Abstract
An acoustic micrometer developed recently by the present authors has been applied to the thickness measurements of copper surface layers in copper and glass-epoxy multi-layered printed circuit boards, for which no non-destructive method was available previously. In the measurements, Lamb waves of S0 mode were excited in the surface copper layers by broad band ultrasonic plane waves which were obliquely incident upon the boards. Dips were observed in the frequency spectra of reflected waves, and the frequency for the dip was inversely proportional to the thickness of the surface copper layer. Because the ultrasonic waves were absorbed in the glass-epoxy layers just below the surface copper layers, measurements were not affected by the presence of the intermediate copper layers in the multi-layered printed circuit boards. Thus, a non-destructive method has been established with which the thicknesses of surface copper layers of multi-layered printed circuit boards, could be measured. Measurements for a wide range of thickness from 17μm to 70μm have been achieved by using the ultrasonic waves with a frequencyband from 20MHz to 120MHz.
