Introduction
In printed circuit boards (PCBs), vias provide electrical connections between layers of the multilayer board. While basic vias span the entire board thickness, advanced PCB technologies enable creating buried vias that connect only between internal layers without reaching the outer surfaces. This article provides an in-depth look at buried vias, discussing their definition, fabrication methods, key benefits, design considerations, reliability factors, and differences from other via types.
What is a Buried Via?
A buried via is a conductive hole formed entirely within the inner layers of a multilayer PCB to create interconnects between two or more internal layers without connecting to the top or bottom surfaces. For example, a buried via could join the first and second internal layers.
Key characteristics of buried vias:
- Connect only internal layers together
- Fully encapsulated within the PCB
- Often used for ground and power planes
- Do not require plating or drilling
- Require advanced PCB processes
- Provide isolation from external layers
Buried vias enable connections inaccessible with through-hole vias. They are distinct from blind vias open to the outer surfaces.
PCB Fabrication Methods
Specialized PCB manufacturing processes are needed to produce buried vias:
Sequential Lamination
Individual laminated layers with pre-formed vias are precisely aligned and bonded together into a multilayer board.
Laser Ablation
Lasers can selectively ablate dielectric coatings to create via openings between metal layers.
Plasma Etching
Plasma etching can selectively remove thin dielectric layers to open buried vias.
Photo-Imageable Dielectrics
Photo-via tenting processes use patterned photoimageable dielectrics to create selective openings for buried vias.
These methods require advanced equipment, materials, and process controls compared to standard PCB fabrication.
Benefits of Buried Vias
Compared to basic through-hole vias, key advantages of buried vias include:
- Higher routing density – Removing unused via lengths increases routing channels.
- Better electrical performance – Shorter vias reduce inductance, resistance, and crosstalk.
- Lower layer counts – Fewer layers may be needed to route a given density of signals.
- Reduced costs – Potentially fewer layers and better yields reduce overall fabrication costs.
- Superior isolation – Signals can be completely isolated from other layers.
- No stub connections – Avoiding stubs provides better impedance control.
- Simplified rework – Small laser ablated vias allow easier rework.
- Improved reliability – Removing non-functional via sections reduces thermal stresses.
Key Applications of Buried Vias
Typical uses of buried vias include:
- Grounding vias from inner layers to ground planes without stubs.
- Power plane partitioning for separating analog and digital supplies.
- RF shielding vias around critical high frequency circuits.
- Inter-layer connections in high density interconnect (HDI) technologies.
- Board-to-board connections in stacked module assemblies.
- High speed signal return path vias minimizing loop inductance.
- Mixed signal isolation between critical analog and digital routes.
Design Considerations for Buried Vias
When working with buried vias in PCB layout, important factors include:
Via Size – Buried vias allow very small microvias if needed. Ensure adequate annular ring.
Routing – Account for buried vias when routing internal layers to prevent creating stubs.
Thermal Management – Assess impact on heat dissipation without plated through-hole vias.
Layer Planning – Arrange internal layers to group associated signals needing interconnects.
Manufacturer Capability – Confirm capabilities and follow their specific design rules.
Test Access – Provide test points or additional access vias for probing buried nets.
Rework Considerations – Repair may be impossible for buried nets without complete board disassembly.
Cost Analysis – Weigh benefits vs. added cost when determining use of buried vias.
Reliability Factors for Buried Vias
For robust quality and reliability with buried vias:
Sequential Lamination – Multiple lamination interfaces increase delamination risks.
Laser Alignment – Tight registration is required for laser ablated and tented buried vias.
Plating Coverage – Full plating of blind openings is critical, requiring x-ray inspection.
Via Filling – Filling buried vias improves assembly yield and long-term reliability.
Stress Analysis – Assess thermal cycling stress concentrations at buried vias.
Inspection – Microsectioning or x-ray imaging helps verify buried features.
Rework Limitations – Buried nets cannot be accessed without complete disassembly.
Comparison Between Via Types
Through-Hole Vias
- Lowest complexity and cost
- Highest inductance/resistance
- Requires plating process
- Large diameters/pads can be used
Blind Vias
- Moderate complexity, some cost increase
- Lower inductance, resistance, stubs
- Often requires plating, drilling, tenting
- Small to moderate diameters
Buried Vias
- Highest complexity, greatest cost impact
- Lowest inductance, resistance, crosstalk
- No drilling or plating
- Very small microvias feasible
Conclusion
In summary, buried vias provide many benefits over basic through-hole vias like improved routing density, electrical performance, reduced layer counts, enhanced isolation, and simplified rework. However, buried vias require advanced PCB processes and careful design for manufacturability, thermal management, test access, and reliability. When applied appropriately, buried vias enable superior interconnect solutions for cutting-edge applications in fields ranging from aerospace to consumer electronics.
Frequently Asked Questions
How do buried vias differ from blind vias?
Buried vias only connect internal layers together, while blind vias connect from an outer layer to an adjacent inner layer. Buried vias are fully encapsulated within the PCB.
What kind of PCB technology allows creation of buried vias?
Sequential lamination, laser direct imaging, plasma etching, and photo-via tenting processes permit selectively forming buried vias during multilayer board fabrication.
What are the main advantages of using buried vias?
Key advantages are higher routing density, better electrical performance, reduced layer counts, lower fabrication costs, superior isolation, no stub connections, and improved reliability.
When would buried vias be used instead of through-hole vias?
For interconnects needed entirely internally between layers, buried vias avoid stubs to external layers. This provides the highest level of isolation for critical signals.
What are important design considerations for buried vias?
Critical factors are via size, layer planning, thermal analysis, manufacturer capabilities, test access, rework limitations, reliability studies, and cost-benefit analysis before using buried vias.
