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.
Discontinuously reinforced aluminum alloys are viewed as candidate materials for elevated temperature applications because of their attractive high temperature strength properties and wear resistance. The elevated temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attention in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fibers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Young's modulus of the composite exceeds that of the unreinforced alloy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The primary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the degradation in the matrix stiffness. Reinforcing the alloy with mullite fibers results in only a moderate improvement in strength at room temperature but the elongation to failure is reduced considerably. Increasing the amount of shot, although not appreciably degrading strength, further reduces the ductility. Shot is found to play an important role in the damage evolution by fracturing early in the loading process, and thus, the composite integrity when subjected to slow stable crack growth, as in fatigue, for example, could be adversely affected.
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