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The Effect of Low-frequency Ultrasound on Catalysed Electroless Copper Plating
July 29, 2014 |Estimated reading time: 1 minute
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 industry for decades because it enables the plating of through-holes and vias in PCBs. The non-conductive substrate must first be catalysed (activated) 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 surface is a complex mix of electrochemical and chemical reactions, but a simplified model is shown in Figure 1.
Figure 1: Oxidation/reduction reactions occurring on a Pd catalysed surface.
Formaldehyde (HCHO) oxidises on the Pd catalyst generating electrons (e-) which are then consumed in the reduction of copper ions (Cu2+) to copper metal (Cu0).
There are many potential benefits in applying ultrasound to an electroless process and this was the subject of a recent review by the author. The majority of the previous studies on the effect of ultrasound investigated electroless nickel plating where sonication generally produced a significant increase in plating rate. Studies on the use of ultrasound during electroless copper plating are less numerous and few take into account the catalysation stage, despite the fact that this is a critical part in the process. One notable exception is the papers published by Touyeras et al. They showed that if relatively high-frequency ultrasound (530 kHz) was introduced to the catalyst bath, then an increase in the subsequent electroless copper plating rate could be obtained. Indeed, by applying ultrasound in both the catalyst and the electroless copper solution the plating rate could be almost doubled. These papers clearly demonstrated the importance of the catalysation step in the electroless copper process.Read the full article here.Editor's Note: This article originally appeared in the May 2014 issue of The PCB Magazine.