Evaluation of printed circuit boards using double-exposure holographic interferometry

Abstract

Double-exposure holographic interferometry (DEHI) is an important technique in nondestructive testing and analysis of stress induced by mechanical, thermal or pressure loadings. Some applications of DEHI in electrical systems include the evaluation of thermoelectric cooling module and photovoltaic cells. In these applications, the deformation of the objects is analyzed under thermal stress due to joule heating.
In DEHI, two waves (of the same wavelength) reflected by the same object are recorded consecutively onto the same holographic film. One of the object waves corresponds to the object in its original state and the other in its deformed state. This deformation can be quantified by measuring the point-to-point displacements during the reconstruction process. In this process, the object wave is reconstructed when the interference pattern (produced by the reference and object waves) is illuminated by the reference wave. If the deformation is of the order of the wavelength, we can observe a macroscopic interference pattern, superimposed on the reconstructed object wave, which will be referred to as the holographic interferogram. The actual displacement can be calculated by following the static method of fringe analysis technique. This method utilizes the interferogram and the holographic recording and reconstruction geometries.
In this study, we applied DEHI to evaluate a DC power supply printed circuit board (PCB) under actual operation. We studied the effect of current passing through the individual clectncal components by measuring the PCB deformation. This deformation is due to local temperature changes on the PCB. From the fringe analysis, potential failure sites on the PCB were located and actual displacements were calculated.