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12 PCB007 MAGAZINE I JULY 2019 were extracted after laser drill and after elec- troless copper. SEM analysis provided high- resolution views for comparison. The SEM pictures revealed that microvia structure was influenced by laser type. Figure 1 shows a two-level staggered mi- crovia and a three-level stacked microvia. Both configurations are used in PWBs; how- ever, when both were used on same PWB, the stacked microvia failed during reflow assembly while the staggered microvia did not. The mi- crovias were formed with a UV-CO 2 combo la- ser UV clean followed by desmear, electroless copper, and electrolytic copper fill. Both stacked and staggered microvias must survive reflow assembly. A cross-section of a failed three-stack microvia shown in Figure 2 revealed a separation between the target pad and plated copper fill at both layers 2 and 4. The microvias were drilled with a UV laser fol- lowed by desmear, electroless copper, a copper strike, then copper fill. The UV laser ablation parameter was set to dig into the copper target pad. The UV-drilled microvia failed during re- flow assembly. Figure 3 is SEM photograph of a UV laser- drilled, copper-filled microvia cross-section be- fore assembly reflow. The copper-fill to target pad interface as shown is a complex structure that meanders into the layer 2 plated copper. Arrow (a) points to a UV laser-formed copper burr at the knee of the copper foil. Arrow (b) points to a thin demarcation that follows the copper fill to copper strike interface. Arrow (d) points to the electroless layer between the copper foil and electroplated copper. Arrows (c) and (e) point to interface features that are darker and coarser than the plated copper. The electroplated copper strike to electroless cop- per to target pad interfaces were obscured by the features created by the UV laser ablation. UV laser-drilled stacked microvias failed re- flow assembly more frequently compared to microvias drilled by other laser types. The dif- ference was related to the UV laser created fea- tures present between the copper fill and target pad as shown in Figure 3. The features, only observed in UV laser-drilled microvias formed as copper melted and recast in the presence of air, coincide with the weak points that fracture Figure 1: Cross-section of layer 1 to layer 3 staggered copper filled microvia and layer 1 to layer 4 stacked copper filled microvia. Cross-sections were polished and microetched to reveal copper structure. Figure 2: Cross-section of a three-stack microvia failure. Figure 3: UV-only laser-drilled copper-filled microvia: (a) UV-laser-induced burr formed on copper foil; (b) Demarcation between copper fill and copper strike; (c) Feature with different texture compared to the plated copper; (d) Electroless copper layer between copper foil and electroplated copper; (e) Feature with texture different compared to the plated copper.

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