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20 PCB007 MAGAZINE I JULY 2019 Figure 18 shows views of the CO 2 laser-drilled microvia through the conformal mask process following desmear and electroless copper metal- lization. After desmear, the glass fibers appeared to protrude more from the microvia sidewall. The electroless copper deposit peeled from the melted tips of the glass fibers. No laser-induced copper target pad modification was visible. Figure 19 shows views of the CO 2 LDD mi- crovia process. The microvia was formed by direct CO 2 laser ablation through oxide treated 9 micron thick copper foil. The microvia di- ameter and roundness depended on the CO 2 beam diameter and quality. In this case, the outer layer copper foil showed melted and re- cast copper surrounding the microvia opening, but no CO 2 laser-induced melting and recast of copper on the target pad. Figure 20 shows the CO 2 LDD microvias after mechanical brush scrub, desmear, and electro- less copper metallization. The brushing opera- tion was completed after laser drill and was nec- essary to remove the surface oxide before the next process to prevent contamination of equip- ment and chemistry. Brushing was also the best practical method to remove the melted and re- cast copper that surrounded the microvia. The brushing process left directional brush marks in the copper foil. Brush marks visible on the tar- get pad were created before the current layer was laminated. No other CO 2 laser-induced cop- per target pad modification was visible. Figure 21 shows views of a CO 2 LDD laser- drilled microvia from another supplier. A simi- lar process was used to oxide the copper foil. The roundness of the microvia was directly re- lated to the roundness and uniformity of the CO 2 laser beam. A substantial amount of ab- lated outer layer copper was melted and recast around the rim of the outer layer of the mi- Figure 17: SEM view of CO 2 laser-drilled microvia aligned to a chemically etched conformal copper window at 0° tilt (L) and Figure 17. SEM view of CO 2 laser-drilled micro- via aligned to a chemically etched conformal copper window at 0° tilt (L) and 45° tilt (R). Figure 19: SEM view of CO 2 laser direct-drilled microvia through oxided 9-mm copper foil at 0° tilt (L) and 45° tilt (R). Figure18: SEM view of CO 2 laser-drilled microvia aligned to a chemically etched conformal copper window after desmear and electroless copper at 0° tilt (L) and 45° tilt (R). Figure 20: SEM view of CO 2 laser direct-drilled microvia through oxided 9-mm copper foil after mechanical brush scrub, desmear, and electroless copper at 0° tilt (L) and 45° tilt (R). Figure 21: SEM view of CO 2 laser direct drilled microvia through oxided 9-mm copper foil from another supplier illustrating variation in beam quality at 0° tilt (L) and 45° tilt (R).