HDI microvia design rules that survive fabrication
A microvia that passes DRC and fails in reflow was designed outside the process window. Aspect ratio, capture-pad annular ring, fill and stagger pitch are the four numbers that decide whether a laser-drilled stack survives thermal cycling. None of them are optional at 0.4 mm pitch.
Aspect ratio
Depth over diameter is the rule that governs everything else
A microvia is laser-drilled and stops on an inner capture pad. The beam produces a tapered sidewall, so the hole is wider at the surface than at the bottom, and the plating has to reach the bottom uniformly. The aspect ratio — dielectric depth divided by the surface diameter — decides whether the plating chemistry can circulate.
IPC-6012 Class 3 puts the practical limit at 0.75:1, with 0.60:1 as the number to design to. A 100 µm laser drill therefore implies a dielectric no thicker than about 65 µm. Push past 0.8:1 and the fabricator is likely to refuse the build, or accept it and produce knee voids that crack in thermal cycling.
| Rule | Design target | Process limit | Failure if violated |
|---|---|---|---|
| Aspect ratio (depth:diameter) | 0.60:1 to 0.75:1 | Above 0.80:1 refused by most fabs | Plating voids, barrel cracks |
| Laser drill diameter | 75 to 100 µm | Below 65 µm needs special tooling | Excess taper, poor capture-pad contact |
| Capture-pad annular ring | 75 µm radial minimum | Below 50 µm radial | Drill wander, pad tear-out |
| Via-in-pad fill | IPC-4761 Type VII copper fill | Unfilled or non-planarized resin | Solder wicking, voiding, head-in-pillow |
| Stagger pitch (centre to centre) | 150 µm or more | Below 125 µm | Dielectric breakdown between barrels |
Reliability
Stacked or staggered, and why it matters at reflow
A stacked microvia puts one via directly on top of another so the connection runs straight down. It saves routing area and it concentrates thermal stress in a single column. Laminate expands about 50 ppm per degree Celsius in z while copper plating moves about 17, so a lead-free reflow at 260 °C pulls the via base away from its capture pad. Stacked structures are allowed, but they are a reliability decision that belongs in the fabrication notes.
Staggering offsets each layer's vias so the stack is not continuous. A 150 µm centre-to-centre offset leaves enough dielectric between barrels to survive thermal cycling and prevents conductive anodic filament growth between them. Below 125 µm the dielectric web is too thin and the fabricator will usually reject the design.
Assembly
Via-in-pad needs a fill and a planar surface
Below 0.65 mm pitch there is no room to escape a BGA without putting vias in the pads. An open via in a pad wicks solder away from the joint during reflow, which produces a void or a head-in-pillow defect that passes X-ray and fails in the field.
The fix is IPC-4761 Type VII: copper-filled, plated over and planarized. Specify it in the fabrication notes, confirm the fab can fill and cap the diameter you are using, and design the pad so the fill is inside the land rather than proud of it. If the fab cannot hold planarity, move the via out of the pad and use a short dog-bone instead.
Stackup symmetry and microvia span are set in the Stackup planner; the DFM rules that follow from them are on our DFM/DFA layout service page.
FAQ
Common questions
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