Lamination

The lamination is the build-up sequence: the drill steps the fab runs between presses, in build order, each a copper span and a drill kind (mechanical, laser or controlled_depth). A via is drillable only when its span and drill kind match a step (a stacked microvia: when every hop of the stack is a laser step). A via type a layout places is reported once, by the layout's via-lamination rule, which counts its vias and names the board entry (vias[i]) to fix; Check reports a via type no layout places as a board error. The router leaves such a class via out. Each lists the spans the lamination drills.

Without lamination the sequence comes from the stackup. The build-up layers on each side are the prepreg-only dielectrics on the outside before the first core, counted per side, so a build can be asymmetric (1+4+2); the N layers between are the core sub-stack, pressed once from its cores and prepregs. The steps are:

1. each core of the sub-stack, drilled mechanically before its press (a buried via in one core);
2. the sub-stack, mechanically, after its press;
3. for each press of build-up layers, inside out: a laser step from each new outer layer to the layer under it (and over two dielectrics, a skip via, from that side's second build-up layer on), then a mechanical step through everything pressed so far; the last is the through drill. The side with fewer build-up layers joins the later presses, so both outer foils go on in the last press: on 1+4+2 the first press adds In5.Cu under B.Cu alone, the second F.Cu and B.Cu;
4. controlled depth from F.Cu and from B.Cu to every inner layer, after the last press.

On hdi-6l-1n1 (1+4+1) that is In1.Cu-In2.Cu and In3.Cu-In4.Cu (cores), In1.Cu-In4.Cu (buried, the sub-stack), laser F.Cu-In1.Cu and In4.Cu-B.Cu, the through drill F.Cu-B.Cu, and the controlled depth spans. On hdi-8l-2n2 (2+4+2) the first build-up adds laser In1.Cu-In2.Cu and In5.Cu-In6.Cu and a buried In1.Cu-In6.Cu, the second laser F.Cu-In1.Cu, In6.Cu-B.Cu and the skip vias F.Cu-In2.Cu and In5.Cu-B.Cu, so a stacked microvia F.Cu-In2.Cu is two laser steps, while F.Cu-In2.Cu on hdi-6l-1n1 would cross its first core. A stackup with no core, or a core outermost, is one press: its sub-stack is the whole board. Whether the fab stacks microvias or wants them staggered stays stacked_microvias.

lamination lists the steps instead, for a build the stackup does not show (a sub-stack pressed on one side first, a laser step into a core):

[stackup]
preset = "hdi-6l-1n1"
lamination = [
  { from = "In1.Cu", to = "In4.Cu", drill_kind = "mechanical" },
  { from = "F.Cu", to = "In4.Cu", drill_kind = "mechanical" },   # F.Cu to In4.Cu pressed first
  { from = "F.Cu", to = "B.Cu", drill_kind = "mechanical" },
]

from is above to, a controlled depth step runs from one outer layer to an inner layer. The steps are in build order, and a stack is only built from the inside out: a stacked microvia type whose outer hop is drilled at an earlier step than the hop under it is an error, and stacked-via reports two vias at one spot where the via under the stack is drilled after the one on top (see Via types). The derived sequence is always in build order.