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36 The PCB Magazine • May 2014 by A. J. Cobley, B. Abbas, A. Hussain, and B. Mkhlef Functional materials applied research Group coventry university, u.K. The Effect of Low-Frequency Ultrasound on Catalysed Electroless Copper Plating One of the most advantageous features of the electroless copper process is that it enables the metallisation of non-conductive substrates. It has been employed in the electronics indus- try for decades because it enables the plating of through-holes and vias in PCBs. The non- conductive substrate must first be catalysed (ac- tivated) and this is normally achieved using a palladium (Pd)/tin colloidal solution. The tin is subsequently removed in either an accelerator solution or in the electroless copper electrolyte itself, leaving Pd on the surface. The chemistry of electroless copper plating on a catalysed sur- face is a complex mix of electrochemical and chemical reactions but a simplified model is shown in Figure 1. Formaldehyde (HCHO) oxidises on the Pd catalyst generating electrons (e - ) which are then consumed in the reduction of copper ions (Cu 2+ ) to copper metal (Cu 0 ). There are many potential benefits in apply- ing ultrasound to an electroless process and this was the subject of a recent review by the au- thor [1] . The majority of the previous studies on the effect of ultrasound investigated electroless nickel plating where sonication generally pro- duced a significant increase in plating rate [2–5] . Studies on the use of ultrasound during electro- less copper plating are less numerous and few take into account the catalysation stage, despite the fact that this is a critical part in the pro- cess. One notable exception is the papers pub- lished by Touyeras et al., [6, 7] . They showed that if relatively high-frequency ultrasound (530 kHz) was introduced to the catalyst bath, then an increase in the subsequent electroless cop- per plating rate could be obtained. Indeed, by applying ultrasound in both the catalyst and F e a t u r e

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