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82 The PCB Magazine • June 2017 copper plating outcome in response to apply- ing different bath agitations alongside the MS. The acoustic transducer was set up as indicat- ed in Figure 1, and the PCBs were plated with the SLOTOCOUP-CU110 plating solution. The plating solution was not configured for via fill- ing and so it was not expected from the plating trials that a uniform filling of Cu would be ob- served. Figure 5 shows PCBs plated for 12 hours un- der pulse-reverse—with a forwards/reverse cur- rent density of 1:3 A/dm 2 —and with differing bath agitations showing A) no agitation what- soever; B) panel movement and bubbles; and C) 450 W MS agitation. The results show clearly that with MS-assisted plating the copper thick- ness was the greatest. As expected, a void was formed in the middle of the via due to the non- filling copper plating properties of the used so- lution. Regardless, the plated result shows that, with MS agitation, greater volumes of electro- lyte can be transported and the concentration replenished within the BV over the plated dura- tion, increasing the throwing power of the bath. Conclusions A range of plating investigations were per- formed characterising the plating performance when applying high-frequency 1 MHz acoustic agitation to improve the throwing power of a copper sulphate electroplating bath. Observations were made of a surface artifact unique to MS plating which was characterised by an increase in the copper grain size and re- duction in plated uniformity. These effects were detrimental to PCB fabrication. Further studies into MS plating using an acoustic transducer, which varies the phase of the outputting wave, may be able to reduce the appearance of these features whilst maintaining the desired acoustic streaming properties [15] . When electroplating within a 500-litre tank in a manufacturing setting, the acoustic stream- ing currents produced by the 1 MHz acoustic wave were shown to transport electrolyte down microscale features in a THV. This confirmed the throwing power of the MS agitations and led to further investigations, which showed that throwing power was higher than the standard bath agitations when plating down BVs. The re- sults obtained show however that considerable development steps are still required to bring MS- assisted plating up to a working standard proce- dure for implementation into PCB manufacture. A particular issue in achieving high ar plat- ing down BVs (ar larger than 1.2:1) is the lack of a sufficient copper seed layer provided by an electroless copper process. This is because of the difficulty in introducing fresh electrolyte solu- tion into the microsized regions, due to the ef- fective viscous forces of the fluid medium pre- venting electrolyte flow. It is the opinion of the MEGASONIC ACOUSTIC SURFACE TREATMENT PROCESS Figure 5: Microsections of blind vias of size 0.15 mm diameter ar 1:1, plated with reverse-pulse at 1:3 A/ dm 2 for 12 hours, showing results for processing with different electrolyte solution agitation conditions.