What is a Via: PCB Via Design
When it comes to a multi-layer Printed Circuit Board (PCB), the layers are stacked over each other and are physically fused with each other. But they are electrically separated, with an insulator coating between every two PCB layers. However, these layers have to electrically connect with each other to form a complete circuit. This electrical connection is achieved using a PCB via/circuit board via.
A PCB via/circuit board via is a hole that can be drilled through multiple PCB layers and these layers in order to electrically connect them to each other by lining them with conducting metal. So a PCB via/circuit board via essentially allows us to connect two or more adjacent layers with each other through a hole and conductive metal, which allows power and signal to pass between two or more PCB layers.
These vias connect multiple PCB layers while taking the shortest possible conductive path, which is efficient from both material and electrical characteristics perspectives. It's also important to note that a circuit board via has to punch through the conductive segment of each PCB layer, i.e., traces, to form the electrical connection. The PCB via/circuit board via are typically filled with epoxy, which can be conductive or non-conductive in nature.
PCB Via/Circuit Board Via Types: Blind Via, Buried Via, and Through-Hole Via
There are three common types of PCB vias/circuit board vias that might be found in most multi-layered PCBs. To understand this, it's important to understand that each multi-layered PCB has two main types of layers: A surface layer and inner layer. There can never be more than two surface layers, though the number of inner layers increases or decreases with the total number of layers in a multi-layer PCB. For example, a five-layer PCB will have two surface layers and three inner layers buried between the two surface layers.
Blind Vias - Surface Layer to An Inner Layer Circuit Board Via
A blind via starts from one of the surface layers and ends at an inner layer, hence the term blind in blind via. It possesses an entryway, but no exit, similar to a blind alley. A blind via can penetrate and connect just two layers or more. However, as long as it starts at a surface layer and ends in an inner layer, it will be considered a blind via.
For example, a blind via may penetrate 19 layers of a 20-layer PCB, but even though it almost penetrates the entire depth of a PCB, it would still be considered blind because it's isn’t surface-to-surface.
Blind vias themselves can be divided into at least four types based on how they are drilled and plated with copper. These are sequential lamination blind vias, controlled-depth drilled blind vias, laser drilled blind vias, and photo-defined blind vias.
From a design perspective, blind via PCBs may face several challenges, especially when they have to be drilled after the blind via PCB (i.e., any PCB that includes blind vias) has been laminated. There is also the challenge associated with effective plating of the via with conductive copper, which requires the diameter and depth of the hole to be in proportion for each via in a blind via PCB. Failure to take this into account can lead to cracks in the conductive path, leading to several negative implications.
When done correctly, blind via PCBs can have significantly reduced parasitic capacitance, particularly in blind via PCBs with high-frequency signals, by allowing for shorter (less depth) and smaller (less diameter) layer-to-layer conductive connections.
Buried Vias - One Inner Layer To Another Inner Layer Circuit Board Via
While not as morbid as the name may have made them sound, buried vias are indeed "buried" and, thus, are aptly named. A buried via is a PCB via/circuit board via or a plated hole that is not visible to and has no opening on the PCB's surface layers. It both starts and ends at an inner layer. A buried via technically can traverse almost the entire thickness of a multi-layer PCB, starting from the first inner layer (just beneath the top surface) and ending at the last inner layer (just above the bottom surface).
Buried vias are used to connect one or more inner layers with each other. Buried vias are an important PCB design consideration because they have to be drilled before the PCB is laminated and the outermost layers/surface layers are added to the PCB. Hence, buried vias increase the cost and complexity of the PCB manufacturing process, but the benefit is a more compact PCB.
Through-Hole Vias - Surface Layer to Another Surface Layer Circuit Board Via
A through-hole via penetrates/traverses the entire width of a PCB and connects one surface layer to the other. So, the drilling depth of a through-hole via is essentially similar to the width of the PCB board, instead of the other two types, where the number of layers the vias connect determines the depth of the drilling.
