PCB007 Magazine

PCB007-July2019

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14 PCB007 MAGAZINE I JULY 2019 ures 4–6 provide examples produced by differ- ent laser processes. Figure 4 is a UV-CO2-UV cleaned microvia stack, which failed in six re- flow cycles. Figure 5, a UV-formed stack, failed in the first reflow cycle. Figure 6, an LDD CO2 laser-drilled stack, survived 30 lead-free reflow passes with a peak temperature of 245°C. Figures 7 and 8 are SEMs of a 0.003"-diam- eter microvia at 30° tilt and 1000x magnifica- tion. Figure 7 is after UV laser drill, and Figure 8 after electroless copper. The microvia was UV laser drilled in 0.5-oz. outer layer copper foil and 106 prepreg. Figure 7 shows the target pad condition after UV laser ablation. The copper surface of the target pad was modified. Also visible is a copper burr and scalloping on the rim of the outer layer copper. Spherical cop- per particles are attached to the microvia side- wall. Figure 8 is a picture taken after 40 µin of electroless copper was deposited. An agglom- eration of small spherical copper particles was trapped inside the microvia. Cracks are visible in the 40 µin copper deposit. Features outside the target pad are interesting, but because fail- ures were observed between the plated cop- per fill and the target pad, this investigation focused on the condition of the target pad im- window process consists of an etched copper window diameter that is greater than the CO 2 beam diameter, so the CO 2 laser beam diam- eter defines the microvia diameter. 4. Laser Direct Drill With CO 2 The laser direct drill (LDD) process uses a high-power CO 2 laser to drill through thin, specially coated copper foil and through glass- reinforced dielectric. After piercing the entry copper, the CO 2 laser parameters are adjusted to reflect off the target pad; therefore, it does not melt or recast the copper surface of the tar- get pad. The cut through the thin treated cop- per does leave the characteristic melted and re- cast copper feature around the rim of the outer layer copper foil. The entry copper foil must be less than 9-µm thickness and treated to absorb the CO 2 beam. After laser drill, a series of chemical opera- tions remove the residue from the target pad left by the CO 2 laser. These operations will be covered in future papers. Reflow-induced failures were observed to follow demarcations between the copper fill and target pad. Demarcations in microvias vi- sually vary for different laser processes. Fig- Figure 4: UV-CO 2 -UV laser-drilled microvia stack that failed in six reflow cycles. Red arrow points to failure. Figure 5: UV laser-drilled microvia stack that failed first reflow cycle. Red arrow at layer 3 points to failure. Figure 6: CO 2 laser direct drill microvia stack survived 30 lead-free reflow cycles.

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