An electrical connection between various layers of a printed circuit board is referred to vias (or via hole). A via is essentially a tiny hole drilled through two or more adjacent layers of a printed circuit board (PCB). An electrical connection is made in the insulating material separating the PCB layers by filling the hole from the inside with copper (either through a galvanic process, riveting, or the insertion of a short tube of conductive material). An electronic board may need vias that cross all the layers of the stackup, just one of the two outermost layers, or just two or more inside PCB layers depending on the particular requirements of the application. There is another type of via known as a thermal via, which serves to disperse heat generated by the components rather than forming an electrical connection between various PCB layers, as we will see later in this article. It is also crucial to emphasise that vias and PTH (Plated Through Hole) holes, which are typically utilised for the electrical connection of through-hole components on the PCB, should not be confused. Some components, such connectors, can also be mechanically connected to the board via the PTH holes, which are larger than the vias. Figure 1 depicts a section of a PCB with surface mount components (SMD), in which numerous vias are visible.
Varieties of via
Based on their placement within the stackup and the number of levels they pass through, vias can be divided into three primary classes:
- Through-hole via: these vias go through all the layers of the printed circuit board, including its two outside sides. They are easier to manufacture because drilling does not need to halt at the target point, and they typically have bigger diameters than other forms of vias;
- Buried via: This type of via has no connections to the external layers since it is completely buried inside the printed circuit board. Before applying the two topmost layers to a multilayer board, the internal plating of a hidden via must necessarily be finished. Buried vias are also highly helpful for lowering the PCB aspect ratio, which is the proportion between the PCB’s thickness and via hole diameter;
- Blind vias: These vias begin on one of the PCB’s exterior sides, traverse one or more internal layers, but do not connect to the other side. Blind vias pose certain challenges from a manufacturing perspective because it is crucial to know exactly when to halt drilling in order to reach the correct depth. Blind vias can greatly increase the number of connections between an internal and external layer while potentially reducing the overall number of traces and hence the PCB’s overall cost.
From the foregoing, it is clear that through-hole vias, which allow light to pass through them, can theoretically also be spotted with the naked eye, whereas buried and blind vias can not. Figure 2 depicts the three through classes.
The introduction of via holes applied directly on the BGA (Ball Grid Array) pads, also known as via-in-pad, has been driven by the evolution of electronic components toward an ever larger integration density and the resulting increase in the number of connecting pins. Via-in-pads are advantageous for PCB producers because they reduce the amount of PCB area needed for traditional vias, making for smaller-sized boards. However, via-in-pads can exhibit problems during PCB construction. In instance, soldering may result in the hole filling up and becoming unusable. Because of this, it is normally desirable to reduce the quantity of via-in-pads.
Microvias
Microvias, which unlike the other varieties are only utilised on printed circuit boards with a large number of interconnections, are a class of vias that are distinct from the others (HDI). When the diameter of the hole is equal to or less than 150 m, it is customary to presume that we can speak of microvia. The definition of microvia was changed by the IPC standard in 2013 to mean a hole with an aspect ratio (the ratio between the hole’s diameter and its depth) of 1. Additionally, a microvia’s depth must never be greater than 0.25 mm. It is important to employ laser micro-perforation, a method that is always evolving and may be able to produce microvia with a diameter even smaller than 15 m, in order to get holes of such small dimensions. The layer lamination procedure is typically carried out prior to laser microperforation, which inherently adds additional costs.
The usual cone shape of microvias, as seen in Figure 3, makes the internal wall plating process more easier. A single microvia can only pass through two adjacent layers at most. Microvia have evolved from single units to more sophisticated structures made up of many microvia as a result of the global dispersion of mobile and portable devices, such as smartphones and wearables. In fact, microvias can be stacked and overlapped if the PCB design calls for vias to cross many layers. The staggered layout, in which the microvias used on neighbouring layers are offset from one another, is an alternative to the prior configuration.
Microvias offer the benefit of lowering the PCB size on boards with a high number of connections when compared to regular via holes. As another example, BGA breakout with 0.65 m pitch is one application where microvias are extremely helpful. Microvias improve electromagnetic interference protection in high-speed and high-frequency applications while reducing crosstalk problems in audio systems. The radiant effect, which might behave like a genuine antenna due to the via’s specific geometric structure, is ultimately considerably less in microvias than in regular vias. Microvias are not without flaws or problems, though, and these can jeopardise the electronic board’s dependability. The primary issue is connected to the emergence of ICDs, also known as interconnect defects, close to the interior plating of the holes, which can result in open circuits and other sporadic issues when used in high-temperature environments.
Thermal vias
Active components, such as high-performance microprocessors, power electronic devices (including MOSFETs, diodes, and power modules), and high-frequency components, generate a lot of heat that can be better drained by using thermal vias. The majority of printed circuit boards are made of FR-4, a material that is reasonably cheap but has a high thermal resistance. As a result, heat accumulates close to the component’s bottom, raising the temperature significantly and raising the possibility of hotspot formation. Directly in touch with the active components, thermal vias facilitate improved heat transfer, enabling the component to operate at lower operating temperatures as close to ambient temperature as is practical. Thermal vias must be positioned as close as feasible (ideally below) to the component that produces heat in order to be effective. Placing more thermal channels reduces the structure’s thermal resistance and allows for the transmission of more heat because each thermal path has a better thermal conductivity than the FR-4 substrate. It is desirable to connect more thermal vias in parallel to the ground plane in order to provide a return path with reduced inductance because thermal vias also have some inductance. Thermal vias typically have a through-hole physical structure, an inside coating of conductive epoxy resin, and a plating process after that. As a result, the thermal channel takes on the characteristics of a tube that may carry heat from a component positioned on one of the surface layers to the deepest layers. To prevent solder from passing through the holes in the thermal channels, a solder mask is also placed over such areas of the board. The availability of software tools capable of doing a thermal analysis of the circuit in the various working situations and with varied values of the ambient temperature nowadays facilitates the determination of the number, size, and location of thermal vias.
