HDI PCB Design: Microvias, Via-in-Pad and Fine-Pitch BGA
Microvia stackups, via-in-pad and fine-pitch BGA fan-out, planned with your fabricator
HDI (high-density interconnect) PCB design uses laser-drilled microvias, via-in-pad and sequential lamination to route parts that standard through vias cannot reach, such as fine-pitch BGAs. Electuni designs HDI boards in Altium Designer for product teams whose board size or pin density demands it, and says plainly when a conventional stackup will do.
Design → Prototype → Production
Key Facts: HDI PCB Design
- Via Types
- Microvia, Blind, Buried
- Build Structures
- 1+N+1, 2+N+2
- Pads
- Via-in-Pad, Filled & Capped
- BGA
- Fine-Pitch Fan-Out
- Layout
- Altium Designer
- Lead Designer
- IPC CID+ Certified
What You Get
- Fan-out study showing whether HDI is needed at all
- HDI stackup proposal agreed with the fabricator before routing
- BGA escape and via strategy for each fine-pitch package
- Altium Designer PCB layout with defined drill pairs and via types
- Fabrication drawing listing via structures, fill requirements and lamination sequence
- DFM review against the chosen fabricator's HDI capabilities
How We Approach HDI PCB Design
The first HDI decision is whether you need it. Microvias, via-in-pad and extra lamination cycles add cost and narrow the list of fabricators who can build the board. A 0.8 mm pitch BGA can usually be fanned out with dog-bone through vias on a conventional multilayer board. At 0.5 mm pitch and finer, with very small board outlines, or with high pin-count processors, microvias usually become necessary for anything beyond the outer ball rows. That assessment is made with a fan-out study before the stackup is drawn.
When HDI is the right answer, the stackup is defined with the fabricator first: the build structure (for example 1+N+1 or 2+N+2), the dielectric thickness between microvia layers, and which layer pairs each laser drill connects. Staggered microvias are preferred where space allows. Stacked microvias are used where routing density demands them, copper-filled and kept to as few levels as the design permits. Via-in-pad is specified as filled and plated over, so solder is not drawn into the via during reflow.
Routing follows from the fan-out plan. Each fine-pitch package gets an escape strategy that decides which rows exit on the surface, which drop through microvias, and which inner layers carry them out, while keeping reference planes intact under high-speed signals. The fabrication drawing then states the via structures, fill requirements and lamination sequence explicitly, so the fabricator quotes and builds the board the design assumes rather than a guess at it.
Technical Constraints and Trade-offs
- Laser microvias are generally limited to an aspect ratio of about 0.75:1 to 1:1, so each microvia spans one thin dielectric layer rather than the full board thickness.
- Every additional sequential lamination cycle adds cost, lead time and registration risk. A 1+N+1 build is far easier to source than any-layer HDI.
- Stacked microvias need copper fill and are more sensitive to thermal-cycling reliability problems than staggered ones, so stacks are kept as short as the routing allows.
- Via-in-pad must be filled and capped to stop solder wicking and voiding during reflow, which adds a process step and cost to the bare board.
- Thin dielectrics between microvia layers change trace impedance, so controlled-impedance widths are recalculated for the HDI layers rather than copied from the core.
- Fewer fabricators support HDI, so the design is checked against a specific fabricator's capabilities instead of generic design rules.
Fabrication and Assembly Handoff
- Fabrication drawing with the full stackup, build structure and lamination sequence
- Drill table and separate NC drill files for each laser and mechanical drill pair
- Via fill and cap requirements, including which pads use via-in-pad
- Gerber RS-274X or ODB++ data with an IPC-D-356 netlist for bare-board electrical test
- Impedance table for controlled traces on the HDI layers
- Assembly drawings, pick-and-place file and BOM for the assembler
Typical project
Typical project: fine-pitch processor board
A product team needs a processor in a fine-pitch BGA on a board that has to fit a small enclosure. The work starts with a fan-out study to see whether through vias can reach every ball. Where they cannot, a 1+N+1 stackup with filled via-in-pad on the inner rows is proposed, confirmed with the fabricator, and documented on the fabrication drawing before routing begins.
How an Electuni Engagement Works
Step 1
Discovery
Requirements analysis and feasibility study.
Step 2
Architecture
System design, component selection, and risk assessment.
Step 3
Design
Schematic, layout, firmware, and mechanical CAD.
Step 4
Review
DFM/DFT checks and design review with stakeholders.
Step 5
Prototype
Board fabrication, assembly, and firmware bring-up.
Step 6
Handoff
Production files, BOM, and full documentation package.
Tools & Technologies
- Altium Designer
- Altium Layer Stack Manager
- Altium Draftsman
- Saturn PCB Toolkit
- Gerber/ODB++ Viewer
HDI PCB Design FAQ
What is an HDI PCB?
An HDI (high-density interconnect) PCB uses features finer than a standard multilayer board: laser-drilled microvias, blind and buried vias, via-in-pad and narrower lines and spaces. These let more connections fit in less area, which fine-pitch BGAs and compact products need. IPC-2226 is the IPC design standard that describes HDI structures and build types.
When is HDI not worth the cost?
When a conventional through-via stackup can route the design. Many 0.8 mm pitch BGAs fan out with dog-bone vias on a standard four to eight layer board. If board size is not tightly constrained and no part forces microvias, HDI adds lamination cycles, cost and fabricator restrictions without a functional benefit. A fan-out study answers the question before you commit.
What is the difference between stacked and staggered microvias?
Stacked microvias sit directly on top of each other, connecting several layers in one footprint, and must be copper-filled. Staggered microvias are offset from one layer to the next. Staggered structures take more area but are generally considered more robust under thermal cycling, so stacks are used only where density requires them and are kept short.
What do 1+N+1 and 2+N+2 mean?
They describe the HDI build structure. N is a conventional core stackup with through vias. Each 1 or 2 is the number of microvia layers added on each side by sequential lamination. A 1+N+1 board needs one extra lamination cycle and a 2+N+2 needs two, which adds cost and lead time. Any-layer HDI uses microvias between every layer.
Why does via-in-pad need to be filled and capped?
An open via in a component pad drains solder during reflow, leaving a starved joint or voids under a BGA ball. Filling the via with resin or copper and plating over it gives a flat, solderable pad. The fabrication drawing must call this out explicitly, because unfilled vias are the default and an open via-in-pad will not be fixed by the fabricator unasked.
Can an HDI board be built by a prototype fabricator?
Some prototype services offer limited HDI features such as blind vias or filled via-in-pad, but capabilities vary widely between shops and change over time. The stackup, drill pairs and minimum features are checked against the specific fabricator before routing, so the design is not tied to a process the chosen shop cannot run. Production may need a fabricator with a full HDI line.