DETERMINATION AND MONITORING OF THROUGH HOLE DIAMETERS IN PRINTED WIRING BOARDS DURING ELECTROLYTIC PLATING USING ACOUSTIC DIFFRACTION

In this paper, we describe a method to monitor and determine the chang e in the diameter of a through hole in a printed wiring board during e lectrolytic plating. Since the change in the radius of such a hole dir ectly relates to the thickness of the deposited layer of material insi de the hole, actual plating thicknesses may be determined in situ, thu s potentially permitting an online optimization of the plating process itself. Fundamentally, our method is based on the evaluation of the d iffraction pattern emanating from a circular aperture in a thin acoust ic baffle, when subjected to an acoustic field. First experiments in a nickel plating bath have shown that thickness changes of a few microm eters in a 3 mm (0.12 in.) through hole can be detected.