It's important to understand that a through-hole via is not the same as a plated through hole, even if they are drilled in the same manner, i.e., surface to surface. One significant difference is that a plated through hole is more than just a surface-to-surface connection. It's also a mechanical path for mounting a component on the PCB. So, its diameter may be larger than it needs to be from a conductive standpoint.
Even though it's more of a design difference than a standard circuit board via type, it's important to understand what a via in-pad is. A pad (regardless of whether or not there is a via in-pad) can be considered a "landing pad" on a PCB, but instead of helicopters, electronic components land on it and are electrically connected to it.
Pads (with or without a via in-pad) on a PCB are conductive areas on the surface layer that allow you to connect/mount electronic components (ICs, chips, etc.) on the circuit. There are two types, through-hole, and surface-mount pads, and for via-in-pad, we are interested in the latter. Surface-mount pads allow the legs of a component to electrically connect (as they are soldered) to the surface layer of a PCB.
Now, if they need to connect to inner layers, they would need the services of a via (typically a blind via). If the vias are located far from these pads, the signal will have to travel further, and the PCB will need to be larger. A via in pad, which basically is a via that's drilled in the pad and directly connects the electronic component (at least one of its legs/conductors) to two or more layers, takes care of both these problems.
A via in-pad allows a shorter path to the signal and keeps the board small. Without a via in-pad in place, a PCB may require more conductive material and higher surface area to accommodate the same number of surface components. A via in pad is usually just a blind via, not a through-hole since it's only relevant to surface-mounted components. A via in pad cannot be buried via for a similar reason because every via in pad needs to have one surface-based connection. The via-in-pad count may be influenced by how many components have to be mounted on the PCB and the number of legs.
PCB Via Design Considerations and Concepts
When it comes to designing PCBs where you need to connect different layers through vias, there are a few important considerations, starting with aspect ratio, which is the ratio that exists between a via's depth and its diameter. A healthy aspect ratio ensures the electrical/conductive integrity of the electroplated conductor and even the conductive PCB via fill.
PCB via fill is another factor to consider. A PCB via fill contributes to the structural and electrical integrity of a circuit. Without the right PCB via fill in place, the circuit board vias may trap moisture or air, which may have an adverse impact on the electrical performance of a circuit board via and by extension, the entire PCB.
PCB via fill also makes sense from a thermal conductivity and dissipation standpoint. The right PCB via fill will effectively dissipate the heat from the circuit. A PCB via fill may also be called a PCB via plug in certain markets. The PCB via fill material you use will depend upon several factors, including the type of via. While epoxy is one of the most common types of PCB via fills available and used in the circuits, the LPI PCB via fill or Liquid Photo Imageable (also called solder resist mark) PCB via fill is also frequently used.
Another important design consideration is the use of microvias. Microvias are just a layer deep and can be used to make all three types, i.e., through-hole, buried, and blind via PCBs. For this explanation, we will focus on blind via PCBs, though the principle will be the same for other types of vias as well and for PCBs leveraging all via types. The essential characteristic of microvias is that they are small and drilled with lasers. To make a blind via PCB with microvias, they can be staggered or stacked.
A blind via PCB using stacked micro vias, i.e., micro vias stacked over one another, may be costlier than a blind via PCB made by staggering micro vias. That's because it requires more accuracy and stricter layer control. It's the same with buried vias. Buried vias can be both stacked and staggered, and stacked buried vias may require more precise tooling (thus increasing the cost) than staggered buried vias. The additional cost factor is not limited to buried vias or blind vias but also extends to through-hole vias.
Staggering micro vias in a blind via PCB (or buried via PCB) is easier because the micro vias are not supposed to be aligned, just connected, which leads to higher tolerances when developing a staggered blind via PCB. The PCB via fill considerations would be the same whether you are designing a buried or blind via PCB.
Conclusion
PCB via design considerations have to take several important elements into account, from the type of via your design will focus on (buried or blind via PCB), PCB via fill, via in-pad design requirement, etc. Some of these requirements, particularly the via-in-pad one, will be influenced more by external factors like surface-mounted components that would need the via-in-pad design to remain compact. Almost all via-in-pad PCBs are also blind via PCBs since a via-in-pad is blind via by definition.
