Nondestructive Inspection and Quantification of Soldering Defects in PCB Using an Autofocusing Digital Holographic Camera

The essential requirements for any digital holography-based system to get employed as an inspection device in electronic industry are portability and ability to automatically focus on the reconstruction plane. Any deviation from the exact propagation distance between the hologram plane and the reconstruction plane may result in erroneous measurement of the surface features under inspection. We report for the first time, to the best of our knowledge, the autofocus capability in a digital holography-based portable camera for full-field quantitative inspection of solder joint defects in a printed circuit board (PCB). An intensity gradient-based algorithm is developed and applied to the digital hologram to automatically locate the reconstruction plane and determine the propagation distance. For a given range of distance, the algorithm calculates the maximum value of high-frequency components in the reconstructed image, and the corresponding propagation distance is used in the reconstruction. The autofocus algorithm along with the double exposure digital holographic interferometry (DHI) is applied on the PCB under thermal loading. 2-D and 3-D deformation maps are analyzed to detect different kinds of solder joint defects, such as holes, irregular shapes, excess soldering, and bridging. The work presented in this article may further stimulate the development of digital holographic techniques for several applications in metrology and inspection in the electronics manufacturing industry.