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Free tool · HDI
HDI microvia planner: aspect ratio, stacked span and capture pad
Check a build-up against laser-drill aspect ratios, see what a stacked span does to them, and confirm the capture ring and the core via before the stackup is frozen.
The short answer
A microvia is a laser-drilled hole through one build-up dielectric, and its limit is the depth divided by the diameter: about 0.75 to 1 is comfortable and 1 to 1 the common ceiling. A 0.1 mm via in 0.06 mm of dielectric is 0.6 to 1 and ordinary. Stack the same via through three layers and the span is 1.8 to 1, which is not buildable; staggering keeps each via at one layer. The capture ring and the core via's own ratio are the other two limits.
Calculator
HDI microvia and build-up planner
Enter the build-up dielectric, the via diameter and how many layers each side. The tool returns the aspect ratio of one via and of the whole span, the capture ring and the core via's ratio, then says whether the build is ordinary, needs the fabricator's confirmation, or is not buildable. The limits are the ordinary industrial ones, so confirm them with the shop.
A build fills the layer count, dielectric and via size together.
Build-up
Dielectric thickness of one build-up layer. Open the button for its units, why it matters and a typical value.
Laser-drilled microvia diameter. Open the button for its units, why it matters and a typical value.
Build-up layers on each side of the core. Open the button for its units, why it matters and a typical value.layers
Capture-pad diameter on the target land. Open the button for its units, why it matters and a typical value.
Stacked or staggered microvias. Open the button for its units, why it matters and a typical value.
A stacked span passes several layers on one axis and is limited by the same aspect ratio.Core
Core thickness. Open the button for its units, why it matters and a typical value.
Mechanical drill diameter for the core via. Open the button for its units, why it matters and a typical value.
HDI feasibility
Aspect ratios per layer and across the span, against ordinary laser and plating limits.
Per-layer aspect 0.60 to 1, span 0.60 to 1.
- Per-layer aspect
- 0.60:1
- Span
- 0.60:1
- Capture ring
- 75µm
- Core via
- 3.2:1
- Build-up depth
- 0.060mm
- Model note
- IPC-2226
Depth ÷ diameter for one build-up layer.
1 layer on one axis.
Aspect limits follow ordinary laser and mechanical capability.
Capability, not geometry
Aspect limits are what a fabrication line can plate, so they change with the shop. Treat this as a check on the stack before it goes out, and have the fabricator confirm the build-up, the fill and the laser registration on their own capability sheet.
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Want the derivation? Jump to the math behind the calculator.
The math
The formulas, with the working shown
Each equation is rendered with KaTeX, which also emits a MathML twin for screen readers and answer engines, explained in plain language, and copyable as LaTeX from the button in its corner. The variable list says what every symbol means; the model-limit note says where the formula stops being trustworthy.
Microvia aspect ratio
Depth over diameter, and the number a laser process is quoted against.
- t_{\text{buildup}}
- cured build-up dielectric thickness, mm
- d
- laser-drilled diameter, mm
The depth is the cured thickness of one build-up dielectric, because the hole stops on the copper below. The diameter is the finished drill.
A 0.06 mm dielectric with a 0.1 mm via gives 0.6 to 1. Raise the dielectric to 0.12 mm and the same via is 1.2 to 1, past the ceiling.
Model limits
The limit is empirical and line-dependent. A shop with a newer laser quotes more; a general-purpose line quotes less, which is why the page says to confirm it.
Stacked span
A stacked via is limited by the depth of everything it passes, not by one layer.
- n
- build-up layers on one axis
- d
- laser-drilled diameter, mm
A stacked via is drilled and plated through the whole span, so its depth is n times one build-up thickness and the aspect ratio grows with the layer count.
That is why 1+N+1 builds are easy and 3+N+3 builds need either a stagger or a copper-filled stack.
Model limits
A staggered build resets the depth at each layer, so its span ratio equals a single via's. The tool uses one layer when stacked is off.
Capture-pad ring
The ring around the hole is the tolerance budget for laser-to-pad registration.
- d_{\text{pad}}
- capture-pad diameter, mm
- d
- drilled diameter, mm
The laser drills from the top of the build-up to a target pad below, and the two are not perfectly aligned. The copper ring around the hole absorbs that misalignment.
Below about 0.05 mm the ring stops absorbing ordinary registration error and vias start missing pads on some panels — a yield problem rather than a design one.
Model limits
The 0.05 mm floor is a rule of thumb across shops. A fabricator with in-house laser registration will hold less; one using a partner may need more.
Core via aspect ratio
The core is drilled mechanically and plated through, so its ratio is set by the board thickness.
- t_{\text{core}}
- core thickness, mm
- d_{\text{core}}
- mechanical drill diameter, mm
The core via passes the whole core, so its depth is the core thickness and its diameter the mechanical drill. Above about 8 to 1 the plating throw becomes the process risk.
This is the limit that catches a thick HDI stack: the build-up can be fine and the core still unbuildable.
Model limits
Plating capability depends on the line: a backplane shop plates 10 to 1 routinely, a quick-turn prototype house may not.
Process
Depth over diameter is the whole limit
A laser drills a blind hole into one dielectric layer. The beam loses focus and energy as the hole deepens, so the practical limit is depth over diameter: 0.75 to 1 is comfortable and 1 to 1 is the usual ceiling. Because the depth is set by the prepreg, a microvia's aspect ratio is decided by the stackup before the drill is chosen.
That is the difference between HDI and a through-hole build. A through via's ratio is set by the whole board thickness, so 1.6 mm with a 0.2 mm drill is 8 to 1 and already a plating conversation, while a 0.06 mm build-up with a 0.1 mm via is 0.6 to 1 and routine.
- Laser microvia: about 0.75 to 1 comfortable, 1 to 1 the ceiling.
- Mechanical through via: 8 to 1 is a review, 10 to 1 is a refusal.
- Stacked vias multiply the depth; staggered vias reset it.
- The capture ring absorbs laser registration error and has a floor of its own.
Build-up
Stacked or staggered
A stacked via passes several build-up layers on one axis. It saves routing room, and it is limited by the same aspect ratio as a single via because the plating has to reach the bottom of the whole span. Three 0.06 mm layers under a 0.1 mm via is 1.8 to 1, which no laser line will hold.
A staggered build breaks the span at each layer: every via is one dielectric deep and the copper between them carries the connection. It costs a little room and it is the ordinary way to reach more than one build-up layer.
The build-up also has to pass the DFM checks that follow it — via-in-pad fill, mask dams, annular ring — which are on our DFM/DFA layout service page.
FAQ
Common questions
About 0.75 to 1 comfortably and 1 to 1 as the usual ceiling, where the ratio is the build-up dielectric thickness divided by the drilled diameter. A 0.1 mm via in 0.06 mm of dielectric is 0.6 to 1 and ordinary; the same via under 0.12 mm is not laser-drillable.
Want this checked on your board?
Send the stackup and interface list. We will check the HDI build-up against the real layout before the quote is fixed.