High-density interconnect (HDI) boards use fine routing features and specialized interconnections to connect dense components within a limited area. Microvias and build-up layers can make routing possible where a conventional through-hole structure consumes too much space. An HDI decision should begin with routing demand and reliability requirements, not layer count alone.
Start with the package and routing problem
Identify the finest-pitch component, the number of escape routes and the signal/reference layer arrangement. Check the component land pattern, solder mask strategy and available via land before selecting a microvia geometry. A smaller hole does not by itself establish a manufacturable design; capture diameter, registration, dielectric depth and copper filling are part of the same process.
Compare build-up structures
| Structure | Planning meaning | Question to resolve |
|---|---|---|
| 1+N+1 | One build-up layer on each side of an N-layer core. | Can the BGA escape and plane references fit within this architecture? |
| 2+N+2 | Two build-up layers on each side of the core. | Which microvias stack or stagger, and what process evidence is available? |
| Any-layer / ELIC | Interlayer connectivity designed around a broader microvia structure. | Which layer-to-layer connections are actually qualified for this design? |
The shorthand does not specify every drill span or the exact manufacturing sequence. Ask the fabricator for a stackup drawing that distinguishes copper, dielectric, via start/stop layers and fill/cap requirements.
Microvia geometry and reliability
Define aspect ratio as depth divided by the agreed hole diameter. Clarify whether the quoted diameter is drilled, laser-formed or finished. For stacked structures, reliability depends on the complete interconnection, including copper interfaces and process controls. Passing a simple ratio check does not prove resistance to assembly or service thermal cycles.
Staggering changes routing area and pad placement; stacking can preserve routing space but needs appropriate fabrication qualification. Avoid treating either choice as universally best. Specify the test conditions and acceptance evidence appropriate to the product.
Materials and impedance
Use the material data relevant to the operating frequency, temperature and moisture exposure. Glass style, resin content and finished dielectric thickness influence both processing and electrical performance. Match the impedance target to a defined geometry, include tolerances and agree on coupon measurement. A catalog Dk value alone cannot guarantee the finished impedance.
What drives HDI cost?
Build-up sequence, via filling, feature sizes, material selection, panel use, yield requirements and inspection scope all affect cost. Reducing a layer can add process complexity elsewhere. Compare alternatives at the same quantity with the same reliability evidence and acceptance criteria, then separate recurring board cost from tooling and qualification charges.
Prepare an HDI RFQ
- Released fabrication data, drill files and a netlist when required.
- Stackup drawing with copper weights, dielectric thicknesses and controlled impedance layers.
- Via table identifying each start/end layer, hole/land geometry and fill/cap requirement.
- Outline, thickness, material and finish requirements with tolerance notes.
- Required acceptance documents and revisions, electrical test, microsections and reliability evidence.
- Build quantity, assembly thermal exposure and approved substitution rules.
Company capability confirmation
Approved HDI structures, minimum line/space, laser-hole limits, equipment, material range and maximum layers: [NEEDS CLIENT INPUT]. The schematic planner does not imply that these structures are offered by a particular factory.