Effects of Broadband Laser Generated Ultrasound on Array Gain

Array gain is a common parameter used in laser phased array research. This paper will present a new parameter called the frequency modulation of laser phased array (FMLPA). The array gain model for laser phased arrays was derived using an assumption that ultrasound from each array member interferes with each other. This would be always true if laser generated ultrasound is narrow band. However, laser generation of ultrasound is broad band. Broad band ultrasound signals have short duration in the time domain. If the time delay between generated wave fronts from each array member is longer than the duration of the broad band ultrasound signal from each array member, the ultrasound signals from each array member will not interfere with each other. The time delay between generated wave fronts from each array member is 0 s at a laser phased array’s beam steering angle and increases away from the beam steering angle. Therefore, ultrasound from each array member always interfere at angles close to the beam steering angle. However, ultrasound from each array member may not interfere at angles away from the beam steering angle depending on the time delay between generated wave fronts and duration of the broad band ultrasound signal. A theoretical model of the FMLPA was developed and experimentally verified for use when ultrasound from each array member does not interfere with each other. It was experimentally verified that current array gain equations still apply when ultrasound from array members interfere with each other. The FMLPA can be used to create new techniques for measuring weld penetration depth, crack location, and dimensions of objects.     

Reaction Mechanism and Kinetics of 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene and Carbon Dioxide in Alkanol Solutions

Carbon dioxide‐binding organic liquids (CO₂BOL) are a new class of solvents with advantageous properties such as high boiling points, low specific heats, high absorption capacities, and easily reversible reactions. In order to implement these solvents in processes, the reaction characteristics must be determined a priori. This work presents an analysis of the rate constants and activation energies of the reaction between carbon dioxide and 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in 1‐hexanol and 1‐propanol. The reactions were found to comply with a termolecular reaction mechanism and exhibited pseudo‐first‐order behavior in the presence of excess DBU and 1‐alkanol. It was concluded that DBU‐based CO₂BOL are environmentally friendly and easy‐to‐handle solvents that may provide great flexibility and improvements over conventional carbon dioxide absorption processes.     

Online-offline laser ultrasonic quality inspection tool for multilayer ceramic capacitors-Part I

The advent of surface-mounted devices has allowed continued size decreases in electronic packages. However, the decrease in device size has led to other manufacturing problems. Therefore, as devices decrease in size, inspection technology must be developed to maintain consistent levels of quality without increasing manufacturing process time. The focus of most inspection technologies, such as X-ray inspection, in-circuit testing, and acoustic microscopy, is on finding defects with solder joint interconnects, but some devices, such as multilayer ceramic capacitors (MLCC), have failures not relating to the solder connection. Defects in MLCCs, known as flex cracks, are commonly caused by manufacturing processes, and no current means of detection exists, unless the cracks cause a device to fail a functional test. A laser ultrasonic and interferometric system, providing a noncontact, nondestructive, and online approach for microelectronic package quality inspection is designed and presented. A pulsed infrared laser excites a specimen into vibration through laser-generated ultrasound, and the vibration displacement is measured using an interferometer. Differentiation between acceptable and unacceptable devices is achieved using signal-processing techniques that compare waveforms between two devices, where one of the waveforms is a reference waveform from a good MLCC. Results are presented for a case study involving MLCCs that have intentionally induced flex cracks, causing the devices to fail a capacitance leakage test. The system has the capability of detecting open connections and flex cracks in the MLCC packages.     

A comparison of PWB warpage due to simulated infrared and wavesoldering processes

Two types of bare, four layer printed wiring board (PWB) test vehicles were designed and fabricated to replicate simplified versions of typical boards produced by PWB manufacturers. The boards were subjected to simulated infrared and wave soldering reflow processes in a small lab-scale oven integrated with a shadow Moire out-of-plane displacement measurement system. The measured warpages were qualitatively and quantitatively analyzed at selected temperatures during the reflow profiles. The results presented compare and contrast the effects that simulated wave and infrared reflow soldering processes have on the warpage of two different PWB designs