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14 DESIGN007 MAGAZINE I JULY 2021 Heidi Barnes: e RF/microwave world has been using skip vias for at least 20 years, and yes, it did avoid the requirement for sequential lamination with the ability to go from layers 1 to 2 or layers 1 to 3 without having a via stub. is also has the advantage of increasing the routing area below. e challenge with skip vias is that the deeper they go, the wider the diameter needs to be so that the plating can reach down into the via. Typically, one tries to not go less than a 1:1 aspect ratio. A lot depends on the fabricator; if a shop specializes in high- volume sequential lamination with microvias, then they may charge more for the additional laser tuning and plating issues that come with skip vias vs. just doing more of what they do best. As always, it depends. I would consider both electrical performance/reliability and cost before making the decision. Carl Schattke: Skip vias are not common; they add cost to a standard through-hole board because they add an additional drilling step for each side where they are used. However, they will be lower cost than adding an addi- tional lamination cycle. I do find them in use on RF boards where you have GND on top, then the RF, then a GND layer again. You can bring the RF trace to L1 to L2 with a regular microvia, then shield vias around it with the L1 to L3 skip vias. is can save cost on an RF board. e other area where all laser drill vias make sense is when you have no mechanically drilled vias. Laser drilling is seconds per panel vs. mechanical drilling in the minutes or high minutes on larger boards. Skip vias do not cost more than regular microvias; they just use a little more energy to ablate the material away a bit deeper, but far less than buried vias on an extra lam cycle. ey will cost a slightly larger pad size, but I usually just use that size when designing so I don't have to keep track of it, and it is a minimal difference. DESIGN007 Are skip vias common? Do they cost more than regular microvias? Our company thinks it might be worth it if it saves us even one lamination cycle. Q

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