Non-destructive quality testing with photoacoustic imaging and optical coherence tomography

Non-destructive testing (NDT) is vital for product safety, quality, and compliance in various industries. It enables early flaw detection, cost-effective maintenance, and customer satisfaction. NDT encompasses technologies like vision testing, radiation testing, ultrasonic testing, and magnetic testing, which are tailored to specific requirements such as environment, size, and material of the specimen. We present concurrent photoacoustic imaging (PAI) and optical coherence tomography (OCT) for non-destructive inspection of printed circuit boards (PCBs). PCBs are integral to electronic products, contributing to their miniaturization and portability. Due to the small size of RLC components within PCBs, high-resolution NDT methods are essential. PAI combines optical excitation and ultrasonic detection based on light absorption, while OCT is a purely optical imaging technique based on interference of backscattered signals. By combining PAI and OCT, we can simultaneously obtain complementary information on light absorption and scattering. However, the development of a dual-modality system combining PAI and OCT is challenging due to the opacity of conventional ultrasonic transducers. To overcome this challenge, we have developed an optically transparent ultrasound transducer for coaxial combination of PAI and OCT. Our approach successfully detects anomalies such as internal thin layer delamination and internal wire disconnection that are difficult to visually observe within PCBs, providing high-resolution PA and OCT images. In particular, unlike conventional pure optical imaging methods, PAI and OCT are depth-resolved 3D volumetric images so that internal defects can be investigated through comprehensive information in the depth direction of the sample. This dual-modality using PAI and OCT for NDT holds great promise for inspecting various specimens, including PCBs, and contributes to enhanced quality control and reliability.