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40 The PCB Magazine • May 2014 resulting in soil/dirt removal. Microjetting is also beneficial in electrochemical systems, en- hancing mass transport and thinning diffusion layers. However, Table 1 clearly shows that in this catalysed electroless copper process it can have a negative effect by removing catalyst from the surface of the substrate to be plated (Figure 2). These effects were reflected in the practi- cal plating tests that were performed. Using an electroless copper solution operating at 40°C, the application of continuous sonication re- sulted in only a modest 8% increase in plating rate compared to the plating rates obtained us- ing conventional agitation. However when a seven-minute delay time was introduced before the electroless copper solution was sonicated, a significant 38% increase in plating rates was observed. Indeed, the plating rate obtained at 40°C using delay time sonication (3.2 µm/25 minutes) was equivalent to those obtained us- ing conventional agitation at 46°C. A change in the grain structure of the elec- troless copper deposit was also recorded on test coupons that had been sonicated. Figure 3 in- dicates that a much finer grain structure occurs on samples that have been subjected to ultra- sonic irradiation. For electronic applications this modified THE EFFECT OF LOW-FREqUENCY ULTRASOUND continues table 1: the effect of ultrasonic agitation in a simulated electroless copper solution on Pd and sn concentrations on the surface of epoxy test coupons. Figure 2: ultrasonically induced microjetting scrubbing catalysed substrate. grain structure has the potential to bring ben- efits in terms of lower porosity, better coverage, increase conductivity and a reduction in signal attenuation.