The recent innovations in additive manufacturing electronics (AME) have paved the way for printing 3D devices and flexible hybrid electronics (FHE) on the micrometer to millimeter scale, including passive devices, sensors, redistribution boards, packages, and electromechanical systems. These applications demand the characterization of mechanical properties for multi-material and multi-layer structures. The ultrasound-based method is widely used for characterizing the mechanical properties of materials at macroscopic scales. The mechanical properties of AME devices affected by deposition processes, polymerization processes, and strain gradient-induced bows can also be characterized by ultrasound methods, where a specific frequency range (0.1-30 GHz) and spatial resolution (0.1-100 µm) are needed to resolve microscale AME features. This work presents the application of GHz ultrasound imaging for the metrology of material properties, defects, and strain gradient-induced warpage in AME devices. The principle of ultrasound pulse-echo reflectometry at GHz frequencies is used for detecting material properties and dimensional metrologies. The ultrasound echo signals resulting from the acoustic impedance differences at the interface of two or more media, such as the silicon surface of the imager chip and an additively manufactured structure, are detected at each piezoelectric transceiver pixel and used for constructing a digital ultrasound image. The GHz ultrasound imager array in this study consists of 128 by 128, 50-micron pixels that can operate between 1-1.9 GHz for acquiring echo data at ~9-20 fps at full frame. The high spatial resolution resulting from the 50-micron pixel size and the use of GHz frequencies allow for resolving micron scale defects (cracks, scratches, bubbles, etc.), and mapping surface layer mechanical properties. The defects that cause discontinuities on the surface layer due to delamination or misalignment of layers in the additive manufacturing processes can also be determined by forming ultrasonic interference patterns in a thin interfacial liquid film between the imager chip and the AME structures. The discontinuities, as small as one-micrometer difference in the height profile, provide the necessary wave propagation path length difference along the Z-axis for an ultrasound wave in the GHz range to produce interference patterns in thin liquid films that can be captured in the ultrasound image. The interference ring sizes in different regions on the print can also be used to obtain information about this path difference, which can be used to compute the amount of strain. Besides having the ability to surface detect defects, stress gradients, and mechanical properties of AME structures, ultrasound pulse-echo reflectometry, and interference phase measurements also opens up the possibility of imaging multilayered or buried devices for manufacturing inspection, hardware malware detection, counterfeit detection, layer adhesion quality assessment, structural failure analysis, aging assessments, and nondestructive 3-dimensional structural metrology for FHE components.
