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60 PCB007 MAGAZINE I OCTOBER 2020 high-density circuitry for IC substrate manu- facturing utilizes extremely thin or no copper foil on the starting substrate. For comparison, Figure 1 shows the trends in starting copper foil thickness for the various HDI manufactur- ing technologies used in PCB and IC substrate manufacturing. SAP vs. mSAP To achieve the electrical routing require- ments for the highest-density interposers, SAP uses electroless copper to create an initial seed layer for buildup. Comparatively lower densi- ty IC substrate designs (and extremely high- density PCBs like those used in mobile boards) are done on thin copper foil laminate using the modified semi-additive process (mSAP). SAP enables the finest line/space structures by building directly onto an organic laminate substrate material. The process starts on bare organic buildup material, such as ABF, that has been laminated onto the previous layer or core layer. After laser drilling microvias, the substrate is desmeared and metallized with a thin seed layer of electroless copper, pattern electroplated to create the copper required for the layer function, and then etched to remove the extra electroplated surface copper and the initial seed layer. SAP technology allows for 10/10 µm line/ space to be achieved. mSAP is in mass pro- duction today at dimensions below 30/30 µm line/space. The process uses conventional pre- preg and ultra-thin copper foil (~3 µm) as the starting substrate. Microvias are laser-drilled, and then the panels are desmeared, processed through a primary metallization process—such as electroless copper or a carbon direct metal- lization process—imaged, pattern plated, and then etched down to the laminate between the features. In both processes, an anisotropic final etch is utilized, which preferentially etches away the copper at lower areas faster than higher areas, limiting the amount of surface copper removed from traces and other features while Figure 1: Starting copper foil thickness vs. line/space dimensions for various advanced PCB manufacturing applications and commercial metallization processes associated with them.

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