I. Charles Ume

Automated system for laser ultrasonic sensing of weld penetration

An on-line sensor is required for real-time control of penetration depth in robotic welding. Currently, no on-line techniques exist for direct penetration depth measurement. The development of a penetration control system will significantly reduce the costs associated with repairing or scrapping defective welds. Laser array generated ultrasound is a noncontact, nondestructive method that can potentially be used to measure weld penetration. This paper will discuss the design and implementation of an automated system that was used to measure the depth of simulated solidified welds. The system consists of a laser array generation source and an electromagnetic acoustic transducer (EMAT) receiver. Measured penetration results for the simulated weld are compared with the exact results. Experimental results will also be shown for array gain measurements in order to demonstrate the power of the array. This automated system is being modified in readiness for use in actual robotic welding.

Development of automated real-time data acquisition system for robotic weld quality monitoring

Abstract The lack of reliable non-contact, non-destructive, online sensors with the ability to detect defects as they form and with the capacity to operate at high temperatures and in harsh environments is a big obstacle to fully automated robotic welding. This paper presents a non-contact automated data acquisition system for monitoring a robotic gas–metal arc welding process based on laser ultrasonic technology. While a robot welds between two 1040 steel strips, a Nd:YAG Q-switched pulse laser generates ultrasound on one side of the weld by ablation, and a non-contact electro-magnetic acoustic transducer (EMAT) placed on the opposite side of the weld detects ultrasound transmitted through the weld bead. Ablation is employed because high temperature specimens require strong signals to compensate for attenuation within the bulk of the material. The data is then analyzed to determine the time required for ultrasound to travel from the laser source to the EMAT, termed as the time of flight (ToF). When experimental ToF is compared to theoretical ToF, it is determined that surface waves are detected by this system. Therefore, this system can measure weld bead reinforcement distance. In most cases, weld bead geometry is an indication of the weld quality, and can be used as feedback to control a welding process.

Laser generated ultrasound by material ablation using fiber optic delivery

This paper presents a laser/optical fiber ultrasonic probe capable of generating ultrasound by material ablation. In addition to the Q-switched Nd:Yag laser and optical fiber(s), a focusing objective is required to ensure ablation, and provides for a safe distance between optics and test specimen. This paper includes a discussion of the development of the optical delivery system and system calibration. The laser/optical fiber probe is highly suited to industrial applications because it combines the flexibility of optical fiber delivery with strong generation capabilities.

Parametric Studies of Laser Generated Ultrasonic Signals in Ablative Regime: Time and Frequency Domains

A laser pulse incident on a material may generate ultrasound by means of two different phenomena: thermoelastic effect at low power density and ablation effect at high power density. Ablative generation of ultrasound is necessary for some critical applications such as on-line weld quality monitoring in which strong signals are required to compensate the elevated temperature and the long path length. While the waveform in time domain has been discussed extensively in the literature, there is little knowledge about the frequency components of laser ultrasound, although this information is necessary for practical applications. In this paper, analytical results from both thermoelastic and ablative regimes are reviewed. Laser ultrasonic signals (longitudinal waves and surfaces waves) generated by laser ablation are measured in a number of metal samples (2024 A1, 6061 A1, 7075 A1, mild steel, and copper) with a broadband laser interferometer. The frequency spectra are analyzed and compared for different thicknesses (50.8 mm, 25.4 mm, 12.7 mm, and 6.4 mm) and for different power densities. Hanning windowing is applied to the longitudinal pulses in time domain before frequency analysis is performed. The experimental data match the theoretical predictions very well. The results show that the frequency spectrum extends from 0 to 15 MHz, while the center frequency occurs near 2 MHz. The detailed distribution of the spectrum is dependent on the material, thickness, and laser power density.

Thermal stress and fatigue analysis of plated-through holes using an internal state variable constitutive model

Abstract Previous related research on plated-through hole (PTH) fatigue investigations has been based on the so-called effective stress/strain methods, which did not account for the fact that fatigue crack nucleation and growth is observed to occur on planes of specific orientation. Moreover, previous related thermal stress/strain analyses were at most based on bilinear constitutive relations for modeling copper plating along with a linear kinematic hardening assumption, and this cannot capture many aspects of cyclic stress/strain behavior during thermal excursions. In this paper, thermal stress analyses using internal state variable (ISV) models of metallic constituents of PTHs are conducted using the finite element code ABAQUS (1996). Two thermal history profiles having two repeated cycles were applied for the PTHs of a double layered printed wiring board (PWB) uniformly: (1) MIL-T-CYC (between −65°C and 125°C), and (2) IEC OIL-T-SHOCK (between 25°C and 260°C). A critical plane theory was used for purposes of multiaxial fatigue life prediction. The stress/strain results were reported and compared at the PTH corner and barrel. For both cases, the thermomechanical mismatch between the FR4 and copper constituents of the PWB generates nonproportional stress/strain responses. This complicates PTH thermal fatigue investigation.

Graduate Mechatronics course in the school of mechanical engineering at Georgia Tech

The field and application of Mechatronics is broad, encompassing, and interdisciplinary. Teaching Mechatronics to mechanical engineering graduate students is difficult because many students lack undergraduate backgrounds in control theory, digital and analog electronics, and in low to mid-level programming. Therefore, how to teach Mechatronics continues to be an open area for discussion. This paper focuses on the structure of the graduate Mechatronics course in the Woodruff School of Mechanical Engineering at Georgia Tech. An overview of the recently revised graduate Mechatronics course and the laboratory experiences is presented. Student projects are also included to demonstrate knowledge learned in the course.

