Why Don’t UHDI Capability Charts Tell the Whole Story?
Understanding the difference between process capability and design intent enables engineers to maximize UHDI performance while improving manufacturability and yield.
by Anaya Vardya
Ultra high-density interconnect (UHDI) is a PCB fabrication technology that achieves trace and space geometries of 75µm or smaller, as defined by IPC-2226 Class C. These finer geometries enable higher routing density within a smaller board footprint.
A UHDI capability chart defines the minimum feature sizes a fabrication process can achieve, including trace and space, microvia diameter and layer-to-layer registration. What it does not define is which of those minimums are repeatable in production or appropriate for a particular design. Using capability data effectively requires understanding the difference between what is possible, what is repeatable and what is best suited to the application. IPC-2226 defines density levels A, B and C that correspond to these capability tiers, with Class C covering the sub-75µm geometries characteristic of UHDI.
One question comes up early in almost every UHDI discussion: “What is the minimum trace and space that can be used?”
It is a logical place to start. It is also where UHDI design begins to move beyond familiar design practices. In UHDI, those minimum values matter, but they do not tell the whole story.
Capability charts are valuable tools. They define what a fabrication process can achieve and help designers understand where new possibilities exist. UHDI expands those possibilities with finer traces, smaller microvias and higher routing density than traditional PCB technologies.
The challenge is that those values can easily be interpreted as design targets rather than process capabilities. A capability chart defines what is possible. Successful UHDI design comes from understanding what is also repeatable and appropriate for the design.
Learning from a capability chart. When a fabricator publishes values for trace and space, microvia size or outer-layer spacing, it is easy to view those numbers as design rules. In reality, they are better understood as process boundaries.
In reality, they are better understood as boundaries. They represent what the process can achieve under the right conditions: specific materials, specific stackups, specific layer structures. They don’t automatically define the most stable or efficient choice for every part of every design.
That distinction becomes more important as geometries shrink. In UHDI, a single number rarely tells the full story. The effectiveness of any feature depends on its context within the design, including where it sits in the stackup, the routing density around it, the other design constraints nearby and, ultimately, what the design is trying to accomplish. When those factors are considered together, UHDI becomes much easier to understand and apply successfully.
Geometry precision matters more in UHDI. UHDI does more than shrink features. It makes the design space more precise. As geometries become smaller, there is less unused margin, which means design decisions carry greater weight and there is less room to accommodate errors.
That added precision enables cleaner breakouts from fine-pitch BGAs, often reduces layer counts, improves electrical performance and makes more efficient use of available board space. At the same time, it requires a more deliberate approach to layout because there is less flexibility built into the design.
This represents an important shift in design philosophy. A 25µm trace may be achievable. A 50µm microvia may also be achievable. Tight outer-layer spacing may be well within a fabricator’s capabilities. The real question is where those features provide meaningful value and where more relaxed geometries can achieve the same result with greater manufacturing margin.
That is where UHDI design becomes strategic.
The most common UHDI design errors. Most UHDI fabrication challenges do not come from pushing the limits of the technology. They result from applying those limits everywhere without a clear design purpose.
One of the most common mistakes is using minimum trace and space across the entire board. Once a design team knows a fabrication process can support very fine geometries, it is tempting to use them throughout the layout. In reality, most designs do not require that level of density everywhere. Areas such as fine-pitch BGA breakouts or other highly constrained regions often benefit from aggressive routing, while more open areas can use relaxed geometries that improve manufacturability without affecting performance.
Another common misconception is assuming the same design rules should apply across the entire PCB. Different layers, routing regions and structures have different requirements. A dense breakout area may justify one set of design choices, while a less constrained routing region may not. Treating the entire board as though it must meet the most demanding conditions can add unnecessary complexity. Thinking in terms of design zones, rather than one global rule, often produces better results.
It is also important to avoid evaluating individual features in isolation. Capability charts typically list values such as minimum trace and space, minimum microvia size and minimum spacing, but a PCB is built from combinations of these features. Trace width affects pad definition, pad definition influences solder mask requirements, and vias interact with dielectric thickness and layer-to-layer alignment. IPC-2315 provides additional guidance for microvia design. In the highest-density areas of a layout, all of these relationships come together.
