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18 PCB007 MAGAZINE I JULY 2019 the foil copper. A burn mark is visible on the outer layer copper foil surrounding the micro- via. Copper droplets and spheres were also scat- tered on and around the microvia. Some of the tips of the glass fibers were melted and fused to- gether. The target pad was undisturbed because the UV laser never touched the target pad. Figure 14 shows views after desmear and electroless process of a microvia from the same manufacturer. The process did not remove the melted and recast copper burr from the rim of the microvia opening. After desmear, the glass fibers protruded more from the microvia side- wall. The glass fabric influenced the texture of the sidewall because of the localized glass fi- ber density. No visual copper anomalies were seen on the target pad. Figure 15 shows microvias from a manufac- turer that etched a 245-µm oversized copper window in the copper foil and then drilled the microvia with a 100-µm diameter CO 2 laser. The oversized window exposed the laminate to allow the CO 2 laser to drill the microvia. The 245-µm copper window was slightly smaller than the 250-µm capture pad that eventually capped the filled microvia. The copper surface and rim of the chemically etched copper foil were clean and well defined since no UV laser was used. This process exposed a substantial amount of bare laminate around the microvia. There was some discoloration visible on the target pad, but no melted and recast copper damage. The discolorization was due to a thin residual film that remained on the target pad surface. This phenomenon occurs in all CO 2 la- ser processes. Shown in Figure 16 are views of the over- sized window with the CO 2 laser-drilled micro- via after desmear and electroless copper. Desmear removed the thin resin film from the target pad, caused the glass fibers to pro- trude more from the microvia sidewall and in- creased the resin texture exposed in the cop- per window. Electroless copper plated directly over the bare laminate inside of the window. Most of the capture pad formed over the bare laminate with only a small portion overlapping the copper foil. Chemically deposited copper- to-laminate adhesion is less than the adhe- sion of laminated copper foil. The diameter of the window was selected to accommodate the cumulative alignment tolerance necessary to make sure the CO 2 laser-drilled the microvia within the window. No target pad melting and recast of copper was visible. Figure 17 shows views of a CO 2 laser-drilled microvia through a conformal copper mask. The copper etched window diameter in this process was the same or slightly smaller than the CO 2 beam diameter. No laminate surface was exposed. The window masked the over- sized CO 2 laser beam and therefore defined the microvia diameter. The edge and surface of the copper foil were clean and well-formed since copper was not melted and recast. No target pad melted and recast copper was visible. Figure 15: SEM view of CO 2 laser-drilled microvia inside chemically etched oversized copper window at 0° tilt (L) and 45° tilt (R). Figure 14: SEMs SE view at 0° tilt of UV-CO 2 laser-drilled microvia after desmear and electroless process at 0° tilt (L) and 30° tilt (R). Figure 16: SEM view of CO 2 laser-drilled microvia inside chemically etched oversized copper window after desmear and electroless copper at 0° tilt (L) and 30° tilt (R).