Resolution of broadband transducers in acoustic microscopy of encapsulated ICs: transducer selection

The lateral resolution of broadband transducers commonly used in acoustic microscopy is discussed in the context of selecting optimum transducers to make images for the best possible resolution of defects in plastic encapsulated integrated circuit (IC) packages. A predictive model to accurately calculate the effective lateral resolution afforded by a transducer is proposed. The model, which explicitly considers the measured frequency dependent attenuation behavior in water and in encapsulant materials, is a practical tool for the acoustic microscopist interested in selecting a transducer with optimum characteristics for inspecting a particular IC package. Experimental data from plastic quad flat packages (PQFP's) molded with two different encapsulants-Nitto MP8000CH and Sumitomo 6300HJ are presented to demonstrate the validity of the modeling approach. Recommendations for optimum transducer selection are presented based on a parametric study of the effect of various transducer and material parameters on the effective lateral resolution and signal loss.