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JULY 2019 I PCB007 MAGAZINE 25 microvias. Figure 37 shows probability plots comparing the two different laser-drilled mi- crovia responses to reflow assembly thermal stress. Samples were subjected to 30 lead-free reflow cycles with peak temperature to 245°C. The Tg of the laminate was 175°C. The prob- ability plot of temperature (L) recorded the temperature when an open was detected. The probability plot of reflows (R) plotted reflow cycles to failure. Both the mean failure tem- perature and cycles to failure for UV-drilled microvias failure were lower than means for UV-CO 2 laser-drilled microvias. These results suggest that UV laser-modified copper surfac- es produced weaker microvia interfaces. This finding is important because many microvias formed by UV-CO 2 laser processes today add a UV laser cleaning step [4] . Unknown is the extent of change in copper morphology by the UV copper surface cleaning step and its effect on reflow survivability. Conclusions Stacked microvias solve PWB circuit conges- tion driven by increased silicon content, den- sity of input/output paths to accommodate functionality, and smaller footprints. It was observed that fabricators' ability to produce reliable stacked microvias varied. Stacked mi- crovias fracture during the reflow assembly process. Stacked microvia fractures were ob- served at interfaces between the copper fill and target pad. Staggered microvias, however, did not fail. Presented material focused on the visual condition of the copper surface of the target pad immediately after laser drill. There is sig- nificant variation in the final surface morphol- ogy of the target pad after laser drill among the different techniques used by PWB fabri- cators. UV lasers created visible copper mor- phology changes not observed with the vari- ous CO 2 laser processes. UV laser-modified Figure 35: SEM view of UV-CO 2 laser-drilled microvia from another supplier at 0° tilt (L) and 45° tilt (R) with a recast copper burr around the rim of the UV-formed copper window. Figure 36: SEM view of UV-CO 2 laser-drilled microvia after desmear and electroless copper at 0° tilt (L) and 45° tilt (R). Figure 35: SEM view of UV-CO 2 laser-drilled microvia from another supplier at 0° tilt (L) and 45° tilt (R) with a recast copper burr around the rim of the UV-formed copper window.