Microvia aspect ratio compares a blind microvia’s depth with a defined hole diameter. The calculation is simple, but the dimension definitions are often not. A tapered opening, an entry diameter and a target-side diameter can give different ratios, while a notation such as 1:0.8 may place diameter first rather than depth first.
This guide shows how to read the convention, calculate a comparable ratio and review a proposed stackup change. It does not present one ratio as a universal manufacturing or reliability guarantee.
What does a microvia aspect ratio describe?
Here, aspect ratio A means depth h divided by reference diameter d: A = h/d. A depth of 60 µm and a diameter of 100 µm give 0.60, often written 0.60:1 when depth is listed first.
The ratio describes a geometry, not every condition needed to fabricate it. The opening shape, bottom condition, material and metallization process still matter. Always state the convention beside the value instead of relying on the word aspect ratio alone.
Which dimensions must be defined?
Use a cross-sectional definition agreed with the fabricator. Nominal CAD dimensions, laser-process dimensions and finished measurements are not automatically identical.
| Dimension | Unit | Definition to confirm |
| Depth h | µm | Start surface to target |
| Entry diameter | µm | At specified process stage |
| Target-side diameter | µm | At bottom of tapered opening |
| Dielectric thickness | µm | Actual laminated construction |
| Outer copper | µm | Whether included in depth |
| Ratio A | None | Named depth and diameter |
Keep units consistent. Dividing a depth in micrometers by a diameter in millimeters without conversion creates a thousandfold error, even if the resulting number is formatted neatly in a report.
Read the order of the ratio before comparing it
Some published guides write diameter first. For example, Würth Elektronik’s HDI design guide expresses its microvia example as diameter/depth = 1:0.8. Expressed as depth divided by diameter, that relationship is 0.8. Check the supplier’s current design rules separately before applying the published example to an order.
This is a supplier-specific published example, not Best Technology’s capability statement or a universal limit. It shows why comparing “0.8:1” and “1:0.8” without reading the axis order can reverse the intended relationship.
A worked example with a tapered hole
Assume a hypothetical depth of 60 µm, an entry diameter of 100 µm and a target-side diameter of 70 µm. The entry-based ratio is 60/100 = 0.60. The bottom-based ratio is 60/70, about 0.86.
Both calculations describe the same hypothetical hole using different reference diameters. You cannot call one supplier more capable by comparing its entry-based number with another supplier’s bottom-based number. First establish the common definition.
Why total board thickness is the wrong depth
A blind microvia connects a limited layer span, so its depth is not normally the complete multilayer board thickness. A thicker central core does not necessarily change an outer microvia’s depth, while a thicker outer buildup dielectric can change it directly.
Identify the start and target layers on the stackup. Read the relevant local construction, including the agreed copper reference, rather than dividing total board thickness by the small via diameter as if it were a through hole.
Compare three geometry changes
If a proposed geometry is outside the agreed process envelope, compare changes in the complete layout and stackup.
| Option | Geometric effect | Other checks |
| Increase diameter | Reduces h/d | Pad size and escape routing |
| Reduce buildup depth | Reduces h/d | Impedance and dielectric requirements |
| Change layer connection | Changes required span | Routing and lamination sequence |
| Use another via structure | Different fabrication problem | Space, cost and qualification |
A smaller numerical ratio is not automatically a better overall PCB design. The revised construction still has to satisfy electrical, mechanical and manufacturing requirements.
Why a stackup change can invalidate the old check
A material substitution or thickness change can alter the actual distance to the target layer. Suppose the same hypothetical 100 µm reference diameter remains, but the agreed depth grows from 60 to 80 µm. The ratio changes from 0.60 to 0.80.
Do not assume the CAD via definition automatically records that construction change. Recalculate against the proposed laminated stackup and ask the supplier to confirm the referenced depth rather than reusing an approval from a thinner buildup.
Keep target-pad capture separate from aspect ratio
Aspect ratio does not prove that the laser opening lands on enough target copper. Registration, target-pad dimensions and material movement affect capture. A via can satisfy a ratio requirement and still have inadequate landing geometry.
