

A past paper discussed the trend of miniaturization in microelectronics and how this trend has driven the development of enhanced methods in acoustic micro imaging. However these methods have been developed in the majority of cases for the analyses of exposed silicon devices prior to encapsulation. Silicon is highly transmissive to the ultrasound allowing for high frequency/high resolution that is needed for the inspection of the small structures in the devices. Conversely encapsulation materials tend to be highly attenuating at high frequencies. Products are typically encapsulated before final quality control evaluation and this presents a challenge to acoustic evaluation methods. Ultrasonic attenuation in molding compounds results from absorption of the acoustic signal in the lower acoustic impedance material and from scattering of the signal from filler particles and voids in the material. Inconsistencies in the encapsulant materials and variations in the molding process can further complicate the acoustic analysis of these parts. To overcome the interference of the molding compound intermediate frequencies are used that provide the best compromise between transmission through the material and adequate detectability of the structures of interest in the devices. In order to test completed units for internal defects some consideration has to be given to the types of materials and processes used in order to render the parts compatible with analysis / inspection technologies. This paper will discuss the properties of encapsulation materials that are problematic in acoustic analysis and the frequencies and techniques that have proven to be effective in overcoming these problems.