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16 PCB007 MAGAZINE I JULY 2019 vias. SEM views of the ablated copper surface of the target pad revealed structures that looked like melted copper, copper flow patterns, sput- tered copper, and recast copper [3] . The struc- tures present in UV-ablated copper target pads are not present on target pads following CO 2 laser ablation. UV-ablated copper filled micro- vias were more fragile than those formed by other laser processes. Figures 10 and 11 show two different UV- ablated microvias that compare different UV beam parameter settings and their effect on the target pad. In Figure 10, the UV beam set- tings produced a smoother target pad surface compared to the UV parameters used in Figure 11. In Figure 10, a concentric circular structure mediately after laser drill. As shown in Figure 7, a significant portion of the copper target pad was ablated away and left roughened by the UV beam. The manufacturer's UV laser pro- cess intentionally drilled into the copper sur- face to remove perceived contaminants before further processing. UV laser parameters are selected by drill- ing test arrays of microvias that are assessed and judged by how visually clean the target pads appeared as viewed by optical micros- copy. The diameter and copper appearance of the exposed target pad were compared to the microvia entry diameter and outer layer cop- per. The test array contained enough micro- vias to establish parameters aggressive enough to account for the variation in laminate glass content. Figure 9 shows examples of typical glass fabric yarn density. A 0.006"-diameter microvia drawn as a red circle is provided for reference. Glass density was greatest at fabric knuckle locations where warp and weft yarns overlapped, and least in the weave openings between the glass yarns. The variation in glass density in the beam path required parameters set aggressive enough to completely clear all the microvias and to accommodate localized material thickness variations typically encoun- tered in multilayer panel fabrication. Material considerations ensure that some microvias re- ceive too much laser energy. The UV ablation of copper produced a vi- sually modified copper target pad surface that failed frequently when used in stacked micro- Figure 7: UV laser-drilled 3-mil diameter microvia at 30° tilt, 1000x magnification. Figure 8: UV laser-drilled 3-mil diameter microvia after electroless copper at 30° tilt, 1000x magnification. Figure 9: Typical glass fabric yarn styles used in microvia construction. The red circle represents a 6-mil diameter microvia for reference.