The BestPCBs microvia target-pad guide covers that separate check. Review both the depth/diameter definition and the target capture evidence when evaluating a dense escape pattern.
Why cleaning and plating still matter
A geometry within the agreed envelope does not establish the condition of the blind bottom or the deposited copper. Residue, interface preparation and plating quality can affect the finished connection. Those conditions need process and inspection evidence, not an arithmetic ratio alone.
An IPC-hosted microvia reliability presentation discusses several interacting failure mechanisms. Treat it as technical background, not a substitute for the applicable current procurement requirements or a universal pass/fail ratio.
Check stacked and staggered structures separately
For multiple buildup levels, evaluate each microvia and the full interconnect structure. A stack is not qualified simply because every individual hole has an acceptable nominal h/d. Interfaces between levels and the fabrication sequence introduce additional review needs.
Do not describe a tall stack as one ordinary microvia unless the construction and calculation explicitly define it that way. Keep the individual layer spans and the intended reliability evidence visible in the stackup review.
Request measurements that match the drawing
A cross section can show the measured depth, taper, bottom and copper condition at the section location. It does not prove every via across every panel has the same geometry. State the sampling and acceptance requirements along with the dimension definitions.
Ask the report to identify the board revision, stackup, specimen location and measurement stage. If the section is offset from the intended hole center, its apparent opening may not represent the diameter you meant to evaluate.
Release the construction, not just the ratio
Approve the actual layer span, material construction, dimension references, via and pad geometry, and required inspection or reliability evidence together. Keep a change trigger for revised dielectric thickness, laser geometry or interconnect structure.
For a Best Technology HDI review, send the stackup, start and target layers, intended microvia dimensions and product requirements to sales@bestpcbs.com. Ask which diameter and depth the fabricator uses before interpreting a quoted aspect-ratio capability.
FAQ
Is a microvia ratio of 1:0.8 the same as 0.8:1?
It can describe the same geometry if the first notation lists diameter before depth and the second lists depth before diameter. It can also mean something different if the conventions are not stated. Read the definition and convert both to the same h/d expression before comparing.
Should I use the entry diameter or the bottom diameter?
Use the diameter defined by the applicable design rule or supplier agreement. A tapered hole has more than one meaningful diameter, and each produces a different ratio. Do not switch definitions to make a borderline geometry appear compliant; clarify the reference and keep the measured evidence consistent.
Does a ratio below one guarantee reliable plating?
No. Geometry is one input to a fabrication process. Bottom preparation, metallization, copper deposition, interfaces and the required qualification evidence also matter. Check the supplier’s actual process envelope and the applicable inspection requirements rather than treating a numerical threshold as a complete reliability test.
Can I use the full board thickness in the microvia calculation?
Not for an ordinary blind microvia spanning only part of the stack. Use the depth between the stated start and target references. Total board thickness is relevant to other structures, such as a through hole, but it can badly misrepresent a shallow outer buildup connection.
Does increasing the hole diameter solve every aspect-ratio problem?
No. It may reduce h/d, but it also uses more pad and routing area and can affect the escape design. Check the target capture, spacing and finished interconnect requirements. Sometimes a thinner buildup or a different layer connection is a better option, subject to the full stackup review.
Can a material change affect the ratio without changing my drill file?
Yes. A change in laminated buildup thickness can change the distance to the target while the nominal opening data remain the same. Recalculate using the proposed actual construction and confirm whether copper is included in the stated depth. Keep the revised approval with the material change.
Does a microsection establish every hole’s exact diameter?
No. It measures a particular section through a particular specimen, and the cutting plane and preparation affect what is visible. Use the agreed sampling plan and interpret the apparent dimensions against the measurement method. A report should identify the location and construction instead of presenting one image as universal proof.
Can I compare different fabricators using only their aspect-ratio numbers?
Not safely. First align the ratio convention, reference diameter, depth definition, material and via structure. Then compare the relevant fabrication and verification scope. Two similar headline numbers can describe different holes or different evidence, so a dimensioned cross section and stackup are more useful comparison inputs.
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Tags: Aspect Ratio, HDI PCB, Microvia, pcb stackup
