Vias are an important part of high-density interconnect strategies in flex and rigid-flex printed circuit board (PCB) designs. As assemblies become smaller and layouts become more complex, the selected via structure affects routing density, component placement, stack-up feasibility, manufacturability, cost, and mechanical reliability.
These considerations are particularly important in space-constrained medical, aerospace, defense, and industrial automation applications. A via structure that works routinely in a rigid circuit board may introduce additional processing or reliability concerns when flexible materials and mechanically active regions are involved.
What Is a Via?
A via is a small hole, typically 0.010 inches (254 microns) or smaller, that creates an electrical connection between PCB layers. In a rigid-flex PCB, vias can route signals between layers in the rigid and flexible portions of the construction.
Different via structures serve different design purposes:
- Blind vias connect an outer layer to one or more inner layers without passing through the entire circuit board.
- Buried vias connect internal layers and are not visible from either outer surface.
- Filled vias are filled with epoxy or copper and usually plated over to create a flat, solderable surface.
- Vias in flex sections are typically blind vias placed within the flexible circuit construction.
A microscope cross-section of a rigid-flex circuit board and through hole.
These structures can improve routing efficiency and support denser layouts. However, they also add manufacturing constraints and potential reliability risks that must be considered during stack-up development.
How Blind and Buried Vias Support Rigid-Flex Routing
Blind and buried vias allow electrical connections between selected layers without consuming routing space throughout the complete PCB stack-up. They can support higher-density BGA routing, power plane optimization, and signal integrity management.
Although blind and buried vias are routinely used in rigid circuit boards, their use in rigid-flex PCBs introduces additional stack-up and manufacturing considerations.
Via Aspect Ratio
One of the primary constraints is aspect ratio, which describes the relationship between a via’s depth and diameter. Supported aspect ratios vary by manufacturer. One manufacturer may use a 10:1 aspect ratio, while another may support only 6:1 or 8:1 for blind or buried vias.
The proposed via structure must therefore be evaluated with the PCB fabricator during stack-up development. If the via depth, diameter, or layer span cannot be supported, the design may require a different layer count, via arrangement, or trace escape strategy.
Via Start and Stop Layers
Via start and stop locations are particularly important in rigid-flex construction. A via should not begin on a rigid layer and terminate on a flex layer. This arrangement can create stress concentrations and may conflict with the intended stack-up construction.
Via starts and stops should align with rigid-layer boundaries rather than transition into mechanically active flexible regions. The number of blind and buried via sets must also be controlled because excessive via-set complexity can make the stack-up more difficult to manufacture.
When Filled Vias Are Used
Filled vias are sealed with conductive or non-conductive epoxy and are typically plated over to create a smooth, flat surface. They are often used in via-in-pad designs, where a component pad is positioned directly over the via.
This approach can help conserve routing and component-placement space in miniaturized or high-density assemblies. Filling and plating the via also helps manage solder wicking and provides a uniform surface for component placement. Improved heat dissipation is a potential benefit.
Filled vias are generally limited to the rigid sections of a rigid-flex PCB. The epoxy filling and plating processes involve temperature and pressure conditions that are incompatible with flexible polyimide substrates. Flexible layers may deform during curing or plating. Sanding or planarization used to produce a flush surface can also distort or warp thin flexible layers.
Epec supports non-conductive filled vias and plated-shut options. Conductive via fill is technically possible, but it is uncommon because of its cost, complexity, and limited benefit in most designs. Fill selection should be reviewed with the PCB manufacturer based on the application and proposed construction.
Why Vias in Flex Sections Require Additional Caution
Placing vias in the flexible portion of a rigid-flex PCB is a specialized design approach. These vias are often blind vias and may be used for:
- Creating a Faraday cage for EMI protection.
- Adding redundant grounding paths.
- Connecting trace layers in multilayer flex designs.
The principal concern is mechanical movement. Flex layers bend during installation or operation, and copper surrounding a via does not respond to this movement in the same way as an uninterrupted flex trace. Flexing near or over a via can weaken the structure, fracture the copper connection, or damage the via barrel. These conditions are a potential cause of intermittent or latent field failures.
IPC-2223 recommends against placing vias in bend areas. When a via in flex is necessary, it should be located in a non-bending area and mechanically supported with a stiffener or encapsulant. You should be using vias in flex as redundant connections rather than primary connections.
Design Considerations for Advanced Via Structures
Advanced via structures can improve routing density and component placement, but they must remain compatible with the materials, stack-up, and fabrication process.
Confirm Fabrication Limits Early
The PCB fabricator should review the proposed aspect ratios, via-fill requirements, layer transitions, and stack-up before the design is finalized. Early review can identify structures that exceed the manufacturer’s processing capabilities or require a different routing strategy.
Keep Vias Out of Flex Bend Areas
Vias should not be placed in areas intended to bend. If a via must be included in a flex section, it should remain within a static, mechanically supported area and, where possible, serve as a redundant connection.
Apply Filled Vias Selectively
Filled vias can support via-in-pad layouts and dense component placement, but the added filling, plating, and planarization requirements can affect fabrication cost and lead time. Their use should be concentrated where the layout requires a flat, solderable surface.
Avoid Rigid-to-Flex Via Transitions
Via structures that transition from rigid layers into flexible regions can complicate lamination and drilling. Via start and stop layers should align with rigid portions of the stack-up.
Limit Via-Set Complexity
Multiple blind, buried, and filled via configurations can increase stack-up complexity, stack-up height, and fabrication cost. Excessive via structures can also decrease manufacturing yield. Each via set should serve a defined routing, grounding, or component-placement requirement.
Selecting a Via Strategy for a Rigid-Flex PCB
Blind and buried vias can create routing space by connecting only the required layers. Filled vias can support via-in-pad designs and dense component placement in rigid regions. Vias in flex sections can address specialized grounding, shielding, or layer-connection requirements, but they introduce greater mechanical risk and should remain outside bend areas.
No single via structure is appropriate for every rigid-flex design. Selection depends on the required layer connections, routing density, component layout, flex movement, stack-up construction, and the fabricator’s supported processes.
Summary
Advanced via structures can expand routing and component-placement options in rigid-flex PCBs, but each structure introduces specific manufacturing and reliability considerations. Blind and buried vias must remain within supported aspect ratios and stack-up boundaries. Filled vias can provide flat surfaces for via-in-pad layouts but are generally limited to rigid regions. Vias in flexible sections carry additional mechanical risk and should not be placed in bend areas.
Early collaboration with the PCB manufacturer is essential for confirming aspect-ratio limits, via-fill capabilities, layer transitions, and stack-up feasibility. Epec’s engineering team can provide stack-up review, impedance planning, and design-for-manufacturing support for rigid-flex PCB designs that use advanced via structures.
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.
