How do you inspect PCB microvia depth accurately?

[Michael He]

As PCB designs continue to evolve toward higher density and smaller feature sizes, microvia quality has become increasingly important for board reliability.
One question I've been thinking about recently is:
How are engineers accurately measuring PCB microvia depth without relying entirely on destructive cross-section analysis?

Cross-sectioning is often considered the gold standard because it provides a direct view of the internal structure. However, it destroys the sample, requires preparation time, and isn't always practical for routine process monitoring.
Optical inspection is non-contact and much faster, but it introduces its own challenges. Anyone who has tried to inspect a deep microvia under high magnification has probably encountered the same issue: only a small portion of the hole remains in focus at one time.

Recently, I examined a PCB via with a diameter of approximately 245 μm and a depth of about 501 μm using a 3D optical inspection system with focus stacking (extended depth-of-field imaging).
What I found interesting wasn't just the measurement itself, but the amount of process information visible inside such a small structure.


The inspection revealed:

✅Clean hole edges with no obvious burrs or chipping
✅Relatively uniform sidewall texture
✅No visible glass fiber protrusion
✅No significant resin residue
✅Consistent wall geometry throughout the hole depth
✅No obvious delamination or cracking

In this case, the depth value was only part of the story.
The sidewall condition, taper characteristics, and overall geometry provided useful clues about drilling quality and process stability.
This got me wondering how others approach microvia inspection.

Do you primarily use:

✅Cross-section analysis?
✅Optical measurement systems?
✅X-ray inspection?
✅Confocal microscopy?
✅Focus stacking / 3D imaging methods?

And at what point does simple visual inspection stop being sufficient?
As via diameters continue to shrink, I suspect more manufacturers will need practical methods for obtaining depth and profile information without sacrificing production samples.
I'd be interested to hear what techniques others are using and what accuracy levels are considered acceptable in real production environments.

For anyone interested in the broader principles behind optical depth measurement, focus stacking, and non-contact 3D inspection, I recently found this overview helpful: Understanding modern 3D measurement methods and how they are applied in industrial inspection:[ https://mcscopes.com/what-is-3d-measurement/]