Via Types — Through-Hole, Blind, Buried, Micro

Section Overview

Chapter 2 opened with the trace, and you saw that when a connection must move from one layer of a board to another, the trace hops between layers through a via (§2.1). This section examines the via itself — the plated hole that carries a connection through the board — and the several types it comes in. Physically, a via is a small drilled hole whose wall is plated with copper; that plated wall is the via barrel, and it is what actually carries the connection between the layers the via touches, ringed on each of those layers by a small pad of copper — an annular ring. The types differ in how far through the board the drilled hole goes. A through-hole via — a plated-through hole — runs the whole way through, from the top face to the bottom, so you can see and probe it on both sides; it is the commonest and the most repairable. A blind via connects an outer layer to an inner layer without passing all the way through the board, so it is visible from one face only. A buried via joins two inner layers and never reaches an outer face at all, so it is hidden inside the board — you cannot see or probe it directly. And a microvia is a very small, laser-drilled via, usually spanning just one layer to the next, used to pack the connections of dense, high-layer-count boards. For a repairer these differences decide almost everything: which vias you can see, reach, and repair (Chapter 7). A through-hole via can be inspected from both sides and reconstructed by hand; blind, buried, and micro vias are hidden and often beyond hand repair, so recognizing which kind you face — from what is visible on each surface — tells you what a fault might be and whether you can fix it (Chapter 7; 10.5). Know the via and its types, and a board's hidden third dimension — the connections that run between its layers — becomes something you can reason about.

Why This Matters

The via is how a board uses its layers, and via failures are a common, easily-missed cause of intermittent and dead faults — so knowing the types is both a diagnostic and a repair skill. This matters because a via is a connection you cannot always see: a trace you can follow across a surface, but a via carries that connection into the board, and knowing whether it is a through-hole, blind, buried, or micro via tells you whether you can inspect it at all (§2.5). This matters because vias fail: a cracked via barrel or a broken plated hole breaks the connection between layers, producing an open that reads as a dead or intermittent circuit, and recognizing a via as the suspect is half of finding it (Chapter 7). It matters because the type decides the repair: a through-hole via can be reached from both sides and rebuilt, while a buried or micro via is often beyond hand repair and forces a different strategy — a jumper around it, say (Chapter 7). It matters because you must not damage the vias you do not repair: drilling, scraping, or over-heating near a via can destroy a good connection, so knowing what is there protects it. And it matters because reading vias lets you follow a board in three dimensions: once you see where a trace dives into a via and comes up elsewhere, a multilayer board stops being a flat mystery and becomes a structure you can trace through (§2.5; 10.5). Learn the via types, and the connections hidden inside a board become something you can diagnose, protect, and — where the type allows — repair.

Required Prerequisites

• Trace Anatomy and Function — Section 2.1 introduced the via as the plated hole a trace hops through to change layers; this section examines that via in full and its types. You should also be comfortable with a board's layers and stackup from Section 1.3, since via types are defined by which layers they connect. This is a knowledge and observation section — no hot work; power only if you choose to probe, with care.

Recommended Consumables

• A few scrap boards with visible vias — a mix of simple double-sided and denser multilayer ones is ideal — to see through-hole vias and, on dense boards, where blind and micro vias sit
• A board's fabrication drawing or stackup notes, if you can get one — to see which via types a board actually uses and which layers they span
• Isopropyl alcohol and a brush — to clean a board so via pads and annular rings are easy to see
• A notebook — to sketch which layers each via connects

Recommended Practice Hardware

• A magnifier or loupe and a bright, angled light — to see a via's annular ring, its barrel, and whether a trace enters it
• A bright backlight — to shine through a thin board and see whether a via passes all the way through
• A multimeter with a continuity beeper — to confirm, unpowered, whether a via connects the points a trace suggests it should
• No iron, hot air, or hot work is needed — this section is reading and reasoning, not procedure

