Issue link: https://iconnect007.uberflip.com/i/834021
80 The PCB Magazine • June 2017 Variations in the thickness of the surface fin- ish were also observed within the SAW features. Shown in Figure 3B are copper ridges formed on top of the MS-plated PCB surface. The ridg- es are again separated by around 0.7 mm and are characterised by peaks and valleys. Ridges occurred on both sides of the board, although during some trials they would appear greater on one side of a panel rather than the other. This was possibly because of the interplay between the absorption and transmission of the wave through the PCB. The ridge orientation was typically vertical to the orientation of the board in the bath, and changes to the angle of inci- dence of the acoustic waves altered accordingly their positions and orientation across the sur- face. The ridges induced a variation of 20 µm in plating uniformity as measured from cross-sec- tions. This variation is large and could produce a manufacturing difficulty when attempting to keep within a customer's specification for cop- per thickness. A typical copper surface is indi- cated in Figure 2C, highlighting more ideal cop- per plating behaviour, which is characterised by high uniformity in height and grain structure. Through-Hole Via (THV) Plating Outcome Plating within a 500-litre tank displays dif- ferent acoustic reflections and characteristics than plating within a smaller 40-litre tank, due to the standing waves and acoustic streaming currents setup within the bath [14] . An experi- ment was performed to observe if the chang- es in acoustic conditions in the new bath, due to bath size and fluid flow in the tank at MCT, would still enable electrolyte replenishment down small via interconnects [5] . Plating was per- formed on a 1 mm thick, FR-4 PCB which con- tained drilled THVs of diameter approximately 0.15 mm and ar 5.7:1. 450 W of MS agitation was applied to the board with the transducer setup as in Figure 2. The board was DC-plated at 0.5 A/dm 2 for a duration of 16 hours. Displayed in Figure 4 are the plating results for with MS agitation during the plating and without any agitation whatsoever, shown in A) and B) respectively. A thicker deposit is clear- ly highlighted down the MS-plated THV, which show that the changes in acoustic conditions due to operating within the new plating tank— such as convective fluid motions and the acous- tic reflections off the walls of the tank—did not interfere with the MS plating performance and that fluid transport down the THV was possible, enabling a higher throwing power. This investigation provided evidence that microfluidic motions induced by the MS acous- tic streaming were sufficient to enable electro- lyte to be replenished and copper deposited. A plating investigation was then performed to look at how the replenishment of electrolyte by MS compared to the standard processing tech- niques used in plating. This investigation is dis- cussed next. Blind-Via (BV) Plating Enhancement The standard agitations applied in PCB man- ufacture are the movement of the PCB in solu- tion at a rate of 3 cm/s and the application of bubbles into the solution by a sparge pipe. The action of these two agitations serves as a meth- od of replenishing depleted electrolyte within the microvia features on a board and enables the bath to display a high throwing power suit- able for the electrodeposition of copper. A plating experiment was performed on a 1.6 mm thick FR-4 PCB drilled with 0.15 mm diameter ar 1:1 BVs, looking to observe the MEGASONIC ACOUSTIC SURFACE TREATMENT PROCESS Figure 4: Plating performance witnessed down through-hole vias of diameter approximately 0.15 mm, ar = 5.7 :1, DC-plated at 0.5 A/dm 2 for a duration of 16 hours showing A) with 450 W of MS and B) without any agitation [5] .