Types of Vias

In electronics and computing devices, the circuitry is conducted by a tiny green board that transfers various signals from the control prompts to the screen. In every smartphone, for example, a printed circuit board (PCB) can be found inside with various chips and components that conduct the signals for thousands of different features and commands. Each time a prompt is pushed on the touchscreen, one of the signals on the internal board is activated. Most of these signals are conducted through PCB vias.

What Is a Via?

In a printed circuit board, vias are holes that pass through the layers of the board for the purpose of conductivity. Each hole functions as a conductive path through which electrical signals are passed between circuit layers. Vias travel through varying levels on a printed circuit board. Depending on the design of the PCB, the board might require a hole that goes through all of the layers from top to bottom. Alternately, some vias penetrate only the top or bottom layer, and some are placed through an inner layer. There are various options when it comes to vias on a printed circuit board.

Vias are among the most crucial features on a printed circuit board. Consequently, they account for a sizable share of the cost involved with board manufacturing.

Despite the fact that these different types of vias serve the same basic purpose, one particular type of via will be more suitable than others on certain PCB designs. This Gold Phoenix PCB Knowledge Center article covers the different types of vias in PCB design and how each can facilitate an electrical connection on a printed circuit board.

Main Types of Vias

Vias can be classified in two ways. By layer penetration, there are two primary categories — the blind hole and the buried hole. By construction and application, vias fall into three main types — through-hole, via-in-pad, and microvia. Both perspectives are relevant to PCB design.

Layer Penetration: Blind and Buried

In a blind hole, the hole penetrates from the top or bottom surface of the board and connects to one or more inner layers, but does not pass through the entire board. Blind holes are named as such because they cannot be seen through when holding a board up to the light. The process involved with blind-hole creation can be difficult because the drilling depth must be precisely controlled.

Another type of via is the buried hole, which might appear through one or more internal layers. Since the buried hole is sandwiched between layers, it cannot be seen by the naked eye. For a multi-layer board to have a buried via, the hole plating on the internal layer must be completed early on during PCB fabrication, before the top and bottom layers are applied to the board.

Construction and Application: Three Main Types

Wherever the via is placed, it is likely to be one of three main construction types — through-hole, via-in-pad, or microvia.

1. Through-Hole

The most obvious type of via is the plated through-hole, which penetrates all the layers in a multi-layer board. Through-holes are typically larger than blind holes and buried holes and are also a lot easier to identify with the naked eye. When a board is held up to the light, the light will pierce right through a plated through-hole. Through-holes are also easier to make because the drill goes right through all the layers, unlike with blind holes, where the drilling depth must be carefully controlled.

Through-hole technology has been around since the mid-20th century when it replaced point-to-point construction. Through-holes were most common between the 1950s and 1980s when nearly all the features found on a printed circuit board were attached to a through-hole.

In the early days of through-hole technology, PCBs had their tracks printed only on the top side. As technology advanced, the printing appeared on both sides. Eventually, multi-layer boards came into use. At this point, through-holes were updated to plated through-holes to enable contacts between conductive layers. Today, plated through-holes are used to connect the different layers in a PCB. Through-holes are usually used to facilitate components with wire leads. Axial-leaded components are placed through these holes and used to connect across short spaces.

Through-holes can often be seen on the large motherboards that were used in computer towers during the 1990s and 2000s. PCBs with through-hole components could also be seen in the old peripheral component interconnect (PCI) cards that were plugged into these older motherboards. For example, the sound cards that connected to PCI slots inside the tower would typically feature components that were attached to the board in through-holes. Similar features could be found on graphics cards in older computers that predate today’s all-in-one flat-screen devices.

With the growing popularity of small, compact, mobile computing devices and electronic gadgets, PCBs are increasingly designed with fewer through-hole components. Large through-holes consume significant board space that could be used more efficiently with microvias or surface mount technology. For applications where board size is a constraint, microvias and surface mount components offer clear advantages in space utilization. Through-hole technology nevertheless remains relevant for power electronics, high-reliability applications, prototyping, and components that require stronger mechanical bonds.

2. Via-in-Pad

One of today’s more popular designs for printed circuit boards involves the application of vias on ball grid array (BGA) pads, alternately known as via-in-pad. In the via-in-pad design, the vias are placed on the BGA pads of the PCB. The design has become popular because it makes it possible for manufacturers to minimize the amount of space required for vias. As such, via-in-pad allows manufacturers to make smaller printed circuit boards that require less space to route signals. Via-in-pad is an optimal technology for today’s compact electronic and computing devices, which are designed to fit into pockets and sometimes around wrists.

