HDI Design Rules for Dense Boards

What Makes a Design HDI

High density interconnect is defined by the use of microvias, laser-drilled holes that are much smaller than conventional plated through holes, combined with finer traces and tighter spaces than a standard board allows. The result is more routing per unit area, which is what makes dense packages such as fine-pitch ball grid arrays routable without adding layers.

The term is also used loosely to describe any dense board. In design terms, the distinguishing feature is the microvia and the sequential lamination that supports it, because those two elements drive the cost, the fabrication process, and the design rules that follow.

HDI is not free routing space. It trades board area, layer count, and material cost against a more complex fabrication process, and the design rules are tighter in every dimension. A design that uses HDI technology without understanding those rules will be difficult or impossible to build.

Via Types and Their Rules

The microvia is the defining feature. It is a laser-drilled hole, typically with a diameter in the range of a few thousandths of an inch or less, that connects an outer layer to the layer immediately beneath it or, in some constructions, through a limited depth. The aspect ratio of a microvia is limited by the laser and plating process, so the hole diameter scales with the dielectric thickness it has to penetrate.

Microvias can be stacked, staggered, or arranged in a variety of patterns. A stacked via places one microvia directly on top of another, which saves space but concentrates stress and is more difficult to plate reliably. A staggered arrangement offsets each via, which is easier to build but consumes more area. The design should follow the fabricator’s rule for the maximum number of stacked vias, because that limit is set by the reliability of the plating rather than by the routing.

Conventional through holes and buried or blind vias are still used in an HDI design, usually for the core layers that carry the through-hole connections. The design rules for those holes are different from the microvia rules, and the two sets should be applied to the correct features.

Trace and Space Limits

HDI allows finer lines than a standard board, with trace width and spacing often reduced to a fraction of what a conventional process can produce. The exact limits depend on the fabricator and on whether the copper is formed by etching or by a semi-additive process, which produces finer features than etching alone.

The relationship between trace width and copper thickness is important. A thick copper layer is difficult to etch into fine lines, so fine-line layers usually use thinner copper. Where a design needs both fine lines and current capacity, the layers are separated by function rather than by making a single layer do both.

Impedance control becomes harder as the geometry shrinks, because the tolerances on trace width and dielectric thickness are a larger fraction of the nominal value. A design that specifies tight impedance control on fine lines should be reviewed with the fabricator to confirm that the tolerance can be held.

Stackup and Layer Sequencing

HDI stackups are built by sequential lamination, where the outer layers are added and drilled after the core is complete. The number of lamination cycles determines the possible via structures and the cost. A single lamination cycle allows one layer of microvias; additional cycles allow deeper structures and stacked vias.

The stackup should be designed so that the via structure matches the routing needs. If most of the density is in the outer layers, a single build-up is enough. If the routing requires connections deep into the board, more cycles are needed, and the cost rises accordingly.

Symmetry matters in the stackup to control warpage. An unbalanced construction with more build-up on one side than the other tends to warp during lamination and assembly, which affects the placement accuracy and the reliability of the assembly. The stackup should be balanced around the core.

Fill, Cap and Plating

Microvias are usually filled and capped with copper before the next layer is added, so that the surface is flat enough for the following lamination and for the pads that sit above. The fill must be complete, because a void in a filled via can outgas during assembly and cause a blowhole or a delamination in the pad above.

Plating quality is the reliability question. The copper in a microvia is thin, and the interface where it meets the target pad is where failures begin under thermal cycling. The fabricator’s process control for microvia plating, verified by cross-sectioning and by thermal cycling tests, is more important than the nominal dimensions in the design data.

Where a via is placed in a pad, the fill and cap must be planar enough that the component can be placed. Via-in-pad is common in HDI designs under fine-pitch packages, and it requires a filled and plated-over structure rather than a tented via, because an open via under a ball grid array will trap air and cause a void in the joint.

Cost and Manufacturability

HDI cost is driven by the number of lamination cycles, the laser drilling time, the total number of microvias, and the tightness of the trace and space rules. Every additional cycle adds a plating step, a lamination step, and additional yield loss, so the stackup should use the minimum number of cycles that meets the routing requirement.

The via count matters for the same reason. Each laser-drilled hole costs machine time and is a potential defect site, so the design should avoid redundant vias and should use the largest via that the routing allows. Where a via can be replaced by a routing change or a component placement adjustment, that is usually cheaper than adding it.

The final consideration is the fabricator’s capability. Fine line widths, small via diameters, and complex stackups should be discussed before the design is frozen, because the rule set varies between suppliers and a design that is comfortable for one may be at the limit for another. Agreeing the rules early avoids a redesign after the first quotation.

FAQ

What is a microvia? A small laser-drilled hole that connects one layer to an adjacent layer in a high density board. Its small diameter and shallow depth distinguish it from a conventional plated through hole.

Can microvias be stacked? They can, within the fabricator’s limit, which is usually set by plating reliability. Staggered vias are easier to build, while stacked vias save more space.

Why does HDI cost more? Because of sequential lamination, laser drilling, via fill and plating, and the tighter process control needed for fine features. Each additional lamination cycle adds cost and yield risk.

Do HDI boards need via-in-pad? Not all of them, but via-in-pad is common under fine-pitch packages. Where it is used, the via must be filled and capped so the surface is flat and no air is trapped in the joint.

How do I know my HDI design is buildable? By reviewing the rule set with the fabricator before the design is released, including the stackup, the via structures, and the trace and space requirements.

Conclusion

HDI design rules exist because the microvia and the sequential lamination process impose limits that a conventional board does not have. Matching the via structure to the routing need, respecting the plating limits on stacked vias, balancing the stackup to control warpage, and agreeing the fine-line rules with the fabricator make a dense design buildable rather than theoretical. For related topics, read our notes on PCB design and layout, PCB manufacturing, PCB capabilities, and quality management for how dense boards are produced in 2026.