This paper describes a system that inspects the inner wall of plated through-holes in high-density printed circuit boards. These through-holes sometimes contain “void defects”, in which part of the copper plating is missing. These voids are detected by means of fluorescence emissions from the base material of the P.C.Bs. The paper describes an optical system for illuminating the inside of a through-holes, and for detecting fluorescence emitted from the void. This system can detect voids as small as φ 150 μm on the wall of a through-hole of diameter of 0.24 mm at depth of up to 3.5 mm.
