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Learn PCB Breadcumb Introduction Breadcumb Vias


Vias are small holes drilled into PCB layers in order to create connections between components and the layers of the board.

There are three main types of vias: through hole, via-in-pad, and microvias.

Depending on the type of via, the via may be made with a mechanical drill or a laser drill. Mechanical drills are used for larger vias and laser drills are used to make microvias.

Vias can be filled with copper, epoxy or metal epoxy. Copper is the most commonly used via fill. Epoxy is non-conductive and will insulate from signals and heat. Metal epoxy is conductive and may be used to spread heat. If the via is too long and narrow, or is filled improperly, voids and other inconsistencies that impact board performance can occur.

Through Hole Vias

Through hole vias go all the way through a printed circuit board. Through holes are mechanically drilled and are typically 8 mil in size, though Sierra Circuits can go down to 5.9 mil through holes. These vias are usually plated with copper, but they can also remain unplated, called non-plated through holes. Through holes allow component wires to go into the hole to be connected to the board.

Example of a through-hole via.

To specify through hole sizes, state the size it should be with a range that the hole can be plated down to. For example, the hole needs to be 8 mil in diameter. If the acceptable range of the via is 5 mil to 11 mil, then the specification will be 8 mil +/- 3 mil.


Vias-in-pad are mechanically or laser drilled into the board, filled, and plated with a pad over the top. The component can then be set on top of the pad with solder and connected to other layers through the via. This save space on the board since the component and pads are stacked on top of each other. In regular through hole, the pads and vias are spread out and take up more real estate. Without a plated via, the solder would melt into the via.


Microvias are defined as any hole equal to or less than 150 microns in size. Microvias are used in HDI boards because they are much smaller than mechanically drilled through holes. The hole connects the layers with copper plating. These are drilled into the board as cone shapes so the sides of the via can be easily and properly plated with copper. The hole can only go from one layer to an adjacent layer, so vias that need to go through multiple layers are stacked on top of each other or staggered in the board.

Stacked and Staggered Microvias

Stacked microvias are laser drilled vias created one at a time and sequentially stacked on top of one another. Two microvias stacked on top of one another is the most common, but up to four vias can be stacked. Four stacked vias is typically not recommended due to higher cost.

Staggered microvias are microvias that are set up like a staircase, one diagonally below or above another.

Buried and Blind Microvias

Microvias can be buried or blind. Blind vias are vias that enter through the outer layer of the circuit board and end in one of the inner layers; the via does not go all the way through the board. Using blind vias increases wiring density. When signal tracks from an outer layer need to be routed to an inner layer, blind vias can be the shortest distance and thus best way to do this. Some microvias go through two layers, called skip vias, but this can cause problems with plating, and so is advised against.


Buried vias are vias that are buried between and connect layers in the inner layers of the board; they cannot be seen from the outer layers. Buried vias are usually manufactured in consecutive laminations, and can be drilled mechanically or with a laser.


Blind vias are a great option to maximize the space of the HDI board. For example, in a four layer board, the via may go from layer 1 to layer 2 or from layer 4 to layer 3.

Microvia Design Guidelines

Hole size

When determining hole size, keep aspect ratio in mind. This will ensure your vias can be properly plated and will not encounter any issues. Consult with your PCB manufacturer if you would like them to look over your design.

Pad size

When determining hole size, keep aspect ratio in mind. This will ensure your vias can be properly plated and will not encounter any issues. Consult with your PCB manufacturer if you would like them to look over your design.

Aspect Ratio

Aspect ratio is the thickness of the PCB dielectric material divided by the diameter of the via. Vias need to be a manageable size to be properly plated. If holes are very small and the board is too thick, plating the via will be an issue and the metal may not make it all the way through the via. Imagine trying to drink a thick milkshake through a very thin straw. Like the milkshake, plating material would have a hard time plating a very thin but deep hole.


Hole Diameter


Maximum Dielectric Layer Thickness

(mils for Aspect Ratios (AR)

AR=0.5 AR=0.75 AR=0.9 AR=1
6.00 3.00 4.50 5.40 6.00
5.00 2.50 3.75 4.50 5.00
4.00 2.00 3.00 3.60 4.00
3.00 1.50 2.25 2.70 3.00
2.00 1.00 1.50 1.80 2.00

Here is a chart you can use to look at examples of aspect ratio, via size, and dielectric thickness. If the dielectric thickness of a board is 3 mils, and the via is 6 mils, then the aspect ratio is 0.5. Another way to look at the chart: the dielectric is 4.5 mils and you want an aspect ratio of 0.9. So the via would be 5 mils in diameter.