Real-World Applications

Recognizing via types shapes how a repairer diagnoses and plans work on almost any multilayer board. A technician chasing an intermittent fault suspects a cracked via when a connection reads open cold but works when flexed, and knowing the via types tells them whether they can even reach it (Chapter 7). A repairer following a signal across layers sees a trace dive into a via and reads the stackup to work out which layer it continues on (§2.5). Someone assessing whether a board is repairable checks whether a damaged connection runs through a reachable through-hole via or a buried one they cannot touch, which decides the whole approach (Chapter 7; §4.5). A builder inspecting a dense board identifies the tiny microvias between the outer layers and knows not to treat them like ordinary vias. And anyone who has drilled or scraped blindly near a via learns that destroying a good plated hole is easy and that knowing what is there is what protects it. The failures this skill prevents: missing a via as the cause of an open, attempting to hand-repair a via that cannot be reached, damaging good vias during other work, and misreading where a connection goes between layers.

Common Challenges

• Assuming every via goes all the way through. Only a through-hole via does — a blind via or buried via stops partway, so a connection may not appear where you expect on the far side (§2.5).
• Trying to repair a via you cannot reach. A buried via or microvia is hidden inside the board — often the fix is to bypass it, not to rebuild it (Chapter 7).
• Mistaking a microvia for an ordinary via. A microvia is far smaller and more fragile — treat it, and the copper around it, with extra care.

Safety Notes

Risk Level: Low. Examining vias on an unpowered board is one of the safest tasks in board work — the cautions apply only if you choose to probe a live board or later cut into one.

Professional Tips Before Starting

• Use both faces and a backlight. Looking at a via from both sides tells you at once whether it is a through-hole via; a backlight through a thin board confirms a hole that passes all the way through — what is visible is the fastest clue to a via's type.
• Read the stackup, not just the surface. A via's type is defined by which layers it connects, so a board's stackup or fab drawing tells you what the surface cannot (§1.3; §2.5).
• Confirm a via by meter. A continuity check across a via — from a trace on one layer to where it should reappear — tells you unpowered whether the via is actually carrying the connection (Volume 3).

Inside the Via, Type by Type

Recap and Frame

Before naming the via types, it helps to fix what a via is and why it exists, because the types are just variations on one idea. In the last section you followed a trace and saw that, on a multilayer board, a connection often cannot stay on one layer — it must cross to another, and where it does, the trace dives into a via (§2.1). A via, then, is the board's way of carrying a connection vertically, from one layer to another, where a flat trace cannot go. That is the whole purpose behind every type you are about to meet: they all move a connection between layers; they differ only in which layers, and how far through the board, they reach. This section does two things. First it looks at what a via is made of — its anatomy — so that later, when you inspect or repair one, you know what you are looking at. Then it walks through the four types named in the title — through-hole, blind, buried, and micro — in order of how much of the board they cross and how visible they are. Throughout, keep the repairer's question in mind: can I see it, can I reach it, and can I fix it? — because that is what the type ultimately decides (Chapter 7). With the purpose clear, start with the anatomy: what is a via actually made of?

Via Anatomy — Barrel, Pad, and Annular Ring

A via is a small, simple structure, and knowing its parts is what lets you inspect one and spot when it has failed. A via begins as a hole drilled through the board where a connection must change layers. That bare hole would not conduct, so its wall is plated with copper: this plated wall — a thin copper tube lining the hole — is the via barrel, and it is the part that actually carries current from one layer to another. Where the via meets each layer it connects, the barrel joins a small ring of copper pad around the hole, called an annular ring (annular ring) — on the outer layers you can see these rings as the little copper doughnuts a trace runs into. The barrel bonds to the trace or pad on every layer the via is meant to connect, so the whole structure — barrel plus its annular rings — is one continuous piece of copper linking those layers. A via may be left open, or it may be filled (with resin or plating) and even capped over so a pad can sit on top of it; a via covered by solder mask is said to be tented. This anatomy is exactly what you inspect: a good via has an intact via barrel and sound annular rings, while a failed one may show a cracked or lifted ring, a broken barrel, or a hole that no longer conducts through (Chapter 7). The plating is thin, so the barrel is the fragile part — thermal stress, flexing, or rough handling can crack it, breaking the connection while everything still looks intact on the surface. Understand the barrel and the rings, and you know both what a via is and where it tends to fail.

