A via or via hole is an electrical connection between different layers of a printed circuit board. It is a small hole that is drilled through the PCB laminates that intersects two or more adjoining layers. The inner part of the hole is covered with copper and forms an electrical connection in the insulating material that divides the PCB layers.
Known in full as Vertical Interconnect Access, a via is a copper plated hole that enables electrical connection between the layers of a printed circuit board. This hole is usually made using a drill or laser and consists of a barrel, a pad, and an anti-pad.
Vias are crucial for designing and producing multi-layered boards, which is usually a demanding process. They serve to provide routes for the sufficient flow of electrical and thermal current between the different layers of the board, and they vary in size and type.
A via is a hole that is used to make an electrical connection between the different layers of a PCB. It is a plated through hole that allows trace routing from the surface layer of a multi-layer PCB to the inner and outer layers to conduct electrical signals between them.
Vias are created by placing copper pads on each layer of a printed circuit board, then drilling a hole through them. A chemical is put in the holes to dissolve epoxy on the internal layers and further uncover the internal layer copper pads. The inner part of the via usually contains a non-conductive material, which is mostly air, while the outer layer has conductive plating that serves to connect each layer of the board.
The hole is made conductive employing electroplating or by placing copper cylinders into the drilled holes. Stacking up and connecting multiple layers of a board through vias creates an avenue for producing smaller, lighter, and high-performing PCBs.
The primary purpose of any PCB via is to provide a conductive path where electrical signals can pass from one circuit layer to another using a plated hole. However, there are different types of vias with different possibilities for how the vias would appear on the surface of the fabricated PCB. Although they all perform virtually the same function, they are chosen according to the particular PCB design requirements, to make for a reliable PCB assembly.
A via is made up of a barrel, a pad, and an antipad.
Barrel: This is a conductive tube that fills the drilled hole, creating a connection between the top and bottom of the board as well as the inner layers, if they are present. The ratio of the hole diameter (D) to the barrel length (L) is known as the aspect ratio and is generally used to determine the strength of the barrel.
Pad: the pad is a circular copper area, also called an annular ring, that connects or links the end of barrels to the traces or components.
Antipad: This is a hole that separates the barrel and the non-connective layer. It is a void area that restricts unwanted signal traces from connecting to a particular plated hole.
Differences in vias manifest in their size, pad shapes, and hole diameters, each one made for specific PCB needs. The major types or structures of vias are through-hole vias, blind vias, and buried vias. Others include skip vias, stacked vias, staggered vias, microvias, and vias in pad.
The choice of vias is dependent on the PCB technology, the circuitry needs, and the budgeted cost of PCB fabrication. A type such as the microvia is much preferred due to its smaller size, but it might not always be the best fit. Fabrication of microvia takes a more complicated process, which makes it more expensive than a mechanically drilled through-hole via. Nonetheless, if a high-density interconnect board is the target, the microvia would be the better choice.
The types of vias are explored in detail below:
Through-hole vias are drilled through the PCB from the upper layer to the bottom layer, using a mechanical drill bit. It forms a conductive path that connects all the layers of a PCB. Though-holes vias are generally plating through-hole (PTH) vias, some can be non-plating through-hole (NPTH) vias. PTH vias are used for electrical connection between the different layers of a PCB, while NPTH is used for mechanical connection with the use of screws or connectors.
Though-hole vias are the most commonly used in printed circuit boards. They are the easiest and lowest cost to fabricate but take much PCB space. Therefore, if a circuit connection is not needed to go through all the layers of a PCB, it is better to use other space-saving PCBs via types. 
Blind vias connect the top or bottom layer of the printed circuit board to one or more of the inner layers. They can be mechanical-drilled or laser-drilled and electroplated, beginning from either the top or bottom of the PCB to an internal layer, but they do not go through the entire board. Hence, the via hole can only be seen from one of the external layers.
Blind vias are often used in high-density interconnect (HDI) PCBs. The quality and functionality of blind vias allow for the enhancement of signal integrity while reducing PCB size. It also offers a range of new routing alternatives and options in contrast to through-hole vias that take up valuable space as they traverse through layers to where they are not connected to. 
Buried vias connect two or more internal layers of the PCB without going through to an outer layer. Thus, they are drilled and electroplated between the inner layers of the PCB, where they are buried or covered, and cannot be seen from the outside. It is useful for PCBs with very dense routing.
In contrast to the blind via, if a buried via connects 3 or more inner layers, it can not be drilled on the PCB directly. It is done by drilling holes on the required PCB layers and then piling them up to electroplate the hole wall.
In the course of drilling and piling for either blind or buried vias, there can be one of two resulting cases: the holes to be connected overlap or they don't. This distinction creates two other via types, namely, stacked or overlapping vias and staggered vias. 
Stacked vias are positioned on PCB layers in a way that one is directly above or below another, to connect the circuits usually on 3 or more layers. They can either be blind or buried.
Stacked vias are easier to design than staggered vias but they cost more to manufacture. The reason for this is that the holes on different layers must be drilled at the same spot, so when the layers are put together, the holes combine to make a complete stacked via with a flat hole wall. This process must be done with extreme accuracy. When there is a PCB layer stack-up, the manufacturer electroplates the stacked hole wall and then deals with other layers. 
Vias of different PCB layers that are connected but do not overlap are referred to as staggered vias. Unlike stacked vias, staggered vias have more complexity in design. Also, since vias of different PCB layers do not overlap when compiling staggered vias, it requires much less precision, resulting in a lower manufacturing cost than that of stacked vias. 
