Specify the complete opening
Identify the entry layer, target layer, dielectric thickness, and dimensions used to characterize the opening. Because laser microvia geometry may taper, an entry diameter and a target-contact diameter are not interchangeable. Confirm the supplier's measurement convention before comparing numbers or calculating aspect ratio.
Also identify the copper conditions at the entrance and target. The process has to remove the intended material while preserving a suitable landing region; the CAD circle alone does not describe these conditions.
Connect material choice to drilling
ESI describes rigid-board laser applications across different copper-clad and dielectric materials. The relevant lesson is that laser processing is a material interaction, not a geometry-only operation. A stack substitution should therefore be checked against drilling and metallization assumptions as well as impedance.
Ask for feedback on the chosen dielectric construction in the actual dense region. Avoid selecting the thinnest available material solely to achieve a favorable ratio if it makes the electrical or mechanical design less suitable.
Distinguish registration from hole formation
A cleanly formed opening can still land in the wrong place relative to its target. Conversely, a centered opening may have an unacceptable landing condition. Review positional allowance and feature quality as separate requirements, then combine them when selecting the land size.
- Confirm the datum and layer pair for registration evaluation.
- State the geometry and interface features requiring inspection.
- Include representative dense and sparse areas in process review.
- Tie permitted deviations to the applicable acceptance agreement.
Use a practical drawing review
For an illustrative BGA escape, overlay the proposed microvia entry, target land, and adjacent conductors at their expected tolerances. If the margin is inadequate, consider a larger target, a shorter depth, or a different escape strategy. Reducing the nominal hole again is only one option and may create a new process problem.
Adjacent-layer access, one connection at a time
Dielectric · geometry + material
Core or build-up dielectric
Illustrative only. Copper thickness, dielectric construction and via spans require a reviewed stackup.
Common questions
Can I quote a machine specification as board tolerance?
No. Finished-board results include material movement, registration, preparation, drilling, and subsequent processing. Obtain the agreed capability for the complete construction.
Is the smallest laser hole the best choice?
Only when it resolves a constraint with acceptable process margin. A larger approved feature may be preferable where space allows.
References & further reading
Method & assumptions · General design guidance. Validate the proposed construction and acceptance criteria for your project.