The Through-Hole Via

The first and most common type is the one that runs the whole way through the board, and it is the type you will meet — and repair — most often. A through-hole via is a plated-through hole that goes all the way from the top face of the board to the bottom, its via barrel passing through every layer in between (plated-through hole). Because the hole is open on both surfaces, a through-hole via is the most visible and accessible type: you can see its annular ring on the top and the bottom, probe it from either side, shine a light through it, and — crucially — reach it for repair (Chapter 7). It connects the two outer layers, and on a multilayer board it can also connect to inner layers whose copper meets the barrel along the way. It is the type used by default on simple double-sided boards, where there are only two layers to join, and it remains common on multilayer boards for connections that run between the outer faces. For a repairer it is the friendliest via: its accessibility from both sides means a cracked barrel or a broken connection can often be diagnosed with a continuity check and rebuilt — with conductive epoxy, a via rivet, or a jumper — using techniques you will meet in Chapter 7 (Chapter 7). When you see a plated hole that you can look straight through and reach from both faces, you are looking at a through-hole via — the one you can most likely fix.

Blind and Buried Vias

As boards grow denser, designers use vias that do not run the whole way through, and these — blind and buried vias — are far harder to see and to repair. A blind via connects an outer layer to one or more inner layers but stops partway, without reaching the opposite face: it is "blind" because, looking at the board, you can see it from the one outer surface it touches but not from the other side, where nothing of it appears. A buried via goes further out of reach: it connects two or more inner layers only and never reaches either outer face, so it is completely hidden inside the board — you cannot see it, probe it, or shine a light through it from outside at all. Both exist for the same reason: on a board with many layers and little room, letting some vias span only the layers they need — instead of drilling every hole through the whole stack — frees space and lets the routing be much denser (§1.3; routing, §2.1). For a repairer they are a different world from the through-hole via. A blind via, reachable from one side, may sometimes be inspected or worked on from that face, but a buried via is beyond direct reach entirely: you learn of it only from the board's stackup or fab drawing, or by X-ray, and you diagnose a fault through it by continuity — an open between two layers that should connect (§2.5; 10.5). When such a via fails, the repair is usually not to rebuild it but to bypass it — running a jumper to make the connection another way (Chapter 7). The lesson is to recognize, from a board's density and its stackup, that not every connection is one you can see or touch.

Microvias and HDI

The smallest via type appears on the densest modern boards, and it demands the most respect because it is so small and fragile. A microvia is a very small via — typically a fraction of a millimetre across — formed not by mechanical drilling but by a laser, which can make far smaller and shallower holes. A microvia usually spans just one layer to the very next one — an outer layer to the first inner layer, say — rather than crossing the whole board. These vias are the hallmark of HDI boards — high-density interconnect — the tightly-packed boards found in phones, laptops, and other compact modern electronics, where thousands of connections must fit under fine-pitch chips. To reach deeper layers, microvias are sometimes stacked or staggered on top of one another, layer pair by layer pair (§1.3). For a repairer, a microvia is at the edge of, or beyond, what hand tools can do: it is far too small to rebuild with ordinary methods, its copper is thin and easily destroyed by heat, and the parts around it are packed tight. Diagnosis leans on X-ray and the board's files, and a fault involving a microvia is often handled — if at all — by bypassing the affected connection rather than repairing the via itself (Chapter 7; 10.5). When you meet the tiny, laser-drilled vias of a dense modern board, treat them and the copper around them with extra care, and recognize that they mark the limit of hand repair.

