Introduction#
In the engineering world, particularly in the design and manufacturing process of multi-layer Printed Circuit Boards (PCBs), the term “via” has become widespread and essential. A via, a Latin word which means “way”, forms an integral part in the functionality of PCBs. Specifically, it forms the pathway that allows for the transmission of electrical signals from one layer of the PCB to another.
However, for the uninitiated, the term may remain ambiguous. The lingering question remains – what exactly does a via entail in the scope of multi-layer PCBs, and how is it leveraged in designs?
In this article, we aim to unravel the complexities surrounding PCB vias. We will peel back the layers of technical jargon, deliberating on what they are, their functions, and the multiple types available.
What is a Via?#
A via is a conductive connection formed in a hole between selected copper layers. The hole is typically drilled mechanically or by laser and then metallized. Its layer span determines which layers it can reach; pads and clearances determine which conductors actually connect to it. Vias carry signals or power between otherwise separate layers.
Vias are commonly classified into three types based on their functionality—blind via, buried via, and through-hole via.
Types of Vias
Blind Via
A “blind via” in the context of PCB (Printed Circuit Board) design is a small hole that starts at one surface of the board—either the top or bottom—but doesn’t go all the way through to the other side. Instead, it only connects to one or more layers inside the board. As a result, you can only see and access a blind via from one side of the PCB, since it is not penetrating fully through the entire board.
In the design of PCBs, blind vias have a unique and beneficial usage. One of their main advantages is that they create extra room on the board as they do not traverse all layers. This can be critical in compact designs where space is at a premium. For this reason, you’ll find them frequently used in highly dense circuit assemblies such as BGA (Ball Grid Array) and HDI (High-Density Interconnect) PCBs, where they contribute to achieving a more compact and efficient layout.
Buried Via
A “buried via” is a particular type of via used in Printed Circuit Board (PCB) design. Unlike other vias, it doesn’t reach out to either the top or bottom surface. Instead, it interconnects at least two inner layers of the board, making it invisible from the outer layer. This inward linking nature of buried vias makes them completely hidden from view when you examine the PCB from the outside.
Buried vias connect internal layers without occupying the outer surfaces. This can free routing space in a dense layout. Improved signal integrity is not automatic: return paths, impedance discontinuities, coupling and the full via geometry still require review. Buried vias are particularly useful when an HDI layout needs internal connections without extra surface pads.
In such dense board designs where there’s a critical need to conserve space and maintain high performance, the application of buried vias becomes exemplary.
Through-Hole Via
The “through-hole via” stands as the most frequently used type of via in Printed Circuit Board (PCB) production. It creates a link between the inner and outer layers of the PCB. Unlike other types of vias like blind and buried vias, a through-hole via cuts all the way through the entire board. This is the reason behind its name – it literally creates a ‘through-hole’.
Through-hole vias possess a couple of significant utilities in the scope of PCB manufacturing. Primarily, they serve as channels for internal interconnections within the board, thus allowing electrical signals and power to flow from one layer to another freely.
A through-hole via is an electrical interconnect, not a general mounting hole. Component leads may use plated through-holes designed specifically for their dimensions and assembly requirements; mechanical mounting holes have their own specifications. Do not interchange these features merely because all pass through the board.
Via Covering Processes#
Common surface-treatment choices include open vias, tented vias and plugged vias. Filled and copper-capped vias are another distinct option. Specify the intended construction rather than treating every covering method as interchangeable.
Tenting Vias
Tenting covers a via opening with solder mask. It is different from filling the hole. The mask can protect the external conductive area, but coverage depends on hole size, material and process.
Ask the fabricator which hole sizes can be tented and how coverage is checked. A nominal tent can contain an opening or fail to close completely. Do not assume an impermeable seal or a guaranteed insulation barrier without an agreed requirement.
A one-sided or incomplete closure can trap process chemicals or residues. Agree which side or sides are covered and the acceptance criteria for the selected process. More mask thickness alone is not a complete solution to coverage or cleanliness.
Vias Not Covered
Choosing “vias not covered” as a design approach for your Printed Circuit Board (PCB) means deciding to leave both the hole of the via and its surrounding ring, also known as the annular ring, exposed. They will not have a layer of solder mask, which typically acts as an insulator. This absence of solder mask sealing leaves the vias open and bare.
Open vias may provide access for measurement or debugging when probe access and electrical clearance are intentionally designed. Removing solder mask from a via is not a substitute for a thermal design; the copper connection, via geometry and heat-transfer path govern its thermal role.
The exposed metal also creates a possibility of unintended contact or solder entry. Consider nearby pads, conductive enclosure parts, contamination and the assembly process when deciding whether a via should remain open.
Plugged Vias with Solder Mask
Plugging introduces material into the via opening rather than merely covering it. The specified plug material and process determine whether the hole is partially or fully closed. This must not be confused with a fully resin-filled, planarized and copper-capped via.
A defined plugging process may be used to manage solder entry and residue risks during assembly. It does not automatically remove flux or guarantee a void-free seal. Agree cleaning, coverage, material compatibility and inspection requirements with the fabricator and assembler.
A BGA or IC does not automatically require solder-mask-plugged vias. Via-in-pad applications need a suitable solderable pad surface and control of solder loss into the hole; resin filling and copper capping may be appropriate. Confirm the required process with the fabricator. Eurocircuits distinguishes these constructions in its via-covering guidance.
Blind vias connect an outer surface to internal layers; buried vias connect internal layers without reaching an outer surface. Availability depends on the actual stackup, layer pairs and fabrication process, not a universal four-layer rule. These structures can improve routing density but may add lamination and processing steps. Through-hole vias are usually simpler and more economical to manufacture. Discuss the required construction with PCBPit before finalizing the design.
Before production, place the critical requirements for What Is a PCB Via? Types and Design Guide in controlled design data, review them with the fabricator and validate the finished assembly under its real electrical, thermal, mechanical and environmental loads.