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.