Application of the pressure wave propagation method for adhesion defects detection and quantification in bilayer structures

Abstract

The pressure wave propagation method, usually used to study electrical behavior of dielectric materials, is applied here to nondestructive detection and quantification of adhesion defects in a bilayer structure. This method relies on a very simple idea: as an electric field is created in a bilayer dielectric sample placed between short-circuited electrodes, the propagation of a pressure pulse induces an electric signal. If the field distribution is known, the signal leads to the pressure profile all along its propagation through the sample and therefore gives information on the interface. First, we extend the signal expression already established for a monolayer structure to a multilayer structure and consider particularly bilayer structures. After explaining the signal analysis in perfectly bonded and totally disbonded structures, a model is proposed to describe and analyze the signal in a partially disbonded structure and relate it to the percentage of totally disbonded area in the zone being tested by the pressure pulse. To assess this analysis, measurements were carried out on kapton (130 μm)–adhesive (98 μm) transparent samples. Some samples are perfectly bonded, some are totally disbonded and the others are partially disbonded. The pressure pulse is created by the impact of a laser pulse on an absorbing target coupled to the sample. The excellent agreement between the measurements carried out on perfectly bonded and totally disbonded samples and simulations assesses the correctness of the signal expression for a multilayer structure. In the case of partially disbonded structures, the value of the percentage of totally disbonded area, determined by measurement and simulation, is very close to that deduced from a photograph of the sample, only possible for transparent materials. The spatial resolution of the method is related to the spectrum of the pressure pulse, which extends up to 200 MHz. This method presents an excellent spatial resolution. If the transit time of the pressure pulse in each medium is superior to 11 ns, measurements and simulations show that owing to this method, it is possible to detect and localize in the structure submicron gaps and a percentage of totally disbonded area as low as 5% in the tested zone by analyzing the signals in the time domain in a very simple manner.

Application of the pressure wave propagation method for adhesion defects detection in very thin multilayer structures

“…As we have already seen in Ref. 10, a single measurement of the short circuit current generated by the propagation of a pressure pulse in a dielectric film leads to the transit time of the pressure in the sample and to the pressure profile in front and at the back of the film. Therefore, if the thickness of kapton, adhesive, and PEEK films is known, this measurement leads very simply and accurately to the acoustic velocity , to the acoustic attenuation ␣, and to the dispersion in the material.…”

“…10, we showed for the first time how the PWP method, initially meant to determine space charge distributions in dielectric samples, can be applied with success to the detection and quantification of adhesion defects in insulating bilayer samples. This method relies on a very simple idea.…”

“…The meaning of the coefficient G( r ), already given in Ref. 10, as well as the explanations necessary to understand what is following will be developed in Sec. IV.…”

“…In a recent paper, 10 we proposed an original method for nondestructive testing of interfaces. As in the LBU techniques, information on interface quality is obtained by studying the propagation of laser generated pressure pulses in the samples, but the detection mode is quite different.…”

“…In Ref. 10, this was applied to bilayer dielectric samples. It was shown that it is possible to study very thin structures without mathematical treatment, with an excellent spatial resolution related to the spectrum of the pressure pulse.…”