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78 The PCB Magazine • June 2017 plated at 1 A/dm 2 for one hour with an acoustic power output of 225 W. After plating, the sample was rinsed in D.I. water and dried. A variety of ridge-like features were observed on the plated copper. Shown in Figure 3A is a top-down image of the cop- per surface surrounding a tooling hole shown on the top right of the image. Emanating from the hole were concentric ringlets, observed as a dull finish on the PCB. The distance between the ringlets was approximately 0.7 mm, which corresponds to half the acoustic wavelength (λ) in the electrolyte solution. The appearance of the periodic ringlets suggested the presence of Rayleigh-type surface acoustic wave (SAW), which have scattered off the PCB and become confined within its surface at fixed locations [12] . The ringlets were characterised by a deposition of large grains with a fine grain region in be- tween. Large grain formation was due possibly to low concentrations of additives at the SAW pressure maxima—these are periodic regions in a SAW where particles or organic molecules attached loosely to a surface will be maximal- ly displaced [13] . The additives might concentrate in the pressure nodal regions, where minimum displacement occurs, increasing in concentra- tion and encouraging a shiny, fine grain finish, shown inbetween the matte ringlets. A matte finish characterised by a large grain structure is more structurally unstable than a fine-grain finish, as it demonstrates a great- er brittleness and greater susceptibility to frac- turing. A PCB has to pass thermal shock testing requirements, for example IPC-6012 standard, where the board is exposed to temperatures up to 288 o C. Under these tests a brittle or large grain structuring will be more likely to degrade and cause electrical opens, effectively scrapping the circuit. MEGASONIC ACOUSTIC SURFACE TREATMENT PROCESS Figure 3: Optical images of PCB surfaces after MS-plating with 225 W of acoustic power and at DC 1 A/dm 2 for one hour, showing after plating A) ringlets emanating from tooling-hole comprised of a large-grain texture and B) copper undulating ridge features. C) shows a typical copper surface after plating under the same current density but with standard bath agitations rather than MS [4] .