Four via types, one drill table
An HDI board can carry through-holes, blind vias, buried vias and microvias at the same time. The drill table has to describe each one, because the fabricator prices them differently and the assembly house needs to know which pads are filled.
- Through-holes — mechanical, 0.15 mm minimum, 15:1 aspect ratio, connecting every layer.
- Microvias — laser-drilled at 0.1 mm, connecting adjacent layers only.
- Blind and buried vias — built by sequential lamination; blind reaches an outer layer, buried stays inside.
- Via-in-pad — a microvia placed inside a component pad and filled so the pad can be printed.
Stacked or staggered
A staggered microvia lands beside the one below it. The connection steps sideways through the build, which costs routing space but avoids stacking one via directly on another. It is the cheaper option and the easier one to inspect.
A stacked microvia lands directly on the via below, so a single vertical connection crosses several layers in the space of one pad. That requires the lower via to be copper-filled and planarised, because the next lamination has to bond to a flat surface. Stacked builds save area and cost more; they also concentrate thermal and mechanical load in one column, which is why the fill quality is inspected rather than assumed.
Filling and capping
Via-in-pad turns the fill into a product requirement. Paste printed over an unfilled via drains into the barrel; paste printed over a partly filled via leaves a void under the joint. The fill is therefore specified with a planarisation step and checked by X-ray at assembly.
The same applies to any via that will carry a reflowed joint above it, including thermal vias under power packages.
What to put in the drill table
For every via: type, finished diameter, depth, the layers it connects, and whether it is filled and capped. Add the stack-up intent, the impedance targets and the layer count. That table is what decides whether the build is 1+N+1 or 2+N+2, and it is the difference between a quotation and a guess.
Existing stack-up detail is on the HDI product page, the process ranges on the capability page, and the layer-count trade-offs in the HDI stack-up note.
