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Piezoresistive sensors have been demonstrated to be an accurate and efficient tool for stress measurements on chip surfaces inside microelectronic packaging. In this work, test chips with piezoresistive stress sensors, diode temperature sensors as well as heaters were first designed, fabricated, and calibrated. We next packaged the test chips into low profile, fine pitch ball grid array (LFBGA) packaging with 196 balls and measured the stresses on chip surfaces inside the packaging. After measuring the packaging induced stress as well as the stress under stable environmental temperature rises, it was found that compressive stresses were obtained at room temperature, and the stresses were relaxed as temperature went up at a rate between 0.45 MPa/ °C and 0.60 MPa/ °C. For thermo-stress experiments, the temperatures on chip surfaces at different power levels were measured, and compressive chip stresses were first extracted. As the chip power increased, the compressive stresses became tensions. Since the LFBGA structure is thinner with higher packaging efficiency, different results from our earlier plastic quad flat package stress measurements were observed and discussed., In addition, the final comparisons between the experimental data and the finite element simulations show good consistency. [ABSTRACT FROM AUTHOR]

Moisture is one of the major contributing factors in fracture and reliability issues for microelectronic packaging. To characterize the moisture-induced stress distribution inside the packaging structure, an in situ, quantitative, and nondestructive experimental methodology is needed. This paper proposes the use of piezoresistive sensors to measure moisture-induced stress in a plastic low profile, fine pitch, ball grid array (LFBGA) packaging. The measurements include hygroscopic swelling stress extractions and real-time stress monitoring of the popcorn phenomenon, and the results associated with gravimetric analyses are reported. Postreflow scanning acoustic microscope (SAM) inspection results and cross section observations are used as experimental verification. Comparing with thermal stresses previously measured on the same package, it is found that the hygroscopic mismatch stress is significant and important for package engineers. In addition, piezoresistive sensors were proven useful in this work for recording popcorn occurrence and monitoring the stress drops at the popcorn initiation. [ABSTRACT FROM AUTHOR]

We report a double-transducer technique for more effective generation of picosecond acoustic waves. A tungsten layer, which is buried under a transparent film and a thin top metal transducer, plays the role of a bottom laser-acoustic transducer as well as a high-impedance acoustic reflector. The pulse shape and the induced piezoreflectance response of the acoustic wave launched by the bottom transducer are different from the conventional top transducer. The effect of the bottom transducer depends on the thicknesses and optical constants of the top transducer and the transparent film. The accuracy of velocity measurement can be raised owing to more efficient energy utilization, halved pulse broadening and attenuation of the tungsten-launched waves, and the added signatures on the reflectance curve. [ABSTRACT FROM AUTHOR]