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104 DESIGN007 MAGAZINE I OCTOBER 2021 Types of Microvias • Blind via: Used to connect one surface layer with at least one internal layer • Buried via: Used to create connections of internal layers with no contact to the surface layers • Via-in-pad: A type of blind via in which the via hole is drilled in a surface mount pad, eliminating the need to run a trace and via pad from each SMT pad • Filled vias: Completely filling the microvia with either a non-conductive or conduc- tive paste. Conductive fill is typically used for heat dissipation and non-conductive fill is used to closely match the thermal expansion of the substrate. Microvia Formation Microvias can be formed through a number of methods, primarily mechanical drilling, laser drilling and sequential lamination. • Mechanical drilling: Uses traditional drilling equipment to mechanically form holes, but typically limited to 0.006" diameter and dependent on the depth needed • Laser drilling: Special drilling equipment that utilizes a laser to form the hole and can go down to 0.001" in diameter • Sequential lamination: A process where the microvias are drilled all the way through a sub-panel of the layers that need to be connected by the via, which could require multiple lamination, plating, filling and planarization operations (Figure 2). Stacked vs. Staggered Microvias • Stacked: Microvias that are electrically connected and literally stacked vertically on top of each other through various layers of the PCB • Staggered: Microvias that are electrically connected and offset to one another through various layers of the PCB (Figure 3) Via-in-Pad Microvias e via-in-pad production process allows you to place vias in the surface of the flat lands on your PCB by plating the via, filling it with Figure 2: Sequential lamination.