Mapping of solder mask covered interconnects on high density printed circuit board

As the density of interconnects on printed circuit board is increasing tremendously, the use of optical microscope for checking of open trace is running out of steam. While electron microscope has the required resolution, the presence of solder mask on the top surface of the board hinders its direct application. Recently, the method of capacitive coupling voltage contrast (CCVC) method showed the feasibility for such a checking around a small region where open circuit or broken trace is located. In this work, we extend the CCVC method to obtain a contrast map of interconnects on an entire printed circuit board. With the use of simple image processing on the contrast map, both interconnects and the site of open circuit can be observed, making it a useful quality and failure analysis tool for copper traces integrity test. Finite element modeling of the CCVC is also performed, and the results agree qualitative with the experimental results