N. Guo

Advances in Ultrasound Technology for Environmental Remediation

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Formation of micro protrusion arrays atop shape memory polymer

Utilizing the significant shape recovery ability (in the order of 100% strain) in shape memory polymers (SMPs), we propose two simple approaches, namely laser heating and indentation, to produce micro-sized protrusion arrays. In the former, after local laser heating a pre-compressed SMP, protrusive bumps can be produced. In the latter, through an indentation-polishing-heating process, various shaped protrusive bumps can be produced. It is to demonstrate that indentation is a more convenient and powerful approach than laser heating, since well controlled, different shaped protrusion arrays can be realized.

Non-destructive evaluation of the interface between silicon dies and copper leadframes in integrated circuit packaging

The silicon die and copper leadframe in integrated circuit (IC) packaging are bonded together by die attach adhesive, and the quality of the interface is a critical issue in the reliability of IC packaging. Common defects such as cracks and delaminations can be detected using the C-scan ultrasonic microscopy method with sufficient confidence. However, weak interfaces due to weak adhesion and poor cohesion have often gone undetected, to subsequently become potential defective areas. In this paper we present experimental work to evaluate the quality of the interfaces that typically exist in IC packages using longitudinal ultrasonic wave propagation with contact transducers. Three different conditioning processes, varying curing, moisture exposure and pre-curing moisture contamination, are used to degrade the interface bonding the silicon die and copper leadframe. Ultrasonic reflection coefficients from the interface are then measured. The results show that the reflection coefficient depends strongly on the interface quality, and can be used as a quantitative indicator to characterize the bond quality.

Automatic Defect Characterization using Artificial Neural Networks and Deconvolution Techniques

An automatic defect testing system is dealt with in this article. It includes two parts, defect feature extraction, and defect classification and sizing. Defect feature extraction is carried out by adaptive filter deconvolution. The time delay between two consecutive taps of the adaptive filter is one half cycle of the ultrasonic echo to be processed. Wideband ultrasonic defect echoes of center frequency 1.2 MHz generally have 2-4 cycles and 100-200 data points for a 50 MHz sampling rate. After deconvolution, data are reduced to 4-8 points, and the frequency bandwidth is greatly extended. As a result, the defect features stand out. The deconvolved defect echoes are presented to an artificial neural network (ANN) for automatic defect classification and sizing. Two application examples are given in this article, exact classification and reasonable sizing accuracy have been achieved.

Lamb-wave-based damage detection using wave signal demodulation and artificial neural networks

The interaction between Lamb wave and damage will modify the response wave signal from which information related to damage can be extracted for automated damage detection. However, the interpretation of the response wave signal is not easy due to the complex nature of the wave-damage interaction. This paper discusses a damage detection algorithm based on wave signal demodulation and artificial neural networks (ANNs). The response wave signal is considered as a low-frequency signal modulated by a high-frequency carrier signal. After baseline subtraction, frequency domain convolution and filtering, the original signal is demodulated and transformed into a new simplified signal related to the energy change due to damage. Subsequently feature extraction is carried out by finding the local maxima in the new signal and the obtained peak values and locations are used as inputs into the ANNs for damage characterization. The validity of this damage detection algorithm is then verified using a finite element (FE) model of a composite laminate with notch defects. The response wave signals of different notch depths and locations are acquired from the simulations and used as the training and testing samples. Finally the assessment of the networks accuracy and generalization ability is performed and the result is satisfactory.

SELECTIVE MODE EXCITATION OF LAMB WAVE IN COMPOSITE LAMINATES

Lamb wave has a great potential for structural health monitoring of large area structures due to its long range propagation. The piezo array configurations and array pitch for selective mode excitation are first studied with 2D FE models in a composite plate. A single PZT actuator bonded on the top surface of the laminate in unimorph configuration excites both symmetric mode (S0) and anti-symmetric mode (A0). The A0 mode is predominant and four times stronger than the S0 mode. The second configuration is a bimorph where the PZT actuators are bonded on either side of the laminate. The in-phase excitation of PZT actuators suppress the A0 mode and excite only the S0 mode, while the out-of-phase excitation eliminates the S0 mode and launches the A0 mode in the laminate. The third configuration is the unimorph array configuration where PZT actuators are bonded on the top surface of the laminate. By keeping the array pitch equal to half wavelength of the S0 mode, the S0 mode gets suppressed by twelve times. The FE results are checked with experiments.

Characterization Methodology of the Interface in Multilayer Composites

The quality of the interfaces in multilayer composites is a critical issue in the reliability testing of the composite product during the manufacturing process and in-service. Weak interfaces have often gone undetected and may become potentially defective at a later stage. One example is the interface between mold compound and silicon (MC/Si) in IC packaging. There is a desire to study the interface quality quantitatively, so the potential defective area can be evaluated and identified early. In this paper, a nondestructive evaluation methodology is proposed to measure the available strength of the interface by using ultrasonic reflection coefficients. It is known that interface degradation can be either due to poor manufacturing process and stress loading. Characterization of the interface quality of the MC/Si interface is first conducted by measuring longitudinal ultrasonic wave reflections from the interface samples fabricated under varying conditioning processes that simulate the degradation. A combined test that measures the reflection coefficient of the interface under stress load has also been conducted to quantify the effect of the load. Finally, it is shown that the overall effect on the reflection coefficient and available strength of the interface is derived and can be used as a quantitative indicator.

Active Vibration Control of the Print Circuit Boards Using Piezoelectric Bimorphs

Piezoelectric bimorphs were assessed for their capabilities to be used as control actuators for vibration suppression of the print circuit boards (PCBs). Plate structures made of FR-4, a widely used industrial-grade material for manufacture of PCBs, were considered. An advanced and structured control algorithm, linear quadratic regulator with output feedback (LQROF), was used for active vibration control of the PCB structures. Experimental results showed that the LQROF control is a more robust algorithm than the classic control using the direct velocity feedback, and piezoelectric bimorph actuators present a great potential for active vibration control of the PCBs, and smart composites with embedded actuators.