Introduction to PCB Vias
A PCB (printed circuit board) via is a small hole drilled through a PCB that allows electrical signals to pass from one layer of the board to another. Vias are a critical component in PCB design, enabling complex circuits to be routed across multiple layers while keeping the board compact.
Types of PCB Vias
There are several main types of PCB vias:
Via Type | Description |
---|---|
Through Hole | Goes completely through all layers, from top to bottom |
Blind | Connects an outer layer to an inner layer, but does not go through the entire board |
Buried | Connects inner layers without being visible on the outer layers |
Microvia | A small blind via with a diameter ≤0.15mm, used in HDI PCBs |
Through Hole Vias
Through hole vias are the most common type. They are drilled through the entire thickness of the PCB, from the top layer to the bottom layer. Through hole vias provide strong mechanical connections and are used for mounting through-hole components. However, they constrain routing space on inner layers.
Blind Vias
A blind via connects an outer layer to one or more inner layers, but does not go all the way through the board. Blind vias allow greater flexibility in routing, as they free up space on layers they don’t pass through. They are formed by drilling only through certain layers during fabrication.
Buried Vias
Buried vias connect two or more inner layers without being visible on either the top or bottom of the PCB. They are created by drilling the via hole, plating it, then laminating additional layers on top. Buried vias save even more routing space on outer layers compared to blind vias.
Microvias
Microvias are small blind vias with diameters of 0.15mm or less. They are laser drilled and are a key technology in high density interconnect (HDI) PCB designs. Microvias enable very fine pitch components and dense routing of circuitry. Multiple layers of stacked microvias can connect several layers.
PCB Via Hole Size and Pad Size
The size of a via hole is determined by the drill bit used to create it. Standard via hole sizes range from 0.2mm to 0.5mm. Smaller via holes enable denser routing but are more expensive to manufacture. The pad size is the diameter of the annular ring of copper around the via hole.
Typical via dimensions for an 8-Layer PCB are:
Via Hole Diameter (mm) | Pad Diameter (mm) |
---|---|
0.2 | 0.5 |
0.3 | 0.6 |
0.4 | 0.7 |
0.5 | 0.8 |
Microvias have hole sizes of 0.075-0.15mm and pad diameters of 0.175-0.3mm. The aspect ratio of a via (depth to diameter) should generally be kept below 10:1 to ensure reliable plating of the hole walls.

PCB Via Plating and Filling
After a via hole is drilled, it needs to be plated with a conductive material, usually copper, to electrically connect the layers it passes through. This is done through an electroplating process. The via hole walls are first coated with a thin conductive seed layer, then thicker copper is plated on top.
Some vias need to be filled after plating for various reasons:
– To create a flat surface for mounting components over the via
– To prevent solder from wicking into the via during reflow
– To improve thermal conductivity for heat dissipation
– To prevent contaminants from getting inside the via hole
There are several methods for filling vias:
1. Conductive via filling with copper or silver epoxy
2. Non-conductive filling with dielectric epoxy
3. Capped vias where a plated cap seals the top of the via
Via filling adds cost and complexity to PCB manufacturing, so it is only used when necessary. The filling material and process must be compatible with the board and not cause reliability issues.
PCB Via Stub and Back-Drilling
A via stub is the unused portion of a through hole via that extends beyond the last layer it needs to connect. Via stubs cause signal integrity problems at high frequencies, acting as unterminated transmission lines that reflect signals back. The longer the stub, the worse the impact.
Back-drilling is a process that removes the stub portions of through hole vias. A drill bit slightly larger than the via drill size is used to remove the plating up to the desired depth, eliminating the stub. Back-drilling improves high speed signal performance but adds fabrication cost and time.
The maximum stub length that can be tolerated depends on the signal rise time. A good rule of thumb is to keep stubs less than one-quarter of the rise time. For a 100 ps rise time signal, the stub should be under 25 ps (50 mil on FR-4). Signals with faster rise times need proportionally shorter stubs.
PCB Via Current Capacity and Thermal Resistance
The current carrying capacity of a via is determined by its cross-sectional area. Smaller diameter vias have less copper and can handle less current. IPC standards specify the maximum current for a given via size and copper thickness. Exceeding the current limit will cause excessive heating and potential reliability issues.
Thermal vias are used to transfer heat from power components to other layers or to an external heatsink. They are typically placed in an array under the component. The thermal resistance of a single via can be calculated from its dimensions and the thermal conductivity of the PCB material. More vias in parallel provide a lower total thermal resistance.
Example current limits for 1 oz copper vias:
Via Hole Diameter (mm) | Maximum Current (A) |
---|---|
0.2 | 0.5 |
0.3 | 1.0 |
0.4 | 1.5 |
0.5 | 2.0 |
PCB Via Placement and Routing Guidelines
Proper via placement and routing techniques are essential for a reliable and manufacturable PCB design. Some key guidelines include:
– Avoid placing vias under components, especially BGA pads
– Maintain adequate clearance between vias and other features like traces, planes, and edge of board
– Avoid via-in-pad unless absolutely necessary for dense BGA breakout
– Route high speed signals through vias with minimal stubs
– Use enough vias to handle the required current in power nets
– Provide thermal relief connections to planes
– Minimize layer transitions and use direct vertical vias when possible
Following standard via dimensions, drill pairs, and capture pad sizes for your manufacturer will reduce cost and improve yield. Consult your fab house early in the design process to understand their capabilities and design rules.
PCB Via Reliability Concerns
Vias are subject to various reliability concerns that must be considered in PCB design and manufacturing:
– Barrel cracking due to thermal or mechanical stress
– Separation of the via barrel from the target pad
– Fracturing of small via drill bits during fabrication
– Insufficient plating in high aspect ratio vias
– Outgassing and contamination in non-filled vias
To mitigate these risks, use larger via sizes when possible, keep aspect ratios below 10:1, and fill vias when needed. Ensure proper via plating thickness and use rounded via caps to reduce stress concentration. Control drill parameters and inspect vias for quality during manufacturing.
Conclusion
Vias are an essential building block of multi-layer PCBs, providing electrical connectivity and thermal management. Choosing the right via type, size, and placement is critical for optimizing performance, manufacturability, and reliability. By understanding the characteristics and trade-offs of different via structures, PCB designers can create high-quality boards that meet their application requirements.
Frequently Asked Questions
1. What is the difference between a through hole via and a blind via?
A: A through hole via passes through all layers of the PCB, while a blind via connects an outer layer to one or more inner layers without going through the entire board.
2. How small can a microvia be?
A: Microvias can have hole diameters as small as 0.075mm (3 mil) and pad sizes of 0.175mm (7 mil). These dimensions enable very high routing density in HDI PCB designs.
3. Why are via stubs a concern for high speed signals?
A: Via stubs act as unterminated transmission lines that reflect signals back, causing impedance discontinuities and degrading signal integrity. The impact worsens with longer stub length and faster signal rise times.
4. How do you calculate the current carrying capacity of a via?
A: The current capacity of a via depends on its cross-sectional area, determined by the hole diameter and plating thickness. IPC standards provide tables of maximum current for different via sizes and copper weights.
5. What are some common via reliability failure modes?
A: Vias can suffer from barrel cracking, separation from target pads, fracturing during drilling, insufficient plating, and contamination. These issues are mitigated through proper design, material selection, and process control.