PCB007 Magazine

PCB007-July2019

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24 PCB007 MAGAZINE I JULY 2019 The SEMs in Figure 31 are of the UV laser- drilled microvia from another supplier. The im- ages show a significant amount of melted and recast copper on the target pad. The rim of the outer layer copper foil was melted and recast. The laser beam pitch was visible on the rim and sidewall of the microvia. Figure 32 shows the UV laser-drilled mi- crovia after desmear and electroless copper. Some melted and recast copper is visible around the rim of the microvia. Also visible is the extent of melted and recast copper on the target pad. Figure 33 shows pictures of a UV-CO 2 laser- drilled microvia. Melted and recast copper was visible around the rim of the outer layer cop- per foil and at the target pad. The tips of the glass fibers were melted and some were fused together. Figure 34 shows the UV-CO 2 laser-drilled mi- crovia after desmear and electroless copper. The UV laser did not drill a perfect circle. Also, there was some misregistration between the UV and CO 2 laser beams. The electroless cop- per deposit appeared to be cracking and flak- ing off the glass fibers. Some melted and recast copper was visible on the target pad, which suggested either very aggressive initial UV pa- rameters were used that allowed the UV beam to pierce the dielectric and hit the target pad, or a UV clean-up process was used after CO 2 ablation. Figure 35 shows a UV-CO 2 laser-drilled mi- crovia from another supplier. A melted and re- cast copper burr was formed around the rim of the microvia, but no melted or recast copper was observed on the target pad. Figure 36 shows the UV-CO 2 laser-drilled af- ter desmear and electroless copper. Electroless copper cracked and peeled off the melted tips and fused glass fibers. The SEMs show that copper target pad sur- face structure is different for UV and CO 2 la- sers. The UV laser leaves melted and recast copper on the target pad, but the CO 2 does not. The high-power CO 2 LDD laser does this only on the entry copper that is not the source of failures that form during reflow assembly. Reflow-tested samples compared UV-CO 2 (supplier A) and UV (supplier B) laser-drilled Figure 31: SEM view of UV laser-drilled microvia from another supplier with parameters that modified the morphology of target pad 0° tilt (L) and 15° tilt (R). Figure 33: SEM view of UV-CO 2 laser-drilled microvia at 0° tilt (L) and 45° tilt (R) with recast copper around the rim of the UV-formed copper window. Figure 32: SEM view of UV laser-drilled microvia from another supplier after desmear and electroless copper at 0° tilt (L) and 15° tilt (R). UV parameter modified the morphology of the target pad. Figure 34: SEM view of UV-CO 2 laser-drilled microvia after desmear and electroless copper at 0° tilt (L) and 45° tilt (R). Example of slight misregistration between UV and CO 2 beams.

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