HDI Stack-up Design: Microvia Types and When to Use Them
HDI is a cost decision as much as a routing one. How 1+N+1, 2+N+2 and any-layer stack-ups differ, which microvia structure fits your BGA, and where the cost actually goes.
Start with 1+N+1 and move to 2+N+2 or any-layer only when routing density forces it. Each additional sequential lamination cycle adds cost and reduces yield, so the cheapest HDI board is the one with the fewest build cycles that still routes cleanly.
HDI becomes necessary in two situations: a fine-pitch BGA that cannot be escaped with through-hole vias, or a board that must shrink below the area a mechanical drill allows. If neither applies, a conventional multilayer with through-hole vias is cheaper and more robust.
01Microvia structures
A microvia is a laser-drilled hole, typically 0.1 mm to 0.15 mm finished diameter, connecting two adjacent layers. Because it is drilled after the outer layers are laminated, it never has to go through the whole board, which is what makes the fine pitch possible.
| Single microvia | One laser-drilled hole spanning two adjacent layers. The baseline for 1+N+1 construction |
|---|---|
| Stacked microvia | Two or more microvias drilled and plated on top of each other across successive cycles. Saves area but is the hardest structure to yield |
| Staggered microvia | Vias on successive layers offset laterally. Easier to manufacture than stacked, uses more routing area |
| Via in pad | A via inside an SMD pad, resin filled and copper capped. Required for BGA pitch below 0.5 mm, adds a plating step |
02Reading the stack-up notation
1+N+1 means one build-up layer on each side of an N-layer core. 2+N+2 means two on each side, built in two sequential cycles. Any-layer means every layer pair can carry microvias, which requires a full sequential build with no conventional core.
The number of sequential lamination cycles is what drives cost. A 1+N+1 board has one extra cycle beyond a conventional multilayer. A 2+N+2 has two. An any-layer board with ten layers can have five or more, and each cycle carries its own yield loss.
03BGA escape rules of thumb
These are the pitch thresholds we use when reviewing whether a design needs HDI at all.
- 1.0 mm pitch and above: escape with through-hole vias placed outside the pad, no HDI needed
- 0.8 mm pitch: through-hole vias are tight but possible with a small drill and reduced annular ring
- 0.65 mm pitch: via in pad or dog-bone with microvias becomes the practical choice
- 0.5 mm pitch and below: via in pad, resin filled and copper capped, is effectively mandatory
- 0.4 mm pitch: any-layer construction with stacked or staggered microvias
04Reliability considerations
Microvia reliability is dominated by the interface between the plated copper and the via bottom. During thermal cycling, the Z-axis expansion of the laminate pulls the barrel away from the target pad. This is why stacked microvias, which put that interface under load repeatedly, are the structure most likely to fail in a temperature-cycled product.
If the product will see thermal cycling in service, prefer staggered microvias over stacked, keep the aspect ratio of each microvia at or below 1:1, and ask for micro-sections on the first article so you can see the barrel and the target interface before the run is released.
Key takeaway
Use the fewest sequential build cycles that route your BGA cleanly, prefer staggered over stacked microvias in temperature-cycled products, and verify the via structure on a first-article micro-section.
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