Blind and Buried Vias: An In-Depth Guide For PCB Design

Introduction

Blind and buried vias are the most popular via type for designing PCBs, especially the high-density ones. They connect different PCB layers to form a compact board, ensuring proper signal flow. 

Blind vias are used to join one or more interior layers to the outermost PCB layer. Buried vias, on the other hand, deal with the interior layer connection; they pass neither of the exterior PCB layers. Let’s explore more about these two via the types of PCB design. 

What Is Blind Vias?

Blind vias join the outermost PCB layer to one or multiple inner layers without passing through the other end. So, the joining is visible only from one side, which is why it is called blind vias. 

You can drill blind vias using mechanical or laser drilling techniques. This is suitable for designing compact PCBs, allowing denser layouts and saving space. Blind vias are used in circuits of mobile devices, wearables, and high-speed electronics.

Key Features Of Blind Vias

• Visible from only one side of the board
• Mostly used in PCBs with more than four layers 
• Suitable for high-density interconnect (HDI) routing
• Has a smaller diameter compared to through-hole vias 
• Complex and expensive manufacturing process 

How Are Blind Vias Created?

Blind vias are created on the outermost PCB layer and the other specific inner layers, which are to be connected. You can use either mechanical or laser drilling. Once done with drilling, add pads on the outermost layer where the vias will connect. Next, the drilled holes, including the via pads, are plated with copper using electroless copper deposition. This ensures the electrical conductivity of the vias. 

For correct positioning and integration, the PCB layers, pads, and via holes are stacked together with prepreg material. Once the vias are in place, they are copper-plated to enhance conductivity. However, in some cases, vias are filled with epoxy for improved mechanical strength and stability.

Pros & Cons Of Blind Vias

Application of Blind Vias

Blind vias are used in applications that require high-density PCB layouts. This way, multiple layers are packed in a compact format. The common application of blind vias includes the following:

• Smartwatches
• Smartphones
• GPS trackers
• Fitness trackers
• Laptop and other portable gadgets

What are Buried Vias?

Buried vias join the two or more inner layers of the PCB without passing through either of the external layers. That is, buried vias are entirely enclosed within the PCB; unlike blind vias, they don’t connect with the other layer. These vias are ideal for high-density and complex multilayer PCBs.

You can’t see the buried vias holes from the outer source. This allows PCB designers to use the outer surface for other purposes, such as routing traces or placing other components.

Key Features Of Buried Vias

• Does not occupy the outer surface space of the PCB
• Allows for denser component placement and more efficient routing
• Must be drilled and plated before lamination
• Enable multi-layered, high-performance circuit designs.

How are Buried Vias Created?

Mechanical drilling is done on the inner PCB layers with appropriate hole sizes. The via walls are then electroless plated with copper to establish conductivity. After plating the buried vias, the inner layers are laminated together with prepreg and core materials. This makes the buried vias inaccessible as they are enclosed within the PCB. However, after lamination, you can drill blind vias or through-hole vias in subsequent layers.

After lamination and layer stacking, the PCB goes through a final copper plating to ensure seamless electrical connections. 

Pros & Cons Of Buried Vias

Application of Buried Vias

Buried vias are used in designing PCBs for the following applications: 

• High-density computing
• Automativ electroniucs
• Telecommunications equipment
• Aerospace systems 
• Industrial control systems
• Robotics and autonomous systems
• Medical devices

Importance of Blind and Buried Vias In PCB Technology

Space Optimization

Unlike through-holes, blind and buried vias don’t pass through all layers. Due to such an efficient vertical connection, the PCB becomes more compact, shrinking board thickness and size.

This makes blind and buried vias suitable for applications with space constraints, such as smartphones, IoT devices, etc. Besides, it also leaves enough surface space to enable more traces and components.

High-Density Interconnects (HDI)

Blind and buried vias are foundations of high-density interconnects or HDI technology that supports modern electronics. They allow 15-20% higher routing density compared to traditional PCBs. Blind and buried vias further support multilayer stacking by allowing step-by-step layer addition, avoiding excessive drilling. 

