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JULY 2019 I PCB007 MAGAZINE 17 is visible at location C that looks like copper waves propagating from the smoother copper B region. These features were produced by the action of the UV beam repeatedly trepanning in pulses over the copper target pad. Copper waves propagated from the center of the tar- get pad as the beam repeatedly punched the central region. A chain of copper craters at ar- row A located at the perimeter of the target pad follows the UV beam pitch pattern. Both pictures show that copper melted, flowed, and solidified to form these patterns. Similar cop- per melt structures were also present on the rim around the copper edge of the outer layer copper around the entrance of the microvia. The change in copper morphology by the ac- tion of the UV laser beam shown in the SEM pictures are the unusual features seen at the demarcation of Figure 3 associated with frac- tures. These features were only present on the target pads of microvias touched by the UV la- ser beam. SEM analysis revealed more detail than op- tical microscopy. Additional SEM photographs were taken of samples fabricated by the UV- CO 2 laser, the CO 2 laser used in the etched oversized window and conformal mask pro- cesses, the CO 2 laser used in LDD, and the UV laser. The pictures are provided to inspire fur- ther discussion. Figures 12 shows views of UV- CO 2 laser-drilled microvias at 0° and 30° tilt. The UV laser initially opens a window in the copper foil and ablates a little bit into the ep- oxy glass (Figure 13). The CO 2 laser then removes the remaining epoxy glass material and leaves a very thin res- inous film as it reflects off the copper target pad [2] . The initial UV laser step leaves a copper burr of melted and recast copper as it clears Figure 10: SEM SE (secondary electron) view of UV laser- drilled microvia with gentle UV ablation parameters. Figure 11: SEM BSE (backscattered electron) view of UV laser- drilled microvia with aggressive UV ablation parameters. Figure 12: SEM SE view of UV-CO 2 laser-drilled microvia at 0° tilt (L) and 30° tilt (R). Figure 13: SEM SE view at 0° tilt of UV-CO 2 laser-drilled microvia after the UV laser drill step opened a window in the copper foil and before the CO 2 laser completes the microvia ablation to the target pad.