Introduction

In printed circuit boards (PCBs), vias are plated-through holes that provide electrical connections between layers of the board. There are several types of vias, including blind vias, buried vias, and through vias. Blind vias and buried vias are often confused with each other, but there are some key differences between the two.

What are Blind Vias?

Blind vias are vias that connect only two or more internal layers of a PCB without going through the entire board. A blind via does not have access to the outer layer(s) of the board. Blind vias are commonly used for signal routing between internal layers.

Some key characteristics of blind vias:

• Connect internal layers of a multilayer PCB, but do not go through the entire board.
• Allow connections between internal layers without using up routing space on the outer layers.
• Often used for signal routing between internal layers.
• Require careful layer planning to ensure the blind via connects the necessary layers.
• Can be filled and plated.
• Available in various shapes like round, rectangular, octagonal etc.

What are Buried Vias?

Buried vias are vias that connect two or more internal layers of a PCB without connecting to the outer layers. The key difference from blind vias is that buried vias are covered by copper on the outer layers.

Some key characteristics of buried vias:

• Connect only internal layers of a multilayer PCB.
• Completely encapsulated within the board layers.
• Often used to route power or ground connections between internal layers.
• Do not require drilling access holes in outer layers.
• Allows higher component density on outer layers.
• Require careful layer planning.
• Can be filled, but not plated since they are enclosed.

Differences Between Blind and Buried Viasv

Although blind vias and buried vias serve similar functions in routing between internal PCB layers, there are some important differences between the two:

Access to Outer Layers

• Blind vias can connect to the outer layers of the PCB through access holes drilled in the outer copper layers. This allows them to connect signals to components on the outer layers.
• Buried vias are completely encapsulated within the PCB layers and do not connect to the outer layers at all. They are “buried” inside the board.

Plating

• Blind vias can be plated since they have access to the outer layers. This allows them to carry signals between layers.
• Buried vias cannot be plated since they are fully enclosed. They typically carry power or ground connections.

Routing Density

• Buried vias allow for higher routing density on the outer layers since they do not require access holes.
• Blind vias use up some routing space on the outer layers for their access holes.

Common Uses

• Blind vias are commonly used for routing signals between internal layers and components on the outer layers.
• Buried vias are typically used for power delivery network connections between internal layers.

The Benefits of Using Blind and Buried Vias

Using blind and buried vias in multilayer PCBs provides several benefits compared to only using through vias:

Increased Routing Channels

• Blind and buried vias help avoid congestion on the outer layers by routing signals internally. This frees up space on the outer layers for increased trace routing.

Signal Integrity

• Routing signals internally using blind/buried vias keeps them isolated from noise sources on the external layers. This improves signal integrity.

Higher Component Density

• Buried vias enable higher component density on the outer layers since they do not require access holes.

Layer-to-Layer Shielding

• Using blind/buried vias to route signals internally provides shielding from adjacent layers. This helps minimize crosstalk.

Lower Cost

• PCBs using blind/buried vias may have lower costs compared to boards that rely solely on through vias between all layers. This is due to reduced drilling and plating needs.

Design Considerations When Using Blind and Buried Vias

Here are some important considerations when working with blind vias and buried vias in your PCB design:

Layer Planning

• Careful layer planning is needed to ensure blind/buried vias connect the required layers. This requires knowledge of the layer stackup and component layout ahead of time.

Via Access

• Blind vias require access holes in the outer copper layers that can impact routing density. Minimize access holes to free up space.

Thermal Management

• Buried vias are enclosed in the PCB so heat dissipation needs to be considered in the design. Filling them can help conduct heat.

Aspect Ratios

• High aspect ratio blind/buried vias can be prone to defects like poor plating. Stay within recommended aspect ratios when possible.

Testing and Inspection

• Since blind/buried vias are not accessible from the outer layers, testing them requires specialized techniques like x-ray inspection.

Fabrication Factors

• The PCB fabricator needs to be made aware of any blind/buried vias to ensure they can fabricate the board properly.

Via Structures and Shapes

• Blind/buried vias are available in different shapes like round, rectangular, octagonal, etc. The shape impacts cost, routing, and drill access.

Frequently Asked Questions

What are some typical aspect ratios for blind and buried vias?

Typical aspect ratios (ratio of length to diameter of the hole) for blind and buried vias are:

• Blind vias: Up to 8:1 aspect ratio
• Buried vias: Up to 10:1 aspect ratio

Higher aspect ratios are possible but can increase fabrication difficulties and defects.

Can blind and buried vias be filled?

Yes, both blind vias and buried vias are often filled with epoxy or other materials. Filling blind vias helps with soldering and heat dissipation. Filling buried vias is required since they cannot be plated.

Can buried vias be used to carry signals?

It is not recommended to use buried vias for critical signals since they cannot be plated and may have reliability issues. They are better suited for carrying power and ground connections. For signals between internal layers, use blind vias.

How are blind/buried vias tested and inspected?

Since they are not accessible from the outer layers, blind/buried vias cannot be probed or visually inspected. They require x-ray inspection and techniques like flying probe testing. Impedance testing may also be used.

Can microvias be used as blind/buried vias?

Yes, microvias (small vias with diameter less than 0.15mm) can be implemented as blind and buried vias. This allows high-density routing between layers, but increases fabrication complexity.