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72 PCB007 MAGAZINE I JULY 2023 plate agitation used in wafer plating tools. is style of agitation uses a reciprocating grating near the panel to generate turbulence at the surface and maximize transport efficiency of metal ions and other critical reactants 5 . Vacuum Pre-wet When the part to be plated includes deep or high aspect ratio features, it is possible for air bubbles to become trapped in the vias or trenches when it is inserted into the plating bath 4 . ese air bubbles, held in place by sur- face tension, can delay or even prevent plat- ing from occurring in those features by pre- venting the plating solution from making con- tact with the active surface. e most effective method for eliminating air bubbles is to bring the part under vacuum, and fill the chamber with degassed, deionized water. With no air present, the water fills all the recesses, and now surface tension acts to exclude air as the part is transported to the plating cell 5 . Examples Many OEMs, PCB fabricators, and OSATs have proposed and created prototype flush circuit ultra-HDI substrates, including Siemens, Samsung , WUS, Unimicron, and Amkor (Figure 6). Further examples are supplied by WUS in Taiwan 6 . Figures 7 and 8 show the traditional mSAP subtractive process compared to the Figure 6: Standard mSAP fine-line circuits compared to the flush circuits. 2