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38 The PCB Magazine • May 2014 the electroless copper solution the plating rate could be almost doubled. These papers clearly demonstrated the importance of the catalysa- tion step in the electroless copper process. Studies at Coventry University The Functional Materials Applied Research Group at Coventry University has carried out a three-year project funded by the Innovative electronics Manufacturing Research Centre (IeMRC), which investigated the effects of ul- trasound on electroless and immersion plating processes. This work mainly utilized sonication at low frequency (40 kHz) which, for industrial applications, is preferred due to cost, availabil- ity and its more uniform effects in a bulk solu- tion. These studies quickly indicated that the effect of ultrasound on the catalysed surface must be considered if the beneficial effects of sonication are to be realised. X-ray photoelec- tron spectroscopy (XPS) analysis of the surface of the catalysed substrate after immersion in a simulated electroless copper solution (Table 1) showed that the application of ultrasound had a dramatic effect on the concentration of Pd on the surface. It can be seen that the catalysed substrate that had been immersed in the soni- cated electroless copper solution had almost no Pd remaining on the surface after 10 minutes. When ultrasound is applied to a liquid medium, a phenomenon known as acoustic cavitation occurs that leads to the creation of cavitation bubbles. The collapse of these cavi- tation bubbles near a solid surface produces microjetting, which is the driving force behind ultrasonic cleaning, as these microjets have a scrubbing action on the surface of material THE EFFECT OF LOW-FREqUENCY ULTRASOUND continues Figure 1: oxidation/reduction reactions occurring on a Pd catalysed surface.