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50 PCB007 MAGAZINE I JUNE 2021 Step 3: Via holes are laser ablated through the PET film and prepreg using the copper cap plate as a stop for the laser. is is the second point of differentiation from standard PCB fabrication practices. Although it is common to laser ablate microvias into C-staged lami- nate for sequential-build-up plating, tuning the laser parameters to cleanly cut the PET, the underlying B-staged prepreg (or film adhe- sive), and removing any organic residue from the pad surface takes some learning. e use of low- or no-flow prepregs, thinner glass styles, and a combination of a CO 2 laser to do the ma- jority of the ablation, followed by a quick burst of a UV laser to desmear the bottom of the hole, is highly recommended. If a laser is not available, or if the feature sizes and registration tolerances are forgiving, the via holes could be achieved by punching, or mechanically drilling prior to attachment of the prepreg to the core. Step 4: TLPS paste is then deposited into the vias using the PET film as a conformal stencil mask. e use of automated stencil or screen printers improves the consistency of the filling process and an "open" stencil that frames the active area of the circuit layer can be used as a "parking lot" for the paste between prints. Al- though automated stencil printers are not com- monly found at most PCB fabricator shops, screen printers are relatively common from the days of printing solder mask and legend ink. Once the vias have been filled and topped off, the PET film is carefully removed by peeling, leav- ing slight protrusions of paste on the surface. e subassemblies, now bearing prepreg and paste- filled vias, are placed in a standard box oven for 30 minutes at 90air environment) to evaporate the solvent and stabilize the protruding paste for handling in layup of the lamination book. e protruding paste stubs are compressed into the vias during lamination resulting in sintered inter- connects with very high metal density. Step 5: e subassemblies bearing the inter- connect layers are aligned in a lamination book with the usual stiffeners, pressure distribution materials, and release layers. Lamination is per- formed under the standard lamination condi- tions recommended for the prepreg selected and the TLPS pastes as the prepreg adhesive cures. When the PCB is removed from the lamination press, all of the electrical interconnections have been formed and the PCB is complete. ere are an enormous number of potential variations to the process and ultimate construc- tion of the PCB. Figures 6, 7, and 8 depict some of the more common variations, with typical dimensions and increasing level of reliance on the TLPS Z-axis interconnect layers as an alter- Figure 6: "Core-to-core" implementation method for installing TLPS paste vias in high-layer-count PCBs.