# A Guideline for Implementing a Robust Optimization of a Complex Multi-Stage Manufacturing Process
## 3 The Methodology for Continuous Improvement: Robust Process Optimization
### The Scanning Acoustic Microscopy (SAM) Inspection  
The techniques able to contribute to the optimization of the manufacturing process, in general, are based on the disassembly of the product, in which the reverse flow does not impact the quality improvement. In fact, the reverse engineering can help for viewing the defects [21], but in many process this is no possible due to the destruction of the product. The US probes is the case in which the quality inspection requires non-destructive testing techniques (Figure 4).  
The infrared thermography is influenced by ambient factors (i.e., moisture, temperature) and by different reflection coefficients of the materials, especially when the components are embedded in or bonded with each other in micro-metric range.  
The optical microscopy is strongly limited to a surface investigation and X-ray computed tomography (XCT) is non-effective in case of layers of micrometers thickness, in which the resolution is a negative aspect to consider when the defects with dimension in the micrometric order must be detected. Furthermore, the composition of the PMN-PT material limits the potentiality of the X-ray-based technique due to the high content of Pb. In fact, lead acts as a shield and the result is an image without information. The authors implemented a large campaign of measurement by means of XCT analysis in order to test the effectiveness of high-resolution 3D X-ray microscopy by means of SKYSCAN 1272 (14,450 \(\times\) 14,450 pixels)--BRUKER. The phase-contrast and the resolution did not distinguish the virtual slices through samples and, therefore, the defects. The benchmark led the authors to the synchrotron X-ray imaging, but the timing of scan and the data processing for an area with dimension \(\sim\)900 mm\({}^{2}\) and thickness of 1 mm have been calculated more than two days per piece and the conditions for the promising in-process inspection becomes very prohibitive for the manufacturing of the products. However, the X-ray-based techniques can be effective for smaller samples under test and in case of layer-by-layer inspection.  
On the other hand, the visualization of the internal structure in a non-destructive manner, and the detection of the potential effects of the failure root causes make SAM an important tool in a wide range of manufacturing process. SAM has high potential in microelectronics packaging [63] by detecting voids [64], delamination [65,66], and micro-cracks [67]. Defect inspection proved high confidence levels of failure localization and the reliability assessment in MEMS (Micro electronic mechanical systems) sensors [68].  
SAM is an in-line inspection investigated in the previous study by the authors [69]. SAM analysis has been effective in non-destructive testing for the detection of failure during the manufacturing process of US probes. Therefore, this method becomes very attractive combined with less preparation time for the inspection aiming to reduce the costs of scrapped material and to determine a priori the quality degree of a product.  
The challenge is to detect the potential defects on a multilayered structure with dimension of 60 mm in length and 15 mm in width. The generation of internal defects, e.g., micro-cracks and delamination, or the realization of non-homogeneous layers of epoxy adhesive in the ultrasound transducers for medical imaging are not tolerable.