Reading Vias and What They Mean for Repair

All of this comes together in a single practical read: from what you can see, work out a via's type, and from the type, work out what you can do. Start with what is visible. Look at the board from both faces: a plated hole you can see and probe on both sides, and look through, is a through-hole via — the accessible, repairable kind. A hole or pad that appears on one face but has no counterpart on the other is likely a blind via, reachable only from the side you can see. A connection that the stackup or a continuity check says must exist between layers, but which shows no hole on either face, points to a buried via hidden inside. And the tiny, laser-fine holes clustered under a fine-pitch chip on a dense board are microvias. Where the surface cannot tell you enough, two things can: the board's stackup or fabrication drawing, which states which via types it uses and which layers they span (§1.3), and X-ray, which sees the barrels and inner rings the surface hides (10.5). Then turn the type into a plan. A failed through-hole via is often repairable by hand, reached from both sides (Chapter 7). A failed blind via, buried via, or microvia is usually not: the connection is bypassed with a jumper to reachable points instead (Chapter 7; §2.5). In every case, confirm the fault the same way — an unpowered continuity check across where the via should connect, an open where a connection is expected (Volume 3). Read the via, name its type, and you know in advance whether a fault is one you can rebuild or one you must route around.

Common Mistakes

• Assuming a via passes all the way through. Only a through-hole via does — a blind via stops at an inner layer and a buried via never reaches the surface, so a connection may not appear on the far side (§2.5).
• Trying to hand-repair a hidden via. A buried via or microvia cannot be reached — plan to bypass the connection with a jumper rather than rebuild it (Chapter 7).
• Damaging a good via during other work. Drilling, scraping, or over-heating near a via can crack a sound via barrel — know what is there and protect it.
• Ignoring the stackup. A via's type is set by which layers it connects, which the surface may not show — read the board's stackup or fab drawing (§1.3; §2.5).
• Probing a via on a powered board. That gives false continuity readings and risks the meter — check a via unpowered (Volume 3).

Troubleshooting Guidance

Via problems come down to whether a via still carries its connection, and whether you can reach it. If a connection reads open between two layers: suspect a cracked via barrel on the via that should join them — confirm with an unpowered continuity check (Chapter 7). If a fault is intermittent and changes with flexing or heat: a cracked via barrel that opens and closes is a classic cause (Chapter 7). If you cannot see a via on the far side: it is a blind via or buried via — read the stackup or X-ray to find its span (§1.3; 10.5). If a via is tiny and clustered under a fine chip: it is a microvia — treat it as beyond ordinary hand repair (Chapter 7). If a hidden via has failed: bypass the connection with a jumper to reachable points rather than trying to rebuild it (Chapter 7). If you cannot tell which layers a via joins: the board's stackup or fabrication drawing states it (§1.3). If you must probe a live board: do it carefully and only when necessary — otherwise power down and check continuity cold (Volume 2). The throughline: find the via that carries the failing connection, confirm it unpowered, and let its type decide whether you rebuild it or route around it.

Verification & Testing Methods

Use this as a check that you can read a via and its type, not a hot procedure:

• [ ] I can describe a via's anatomy — a copper-plated via barrel through a drilled hole, joined to an annular ring on each layer it connects (Chapter 7).
• [ ] I can identify a through-hole via and say why it is the most inspectable and repairable — full-depth and visible on both faces.
• [ ] I can tell a blind via from a buried via by span and by what is visible: one outer face for blind, nothing for buried (§2.5).
• [ ] I can explain what a microvia is — a tiny, laser-drilled via used on dense HDI boards — and why it is at the edge of hand repair.
• [ ] I can read a via's type from the board and its stackup and judge whether a fault through it is within hand repair (§1.3; Chapter 7).

Then try the practice exercises below — observation and reasoning practice; scenarios differ from the quiz.

Practice Exercises

1. Classify vias by visibility (5 minutes, observation). On real boards, find several vias and classify each by what is visible: through-hole (both faces), blind (one face), or hidden, sketching your reasoning.
2. Follow a trace through a via (5 minutes, observation). Find where a trace enters a via and, using a backlight and both faces, work out whether it reappears on the far side or dives to a hidden layer (§2.5).
3. Sketch a via's span (4 minutes, reasoning). For each via type, sketch a simple layer stack and draw which layers a through-hole, blind, buried, and microvia would connect (§1.3).
4. Reason about repairability (4 minutes, reasoning). For a given via type, decide whether you could inspect and repair it by hand or would need to bypass it, and explain why (Chapter 7).