Understanding how these features interact, rather than evaluating each one independently, is what distinguishes a well-executed UHDI design from one that simply pushes minimum dimensions.
Another misconception is assuming all UHDI fabrication processes perform the same way. Capability depends on the fabricator’s equipment, materials and manufacturing experience. Two suppliers may publish similar capability charts but achieve those results using different processes and process windows. That does not make one approach better than another, but it does mean those published values should not be viewed as interchangeable.
This is why early collaboration with the fabricator is so important. Working together early in the design process ensures the layout is aligned with a proven manufacturing process rather than a generic assumption.
How to think about UHDI capability. One useful way to think about UHDI capability is in three stages. First, determine what is possible, meaning the feature can be produced under controlled conditions. Next, consider what is repeatable, or what can be manufactured consistently. Finally, evaluate whether that feature is well suited to the design by supporting manufacturability, reliability and yield.
That final step is where the strongest UHDI designs are developed. The goal is not to avoid the edge of process capability, but to understand when operating at that edge is necessary and when a more forgiving geometry will achieve the same result.
Using capability data more effectively. Capability charts become far more valuable when they are treated as design tools rather than rigid rules. The smallest trace widths, microvias and spacing should be reserved for the areas that truly require them, such as fine-pitch BGA breakouts, dense escape routing and other localized high-density regions. Elsewhere, allowing a little more design margin can improve manufacturability without sacrificing performance.
It is equally important to ask questions early in the design process. Rather than concentrating only on minimum feature sizes, discuss which geometries are most comfortable for the selected stackup, where the process becomes more sensitive and which areas of the board would benefit from additional design margin. These conversations help shape a design that is aligned with the manufacturing process.
Designers should also think in terms of zones rather than applying one set of rules across the entire board. Different regions often have different requirements, and UHDI provides the flexibility to apply tighter geometries only where they provide meaningful value.
Ultimately, successful UHDI designs are developed within a proven process window rather than at the limits of capability simply because those limits exist. Designing within that process window typically results in more predictable fabrication, improved yields and a smoother transition from design to manufacturing.
UHDI is not higher risk than traditional PCB design; it offers more control. Tighter geometries enable more efficient routing, reduced layer counts, and improved electrical performance when applied with intent. It’s easy to look at fine geometry and assume higher risk; in practice, the opposite is true when the design is aligned to a real process window.
It gives designers the ability to:
- route more efficiently
- reduce layer counts
- improve electrical performance
- and solve problems that were difficult or impractical with traditional approaches.
Like any advanced technology, it rewards understanding. But it does not require perfection.
Designers don’t need to memorize every process detail. They need to understand how to interpret the information they already use. Capability charts are still part of that picture and they work best when paired with intent. Because in UHDI, the advantage doesn’t come from using the smallest number everywhere. It comes from knowing where that number matters, and where it doesn’t.
The most important thing to remember is that capability charts define what is possible under controlled manufacturing conditions, not what is optimal for every design. Successful UHDI designs apply the smallest geometries only where higher routing density is truly needed and use more relaxed dimensions where they provide the same electrical performance with greater manufacturing margin.
Understanding the difference between the limits of a fabrication process and the right geometry for a specific application is what transforms UHDI from a collection of capability numbers into a practical manufacturing advantage.
Key Takeaways
Capability charts establish process boundaries, but they should not be treated as universal design rules. Minimum feature sizes describe what a fabrication process can achieve, not necessarily what can be manufactured most consistently or what is best suited to every design.
Applying the smallest geometries only where they add value, such as fine-pitch BGA breakouts and other localized high-density routing areas, improves manufacturability without compromising performance. Likewise, treating different areas of the board according to their individual requirements, rather than applying one aggressive rule set everywhere, creates a more balanced design.
Early collaboration with the fabricator ensures the layout is matched to a proven process window instead of relying on generic capability data. When approached this way, UHDI provides designers with greater flexibility and control, enabling higher-density designs while maintaining reliable, repeatable manufacturing.End of article content
Anaya Vardya is CEO of American Standard Circuits and ASC Sunstone Circuits. ASC works closely with customers and design teams on advanced PCB technologies, including RF, HDI, rigid-flex and ultra HDI, with a focus on turning complex fabrication realities into practical design guidance.