HDI — High-Density Interconnect — is not a single technology. It is a family of PCB fabrication methods that use microvias (laser-drilled holes under 0.15mm), buried vias, and sequential lamination to pack more interconnections into less board area. At Huaxing PCBA, we manufacture HDI boards from simple 1+N+1 structures up to any-layer (every layer pair connected by microvias) on boards with 32 layers. But the engineering question most designers face is simpler: "Is this board dense enough to justify HDI?"
Decision threshold: If your board design has more than 0.4 BGA components per square centimeter or requires trace widths below 4 mils to fit routing, HDI is almost certainly justified. Below that threshold, a standard through-hole design on the same layer count almost always costs less.
Microvias vs Through-Holes: The Physics of Density
A standard through-hole via goes all the way through the board — every layer, top to bottom. On a 12-layer board, that via occupies real estate on all 12 layers, even if it only connects layers 1 and 2. On a dense BGA breakout, those wasted pads on layers 3–12 block routing channels that you cannot afford to lose.
A microvia solves this. Laser-drilled from the outer layer to the layer directly beneath it, a microvia occupies space on exactly two layers — the ones it connects. Layers below remain untouched, free for routing. The result: on a board with a 0.8mm pitch BGA, moving from through-hole vias to microvias typically frees 30–40% of routing channels on internal layers.
0.15mm
Minimum mechanical drill diameter for standard through-hole vias. Below this, you need laser-drilled microvias — typically 0.075mm to 0.1mm. The transition from mechanical to laser drilling is the engineering boundary between standard and HDI fabrication.
HDI Structures: 1+N+1 to Any-Layer
HDI complexity is defined by the number of sequential lamination cycles and microvia layers. The notation is standard across the industry:
1+N+1 (One Build-Up Layer)
One microvia layer on top, one on bottom, with a standard core of N layers in between. The microvias connect the outer layers to the first inner layer beneath them. This is the most common HDI structure — used in smartphones, tablets, and automotive ADAS modules where board space is constrained but not extreme. At Huaxing, a 1+6+1 board (8 layers total) adds roughly 15–20% to the fabrication cost compared to a standard 8-layer through-hole board.
2+N+2 (Two Build-Up Layers)
Two stacked microvia layers on each side, enabling blind connections from the surface to the second internal layer. Common in high-performance computing modules, FPGA carrier boards, and aerospace where multiple BGA packages with different pin densities share the same board. The sequential lamination adds a second press cycle, which increases lead time by 3–5 business days and fabrication cost by 30–40% over a 1+N+1 structure.
Any-Layer HDI
Every layer pair is connected by microvias. There are no through-hole vias, no mechanical drills — every interconnection is laser-formed. On a 10-layer any-layer board, that means 9 microvia layers. This is the highest-density HDI structure, used in premium smartphones, military radar modules, and medical implantables. The process requires a separate lamination cycle per layer pair, which pushes lead time to 15–20 business days and fabrication cost to 2–3× that of an equivalent-layer-count standard board.
+15–20%
Typical cost adder for 1+N+1 HDI over standard through-hole on the same layer count. For 2+N+2, the adder is 30–40%. For any-layer, it is 100–200%. The decision to use HDI should be driven by whether the board area savings or performance gains justify that premium — not by default.
When HDI Is Worth the Cost
HDI is not always the right choice, even when a board has high component density. The economics depend on what the denser routing actually achieves:
Board Area Reduction That Eliminates a Layer Pair
The most common justification: a 10-layer standard board can be reduced to an 8-layer 1+N+1 HDI board because the microvias free enough routing space to eliminate two internal signal layers. Removing two layers reduces raw material cost, lamination cycles, and drilling time — partially offsetting the HDI premium. The net cost difference is often under 10%, and the thinner board improves thermal performance and reduces weight.
BGA Escape Where Standard Vias Fail
When a BGA package drops below 0.8mm pitch, standard through-hole vias between pads become geometrically impossible — the annular ring requirements consume the available space between pads. At 0.5mm pitch and below, microvias are not a choice; they are the only routing option. The cost question is moot because the alternative is an unmanufacturable board.
Signal Integrity at Speed
Through-hole vias create stubs — unused portions of the via barrel that extend past the signal layer. At frequencies above 5 GHz, these stubs become quarter-wave resonators that degrade signal integrity. Microvias eliminate stubs entirely because they only span the layers they connect. For high-speed digital designs — DDR5 memory interfaces, PCIe Gen 5, 100G Ethernet — microvias improve eye diagram opening by 15–25% compared to back-drilled through-hole vias on the same stackup.
Huaxing HDI Capability: What We Actually Ship
Our HDI line in Shenzhen handles 1+N+1 through any-layer on boards up to 32 layers and 600×800mm panel size. Here are the specs our engineering team works to daily:
Laser microvia diameter: 0.075mm minimum (3 mil)
Microvia aspect ratio: 1:1 (depth = diameter)
Stacked microvia reliability: Passes 6× reflow at 260°C with IST (Interconnect Stress Testing) to 150°C
Registration tolerance: ±0.05mm layer-to-layer — critical for stacked microvias where each laser drill must land precisely on the pad below
For designers evaluating HDI for the first time, we provide a pre-production stackup review that models the cost and lead-time impact of each HDI structure against the board's routing requirements. Often we find that a 1+N+1 structure with slightly relaxed design rules achieves 90% of the density benefit at 60% of the cost of a 2+N+2 approach.
Conclusion: Let Density Drive the Decision
HDI is a precision tool, not a default setting. When board density genuinely requires microvias to escape BGA packages or meet layer-count constraints, HDI pays for itself in board area savings, layer reduction, and signal integrity. When a standard through-hole design fits the routing comfortably, adding HDI adds cost without adding value. The best designs apply HDI surgically — a 1+N+1 structure where needed, keeping the rest of the board standard — rather than committing to any-layer across the entire stackup.
If you are working on a design that is pushing the limits of standard via routing, send us your stackup. Our engineering team will review it and return a specific recommendation — which HDI structure, which layer count, and what the cost and lead-time impact will be — within 48 hours. No commitment, just data.
