PCB Via Structures: Through, Blind, Buried and Microvia

The Smallest Feature With the Largest Influence

Holes are the most basic structure on a printed circuit board and, in a high density design, the one that constrains everything else. A hole either connects layers or it does not; it either accepts the component lead or it refuses it; it either presents a controlled impedance to a fast signal or it disrupts it. The choice between a through hole and a microvia determines how much routing space is left on the board, how many lamination cycles the fabricator has to run, and a significant fraction of what the board costs.

Understanding the family of hole structures, and what each is for, is a prerequisite for any dense layout.

What Holes Do

Holes serve two functions, and the distinction matters because the two are manufactured differently. Plated holes provide electrical connection: once the wall carries copper, the hole connects the layers it passes through, which is how signals, power and ground move vertically in a multilayer board. Non-plated holes provide mechanical fixing and are used for mounting screws, heatsink attachment, connector location and similar hardware.

Either way, the accuracy of the hole, its diameter and the quality of its wall directly affect the reliability and the electrical behaviour of the finished board. A hole that drifts off position reduces the annular ring. A rough wall compromises plating. A void in the plating breaks a connection that may only fail in the field.

The Four Via Structures

• Through hole. Drilled mechanically through the entire board and plated. It is the most common structure and the most mechanically robust, and it suits high power circuits and conventional component mounting. Its cost is space: a through hole occupies routing real estate on every layer it passes, which in a dense board is expensive territory.
• Blind via. Connects an outer layer to one or more inner layers without passing through the board. Because it does not block the layers beneath, it frees routing space and raises the achievable density. It is fundamental to HDI design and appears throughout smartphones and communications equipment.
• Buried via. Connects inner layers only and is invisible from the outside. It is formed during sequential lamination, which means the board is built, laminated, drilled, plated and then laminated again. Buried vias add density in high layer count boards and are common in advanced communications hardware.
• Microvia. A laser drilled hole, typically 50 to 150 micrometres in diameter, connecting adjacent layers. It is the core technology of HDI boards and the structure that makes fine pitch ball grid array escape routing possible. It appears in smartphones, wearables and 5G equipment, where the density requirement would otherwise be unachievable.

Choosing between them is a density and cost decision. Our notes on HDI PCB technology describe how microvias are used in stacked and staggered arrangements to build the interconnect structures that high density designs require.

Plated and Non-Plated Holes

Plated through holes carry copper on the barrel, typically twenty to twenty-five micrometres thick, and are the electrical connections. Non-plated holes have no plating and exist purely for mechanical purposes. Distinguishing between them on the fabrication drawing is a basic requirement, and confusing the two is a common and expensive error: a mounting hole that is plated may short a plane, and a signal via that is not plated simply does not work.

Where a hole must be plated but also lie flush with a pad for a component to sit on it, the hole is filled and plated over, a process known as via in pad. It is standard practice under fine pitch packages, and it imposes its own process requirements because the fill has to be void free and the plated cap has to be flat.

Hole Sizes and the Numbers Around Them

Typical hole diameters divide into four bands. Microvias run from 0.15 to 0.30 millimetres, with HDI processes reaching 0.10 to 0.15 as the practical minimum. Signal vias run from 0.30 to 0.50 millimetres. Component lead holes run from 0.60 to 1.00 millimetres, sized to the lead with clearance for insertion and solder. Mounting holes are above one millimetre.

Three numbers govern whether a hole is manufacturable. The annular ring, which is the copper pad remaining around the hole, should generally be at least 0.10 to 0.15 millimetres, because it absorbs drill position error and provides the connection. The drilling tolerance is typically plus or minus 0.05 to 0.075 millimetres, and a hole specified tighter than the process can deliver will either be scrapped or will fail to accept its component. And the aspect ratio, the ratio of board thickness to hole diameter, should generally be kept between 8:1 and 10:1 for through holes, because a deeper and narrower hole is difficult to plate uniformly and the plating at the centre of the barrel becomes unreliable.

