Plated Through Hole, Blind Via and Buried Via Compared

A via looks like a hole and behaves like a very short piece of wire, but its real cost is space. Every via drilled through the whole board consumes a cylinder of routing area on every layer it passes through, and on a dense design that cylinder is often the reason a signal cannot reach its destination.

The three via types differ in exactly that respect. This article compares the plated through hole, the blind via and the buried via in terms of fabrication, the routing space they consume and the cost they add, and sets out how to decide which one a stack-up actually needs.

Why Vias Exist and What They Cost

Copper layers are connected by drilling a hole through the laminate, plating the wall with copper and filling or capping it. The plating is what makes the connection, because the drilled wall itself is resin and insulates. Once plated, the barrel carries the signal from one layer to another, and the annular ring around it on each layer provides the landing pad.

The cost has three parts: the drilling operation, the plating operation and the space. Drilling time scales with the number of holes and their depth, plating quality is limited by the aspect ratio of the hole, and space is lost because the hole blocks routing on every layer it crosses. Of the three, the space cost is usually the one that decides whether a design can be built at the layer count that was planned.

The Plated Through Hole

A plated through hole runs from the top layer to the bottom, so it consumes routing area on every layer. It is the cheapest option because the drilling happens once, after all the layers have been laminated together, and no special registration between sub-assemblies is required. For a four or six layer board, this is almost always the right choice.

The design rules that apply to vias usually constrain the annular ring and the drill size rather than the via type itself.

The advantage is also the limitation. On a twelve-layer board, a through hole that only needs to connect layers two and three still blocks layers four to twelve. Where a design has a dense area such as a ball grid array, that blockage accumulates quickly, and the designer ends up adding layers to recover routing space that the vias themselves are consuming.

The Blind Via

A blind via connects an outer layer to an adjacent inner layer, or to one a few layers in, without passing through the entire board. It is drilled after a partial lamination, so the depth has to be controlled precisely: the hole must reach the target layer without penetrating the layers below it.

Because the via ends inside the board, the layers beyond it are free for routing. In a dense package escape, that recovered space is often what makes the difference between an eight-layer and a ten-layer stack. The price is process complexity. The hole depth must be held within a tolerance that is small compared with the dielectric thickness, and the plating of a shallow, wide hole has its own challenges, so the fabrication cost of a blind via is several times that of a through hole.

The Buried Via

A buried via connects two inner layers and does not reach either outer surface. It cannot be drilled after lamination, because the drilling tool can only reach the outside of the panel. Instead, the vias are formed on the inner sub-assembly, plated and then laminated over, which requires the layers above and below to bond to a surface that already contains plated holes.

That sequence makes the buried via the most expensive of the three, and it is used only where the density genuinely requires it. The benefit is the same as for a blind via, taken further: a buried via frees routing space both above and below, so it is the tool that makes a high-density structure possible at all rather than an incremental refinement.

Aspect Ratio and Drilling Depth

The aspect ratio is the hole depth divided by its diameter, and it is the parameter that limits how small a via can be for a given board thickness. A deeper hole with the same diameter is harder to plate uniformly, because the plating chemistry must reach the middle of the barrel where the flow is poorest. Thick boards and small holes therefore conflict.

This is why moving to blind and buried vias is often described as a way to reduce the aspect ratio rather than as a way to save space. A blind via from the top layer to layer three is short, so it can be small while remaining platable. The same small diameter drilled through a thick board would be beyond the process window, which is a hardware limit rather than a cost preference.

Stack-Up Choices: Sequential Lamination

Blind and buried vias require the board to be built in stages, pressing and drilling some layers, then pressing on the rest. Each additional stage is a sequential lamination cycle, and the number of cycles is what the fabricator will quote against. A single-stage stack supports through holes only. Two stages allow one level of blind vias. More stages support deeper blind and buried structures at correspondingly higher cost.

The stack-up should therefore be designed around the smallest number of laminations that carries the routing. It is worth testing whether a design that seems to need buried vias can be built with blind vias only, or whether a design that needs blind vias can be rearranged to use through holes plus one extra layer. The answer is not always the cheaper option on paper, because the layer count, the yield and the tooling all move together.

Cost and When It Is Worth It

The multiplier is significant. Blind vias often cost several times an equivalent through-hole build, and buried vias cost more again, because both add processing steps, tighter registration and lower yield. On a prototype order that cost is visible immediately; in volume it is partly offset by the smaller layer count the denser structure allows.

The decision therefore hinges on whether the routing genuinely cannot be completed otherwise. A useful test is to attempt the layout with through holes only and record exactly where it fails. If the failures are localised to one escape area, a small number of blind vias in that region may solve the problem at far less cost than rebuilding the whole stack as a high-density structure. The relevant rules for these structures are collected in the guide to selecting a blind and buried via stack.

Design Rules to Give the Fabricator

Whatever the mix, the fabrication drawing has to state it unambiguously. The drill table should list each via type with its diameter, its start and end layers and its tolerance, and the stack-up diagram should show which laminations are involved. Where a via is filled and plated over, that should be stated as well, because an in-pad via behaves differently from a plated-through hole during assembly and the fabricator needs to know which is intended.

The plating and via filling steps used in HDI construction are the reason the cost scales with the number of lamination stages rather than with the number of holes.

Two rules of thumb are worth applying at review. Keep the aspect ratio within the fabricator’s stated capability rather than at its limit, so that plating quality is not marginal on every board. And keep the number of distinct via types small, because each one is a separate drilling operation and a separate opportunity for a registration error between stages.

FAQ

Can a blind via be drilled from either side? Yes. A blind via may be drilled from the top or the bottom of the stack, and the choice depends on which layers the sub-assembly structure makes accessible. The fabrication drawing should state the direction explicitly rather than leaving it to the fabricator.

Are buried vias only used on HDI boards? They are most common there, because the density justifies the processing cost, but any design with a deep stack and a dense escape area can benefit. The deciding factor is whether the layers above and below the connection are needed for routing.

What aspect ratio should be specified? Whatever the fabricator states as standard for the board thickness in question, and no more. Staying inside the comfortable range improves plating uniformity and yield, and moving to a blind via is often a better answer than pushing the aspect ratio of a through hole.