Improved Electrical Performance

Blind and buried vias have shorter lengths compared to through-hole vias. This minimizes parasitic inductance and capacitance, which is essential for maintaining smooth signals in high-speed PCB applications. Blind and buried vias further reduce traces between EMI and crosstalk, contributing to improved signal performance.

Comparison Table: Blind Vias vs. Buried Vias

Guidelines For Designing Blind and Buried Vias In PCB

Aspect Ratio

Aspect ratio refers to the ratio between drill depth and hole diameter. You must maintain the exact ratio based on the application; exceeding the limit causes manufacturing defects like incomplete plating. The aspect ratio of blind and buried vias for different application cases is as follows: 

Drilling and Plating

Mechanical drilling must be done before lamination for buried vias. However, with blind vias, you have both mechanical and laser drilling options. For larger drills, choosing a mechanical drill is ok. But if you are dealing with complex and smaller holes, laser drilling is best for accuracy. Besides, for reliable connection and stable conductivity, the plating process (electroplating or electroless copper plating) must be compatible with the drilling method.

Via Pad Size

Remember, the pad diameter should be at least 0.1 mm larger than the via diameter to ensure a reliable connection. For example, if the via diameter is  0.25 mm, the pad diameter should be at least 0.35mm.

According to IPC-2221, the calculation for pad diameter is as follows:

Below we have added min pad size for mechanical and laser blind and buried vias: 

Cost and Complexity

Buried vias are more costly than blind and buried vias. This is because the manufacturing process here is complex and requires more precise equipment for the inner layer connection. 

However, blind vias are more affordable and simpler than buried ones. But, the laser blind vias are expensive yet cost-effective for PCB space optimization and high-density interconnects (HDI). 

NB: The cost varies for a number of board layers, density, drilling method, and signal speed. 

Copper Layer Considerations

To ensure proper alignment and reliable electrical connection, you should always span vias in an even number of copper layers. If the vias span an odd number of layers, they misalign the layers and make the manufacturing process challenging. 

Reliability Factors for Blind and Buried Vias

Plating Coverage 

Insufficient plating directly affects the electrical performance and mechanical strength of the PCB. Therefore, full coverage plating is a must for reliable signals; x-ray imaging is an excellent option to inspect plating coverage. 

Bonding

Due to multiple layer additions in the lamination process, delamination risk increases in blind and buried vias. Therefore, to maintain PCB layer integrity and avoid weak spots, the bonding should be done carefully. 

Via Filling

The filling is essential in blind and buried vias to avoid voids and improve mechanical strength. In this case, you can use conductive or non-conductive material (such as epoxy) for filling. This enhances PCB durability and minimizes the risk of cracks.

Stress 

Vias in high-density PCB areas often experience stress due to thermal expansion caused by soldering or mechanical vibrations. This can cause cracking in the PCB, which questions its reliability. So, while designing the PCB, considering stress distribution is essential. 

Inspection

Using advanced technologies like microsectioning or thermography, inspect the voids, filling, and plating of the PCB. 

Rework

Repairing vias, especially buried vias, is very challenging; you need to completely disable the PCB for reworking. 

The Bottom Line

Whether you need blind or buried vias PCB, considering the manufacturer is a must to ensure reliability. In this case, PCBTok is your ultimate solution. We offer a wider range of customizations for our blind and buried vias PCB. 

All our circuit boards meet IPC class 2 and class 3, and there is no MOQ for new orders. We also offer OEM and ODM facilities for blind and buried PCBs. So, check our blind and buried vias capability and contact us soon to place your order! 

FAQs

What Are the Limitations of Blind Vias?

As blind vias don’t pass through the PCB, there is a depth restriction that can limit deep interconnections. Besides, compared to through-hole vias, blind vias usually have smaller diameters. This can limit their use in high-current applications.

Are Blind and Buried Vias More Expensive Than Traditional Through-Hole Vias?

Yes, blind and buried vias are more expensive than traditional through-hole vias as they go through a more complex manufacturing process.