Enhanced robustness and extended dynamic range phase-sensitive SDOCT for reliability monitoring of PCB reflow soldering process.

For printed circuit board (PCB) component assembly, the reflow soldering process is widely adopted in the electronics manufacturing industry to solder and secure electronic components onto PCB surfaces. Owing to mismatches in the coefficients of thermal expansion and Young's modulus across the heterogeneous material layers of PCBs, PCB warpage may occur during reflow soldering. Such deformation further induces tombstoning defects and compromises the reliability of electrical interconnections between components and PCBs. This paper proposes a phase unwrapping method with large dynamic range and high robustness for phase-sensitive spectral domain optical coherence tomography. The method simultaneously retrieves the spectral frequency and the wrapped phase in the spectral domain, and determines the fringe orders from the difference between the absolute phase and the wrapped phase. Compared with conventional phase unwrapping algorithms, the proposed method features a wider dynamic range and higher robustness, enabling the simultaneous detection of micrometer-scale PCB warpage and component out-of-plane displacements on the order of hundreds of micrometers. Experimental results demonstrate that the proposed method holds great potential as a practical and highly robust approach for reliability assessment in the reflow soldering process.