These core ideas — via anatomy, the through-hole via, blind and buried vias, microvias, and reading a via's type for repair — are tested in the Chapter Quiz at the end of this chapter, where a score of 80% is required to continue.

Key Takeaways

• A via is a copper-plated hole that carries a connection between a board's layers; its plated wall is the via barrel, joined to an annular ring of pad copper on each layer it connects, and the barrel is the fragile part that tends to crack (Chapter 7).
• A through-hole via runs the whole way through the board, is visible and probeable on both faces, and is the most inspectable and repairable type — the default on double-sided boards.
• A blind via connects an outer layer to an inner one and shows on one face only, while a buried via joins inner layers and is hidden inside the board entirely — both are usually bypassed rather than rebuilt (§2.5; Chapter 7).
• A microvia is a tiny, laser-drilled via used on dense HDI boards, usually spanning one layer to the next; it is fragile and at or beyond the limit of hand repair (Chapter 7).
• Read a via's type from what is visible on each face plus the board's stackup or X-ray, then let the type decide the plan: rebuild an accessible through-hole via, or route around a hidden one, confirming the fault with an unpowered continuity check (§1.3; 10.5; Volume 3).

Skills Learned

• You can now describe a via's anatomy — barrel, annular ring, and the layers it connects.
• You can now identify a through-hole via and say why it is the most inspectable and repairable.
• You can now tell a blind via from a buried via by span and by what shows on each surface.
• You can now explain what a microvia is and where it is used.
• You can now read a via's type from the board and judge whether it is within hand repair.

Glossary Additions

• via barrel — the thin tube of copper plated onto the wall of a drilled via hole; it is the part of a via that actually carries current from one layer of the board to another, bonding to the annular ring of pad copper on each layer the via connects. Because the plating is thin, the barrel is the fragile part of a via: thermal stress, flexing, or rough handling can crack it, breaking the connection between layers while the board still looks intact on the surface — which makes the barrel the usual suspect when a connection reads open between layers.
• blind via — a via that connects an outer layer of a board to one or more inner layers but does not pass all the way through to the opposite face. It is "blind" because it is visible from only the single outer surface it reaches; from the other side of the board, nothing of it appears. Blind vias let designers pack connections more densely on high-layer-count boards, and because they are reachable from only one face (if at all), a failed blind via is usually bypassed with a jumper rather than rebuilt.
• buried via — a via that connects two or more inner layers of a board and never reaches either outer face, so it is completely hidden inside the board and cannot be seen, probed, or lit through from outside. Buried vias free space for dense routing on multilayer boards. Because they are beyond direct reach, they are found from the board's stackup or fabrication drawing or by X-ray, diagnosed by continuity (an open between layers that should connect), and, when failed, bypassed rather than repaired.
• microvia — a very small via, typically a fraction of a millimetre across, formed by a laser rather than a mechanical drill, and usually spanning just one layer to the next. Microvias are the hallmark of HDI (high-density interconnect) boards — the tightly-packed boards in phones, laptops, and other compact electronics — and are sometimes stacked or staggered to reach deeper layers. Being tiny, thin-copper, and packed among fine-pitch parts, a microvia is at or beyond the limit of hand repair; faults involving one are diagnosed by X-ray and the board files and usually bypassed rather than rebuilt.

Suggested Next Sections

Must read next:

• Pad Types and Geometry — you now know the trace that carries a connection and the via that passes it between layers; the next section examines the third feature you repair — the pad where a component solders to the board — its types, geometry, and how its shape and annular ring bear on soldering and repair.

Recommended:

• Trace Anatomy and Function — the trace that dives into a via to change layers, and how to follow it pad to pad.
• Layer Stackup Conventions — the arrangement of a board's layers that defines which layers a via spans.