Introduction
In printed circuit boards (PCBs), vias provide electrical connections between layers of the board. While basic through-hole vias span the entire board, more advanced via 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 definitions, PCB technology enabling their fabrication, key benefits and applications, design considerations, reliability factors, and comparison to other via types.
What are Blind Vias?
A blind via is a hole that connects one layer of a PCB to an internal layer immediately adjacent to it, without spanning the entire board thickness. For example, a blind via could connect the top layer to the first internal layer. The key characteristics of blind vias:
- Connect between adjacent layers only
- Do not pass completely through the PCB
- Often used to route high density interconnects
- Allow connections avoiding inner layer vias
- Require specialized fabrication processes
Blind vias are distinct from through-hole vias that penetrate the full board. They provide connectivity similar to buried vias but are accessed from an external layer rather than being fully encapsulated internally.
What are Buried Vias?
A buried via connects two or more internal layers of a multilayer PCB without connecting to the outer surfaces. For example, a buried via could join the first and second internal layers. Key characteristics:
- Connect only internal layers
- Fully encapsulated within the PCB
- Often used for grounding and power planes
- Do not require plating or hole drilling
- Also require advanced processes to produce
Buried vias have no openings on the top or bottom surfaces of the finished board. They create isolation between layers by avoiding connections to the external layers.
PCB Technology for Blind and Buried Vias
Several advanced PCB fabrication techniques enable creating blind and buried vias:
Sequential Lamination
Each internal layer is laminated together with pre-formed vias aligned between layers. Allows great flexibility in via structures.
Laser Ablation
Lasers can selectively ablate conformal dielectric coatings to open blind/buried via connections where needed.
Plasma Etching
Plasma etching through thin dielectric layers can selectively expose metal pads to open blind vias.
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.
Benefits of Using Blind and Buried Vias
Compared to through-hole vias, blind and buried vias provide several advantages:
- Higher routing density – Removing unused sections of vias allows more routing channels.
- Better electrical performance – Shorter vias have less inductance, resistance, and crosstalk.
- Reduced layer count – Fewer layers may be needed to achieve a given routing density.
- Lower cost – Potentially lower layer counts and improved yield can reduce costs.
- Better impedance control – Avoiding stubs allows closer impedance matching for high speed signals.
- Mixed signal isolation – Digital and analog signals can be better isolated on specific layers.
- Improved reworkability – Smaller pad sizes and access from outer layers allows easier rework.
- Enhanced reliability – Eliminating non-functional via sections improves thermal stress performance.
For maximum benefit, designs should minimize use of through-hole vias whenever possible. However, some may still be needed for mechanical anchoring.
Key Applications of Blind/Buried Vias
Some typical use cases that leverage blind or buried vias:
- Dense BGA routing under packages, enabled by fan-in blind microvias.
- Signal layer changes under components rather than requiring traces to escape around components.
- Grounding vias from outer layers directly to internal ground planes without through-hole stubs.
- 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.
Design Considerations for Blind/Buried Vias
Some key factors to consider when applying blind or buried vias in PCB design:
Via Size – Blind/buried vias can use smaller diameters and pads vs through-hole, but ensure adequate annular ring for plating thickness.
Routing – Account for vias when routing signals to avoid creating stubs, mismatches, and crosstalk.
Thermal – Heat dissipation may be reduced compared to continuous plated through-hole vias. Consider thermal vias to inner layers.
Stackup Planning – Plan internal layers to group related signals needing interconnects with buried vias.
Fabrication Capability – Confirm capability with your PCB manufacturer, and their design rules for these special vias.
Test Access – Ensure buried nets can be properly tested and probed by including test points or vias to the outer layers.
Cost Analysis – Weigh benefits vs. cost when determining use of blind/buried vias for your design needs.
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.
Registration – Tight layer-to-layer registration is needed for aligning laser ablated or tented vias.
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.
Comparison Between Via Types
Through-Hole Vias
- Least complex and lowest cost
- Highest inductance and resistance
- Requires plating process
- Can use large diameters and pads
Blind Vias
- Moderate complexity and cost increase
- Lower inductance, resistance, and stubs
- Often requires plating, drilling, and tenting/filling
- Small to moderate diameter range
Buried Vias
- Highest complexity and cost adder
- Lowest inductance, resistance, and crosstalk
- No drilling or plating required
- Smallest possible diameters down to laser ablated microvias
Selecting the simplest via structure that meets design requirements optimizes cost and manufacturability.
Conclusion
In summary, blind vias and buried vias enable increased PCB routing density, better electrical performance, reduced layer counts, improved impedance control, and mixed signal isolation compared to basic through-hole vias. However, incorporating them requires advanced fabrication processes and design considerations for thermal management, reliability, assembly, and test access. When applied appropriately with these factors in mind, blind and buried vias provide superior interconnect solutions for multilayer boards across a wide range of applications from consumer electronics to high speed digital and microwave circuits.
Frequently Asked Questions
What are the main differences between a blind via and a buried via?
Blind vias connect between an outer layer and an adjacent inner layer, while buried vias connect only between internal layers without reaching the outer surfaces. Blind vias require hole drilling and plating, while buried vias do not.
What fabrication process is typically used to create blind and buried vias?
Sequential lamination, laser direct imaging, plasma etching of dielectrics, and photo-via tenting processes allow selectively creating openings to form blind and buried vias. The methods require tighter process controls than basic PCB fabrication.
When would buried vias be used instead of blind vias?
For interconnects needed entirely internally, buried vias avoid any stubs to the external layers. This provides better isolation for critical signals like RF circuits. Buried vias also allow connections where external layer access is not feasible.
How do blind/buried vias helpreduce layer count?
By providing direct inner layer connections, blind/buried vias avoid the need to route some signals externally between layers. This removes layers previously needed only for vertical transitions, condensing the stackup.
What are some key considerations when designing with blind/buried vias?
Important factors include via size/spacing, layer planning for connections, matching fabrication capabilities, reliability analysis, thermal dissipation, test access, reworkability, and cost-benefit analysis.
