Scanning Acoustic Microscopy (SAM): A Robust Method for Defect Detection during the Manufacturing Process of Ultrasound Probes for Medical Imaging.

The main aim of this paper is to provide the feasibility of non-destructive testing (NDT) method, such as scanning acoustic microscopy (SAM), for damage detection in ultrasound (US) probes for medical imaging during the manufacturing process. In a highly competitive and demanding electronics and biomedical market, reliable non-destructive methods for quality control and failure analysis of electronic components within multi-layered structures are strongly required. Any robust non-destructive method should be capable of dealing with the complexity of miniaturized assemblies, such as the acoustic stack of ultrasonic transducers. In this work, the application of SAM in an industrial scenario was studied for 24 samples of a phased array probe, in order to investigate potential internal integrity, to detect damages, and to assess the compliance of high-demanding quality requirements. Delamination, non-homogeneous layers with micron-thickness, and entrapped air bubbles (blisters) in the bulk of US probe acoustic stacks were detected and studied. Analysis of 2D images and defects visualization by means of ultrasound-based NDT method were compared with electroacoustic characterization (also following as pulse-echo test) of the US probe through an ad-hoc measurement system. SAM becomes very useful for defect detection in multilayered structures with a thickness of some microns by assuring low time-consuming (a limit for other NDT techniques) and quantitative analyses based on measurements. The study provides a tangible contribution and identifies an advantage for manufacturers of ultrasound probes that are oriented toward continuous improvement devoted to the process capability, product quality, and in-process inspection.

Experimental Investigation of Inter-element Isolation in a Medical Array Transducer at Various Manufacturing Stages.

This work presents the experimental investigation of vibration maps of a linear array transducer with 192 piezoelements by means of a laser Doppler vibrometer at various manufacturing finishing steps in air and in water. Over the years, many researchers have investigated cross-coupling in fabricated prototypes but not in arrays at various manufacturing stages. Only the central element of the array was driven at its working frequency of 5 MHz. The experimental results showed that the contributions of cross-coupling depend on the elements of the acoustic stack: Lead Zirconate Titanate (PZT), kerf, filler, matching layer, and lens. The oscillation amplitudes spanned from (6 ± 38%) nm to (110 ± 40%) nm when the energized element was tested in air and from (6 ± 57%) nm to (80 ± 67%) nm when measurements were obtained under water. The best inter-element isolation of -22 dB was measured in air after cutting the kerfs, whereas the poorest isolation was -2 dB under water with an acoustic lens (complete acoustic stack). The vibration pattern in water showed a higher standard deviation on the displacement measurements than the one obtained in air, due to the influence of acousto-optic interactions. The amount increased to 30% in water, as estimated by a comparison with the measurements in air. This work describes a valuable method for manufacturers to investigate the correspondence between the manufacturing process and the quantitative evaluations of the resulting effects.