The via-in-pad design is especially convenient for routing because the holes connect straight to the layer that underlies the component, making it possible to route signals without the risk of going outside the perimeter of the device’s footprint.

Granted, not all manufacturers have embraced the practice of placing vias on BGA pads. One of the main drawbacks is that the pad must be filled with pure copper or some non-conductive material covered with copper. Otherwise, the solder will bleed off the pad and cause the PCB to lose its connectivity.

Another prohibitive factor is that vias require an additional step that can often be costly and time-consuming. Some designers simply do not wish to add to the cost of PCB creation. Moreover, placing vias in a pad alters the required drill diameter.

Despite these issues, there are various advantages of placing vias on BGA pads. Costs aside, via-in-pad makes it possible to produce smaller boards and ultimately smaller devices. For certain modern gadgets and mobile devices, the via-in-pad design might be the only option.

In a standard via layout, solder mask can be applied to stop the flow of solder into the via. With the via-in-pad design, the barrel cannot be left free of filling because air could get trapped, causing out-gassing while the PCB is in production. Therefore, the vias need to be filled when placed on BGA pads. For the design to work, a flat planar surface is also required, as this will make it possible to connect fine-pitch BGAs with little, if any, complications.

In-pad vias can be filled with epoxy. This should be done after the drilling and plating steps. Another option is to fill the vias with copper. This would work if the vias are laser-ablated. Whichever method is chosen, the pad must be large enough for the diameter of the hole and must also have sufficient tolerances. Furthermore, the design would need to fulfill the IPC Class 2 and 3 standards.

3. Microvias

In PCB design, vias that are under 150 microns are known as microvias, which are used on many high density interconnect (HDI) boards. Designers prefer microvias because of the small size of the hole, which consumes far less space on the board than the holes that require more drilling. In a microvia, the layers are connected to one another with copper plating.

Microvias are created in the shape of cones, which makes it easy for the via sides to be copper plated. A microvia can go through two adjacent layers but not any further. If a board design requires a via through multiple layers, multiple microvias would need to be stacked accordingly.

From a manufacturing standpoint, creating stacked microvias can be a costly and time-consuming process. On boards that require one microvia over another, the most common design will feature two microvias. As far as stacking is concerned, the ultimate limit is four microvias. However, four microvias are rarely applied in PCB manufacturing because of the high costs.

An alternative option to the stacked microvia is the staggered microvia, which is set up like a staircase where a second or third microvia is placed a step forward below the one above. As with stacked microvias, staggered microvias can be difficult and costly to produce on a multi-layer PCB.

If a microvia enters through the outer layer of the board and cuts to an inner layer before stopping, that would qualify as a blind microvia. With a blind microvia, the density of the wiring on a printed circuit board can be increased. Blind microvias are especially advantageous if a signal on the outer layer must be routed to an underlying layer, as the blind microvia will offer the shortest distance. On an HDI board, blind microvias make it possible to optimize space in a multi-layer board, such as in a PCB with four layers where the microvia could be placed on either the top-two or bottom-two layers.

In some cases, a microvia will penetrate two whole layers, skipping an intermediate layer. Vias that pass through one or more layers without connecting to them are known as skip vias. However, skip vias are not always recommended because the nature of the hole can result in complications with plating.

A microvia that connects two inner layers in a PCB is known as a buried microvia. For a buried microvia to be included in a PCB design, the layers that contain the holes must first be drilled before the outer layers are applied. With a buried microvia, two inner layers on a printed circuit board can be connected. The drilling can be accomplished with a mechanical tool or, alternately, with a laser.

When placing a microvia on a PCB, it is crucial to pay attention to the aspect ratio of the hole size; otherwise, it might be impossible to plate the board properly.

How to Determine the Right Via Requirements for a PCB Design

The type of vias chosen for a PCB design should be based on the size and purpose of the board. If the board is made for use in an older, larger computing device, the PCB design will probably need to conform to older standards, as anything newer would likely be incompatible with the device in question. For some of these older devices, the only option would likely be a larger PCB with through-hole components.

If designing a PCB for a smaller device, space constraints often favor microvias or surface mount components over through-hole components. For example, if the PCB design is merely a square inch in size, the available space may not accommodate large through-holes given the number of signals that need to be routed. Blind microvias can provide effective signal routing between layers on compact boards while minimizing the board footprint.

The process of manufacturing smaller PCBs with microvias may require additional investment due to the precision and labor involved. However, the space savings and performance benefits of HDI designs can justify the added cost, particularly for compact consumer electronics and high-density applications.

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