What are PCB Vias?

Printed circuit board (PCB) vias are small holes drilled through a PCB that electrically connect different layers of the board. Vias allow signals and power to pass vertically between the layers of copper on a multi-layer PCB. They are essential for creating complex circuits with limited board space.

There are several types of PCB vias:

Via Type	Description
Through Hole	Goes completely through the PCB, requires leads to be soldered on both sides
Blind	Connects an outer layer to an inner layer, does not go through the entire board
Buried	Connects two or more inner layers, does not reach either surface of the PCB
Micro	Extremely small vias, typically less than 6 mil (0.15mm) in diameter

The size of a via depends on the current it needs to carry and the manufacturing capabilities. Typical via sizes range from 8 mil to 28 mil in diameter.

Why Use Vias in PCB Design?

Conserve Board Space

The main reason to use vias in a PCB design is to conserve board space. Modern electronics require increasingly complex circuits to be fit into small form factors. By utilizing multiple layers connected by vias, PCB designers can route more traces in a limited area.

For example, consider a simple 2-layer PCB for an Arduino shield. The component-side of the PCB might look like this:

        +----+
+-------+ IC +------+
|       +----+      |
|                   |
|  +---+       +---+|
|  |USB|       |PWR||
|  +---+       +---+|
|                   |
+-------------------+

The bottom layer could be a solid ground plane. Vias would be used to connect the ground pins of the IC and connectors to the bottom layer, freeing up space for signal traces on the top layer.

Create High-Density Interconnects

Vias also enable high-density interconnects (HDI) between fine-pitch components like ball grid array (BGA) devices. The pins under a BGA chip are not accessible for probing or soldering, so vias are used to bring the connections to other layers.

A 4-layer PCB Stackup for a BGA might look like this:

Layer	Purpose
Top	Signal, BGA pins
L2	Ground plane
L3	Power plane
Bottom	Signal

There would be many small vias connecting the BGA pins on the top layer to signal traces on the bottom layer, passing through the power and ground planes.

Improve Signal Integrity

Vias can also be used to improve signal integrity in high-speed designs. At higher frequencies, the path a signal takes becomes very important. Factors like the loop area, impedance discontinuities, and stubs created by vias can cause reflections and degrade signals.

Careful placement of ground vias near signal vias creates a controlled return path and minimizes crosstalk. Removing unnecessary stubs on vias carrying fast signals prevents unwanted resonances.

For example, a differential pair routed between layers might have this pattern of vias and antipads in the ground planes:

 +----+  +----+
 | S+ |  | S- |
 |    |  |    |
 | G  |  | G  |
 |    |  |    |
 +----+  +----+

The ground vias flanking the signal vias maintain a reference and reduce the loop area. The antipads, the clearance holes in the planes, prevent shorting and control impedance.

How to Create Vias in PCB Design Software

The exact steps to create vias will vary between different PCB design tools, but the general process is:

1. Define the layer stackup, specifying the number, thickness, and purpose of each layer
2. Place components and route traces on the top and bottom layers
3. Change layers and route traces on the inner layers as needed
4. Place vias connecting traces on different layers, either manually or using an autorouter
5. Specify the size, type, and plating of the vias
6. Run design rule checks (DRC) to verify that vias meet manufacturing constraints

Some PCB design tools have additional features to help with via placement and optimization:

• Via stitching – Automatically places a pattern of vias to tie copper pours to power or ground planes
• Via shielding – Surrounds signal vias with ground vias to control impedance and reduce crosstalk
• Unused pad removal – Eliminates stubs on vias transitioning between layers
• Teardrops – Increases the copper area around a via to improve reliability

It’s important to follow the design rules and recommendations from the PCB manufacturer when using vias. The capabilities and costs can vary significantly depending on the via type, size, aspect ratio, and plating.

How to Manufacture PCBs with Vias

The process of manufacturing a PCB with vias involves several steps:

1. Drilling – Small holes are drilled through the PCB substrate at each via location
2. Plating – A conductive layer, typically copper, is deposited onto the walls of the drilled holes to form the electrical connection
3. Filling – For blind and buried vias, the holes are filled with a non-conductive epoxy to create a level surface
4. Laminating – The different layers of the PCB are aligned and bonded together under high temperature and pressure
5. Etching – Unwanted copper is removed to form traces, leaving just the vias and pads

Some advanced PCB fabrication techniques for vias include:

• Sequential lamination – Builds up a PCB through multiple lamination cycles to create blind and buried vias
• Microvias – Uses lasers or plasma etching to create extremely small vias between layers
• Via-in-pad – Places component pads directly over vias to save space, requires precise registration

The manufacturing process is highly automated, but there are still many variables that can affect the quality and reliability of vias. Some key factors are:

• Hole size and aspect ratio – Smaller and deeper holes are more difficult to plate evenly
• Plating thickness – Thinner plating can lead to higher resistance and failures over time
• Registration – Misalignment between layers can cause open or short circuits at via locations
• Contamination – Dirt, moisture, or chemicals can interfere with plating and lamination

Choosing a reputable PCB manufacturer and clearly communicating requirements is essential for getting reliable vias on a PCB.

Troubleshooting and Repairing PCB Vias

Even with a well-designed and manufactured PCB, vias can sometimes cause problems. Some common issues are:

• Open vias – A via that is not electrically connected on one or both sides, due to drilling, plating, or filling problems
• Shorted vias – A via that is unintentionally connected to another conductor, like a plane or adjacent via
• Stub resonances – Unused portions of a via that can cause signal reflections and noise
• Thermal stress – Repeated heating and cooling cycles that can crack vias or cause them to separate from the PCB

Troubleshooting via problems often starts with a visual inspection under a microscope. Checking for proper filling, plating, and registration can identify many issues.

Electrical tests using a multimeter or continuity tester can reveal open or shorted vias. More advanced techniques like time-domain reflectometry (TDR) can locate and characterize stub resonances.

Repairing via problems depends on the type and severity of the issue. For prototypes and small-volume rework, manual methods can be effective:

• Adding a wire or conductive paint to bridge an open via
• Carefully scraping away a short circuit around a via
• Drilling out a via and replacing it with a piece of wire
• Filling unused vias with epoxy to eliminate stubs

For larger-scale production, it’s usually more economical to scrap defective boards and identify and correct the root cause of the via problem. Working closely with the PCB manufacturer and having robust testing procedures in place can minimize the need for repairs.

Frequently Asked Questions

1. What is the difference between a through hole and a via?

A through hole is a larger hole drilled through a PCB to mount a component like a connector or resistor. A via is a smaller hole used to route signals between layers. All through holes are vias, but not all vias are through holes.

2. Can vias carry high currents?

The current carrying capacity of a via depends on its size and plating thickness. In general, larger vias with thicker plating can handle higher currents. PCB designers often use multiple vias in parallel or connected to large copper pours to increase current capacity.

3. What is a blind via?

A blind via connects an outer layer of a PCB to an inner layer, but does not go through the entire board. It is typically used for high-density designs where space is limited. Blind vias require additional manufacturing steps compared to through hole vias.

4. How small can a via be?

The minimum size of a via depends on the PCB manufacturing process. Standard vias are typically 8 mil (0.2mm) or larger in diameter. Microvias can be as small as 4 mil (0.1mm) or less. Smaller vias are more challenging and expensive to manufacture reliably.

5. Do vias affect signal integrity?

Yes, vias can have a significant impact on signal integrity, especially at high frequencies. Stubs, impedance discontinuities, and coupling between vias can cause reflections, crosstalk, and other problems. Careful design and simulation is necessary to ensure vias do not degrade critical signals.