PCB Via Types

When it comes to the level of sophistication, PCBs lie on a massive spectrum - from simple designs that can be created at home to highly complicated ones that require state-of-the-art tools that not many PCB makers possess. But regardless of the level of sophistication, the basic building blocks are the same in almost all PCBs. This includes substrate, conductor for traces (copper mostly), and vias.

What is PCB Via?

A PCB via is an electroplated hole that allows for one layer of PCB to be connected to another. For single-layer PCBs, that's the top and bottom layers of the PCB. In multi-layer PCBs, vias might penetrate a single tier/level of substrate, connecting to PCB layers or all of them, connecting the top layer from the bottom one.

There are multiple reasons why a via - plated hole is the preferred method of connecting two or more PCB layers electrically. One is that it's the shortest path between two layers, which can significantly reduce the losses (low impedance, better signal integrity). Another reason is that vias can be strategically placed almost anywhere on a PCB layer, and they don't take up a lot of the surface area available, leaving adequate space for components and traces. Other methods of connecting two or more layers electrically are complex and do not possess the resilience and versatility of vias.

There are two main components of a PCB via, one mechanical and another, both electrical and mechanical. The mechanical component is drilling - how a via is drilled into the PCB substrate. The second component is plating of the via. It's electrically critical because it's what allows the two layers to electrically connect. The mechanical element via electroplating is connected to the thickness of the conductive layer and its mechanical integrity. Any cracks or weaknesses in the electroplated material may lead to electrical degradation and even shorts.

PCB vias are an integral part of most PCB designs and can critically influence its dimensions, design complexity, cost, etc.

Mechanical Drilling vs. Laser Drilling

Before we dive into different types of vias (that may prove difficult since the diameter is too small), it's important to look into the two main ways via which holes are made in a PCB, i.e., the mechanical creation of vias in a PCB. The two ways are mechanical drilling and laser drilling.

Mechanical Drilling

Mechanical drilling is the conventional way of making holes in a PCB substrate - creating a via. A drill bit of the adequate size is attached to the drill, the PCB substrate is clamped in place to ensure that drilling doesn’t move the board, which can ruin the via. Then holes are made as per the design requirements.

Mechanical drilling offers certain cost benefits but it has its limitations. Once hole density increases beyond a certain threshold and the wear on the drill bits requires frequent replacement of the bits, mechanical drilling may actually cost more than the alternative.

The most significant limitation of mechanical drilling is the via size. Mechanical drilling cannot create holes that are too small in diameter. The lowest limit differs among PCB makers, but a common lowest limit is six mil (0.15mm).

Another drawback is that mechanical drilling may require deburring of the holes to ensure that the walls are mechanically consistent for electroplating.

Mechanical drilling is the preferred method for most large holes, especially through-hole vias that penetrate the entire width of the PCB (multiple layers).

Laser Drilling

Laser drilling requires a high-powered laser that can "burn" a hole in the PCB substrate instead of drilling one. The burned hole is clean, doesn't require deburring, and is even more precise than mechanical hole drilling using CNC machines. Laser drilling is costlier in most instances, but it allows you to drill vias of incredibly small sizes, which can be critical in designing more compact and higher-density PCBs. Another major advantage of laser drilling is that it works on a wide range of PCB materials, while mechanical drilling might be limited to materials with certain mechanical characteristics.

But it has its limitations as well (apart from cost), like speed, since laser drilling might be slower than mechanical drilling, especially for larger holes.

PCB Via Types

PCB vias can be divided in a number of ways, based on their drilling mechanism, where they are located/drilled, and a number of other factors.

Via types based on location and level of penetration.

Through-Hole Vias

Through-hole vias, also called plated through-hole vias, extend from one surface of the PCB to the other (top to bottom). They go through all the layers of a PCB, and when they are plated, they connect not just the two surfaces but also all the layers of the PCB. In most cases, these holes are drilled mechanically, as laser drilling might not be feasible when your goal is to penetrate multiple layers. 

Blind Vias

Blind vias are single-level vias (penetrating just one level/layer of the substrate) that start at a surface layer (top or bottom) and reach the next internal layer. So in a ten-layer PCB where layer one is the surface and layer ten is the bottom layer, and internal layers are two to eight, blind vias can only exist in layers one and two, and ten and nine (starting at ten and ending in nine).

Buried Vias

Buried vias are single-level vias (penetrating just one level/layer of the substrate) that start from one internal layer and end at the next internal layer. They have no opening on the surface, hence the name. In a 10 layers PCB where layer one and ten are top and bottom layers respectively, buried vias can exist only in layers two to eight.

Skip Via

A skip via is a via that penetrates multiple layers of a PCB but skips one or more of the layers in between. For example, a via that extends from layers one to five and is electrically disconnected from layers three and four would be considered a skip via. The skipped part may be considered a "stub."

Via types based on drilling mechanisms and size.

Mechanically Drilled Vias

Mechanically drilled vias are arguably the most common types of vias. They are made using drill bits made of carbide or, in some cases, tungsten carbide. The width of these vias is uniform across the entire depth of the via. However, the walls of the via may not be as uniform and may require deburring before they are electroplated, so a consistent metal layer can be made. Once you go beyond a diameter threshold, it's usually more practical to opt for mechanically drilled vias, as they tend to be relatively inexpensive and faster to drill.

