Issue link: https://iconnect007.uberflip.com/i/1143487
22 PCB007 MAGAZINE I JULY 2019 Figure 27 shows SEMs of a microvia that was CO 2 drilled within a slightly oversized copper etched window. In this process, the CO 2 beam diameter was only slightly smaller than the copper window diameter therefore the CO 2 beam defined the microvia diameter. The win- dow diameter was smaller than the window di- ameter used in the process shown in Figure 15. Less laminate was exposed therefore a greater portion of the capture pad was formed over the copper foil, which provided a better an- chor into the laminate. The chemically etched windows in copper foil did not require a de- burr process. No melted or recast copper was produced on the target pad. Figure 28 shows the oversized window and CO 2 laser-drilled microvia after desmear and electroless copper. Figure 29 are SEMs of a UV laser-drilled mi- crovia. The UV beam parameters were selected to minimize target pad damage. Just a small amount of UV-induced melted and recast cop- per was visible at 12:00 on the target pad in Figure 45. Substantial brush stroke striations were visible on the outer layer copper foil. A small amount of melted and recast cop- per was visible around the rim of the microvia. No visible glass fibers were melted or fused together. Figure 30 shows the UV laser-drilled microvia after desmear and electroless cop- per metallization. Melted and recast copper was visible around the rim of the microvia. The desmear operation included a glass etch step that etched away a substantial amount of the glass fibers. No glass fibers were observed protruding from the microvia sidewall. Small cracks were visible in the electroless copper deposit on the microvia sidewall. The gentler UV laser parameter left less melted and recast copper on the target pad and produced a greater microvia sidewall taper that substantially reduced the microvia contact ar- ea to the target pad. The UV laser parameters were set aggressively enough to account for the heaviest glass content and thickest dielec- tric variation across the laminate to make sure the microvia was fully formed. Microvia target pads located in the openings between the glass yarns and in thinner dielectric region received too much UV ablation. Figure 27: SEM view of CO 2 laser-drilled microvia inside slightly oversized chemically etched copper window illus- trating degree of alignment at 0° tilt (L) and 45° tilt (R). Figure 29: SEM view of UV laser-drilled microvia with parameters minimizing target pad damage at 0° tilt (L) and 45° tilt (R). Figure 28: SEM view of CO 2 laser-drilled microvia inside slightly oversized chemically etched copper window after desmear and electroless copper at 0° tilt (L) and 45° tilt (R). Figure 30: SEM view of UV laser-drilled microvia after desmear and electroless copper at 0° tilt (L) and 45° tilt (R). Example of microvia formed in location crossing glass yarns.