Multicontrast and Nondestructive Transparent Ultrasound Transducer-Based Photoacoustic and Optical Coherence Imaging of Multilayered Electronics

Flexible printed circuit boards (FPCBs) are extensively employed as core components of modern electronic devices, including mobile and wearable devices and displays, due to their low thickness, flexibility, and low weight. However, as FPCB integration technology advances, the inner layer thickness and internal hole sizes in the FPCBs decrease, and the signal lines become finer, which increases the probability of surface and internal defect formation. We propose a nondestructive dual-modal imaging system that utilizes the light absorption contrast of photoacoustic imaging (PAI) and light scattering contrast of optical coherence imaging (OCI). Dual-modal PAI/OCI visually identifies surface and internal defects on FPCBs that cannot be detected by the current automatic optical inspection (AOI) system in a manufacturing environment. The PAI/OCI feature is seamlessly integrated into a compact and coaxial setup using a cutting-edge optically transparent ultrasound transducer (TUT). Internal features, invisible to optical microscopes, are successfully visualized using the proposed system, which intuitively identifies inner layer delamination and internal defects. Therefore, the dual-modal PAI/OCI system is a feasible tool for controlling the quality of FPCBs during manufacturing.