HDI: microvias and lamination

A through via is drilled once, after the last press, from top to bottom. Everything else, blind and buried vias, microvias, backdrills and controlled depth drilling, depends on how the board is built up. agentee models that build, so it can tell you which vias a fab can drill before you route with them.

#Use an HDI preset

The generic and jlcpcb fab presets build through vias only, and report any other via type as an error. For HDI, set the hdi preset and an HDI stackup:

fab = "hdi"

[stackup]
preset = "hdi-6l-1n1"
finish = "ENIG"

The hdi preset is generic with PCBWay's published HDI capabilities: 1+N+1 to 6+N+6 builds, 0.065 mm trace and space, a 0.1 to 0.2 mm laser drill, 0.15 mm mechanical drill, 14:1 aspect ratio and stacked copper filled microvias. [rules] hdi = true turns HDI on for another preset, with that preset's limits.

The two HDI stackups that ship are generic builds, not one fab's published stackup:

• hdi-6l-1n1: 6 layers, 1+4+1, 0.8 mm, a 0.07 mm 1080 build-up prepreg each side of a core sub-stack.
• hdi-8l-2n2: 8 layers, 2+4+2, 1.0 mm, two build-up layers each side over the same sub-stack.

#Via types

[[vias]]
name = "uvia-top"
type = "microvia"
from = "F.Cu"
to = "In1.Cu"
drill = "0.1mm"
diameter = "0.25mm"
fill = "filled_capped"

[[vias]]
name = "core"
type = "buried"
from = "In1.Cu"
to = "In4.Cu"
drill = "0.2mm"
diameter = "0.45mm"

Without type, the span decides. A microvia spans one dielectric unless it is stacked (one microvia per dielectric, which needs stacked_microvias) or skip (one laser hole over two dielectrics). Check holds each microvia to the laser drill limits and its depth over drill to max_microvia_aspect_ratio, 0.8:1 on the hdi preset.

fill is the IPC-4761 via protection, from tented (type I) to filled_capped (type VII). Via-in-pad wants type VII, and a via in a pad with any other fill is a via-in-pad-fill error.

#Lamination decides what can be drilled

The lamination is the sequence of presses and the drill steps between them. Without a lamination list agentee derives it from the stackup: each core drilled before its press, the sub-stack after its press, then for each build-up press a laser step into the layer below and a mechanical step through everything pressed so far, and finally controlled depth steps from each face.

On hdi-6l-1n1 that gives:

• In1.Cu to In2.Cu and In3.Cu to In4.Cu, in the cores,
• In1.Cu to In4.Cu, buried, through the sub-stack,
• laser F.Cu to In1.Cu and In4.Cu to B.Cu,
• the through drill F.Cu to B.Cu,
• controlled depth from F.Cu and from B.Cu to each inner layer.

A via type is drillable only when its span and drill kind match a step. One that matches none is a via-lamination error, and the message lists every span the lamination drills. F.Cu to In2.Cu, for instance, is drillable on hdi-8l-2n2 as two stacked laser steps but not on hdi-6l-1n1, where it would cross the first core. List lamination steps by hand for a build the stackup does not show.

Stacks build from the inside out. Of two vias meeting at one layer, the one nearer the surface sits on top, and the one under it must be drilled at the same or an earlier step. stacked-via reports a stack in the wrong order, and two vias at one spot through the same dielectric, which would be drilled twice.

#Routing with several via types

A class can list several vias:

[[netclasses]]
name = "Default"
via = ["std", "uvia-top", "uvia-bot"]

The router picks, at each layer change, the cheapest listed via that spans both layers (by its cost times --via-cost, ties going to the via that spans fewer layers), so a change from F.Cu to In1.Cu takes the microvia and a change to In2.Cu the through via. [[fanouts]] takes the first via that reaches the pad's layer, which is how a fine pitch BGA gets a microvia in every pad:

[[fanouts]]
ref = "U1"
via = ["uvia-top"]

agentee route NAME --nets '*' --via-in-pad

#Backdrills and controlled depth

[[vias]]
name = "bd"
drill = "0.3mm"
diameter = "0.6mm"
backdrill = { from = "B.Cu", to = "In2.Cu", max_stub = "0.2mm" }

A backdrill removes the barrel from one face up to just before to, leaving a stub of at most max_stub. A controlled depth via (drill_kind = "controlled_depth") is a blind via drilled mechanically from its face after the last press, stopping on its layer by depth. Its depth over drill is held to 1:1, and nothing may stack on it.

Every via, of every type, only occupies the layers it spans. Clearance, pours, stitching, the DC, thermal and FDTD models, the 2D and 3D viewers and the drill files all follow that.

#What the fab gets

agentee fab writes one Excellon file per blind, buried or microvia span, named like drill-F.Cu-In1.Cu.drl, controlled depth vias in their own -controlled-depth file, and backdrills in drill-backdrill-FROM-TO.drl. fab-notes.txt lists each via type with its span, drill kind, size, count, fill and drill file, and every via in a pad to fill and cap.