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June 2017 • The PCB Magazine 77 multilayer board fabricated with standard pro- cessing on the left, and MS-assisted agitation on the right, with the THVs and BVs indicated by the orange vertical features. A multilayer PCB is constructed by the bond- ing together of its individual layers by ther- mally degrading processes. With MS-assisted processing the number of bonding operations is reduced from six to four and the drilling opera- tions reduced from ten to six, which together re- duce fabrication costs and overall manufactur- ing time, and increase the lifetime of the board due to the reduction in high thermal stresses induced during its fabrication. Increasing the lifetime of a PCB is a highly desirable trait for the PCB assembler, who may perform multiple high temperature operations up to 260 o C, in the reflow soldering of components onto the PCB surface [10] . Manufacturing a PCB with the material pro- cessing and cost savings outlined here would enhance the capability of a PCB fabricator, so a series of investigations were performed look- ing to scale-up the process from a small-scale laboratory setting—processing in 40 litre tanks at HWU—into a medium scale setting with 500 litre processing tanks at MCT. Plating Setup and Experimental Investigations at Merlin The experimental setup reproduced at MCT was comprised of a 500W Sonosys MS acoustic transducer device, submersed within a copper sulphate electroplating bath with a solution provided by Schloetter Ltd., which was SLOTO- COUP CU110 set up for soluble anodes. Before MS-plating, the PCBs were processed through a MacDermid M-System Omega electroless cop- per plating line at MCT, depositing 1−2 µm of copper. When in operation, the acoustic transducer was oriented with its active face towards the surface of the PCB, where the features to be plated—THV and BV—were exposed to the on- coming waves. A 2-D schematic of the arrange- ment is indicated in Figure 2. In the setup, one soluble anode was used to the left of the PCB and one transducer was used to the right of the PCB. This setup was applied for the plating tri- als discussed below. A series of investigations were performed aimed at the following: • The influence of the MS agitation on the surface finish of the plated copper, from observations using optical microscopes and scanning electron microscopes (SEM), at MCT and HWU, respectively • The ability for the MS acoustic streaming to transport solution down microfeatures, specifically small, 0.15 mm diameter THVs • The ability for the MS acoustic streaming to enhance the amount of copper deposited down small BV interconnects compared to industry standard solution agitation techniques, which include panel movement and bubble agitation Megasound Acoustic Effects Observed on the PCB Surface The application of high-frequency acoustic agitation during copper electroplating was seen to form periodic features within copper depos- ited onto substrate surfaces. The periodic fea- tures were characterised by regular lines which decreased in distance with increasing frequen- cy [11] . This behaviour was also witnessed in the MS investigations performed at MCT within the deposited copper [4] . A 1.6-mm FR-4 PCB was DC- MEGASONIC ACOUSTIC SURFACE TREATMENT PROCESS Figure 2: Schematic of experimental setup: acoustic transducer (right); PCB (middle); and soluble anode (left). Acoustic agitation applied from right to left onto the PCB surface facing the transducer and into the via interconnects.