A skip via is a PCB via that goes across multiple circuit layers but makes no electrical connection with any specific layer(s). It can penetrate two or more HDI dielectric layers and cease at one or more different layers.
Skip vias can be made overlapping, blind, or buried. Their similarity to blind vias is in their ability to connect the top or bottom layer to an internal layer of a PCB. Blind vias would connect successive PCB layers but the skip via can make connections through multiple layers.
Regarding their relation to buried vias, skip vias can connect multiple internal layers in a PCB. In this capacity, they are called core vias.
Microvias are extremely small vias, usually with a diameter of fewer than 150 microns. This means that they take up less board space, leaving more room for routing. Microvias connect only two adjacent layers of the PCB which can be on the surface or buried within the stack of board layers.
They are extremely versatile and have a lower parasitic capacitance making them suitable for high-speed circuits. More so, due to their small size and ability to connect one layer to the following one, they allow for the fabrication of denser printed circuit boards with more complex designs. They also enable engineers and board technicians to design the component layout of PCBs with smaller BGA pitches.
Microvias reduce the chances of manufacturing defects since they are drilled using lasers which is virtually the safest way of drilling PCBs with much less likelihood of generating residue. However, the manufacturing process is generally more complex and more expensive than the basic through-hole, blind, and buried types. 
A via-in-pad (VIP) is placed directly on the copper pad of a surface-mounted component or device (SMD) and plated with copper, instead of conventionally routing the signal-carrying trace from the pad to the via. This way, the via-in-pad functions to miniaturize the PCB form factor by lessening the space used to route traces.
They can either be regular through-hole vias or microvias, but what makes them peculiar is how they are positioned in a surface mount pad. The applications of via-in-pads are mostly with ball grid array (BGA) components. 
When a simple circuit board is concerned, vias are not necessary, but when dealing with a multi-layered board, they are very important. Vias help to establish a high component density in multi-layered boards and also boost trace density since they can be passed over and beneath each other in various directions. Vias function as vertical connection factors, enabling the various traces to link with each other.
When vias are not incorporated into the routing process of multilayered PCBs, surface mount components will end up being placed densely on the board. Vias are also important to foster the transmission of signals and power between layers of a PCB.
Discussed below are impacts or uses of vias in PCB design, in relation to some via types:
Signal routing: For most PCBs, a through-hole via placed on a grid is used for signal routing. But for denser boards, blind or buried vias may be required, while microvias are used with extremely dense boards.
Escape routing: To effect the escape routing, also called fanout routing, for larger surface mount (SMT) components, through-hole vias are generally used. But blind vias or microvias can be used in some instances. Meanwhile, highly dense packages with high pin-count BGAs need the via-in-pad.
Power routing: Larger through-hole vias are mostly used here since vias used for power and ground nets are to conduct more current. However, blind vias may also be used.
Thermal regulation: This is where a via is used to conduct heat from a component through the inner plane layer that it connects to, for heat dissipation. To carry out this function, a larger through-hole or blind via is required and vias that are used in this capacity are called thermal vias. 
Multilayer PCBs are the present and future of PCB manufacturing, and to fabricate them, vias are needed to ensure the production of the compact and highly functional devices that dominate modern electronics.
1. Proto Electronics. How and Where to Use Vias in PCB Design. 2021. [Cited 2022 Sep 9] Available from: https://www.google.com/amp/s/www.proto-electronics.com/blog/how-and-where-use-vias-in-pcb-design%3fhs_amp=true
2. Our PCB. PCB Via: What is it, and Which Type Should You Choose? 2022. [Cited 2022 Sep 9] Available from: https://www.ourpcb.com/pcb-via.html
3. PCB GOGO. PCB Vias: Types, Design, Covering. 2022. [Cited 2022 Sep 9] Available from: https://www.pcbgogo.com/Article/PCB_Vias__Types__Design__Covering.html
4. OUR PCB. Blind Via PCB - What Should You Know. 2022. [Cited 2022 Sep 10] Available from: https://www.ourpcb.com/blind-via-pcb.html
5. Circuit Digest. Understanding Blind, Buried and Through-hole Vias in PCB. 2021. [Cited 2022 Sep 10] Available from: https://circuitdigest.com/article/understanding-blind-buried-and-through-hole-vias-in-pcb
6. PCB Directory. What are Stacked Microvias? 2022. [Cited 2022 Sep 10] Available from: https://www.pcbdirectory.com/community/what-are-stacked-microvias
7. PCB HERO. Staggered Microvias and Stacked Microvias PCB. 2022. [Cited 2022 Sep 10] Available from: https://www.pcb-hero.com/blogs/lilycolumn/staggered-microvias-and-stacked-microvias-pcb
8. PCB Online. 8 Types of PCB Vias - A Complete Guide of PCB Vias in 2022. 2022. [Cited 2022 Sep 11] Available from: https://www.pcbonline.com/blog/pcb-vias-and-their-types.html
9. Cadence. Microvias in Printed Circuit Design Are an HDI Necessity. 2022. [Cited 2022 Sep 11] Available from: https://resources.pcb.cadence.com/blog/2022-microvias-in-printed-circuit-design-are-an-hdi-necessity
10. MacroFab. Via in Pad PCB Design. 2022. [Cited 2022 Sep 11] Available from: https://macrofab.com/blog/via-in-pad-pcb-design/#:~:text=Via%20in%20pad%20is%20the,reduce%20the%20PCBs%20overall%20size
11. RayMing. What is the Role of Via in PCB - Printed Circuit Board Manufacturing and PCB Assembly? 2021. [Cited 2022 Sep 11] Available from: https://www.raypcb.com/via-in-pcb/