In the world of flex and rigid-flex PCB design, vias are far more than just holes; they're an integral part of a high-density interconnect (HDI) strategy. As designs continue to shrink in size while growing in complexity, via structures have become critical to achieving high-functioning, compact layouts in sectors like medical, aerospace, defense, and industrial automation.
Whether you're layering logic in a multi-function aerospace control unit or building a space-constrained implantable medical device, choosing the right via technology is crucial.
In this post, we'll explore advanced via technologies, focusing on blind and buried vias, filled vias, and vias in flexible sections, and how they affect both design flexibility and manufacturability in rigid-flex PCB builds.
What Is a Via and Why Are They Getting More Complex?
A via is a tiny hole, typically 0.010 inches (254 microns) or smaller, that allows electrical connections between different layers of a PCB. In rigid-flex boards, vias become even more important; they help route signals across layers in both rigid and flexible regions. However, as the demand for denser, more function-packed layouts increases, so does the need for more sophisticated via structures.
There are several types of vias, each serving a specific purpose:
- Blind Vias: Connect outer layers to inner layers, but do not go through the entire board.
- Buried Vias: Reside entirely between internal layers, invisible from the outer surfaces.
- Filled Vias: Vias that are filled with epoxy and usually plated over flush to the surface to provide a flat, solderable surface.
- Vias in Flex Sections: Typically constituted as a blind via, placed within flexible circuits.
A microscope cross-section of a rigid-flex circuit board and through hole.
Used properly, these structures open new doors in performance and routing efficiency, but they also introduce new manufacturing and reliability challenges, especially in rigid-flex designs.
Blind and Buried Vias in Rigid-Flex
Blind and buried vias are now common in advanced rigid-flex designs. They’re critical for routing signals without occupying space on all layers, allowing for higher-density BGA routing, power plane optimization, and signal integrity management. In rigid circuit boards, these vias are routine, but in rigid-flex PCBs, they present unique challenges and are only just becoming more common.
One major limitation is aspect ratio, or the relationship between the via’s depth and diameter. Each manufacturer may follow different guidelines; some might use a 10:1 aspect ratio, while others may only support 6:1 or 8:1 for blind or buried vias. That means designers must consult early with their manufacturing partner, like Epec, to confirm whether the proposed stack-up and via structure can be supported or if alternatives (like changing layer counts or adjusting trace escape strategies) need to be considered.
Another key consideration is via termination and count. A via should not begin on a rigid layer and terminate on a flex layer. This causes stress concentrations and often violates stack-up logic. Instead, via starts and stops should align with rigid layer boundaries and avoid transitions into mechanically active flexible regions. Additionally, too many blind or buried via sets can create issues in stack-ups or manufacturability.
Filled Vias: A Must for Dense Component Placement
Filled vias are vias that are sealed with epoxy (either conductive or non-conductive) and typically plated over to create a smooth, flat surface. This is often used in via-in-pad designs where components are placed directly over the via, which is increasingly common in miniaturized or high component density applications where space is at a premium.
While filled vias are useful for managing solder wicking, improving heat dissipation, and providing a cleaner surface for component placement, they are typically limited to rigid areas of the PCB. This is because the epoxy fill and plating process involves temperature and pressure extremes that are incompatible with flexible polyimide substrates, which can deform during the curing or plating steps. Beyond that, many via fill processes use a form of sanding or planarization to bring the surface flush, which can distort or warp thinner layers such as flexible layers.
Epec supports non-conductive filled vias and plated-shut options, which are cost-effective and manufacturable in most rigid applications. While conductive fills are technically possible, they’re rarely used due to cost, complexity, and minimal benefit in most designs. If in doubt, consult your manufacturer to select the right fill material for your application.
Vias in Flex Sections: High Risk, Specialized Use
Placing vias in the flex portions of a rigid-flex PCB is an advanced move, and one that should be approached with caution.
These are often blind vias, used in specific scenarios such as:
- Creating a Faraday cage for EMI protection
- Adding redundant grounding paths
- Connecting trace layers in multi-layer flex designs
However, because flex layers bend and move, placing vias in dynamic regions can result in fractured copper connections or intermittent failures. The IPC-2223 design standard explicitly recommends against placing vias in bend areas. Flexing near or over a via significantly weakens the structure, potentially even breaking the “barrel” of the via, and is a leading cause of latent field failures in flex-based designs.
When vias in flex are necessary, they must be placed in non-bending areas and ideally reinforced mechanically with stiffeners or encapsulants to protect the interconnect. If a design requires via in flex, it is always best to have them as redundant vias rather than primary connections.
Design Best Practices for Via Use in Rigid-Flex
Rigid-flex designs benefit tremendously from advanced via structures, but using them effectively requires thoughtful design-for-manufacturing (DFM) practices:
- Collaborate early with your PCB fabricator to confirm via aspect ratio limits, fill capabilities, and stack-up feasibility.
- Avoid placing vias in bend areas of flexible circuits. Route them only in static zones with sufficient mechanical support or as redundant connections.
- Use filled vias wisely, particularly in high-density via-in-pad layouts—but recognize they may increase both cost and lead time.
- Minimize via transitions across rigid-to-flex interfaces, which can complicate the lamination and drilling process.
- Limit via set complexity; too many via structures can increase stack-up height, decrease yield, and significantly drive up fabrication costs.
Summary
As devices continue to shrink and signal demands increase, rigid-flex PCBs will rely more on advanced via structures to meet electrical and mechanical requirements. From blind and buried vias enabling tighter routing to filled vias supporting dense component placement, these technologies can unlock powerful capabilities, but only when used within your manufacturer’s design constraints.
At Epec, we specialize in high-reliability rigid-flex PCB manufacturing, with a deep understanding of via design rules, stack-up optimization, and flex-specific challenges. Whether you're developing a complex medical implant, aerospace control system, or next-gen IoT device, our engineering team can help you design smarter and avoid costly revisions late in development.
Have questions about via structures in your next rigid-flex design? Contact Epec’s engineering team for stack-up review, impedance planning, and DFM support tailored to your application.
Key Takeaways
- Blind and buried vias enable high-density routing in rigid-flex designs but require strict adherence to aspect ratios and stack-up compatibility.
- Filled vias (non-conductive or plated shut) support space-saving features like via-in-pad but are typically limited to rigid areas due to processing constraints.
- Vias in flex sections are high risk and not recommended in bend areas per IPC-2223; use only in static zones with proper mechanical support.
- Via structures can significantly impact manufacturability and cost, especially when too many complex via sets are included in a single design.
- Early collaboration with your PCB manufacturer is essential to ensure via structures align with material capabilities, fabrication processes, and long-term reliability goals.