A board may carry anywhere from a few hundred to tens of thousands of holes depending on its complexity, so each of these values is multiplied across the design. Our PCB design and layout notes set out how these constraints are applied during routing.

Drilling and Plating

Two drilling methods are used. Computer numerical control drilling with carbide tools handles the great majority of holes, and it is fast and economical. Laser drilling produces microvias and other very small features, and it is the technique that makes HDI possible, because a mechanical drill cannot make a reliable hole at 0.1 millimetres.

Plating follows the drilling, and it is a multi-step process: the hole wall is cleaned to expose a surface the copper can adhere to, a thin electroless copper layer is deposited to make the wall conductive, the copper is then electroplated up to the specified thickness, and finally the surface finish is applied. Inspection covers the copper thickness on the wall, the integrity of the plating along the barrel, and the presence of voids, which are the defect that most often turns into a field failure. The process control behind this is the same discipline described in our PCB manufacturing notes.

Defects to Design Against

Four problems recur. Drill wander is lateral deviation of the drill from its intended position, which reduces the annular ring. A rough hole wall compromises plating adhesion. A plating void breaks electrical continuity along the barrel. And layer misregistration shifts the pad relative to the hole. All four are process issues, and all four are mitigated by designing with adequate annular ring, moderate aspect ratio and sensible hole-to-hole spacing rather than by relying on the fabricator to exceed its capability.

What Holes Cost

Drilling is a meaningful line in the board cost, and the numbers scale with difficulty. As a 2026 reference, a standard mechanically drilled hole costs roughly 0.0008 to 0.003 US dollars. A small via costs about 0.002 to 0.006 dollars. A laser drilled microvia costs 0.01 to 0.05 dollars, an order of magnitude more expensive because of the equipment and the sequential processing behind it. The total depends on the number of holes, their diameter, the board thickness, the microvia count and the layer count. In an HDI board, drilling can account for fifteen to thirty percent of the total manufacturing cost, which is why microvia use is planned rather than applied everywhere.

Where the Structures Are Used

The pattern is consistent across industries. Consumer electronics such as smartphones and tablets depend on HDI microvia technology to fit their function into the available volume. Automotive control modules rely on high reliability through hole structures, because the mechanical robustness matters more than the density. Communications equipment needs precisely controlled via geometry because the signal integrity of the interconnect depends on it, as covered in our telecommunications PCB notes. And medical electronics demand the same reliability discipline, with the additional constraint of fine features. Our overview of PCB capabilities describes where each of these structures sits in the range of what can be produced.

Where the Technology Is Going

Four directions are visible. Microvias continue to shrink, supporting higher routing density. Sequential lamination is being used more widely to create complex buried via structures in high layer count boards. Laser drilling equipment continues to improve in both precision and throughput. And automated inspection, increasingly assisted by machine learning, is being applied to detect drilling and plating defects that visual inspection would miss. Together these trends support the next generation of high speed and high density products.

Frequently Asked Questions

What is the smallest hole a board can have? In HDI production the practical minimum microvia diameter is around 0.10 to 0.15 millimetres, produced by laser.

What is the difference between a via and a hole? A via is a plated hole used for layer to layer connection. A hole in general may also be non-plated and used for mechanical mounting.

Do all holes need plating? No. Plated through holes carry electrical connections, while non-plated holes are used for screws, enclosures and alignment.

Why do HDI boards use microvias? Because they raise the achievable routing density, which allows a product to be smaller and lighter for the same function.

How many holes can a board have? From a few hundred to tens of thousands, depending on the complexity of the design.

Conclusion

Via structures are where the vertical dimension of a board is designed, and each type trades space against cost against manufacturability. Through holes are robust and simple, blind and buried vias buy density at the price of sequential lamination, and microvias make high density interconnect possible at a cost per hole that is an order of magnitude higher than a drilled hole. Design them with adequate annular ring, a workable aspect ratio and sensible spacing, distinguish plated from non-plated clearly on the drawing, and the interconnect will be both manufacturable and electrically sound.