Board-Level Solder Joint Reliability Study of Land Grid Array Packages for RF Application Using a Laser Ultrasound Inspection System

Microelectronics packaging technology has evolved from through-hole, and bulk configuration to surface-mount, and small-profile ones. Today’s electronics industry is also transiting from SnPb to Pb-free to meet environmental requirements. Land grid array (LGA) package has been becoming popular in portable electronics in terms of low profile on the printed wiring boards and direct Pb-free assembly process compatibility. With the package profile shrinking and operating power increasing, solder joint quality and reliability has become a major concern in microelectronics manufacturing. The solder joint failure at the package level or board level will cause electronic devices not to function during service. In this paper, board-level solder joint reliability of the LGA packages under thermal loading is studied through thermal cycling tests. A novel laser ultrasound-interferometric system developed by the authors is applied to inspect solder joint quality during the thermal cycling tests. While the laser ultrasound inspection technique has been successfully applied to flip chips and chip scale packages, this study is the first application of this technique to overmolded packages. In this study, it is found out that the LGA packages can withstand 1000 temperature cycles without showing crack initiation or other failure mechanisms in the solder joints. The laser ultrasound inspection results match the visual observation and X-ray inspection results. This study demonstrates the feasibility of this system to solder joint quality inspection of overmolded packages. In particular, the devices constituting the objective of this study are radio frequency modules, which are encapsulated through overmolding and are mounted on a typical four-layer FR4 board through LGA terminations.

Effects of Warpage on Fatigue Reliability of Solder Bumps: Experimental and Analytical Studies

Out-of-plane displacement (warpage) has been a major thermomechanical reliability concern for board-level electronic packages. Printed wiring board (PWB) and component warpage results principally from CTE mismatch among the materials that make up the PWB assembly (PWBA). Warpage occurring during surface-mount assembly reflow processes and normal operations may lead to severe solder bump reliability problems. In this research, the effect of initial PWB warpage on the low cycle thermal fatigue reliability of the solder bumps in plastic ball grid array (PBGA) packages was studied using experimental and analytical methods. A real-time projection moire warpage measurement system was used to measure the surface topology of PWBA samples at different temperatures. The thermal fatigue reliability of solder bumps was evaluated from experimental thermal cycling tests and finite element simulation results. Three-dimensional (3-D) models of PWBAs with varying board warpage were used to estimate the solder bump fatigue life for different types of PBGAs mounted on PWBs. In order to improve the accuracy of FE results, the projection moire method was used to measure the initial warpage of PWBs, and this warpage was used as a geometric input to the FEM. The simulation results were validated and correlated with the experimental results obtained using the projection moire technique and accelerated thermal cycling tests. An advanced prediction model was generated to predict board level solder bump fatigue life based on the initial PWB warpage, package dimensions and locations, and solder bump materials.

Development of a Flexible Laser Ultrasonic Probe

Ultrasonics is a widely used nondestructive testing technique, which is often applied off-line for weld quality inspection. Laser ultrasonic (LU) inspection systems have the potential for on-line application, providing the means to identify unacceptable welds as they are formed. Because LU systems are non-contacting, they can be used for testing moving specimens or for operation in hazardous and/or high temperature environments. A highly versatile system can be created when an optical fiber delivery system is incorporated into the design. Introduction of a focusing objective increases the allowable working distance and permits stronger generation using material ablation as the generating mechanism. This paper describes the development of a laser ultrasonic probe using an optical fiber delivery system with a distal end, focusing objective. The optical fiber delivery system can be configured as a single fiber source, a linear array (fiber bundle) or a phased array. Results include experimentally obtained directivity patterns demonstrating ultrasonic generation using ablation sources. Thermoelastic source results are also included. This paper demonstrates the potential of the fiber tool and presents an overview of the weld control scheme.

Nondestructive Evaluation of Poor-Wetted Lead-Free Solder Bumps in Ball Grid Array Packages Using Laser Ultrasound and Interferometric Technique

A laser ultrasound and interferometer inspection system has been successfully applied to detect solder bump defects, including missing, misaligned, open, and cracked solder bumps in flip chips, land grid array packages and chip capacitors. This system uses a pulsed Nd:YAG laser to induce ultrasound in the chip packages in the thermoelastic regime; it then measures the transient out-of-plane displacement response on the package surface using a laser interferometer. The amplitudes of the out-of-plane displacement responses are usually in the order of nanometer. The quality of solder bumps is evaluated by analyzing the transient responses. In this paper, this system is used to evaluate the quality of lead-free solder bumps in ball grid array (BGA) packages; specifically, BGA packages with poor wetted solder bumps are used as test vehicles. Poor wetting not only decreases the mechanical strength of interconnection at the interface between the solder bumps and substrate, but also deteriorates electrical and thermal performance because of the unreliable connections between the package and the printed circuit board (PCB). Causes of poor wetting vary from the solder materials themselves to the manufacturing process. Here, poor wetting of solder bumps were intentionally created by using an improper reflow profile. The transient out-of-plane displacement signal responses from these packages are compared to a hybrid reference signal generated by signal responses from defect-free samples. Packages with poor wetted solder bumps are distinguished from the normal packages by using the correlation coefficient signal analysis method. Then, laser ultrasound inspection results are also compared with results from continuity test, X-ray inspection and scanning acoustic microscope (SAM). Finally, the cross-section images are further used to confirm the existence of the poor wetting in samples with unusual correlation coefficient. It can be concluded that this laser-ultrasound system is capable of identifying the presence of poor wetting in BGA packages.