Through, Blind and Buried Vias: Choosing a Construction

A via is a drilled hole with metal plated onto its wall, and its only job is to connect copper on one layer to copper on another. The three types differ in where the hole starts and stops, and that difference decides how much the via costs, how much board space it takes, and how difficult it is to make.

This article describes the through, blind and buried via, what each one is used for, and how the choice between them affects a design.

Why Vias Exist and How They Are Made

Drilling a hole through a board exposes bare resin on the wall, and resin does not conduct. The wall is therefore made conductive, first by a thin chemical deposit and then by electroplating, which builds a copper barrel that connects the copper features the hole passes through.

That is why a via is described as a plated hole rather than simply a hole. An unplated hole, such as a mounting hole or a slot, has no connection to any net, and the two are specified differently on the fabrication drawing.

The Through Via

A through via, also called a plated through hole, passes from the top of the board to the bottom. It is the original and the simplest construction: the board is drilled right through after lamination, plated, and the connections are made in one operation. Hold a finished board up to a light and the bright points are through vias.

The cost advantage is real, because the drilling is a single operation on a finished panel. The penalty is space. A through via occupies a position on every layer it passes through, including the layers it does not connect, and on a dense board that wasted area is significant.

The Blind Via

A blind via connects an outer layer to an adjacent inner layer and stops there. It is visible from one side of the board and invisible from the other, which is where the name comes from. Because it does not pass through the whole stack, the layers beneath it are free to be used for other connections.

Making one requires either controlled-depth drilling on a finished panel or drilling before the outer layers are laminated. The second route is more common and more accurate, and it requires the layers to be aligned to each other precisely, which means tooling and registration that a plain multilayer does not need. Depth control is the critical parameter, because a hole that is too shallow will not reach the target layer and one that is too deep will pass through it.

The Buried Via

A buried via connects two inner layers only, and it is invisible from both outer surfaces because the layers above and below it were bonded over it after the hole was drilled and plated. The connection is made within a sub-assembly, and the sub-assembly is then laminated into the finished stack.

This is the most expensive of the three, because it forces the build into separate lamination cycles with drilling and plating between them, and any defect in the sub-assembly is sealed inside the finished board where it cannot be repaired. It is used where the routing density justifies the cost, and where the alternative would be more layers overall.

Space, Cost and What Each One Buys

The trade is easiest to see as a building. A through via is a staircase that runs from the ground floor to the roof and serves every floor on the way, whether or not anyone on those floors needs it. It is cheap to build and it consumes a strip of floor space on every level.

A blind via is a staircase between two adjacent floors, and a buried via is one that exists only inside the building, between two inner floors, invisible from outside. Each is more difficult to build and each frees the floors it does not pass through, which is exactly the space that a dense fan-out needs. The comparison of the three, and the way they are combined in a stack, is set out in the selection rules for blind and buried vias.

Aspect Ratio and the Limits of Drilling

The aspect ratio of a hole is its depth divided by its diameter, and it limits how reliably the plating can reach the middle of the barrel. A deep, narrow hole is difficult to plate evenly, and the copper in the centre of the board may be thinner than the specification requires while the surface looks correct.

A blind via is usually a short hole in a thick layer stack, so it is not limited by the stack thickness, but it is limited by the accuracy of the depth. A buried via inside a sub-assembly has its own limit, set by the thickness of the sub-assembly it passes through rather than by the finished board. Because the same hole can be drilled several times at different stages, the via design rules applied to a stack with buried vias have to name the layers for each drill programme.

The Stacked and Staggered Question

Where several laser vias are needed in sequence through a stack, they can be placed one above the other, so that the column forms a continuous connection, or offset from each other so that no two touch. Stacked vias save the space that a second position would occupy, and staggered vias are easier to fill, plate and planarise.

The choice is usually made with the fabricator, because it depends on which of the two their process handles reliably. Where a design can accommodate either, the staggered arrangement is the safer default, and where the space is not available, the filled stacked column has to be specified as such. The filling process and its requirements are described under via filling by electroplating.

Which One to Specify

The default answer is the through via, because it costs least and the fabricator is most familiar with it. Blind vias are added where the fan-out under a fine-pitch package cannot be completed without them, and buried vias where the layer count would otherwise have to increase to provide the same connectivity.

The decision is therefore made from the routing rather than from a preference, and it is recorded on the stack-up drawing so that the fabricator knows which holes belong to which stage. Where a board uses any of the three in place of a through via, the choice between a via in the pad and a plated through hole is a related decision that follows the same logic.

FAQ

Can a through via be used instead of a blind one to save cost? Often, yes, provided the extra space it consumes on the inner layers does not block the routing. Where it does, the blind via is the cheaper answer overall.

Is a via always plated? A via that carries a connection is plated by definition. Holes that are not meant to connect anything are specified as unplated and are treated as mechanical features.

Do buried vias make a board more reliable? They reduce the number of through holes that pass close to sensitive routing, and they remove stubs from the fastest nets. The cost is that a defect inside the sub-assembly cannot be repaired.