Blind vs Buried Vias: A Comprehensive Guide for PCB Design

A printed circuit board (PCB) is made up of layers of copper foil circuits stacked on top of each other, and the connections between different circuit layers rely on vias. If the holes drilled by a drilling machine or laser are left as is, they will not conduct electricity, because the surface of the original drilled hole is only resin, which is not conductive. Therefore, a layer of conductive material (usually copper) must be plated on the surface of the drilled hole. In this way, current propagates through different copper foil layers. Let's take a look at several different types of vias commonly seen on PCBs.

While basic through-hole vias span the entire board, more advanced structures called blind vias and buried vias connect only between adjacent layers without passing completely through the board. This article provides an in-depth overview of blind and buried vias, discussing their fabrication techniques, design considerations, reliability factors, and applications.

What is a Blind Via:

A blind via is a hole that connects the outermost layer of the board to one or more inner layers, but it does not extend through the entire thickness of the board. It is used in multilayer PCBs where there is a need to connect an external layer to an internal layer without passing through the entire board. It helps save space and is often used in high-density designs. It is "blind" because it is only visible from one side.

However, special attention must be paid to the depth (Z-axis) of the drilling, as it can often cause difficulties with electroplating the inner space of holes and thus is almost no longer used by manufacturers. Take the example of buying a building above as an example. In a six-story house, there are only stairs connecting the first floor to the second floor, or connecting the fifth floor to the sixth floor, which are called blind vias. Blind vias offer several advantages, such as saving space on the board, enabling more compact designs, and improving signal integrity by reducing the length of signal paths.

Key Characteristics of a Blind Via:

● Blind vias are used to connect one outer layer with at least one inner layer.

● The holes for each connection level must be defined as a separate drill file.

● The ratio of drill diameter to hole depth (aspect ratio) must be 1:1 or larger.

● Require specialized fabrication processes.

● Often used to route high-density interconnects.

Advantages of Using Blind Via:

1. Increased density: By linking surface-mounted components to inner layers, blind vias enable more efficient use of board space. As a result, the overall size and weight of the PCB may be reduced.

2. Reduced layer count: Blind vias can assist in minimizing the cost of PCB manufacture by lowering the number of layers required for routing.

3. Improved signal performance: Signal routing over blind vias can reduce crosstalk and EMI between traces, improving signal quality and reliability.

Drawbacks of Using Blind Via:

1. Manufacturing difficulty: Blind vias necessitate accurate drilling and plating operations, which can raise Board manufacturing costs and complexity.

2. Layer count restrictions: Blind vias have the potential to reduce the number of layers that can be employed in a PCB design.

What is a Buried Via:

A buried via is completely internal to the PCB, connecting two or more internal layers without extending to the external layers. Buried vias are useful in high-density PCB designs.  Buried Via’s are used in complex multilayer PCBs where space is at a premium, and surface area on the outer layers needs to be preserved. It allows for more complex routing and higher circuit density.

Using the analogy of buying a building, when only the stairs between the third and fourth floors are connected in a six-story building, it is called buried via holes. Buried via holes cannot be seen from the appearance of the board and are actually located in the inner layers of the circuit board. This feature allows designers to use the outer layers of the board for other purposes, such as placing components or routing traces, without interference from Vias.

Key Characteristics of a Buried Via:

● Buried vias are used to create connections of the inner layers, which have no contact with the outer layers.

● The holes for each connection level must be defined as a separate drill file.

● The ratio of drill diameter to hole depth (aspect ratio) must be 1:12 or larger.

● Often used for grounding and power planes.

Advantages of Using Buried Via

1. Increased density: Buried vias, which connect the inner layers of the PCB without extending to the top layers, can help enhance the density of components on the board.

2. Reduced layer count: Buried vias can assist in reducing the number of routing layers necessary, resulting in lower manufacturing costs.

3. Improved signal performance: Signal routing through buried vias can reduce crosstalk and EMI between traces, improving signal quality and reliability.

