Defining Microvia Depth and Diameter Before Checking Aspect Ratio

A microvia aspect-ratio check can be misleading when two reviewers use different definitions of depth or diameter. The hole is part of a build-up construction, and its usable geometry depends on that construction. Establish the dimensional references before deciding that a via meets the supplier’s published capability.

Read the convention in the source

Würth Elektronik’s HDI microvia design rules explicitly label their aspect-ratio convention and pair via geometry with dielectric constructions. Other documents may express the ratio in the opposite order or use different diameter references. Compare definitions rather than just matching a number.

Ask the fabricator which diameter is used for the process check, which surfaces define depth, and how plating or finished geometry is treated. Identify the connected layers and the target land. A generic drill-table entry may not convey all of this information.

Connect geometry to the actual stackup

• Use the supplier’s proposed finished build-up thicknesses.
• Distinguish copper thickness from dielectric thickness where the definition requires it.
• Include the applicable manufacturing tolerances.
• Check capture and target lands alongside the hole itself.
• Review whether the construction is filled, stacked, or used in a solder land.

As a purely illustrative calculation, a 75-micrometer depth divided by a 100-micrometer diameter gives 0.75. If a document instead writes diameter relative to depth, the same geometry appears in a different order. The calculation establishes only a geometric ratio; it does not establish manufacturability or reliability.

Resolve changes before routing is frozen

A thicker build-up dielectric proposed for impedance control can change the microvia depth. A smaller diameter proposed to improve escape routing can change the same ratio in another direction. Review these requirements together so a stackup adjustment does not silently invalidate the via geometry.

Obtain a written supplier interpretation for the selected construction and keep it with the stackup. If the fabricator proposes a different laser process, land size, or via sequence, review the resulting structure rather than accepting a nominally similar diameter alone.

The release should identify dimension references, tolerances, layer connections, and the agreed manufacturing rules. Preserve cross-sectional or other agreed evidence from representative production where required. Recheck the geometry after stackup changes. A correctly calculated ratio is a useful screening input, but it remains one part of a broader interconnect design and qualification decision.

Use the same units throughout the review and retain the original supplier convention in annotated drawings to prevent later transcription errors.

Record whether quoted dimensions describe nominal artwork, a process target, or an accepted finished feature.