Just the same as rigid printed circuit boards (PCBs), flex PCBs sometimes encounter issues with space constraints. Devices get smaller, components become more numerous, and weight becomes an increasing concern. And while flex can be used to drop down the weight of a design, that only solves one of those three problems.
Many times, changes in a flexible PCB design can be made similar to a rigid PCB design in order to overcome issues with space constraints, such as increasing the layer count, shifting the design into a rigid-flex, and if all else doesn’t do it, creating a rigid-flex PCB using blind and buried vias.
An example of a flex circuit where moving to a multi-layer may decrease trace complexity.
Space constraints in flexible circuit boards tend to surface later in the design cycle, often after mechanical packaging, connector locations, or component density have already been established. This makes resolving them more challenging because changes must balance electrical performance, mechanical reliability, and manufacturability at the same time.
Unlike rigid circuit boards, flex circuits must also maintain bend performance, which places additional limits on thickness, copper distribution, and via usage. Understanding the available design levers early makes it far easier to recover space without introducing new failure risks.
Increasing Layer Count to Recover Routing Area
One of the most straightforward ways to regain space in a constrained flex circuit design is by increasing the layer count. Adding layers creates additional routing channels without expanding the circuit board outline, which can be especially helpful when signal density increases late in development. Extra layers also make it easier to introduce dedicated ground or power planes, which can improve signal integrity, impedance control, and noise performance in high-speed designs.
However, this approach comes with tradeoffs that must be carefully evaluated. Each added copper layer increases the overall thickness of the flexible section, which directly impacts bend capability. As flex circuits get thicker, the neutral bend axis shifts and copper strain increases, making the circuit more susceptible to cracking or work hardening over time. Cost also increases as layer count rises, both due to additional materials and more complex lamination cycles.
There is also a practical upper limit. Once a flexible circuit reaches a certain thickness, it stops behaving like a flex circuit and begins acting more like a thin rigid circuit board. At that point, the design may technically function, but it no longer delivers the mechanical advantages that are justified using flex in the first place. Because of this, increasing layer count is often best suited for static or flex-to-install applications, rather than dynamic flex designs.
Transitioning from Flex to Rigid-Flex
When layer count alone is not enough, converting part or all of the design into a rigid-flex PCB can provide another path to reclaim space. This option is sometimes overlooked because it appears counterintuitive. After all, rigid-flex sounds larger and more complex than a simple flex circuit. In practice, rigid-flex can actually reduce system size by eliminating connectors, cables, and separate rigid circuit boards that would otherwise consume valuable volume.
By integrating rigid sections directly into the flex, components can be mounted closer together, interconnect lengths can be shortened, and routing can be redistributed more efficiently. The space previously reserved for connectors or headers can be repurposed for additional circuitry or mechanical features. This often results in cleaner assemblies with fewer points of failure and improved reliability.
The drawbacks are real and should not be minimized. Rigid-flex PCB designs typically carry higher material costs, longer lead times, and increased design complexity. Stack-ups must remain mechanically balanced to avoid warpage, which can limit layer allocation choices. Weight may also increase due to the added rigid sections.
In addition, shielding options become more limited in flexible areas, where copper planes may not be feasible. Even so, for assemblies where space is constrained and reliability is critical, rigid-flex often provides the best overall system-level solution, even if the PCB itself becomes more complex.
Using Blind and Buried Vias in Rigid-Flex Designs
For designs that push beyond what traditional flex or standard rigid-flex can support, the most advanced option is a rigid-flex PCB incorporating blind and buried vias. This approach allows designers to fully exploit the third dimension of the board, enabling dense interconnections without increasing surface area.
Blind vias connect outer layers to inner layers without traversing the entire stack-up, while buried vias connect internal layers only. When combined with through vias, these structures allow routing to be distributed across multiple layers with minimal disruption to surface real estate. This can dramatically reduce trace congestion, especially in component-dense rigid areas.
In rigid-flex PCB designs, blind and buried vias are typically confined to the rigid sections, where mechanical stability supports more advanced via structures. Flexible sections generally rely on through vias only, and those vias are left unfilled. For many manufacturers, filled vias in flex areas are extremely difficult to manufacture reliably and can create even more significant lead times and costs, in addition to decreasing yields.
Utilizing blind and buried vias as a design strategy offers the highest level of routing freedom, but it is also the most expensive and time-consuming. Sequential lamination, tight registration tolerances, and more complex inspection requirements all add to cost and lead time. As a result, this option is usually reserved for applications where space constraints are severe, and no simpler alternative will meet the requirements.
Balancing Electrical, Mechanical, and Manufacturing Constraints
Across all these approaches, the key challenge is balance. Every method used to reclaim space introduces secondary effects that must be addressed elsewhere in the design. Thicker boards reduce flexibility. Fewer connectors simplify routing but increase design dependency on stack-up symmetry and increase lead time and cost. Advanced via structures improve density but further raise cost and lead time, with added complexity threatening manufacturability.
Successful flex and rigid-flex PCB designs come from understanding these interactions early and selecting the least complex solution that satisfies the system requirements. In many cases, a modest increase in layer count or a rigid-flex conversion is enough to solve the problem without resorting to highly complex via structures.
Summary
While it can be difficult to create a flexible PCB design, there are many options at your disposal to decrease the board size while increasing available routing space. Some situations allow for an elegant, simple solution like increasing layer count or converting to a rigid-flex circuit board, but some other situations become more complex and will eventually involve complicating the design in the form of a rigid-flex PCB with numerous blind and buried vias.
While it can be difficult to determine what is needed for your situation, Epec can help to guide you to a solution that will fit your needs.
Key Takeaways
- Space constraints in flex PCBs are often driven by increasing component density, tighter mechanical packaging, and the need to minimize overall system size.
- Increasing layer count is the simplest way to recover routing space, but it comes at the cost of reduced flexibility, higher material usage, and increased manufacturing cost.
- Converting a flex design into a rigid-flex can eliminate connectors and interconnects, freeing valuable board area while improving reliability in many assemblies.
- Blind and buried vias in rigid-flex designs provide the highest routing density and space savings, but they also introduce higher cost, longer lead times, and added design complexity.
- The best approach to overcoming space constraints is choosing the least complex solution that meets electrical, mechanical, and manufacturing requirements, ideally with early input from the PCB manufacturer.