Laser Drilled Vias

Laser-drilled vias are preferred for small hole diameters. The walls of these vias don't require any additional treatment, and they are highly precise, with minimal margin of error in positioning. They can be a bit more expensive and may take more time to make compared to mechanically drilled holes.

Microvias

A microvia is essentially just a very small via. The official definition of or, more accurately, the conditions for a via classifying as a microvia are:

• The depth of a microvia cannot be more than 0.25 mm.
• The maximum aspect ratio of the via is 1:1. An aspect ratio (in a via) is the ratio between the width and depth of the via. An exact ratio of 1:1 is difficult to achieve, even with laser drills, so anything close to it (as long as it complies with the other condition) can work.

While it’s not technically a condition for a via to be considered a microvia, most microvias are products of laser drilling instead of mechanical drilling. Microvias have become a critical part of most modern PCBs as they contribute significantly to the “compactness” of the PCBs.

Backdrill Vias

A backdrill via is not technically a via. It's a via-making/drilling technique that is used to limit or reduce the size of a plated-through hole that goes from surface to surface (top to bottom layer) of a PCB, penetrating all internal layers. Let's say you have a 12 layers PCB with a plated through-hole. Electrically, the via would be connecting all 12 layers of the PCB. However, from a signal perspective, you don't need the bottom four layers to be connected. In fact, their being connected may compromise signal integrity, so you can remove this unused portion of the plated through-hole (also called the stub) via backdrilling.

Backdrill vias are drilled from the back surface of the PCB (hence the name) and are slightly larger in dia than the plated through hole they are drilling in. This allows them to cut the plated-through hole short and limit it to the desired number of layers.

Vias based on different configurations, arrangement, and physical overlap.

Via in Pad

The question to answer before defining via in pad is what is a PCB pad or what is a pad in a PCB? A pad is where a component attaches to the PCB. It can be a surface-level pad for Surface Mount Devices (SMDs) or a through-hole pad that goes through all the layers of the PCB. On the surface level, these pads are often connected to the traces of the PCB to establish the electrical connection between a component and PCB. But if it has to connect to an internal layer, a via in pad is used.

A via in pad (as the name suggests) is a via that's embedded inside a pad that's created to mount a device/component on the PCB. Via in pads are only found for SMD pads because through-hole pads already penetrate the internal layers. By embedding a via directly in a pad, a component can be connected to the internal layer via the shortest possible path, and it also limits the number of vias that blind vias need to occupy space on the surface of the PCB. Via in pads can be challenging to design and fabricate because landing pads for components because drilling and plating have to be precise to get the desired results. It may require supporting elements like an annular ring and additional solder mask clearance.

Stacked Via

Stacked vias are blind and buried vias stacked on top of one another to establish a multi-layer connection. These vias are drilled, electroplated, and filled separately before the layers are stacked. If these vias are covering all the layers of the PCB, a stacked via can essentially be an alternative to a plated through hole via. However, it’s costlier and more difficult to design and fabricate.

Staggered Via

Staggered vias are also used to connect multiple PCB layers through single-layer vias but instead of sitting on top of each other (as they are in stacked vias), staggered vias make up the shape of steps, where a via on one layer is connected to the via on another through the single point of contact, even though the two are horizontally misaligned. Staggered vias have two benefits over stacked vias. They are not as expensive to make, and they don't need to be filled with copper to ensure mechanical integrity. 

What is PCB Surface Finish and Via Plating

PCB surface finish is the term used for both the material that is used to cover/coat a PCB layer so that its copper/conductor remains safe for oxidation and the process of applying this coating. However, the PCB surface finish is also connected to vias in a way, or more accurately, via plating. The choice of a PCB surface finish may influence via plating. It's important to choose via plating material and processes that are compatible with the PCB surface finish you have chosen. There is chemical compatibility as well as compatibility when it comes to the adhesion properties of the two. In some cases (especially high-frequency PCBs), the two choices (PCB surface finish and via plating) may also influence the electrical characteristics of the PCB.

The most common PCB surface finish method used to be HASL (Hot Air Solder Leveling), which used a solder mix made of tin and lead. However, certain environmental concerns have pushed it down in popularity, and now, the ENIG PCB (or Immersion gold PCB) finish is rapidly rising in popularity.

ENIG PCB (or Immersion gold PCB) Finish

ENIG PCB or Electroless Nickel Immersion Gold PCB finish is an amazing PCB finish option that relies upon two metals - nickel and gold. It's also sometimes referred to simply as immersion gold PCB finish.

ENIG PCB (or Immersion gold PCB) finish has emerged as the top PCB Surface Finish for a number of reasons. It offers exceptional corrosion resistance (in copper traces on a PCB), which is the primary goal of a finish. It also facilitates good solderability, which is a significant benefit from an assembling perspective. It's also compatible with a wide range of via plating materials, allowing it to accommodate versatile configurations.

The electroless in the ENIG PCB (or immersion gold PCB) refers to the primarily chemical nature of the process. The PCB or PCB layer is dipped in a solution so that nickel can be deposited on the copper surfaces (activated in nature). Afterward, the PCB is dipped in a solution containing gold, which deposits a thin gold layer over copper.

Final Words

Understanding different via types, the design considerations associated with them, and their strengths and limitations can help you make smarter design and fabrication choices. Understanding the role different drilling methods play can also be crucial in managing your PCB's costs without compromising on quality. This is also true for the surface finish types and the plating choices.