Drawbacks of Using Buried Via

1. Manufacturing difficulty: Buried vias necessitate accurate drilling and plating operations, which raises the cost and complexity of PCB production.

2. Accessibility issues: Because buried vias are not visible from the board’s surface, they might be challenging to troubleshoot and repair if they fail.

Blind/Buried Vias Manufacturing Considerations

Blind through-hole production challenges include the necessity for accurate drilling and plating operations, which might raise manufacturing costs. Although laser drilling technology can enhance precision and save costs, it may only be appropriate for some PCB designs.

The aspect ratio of the vias, which is the ratio of the diameter to the depth of the via, must also be considered by designers. Larger aspect ratios might be more challenging, increasing costs and possible manufacturing problems.

PCB Technology for Blind and Buried Vias

Several advanced PCB fabrication techniques enable the creation of blind and buried vias:

1) Sequential Lamination: Each internal layer is laminated together with pre-formed vias aligned between layers. Allows great flexibility in via structures.

2) Laser Ablation: Lasers can selectively ablate conformal dielectric coatings to open blind/buried via connections where needed.

3) Plasma Etching: Plasma etching through thin dielectric layers can selectively expose metal pads to open blind vias.

4) Photo-Via Tenting: Photoimageable dielectric layers allow selective tenting over vias, leaving openings only where blind/buried vias are desired.

The use of any of these processes requires specialized PCB facilities and procedures when working with blind or buried vias. The costs are higher than standard multilayer boards as a result. To know more about these steps visit our Ultimate Guide to PCB Manufacturing.

Applications of Buried/Blind Via:

● Dense BGA routing under packages, enabled by fan-in blind microvias.

● The signal layer changes under components rather than escaping around components.

● Grounding vias from outer layers directly to internal ground planes without through-hole.

● Power plane partitioning for separating digital and analog power distribution.

● Effective shielding around RF circuits and antennas using perimeter ground vias.

● Board stacking interconnects between adjacent PCBs in a module.

● Mixed-signal isolation between critical digital and analog routes.

● High-density interconnects in HDI, microwave, and flex PCB technologies.

Blind Vias and Buried Vias Costs

Tented vias are a type of via in printed circuit boards (PCBs) in which the via holes are covered or “tented” with a solder mask during the PCB manufacturing process. This means that the via holes are fully coated with solder mask material, a protective layer applied over the copper traces on the PCB to prevent solder bridging, short circuits, and other manufacturing and assembly issues. Before deciding on the type of material to be used in designing a printed circuit board (PCB), it is crucial to consider numerous distinct issues seriously. This choice is required before the PCB can be manufactured. Such considerations have been implemented on the PCB:

● The required density

● The number of board layers

● Signal speed, and the available budget

● Blind and buried vias are more suited to high-density systems with many layers.

Reliability Considerations for Blind and Buried Vias

Several factors related to reliability and PCB assembly should be assessed when applying blind or buried vias:

Plating Coverage: Full plating coverage inside blind/buried holes is critical for reliability. Inspect with x-ray imaging.

Bonding: Delamination risk increases with more interfaces created by sequential lamination.

Via Filling: Filling blind/buried vias improves assembly yield and reliability.

Stress: Thermal stresses concentrate at vias, requiring analysis for high-reliability applications.

Inspection: Advanced techniques like microsectioning or thermography help verify buried features.

Rework: Repairing buried nets may be difficult or impossible without complete board disassembly.

Conclusion:

In summary, Blind and buried vias are essential tools in the PCB designer’s arsenal, enabling the creation of compact, high-performance, and high-density boards. However, incorporating them requires advanced fabrication processes and design considerations for thermal management, reliability, assembly, and test access. And may come with additional manufacturing costs, these specialized vias are indispensable in meeting the demands of modern electronics. By understanding the differences and applications of each type of via, designers can optimize their PCB designs and achieve the best performance in their electronic products.