Ultrasonic evaluation for residual stress measurement has been an effective method owing to its easy implementation, low cost and intrinsically being nondestructive. The velocity variations of acoustic waves in materials can be related to the stress state in the deformed medium by the acoustoelastic effects. In this study, a laser/EMAT ultrasonic method is proposed to evaluate the surface/subsurface longitudinal residual stress distribution generated by gas metal arc welding (GMAW). The velocity variation ΔV/V of Rayleigh wave, which is a surface wave, will be experimentally measured. Q-Switched Nd:YAG laser is used to generate a broadband ultrasonic wave. An electromagnetic acoustic transducer (EMAT) is attached to the welding plate for Rayleigh wave pick up. As the ultrasound receiver, the EMAT is used to measure time of flight (ToF) of the Rayleigh waves traveling along a specific path parallel to the direction of the welding seam. ToF measurements are obtained by changing Rayleigh wave path to welding zone center distance from 0 to 45 mm. A 3D thermomechanical-coupled finite element model is then developed to validate the capability of the proposed technique for welding-induced residual stress evaluation. The distributions of the normalized velocity variations from ToF experiments are compared with the distribution of the normalized longitudinal residual stresses from finite element analysis (FEA). It has been shown that there is a good correlation between these two distributions. The proposed technique provides a potential nondestructive avenue for surface/subsurface residual stress evaluation for welding parts.
Laser Ultrasonic Inspection (LUI) is a non-destructive and non-contact technique to evaluate the quality of solder ball interconnections in area-array microelectronic packages. Dual-Fiber Array Laser Ultrasonic Inspection System was demonstrated identifying defects and failures in chip-scale packages, ball grid array packages, and flip-chip ball grid array packages. The location and severity of the defects and failures in packages have been identified accurately using this system. Further, it is important to establish the correlation between LUI results and the severity of the failures for failure mode analysis, which will enable us to eliminate the need for destructive testing and allow the study of failure evolution in a given sample under continued reliability testing. This paper discusses correlation studies between experimental LUI results and finite-element simulation results from the flip-chip ball grid array packages subjected to thermal cycling reliability testing. The correlation equations will help in predicting the severity of the failures at a given number of thermal cycles based on LUI results. Furthermore, the life of the microelectronic packages can be predicted accurately from LUI results at a fewer number of thermal cycles.
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