In short: The via types in PCB design fall into four structural families — through-hole, blind, buried and microvias — plus two technique-level ideas, via stitching and the via stub. Each one is a plated copper tunnel that carries signal, power or heat between copper layers, and choosing the right type is a trade-off between layer count, fabrication cost and high-speed signal integrity.
A via is a plated hole that electrically connects copper on one layer of a printed circuit board to copper on another. Without vias, every signal would have to stay on a single plane, making anything beyond a trivial two-layer board impossible. Once you move to four, eight or sixteen layers, the way those vertical connections are formed becomes one of the most consequential decisions in the whole layout.
The reason engineers distinguish between so many via types in PCB work is that a via is never free. Every hole costs drilling time, consumes routing space, adds a tiny stub of unused copper, and — above a few hundred megahertz — behaves like a small capacitor and inductor rather than an ideal wire. The six types below exist to give designers the right tool for a given layer stack, density and frequency.
Structural vias are classified by which layers they start and stop on. The diagram below shows all four in a single six-layer stack so you can see how they differ at a glance.
The through-hole via is drilled completely through the board, connecting the top layer to the bottom and passing through every layer in between. It is the cheapest and most robust to manufacture because a single mechanical drill bit does the job, and it is the default for most consumer and industrial boards. The downside is that it occupies real estate on every layer — even the ones it does not electrically use — which wastes routing space on dense designs.
A blind via connects an outer layer to one or more inner layers but does not go all the way through. Because it stops partway, it frees up the opposite side of the board and the layers it never reaches. Blind vias require controlled-depth drilling or sequential lamination, so they add cost, but they are essential when a dense BGA component needs to escape its pins without blocking the rest of the stack.
A buried via connects two or more inner layers and is completely enclosed inside the board — it touches neither outer surface. Buried vias let designers route high-density interconnect on inner layers while keeping the surfaces clear for components. They are the most expensive of the three mechanically drilled types because they must be formed before the outer layers are laminated on top.
Microvias are very small laser-drilled vias, typically 150 µm or less in diameter, used in HDI (high-density interconnect) boards and modern smartphones. They usually span a single dielectric layer, and designers stack or stagger them to climb through multiple layers. Their tiny footprint is what makes fine-pitch BGAs and compact wearables possible.
Beyond connecting signals, vias are used in arrays to bond copper regions. Via stitching places many vias along the edge of a ground pour or between two ground planes. This shortens the return-current path, suppresses electromagnetic interference, improves thermal spreading, and creates a "via fence" that contains radiation along board edges. A closely spaced row of stitching vias effectively behaves like a shielding wall for high-speed signals.
When a signal enters a through-hole via on, say, layer 2 but the via continues down to layer 6, the leftover length from layer 2 to layer 6 is an unused via stub. That stub acts as an unterminated transmission line that resonates and sucks energy out of the signal at specific frequencies, causing notches in the channel's frequency response. The animation below shows a signal travelling down a via and the reflection that the dangling stub sends back.
Two fixes dominate practice. Back-drilling uses a slightly larger drill from the far side to remove the unused copper barrel after plating, physically deleting the stub. On HDI boards, blind and buried vias avoid the problem entirely because the via only spans the layers it actually uses, leaving nothing to resonate. Both approaches matter once channels exceed roughly 5–10 Gbps.
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Explore CourseTron Courses →A blind via connects an outer (surface) layer to one or more inner layers and is visible from one side of the board. A buried via connects only inner layers and is completely enclosed, so it cannot be seen from either surface.
Microvias are typically 150 µm or smaller — too fine for a mechanical bit to drill reliably at scale. Lasers can ablate these tiny, shallow holes precisely through a single dielectric layer, which is why they are standard in HDI and smartphone boards.
No. Simple low-speed boards work fine without it. Stitching becomes valuable when you need stronger ground return, better thermal spreading, or EMI containment — common in RF, high-speed digital and power designs.
Back-drilling removes the unused portion (stub) of a through-hole via. That stub resonates and degrades high-speed signals, so removing it widens usable bandwidth on multi-gigabit links.
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