An HDI PCB uses high-density interconnect construction, including microvias and fine routing features, to connect components where conventional routing becomes difficult. It is not defined solely by having many layers. A useful design review connects the electrical stackup to fabrication capability and then checks the assembled board against the enclosure.
This guide addresses engineering handoffs. It does not imply that MTSPi fabricates PCBs or provides a particular electronic assembly certification. Confirm electronic fabrication requirements with the chosen board manufacturer and mechanical component requirements with the machining supplier.
Establish Whether HDI Solves a Real Routing Constraint
Start with the component package, pin map, board outline, routing density, and electrical requirements. A fine-pitch BGA may need a different escape strategy from a larger package with the same logical function. Moving a connector or increasing the board outline may sometimes avoid a costly construction, but those decisions affect the product enclosure and assembly.
Ask the layout engineer to identify the actual bottleneck: insufficient escape channels, excessive layer transitions, limited routing area, or a package geometry that the current via pattern cannot accommodate. “Make it HDI” is not a complete design instruction.
Read the Via Map Alongside the Stackup
| Feature | Basic role | Question to settle |
|---|---|---|
| Through via | Connects through the board thickness | Where does its occupied area restrict routing? |
| Blind via | Connects an outer layer to an internal layer | Which layer pair is connected? |
| Buried via | Connects internal layers without reaching an outer surface | How is it incorporated into the manufacturing sequence? |
| Microvia | Provides a small, shallow interlayer connection | What geometry, filling, and reliability requirements apply? |
Do not treat every blind via as a microvia. Supply a layer-pair drawing and define the intended structure. Staggered microvias use offset positions between successive connections; stacked microvias align vertically. They are not interchangeable layout shortcuts. Discuss the specific construction, process capability, and qualification evidence with the fabricator.
Industry HDI design guidance, including Altium’s manufacturing guide, emphasizes that the via structure and manufacturing sequence belong in the design discussion. A visually complete CAD layout does not by itself establish that a shop can manufacture and inspect the intended interconnect reliably.
Select a Manufacturable Build-Up Before Detailed Routing
A notation such as 1+N+1 generally describes one build-up layer on each side of an N-layer core structure. It is a shorthand, not a complete procurement specification. Copper weights, dielectric materials and thicknesses, layer functions, via connections, finished thickness, and impedance requirements still need definition.
Eurocircuits’ published HDI guidance illustrates an important approach: begin with an available build-up and its design rules. Its partial-HDI examples also show that fine routing can be concentrated in part of a construction rather than assumed to require the most complex stack everywhere. Those are supplier-specific offerings, not universal dimensions to copy into an unrelated purchase order.
Agree impedance targets with the actual material and layer structure. Track width alone is insufficient: reference-plane spacing, dielectric properties, copper thickness, and differential-pair spacing all contribute. The board fabricator’s stackup proposal and the designer’s electrical constraints must refer to the same revision.
Check the Mechanical Envelope in Three Dimensions
A smaller board is not automatically a smaller finished product. Connectors, cable bends, component height, thermal hardware, fastening access, and assembly order may control the envelope. A dense board can fit its outline perfectly and still interfere with the enclosure lid.
- Define a common coordinate origin and mounting-hole datums.
- Check maximum component height on both sides of the PCB.
- Include connector mating travel and cable exit space.
- Keep fasteners and standoffs clear of copper and components as required by the electrical design.
- Review thermal-pad contact without bending the board through excessive compression.
- Check access for assembly tools and any required rework.
For an illustrative fit study, export the populated board into the mechanical assembly, add the intended screw heads, and sweep the connector’s mating motion. This can expose a conflict that a bare-board outline check misses. Measure the critical physical interface on first articles instead of relying only on nominal CAD alignment.
Programs using flexible interconnects should also review bend-zone and enclosure-fit considerations. These are separate mechanical constraints, not benefits automatically provided by HDI construction.
Prepare Two Coordinated Manufacturing Packages
The PCB package should contain the agreed fabrication data, layer map, drill and via definitions, stackup, material requirements, finish, impedance targets where applicable, and inspection or test requirements. The mechanical package should contain the enclosure model, drawing, datums, finish, mounting geometry, and approved board envelope.
Use a release table linking both packages to the same assembly revision. If the board thickness changes after quotation, notify the mechanical team: connector height or a retaining feature may depend on that value. Likewise, an enclosure standoff change should not be implemented without checking the populated board.
Discuss machined housing features with MTSPi using the board envelope and interface drawing. Use the project contact page to confirm mechanical scope and required evidence.
Compare Supplier Proposals on Evidence
Request explicit confirmation of the proposed via construction and any departures from the submitted stackup. Compare inspection, electrical testing, documentation, change control, and lot identification rather than accepting “HDI capable” as the complete answer. Acceptance standards and their applicable editions should be agreed in the purchase specification.
Keep electrical continuity, structural reliability, and assembly fit as separate verification questions. A board that passes one check has not necessarily passed the other two.
Frequently Asked Questions
Is every multilayer PCB an HDI board?
No. Layer count alone does not establish HDI construction. Interconnect technology and routing requirements matter.
Can I select microvia sizes after routing?
That risks significant rework. Select the fabricator’s supported stackup and rules before detailed routing.
Does HDI guarantee better signal integrity?
No. Layout, reference planes, materials, transitions, and manufacturing tolerances still require electrical analysis.
Why involve the enclosure supplier early?
The mounting interfaces, component heights, connector positions, and thermal contacts must work together in the finished assembly.
Technical reference basis: Eurocircuits HDI build-up and design guidance; Altium HDI PCB Design and Manufacturing Guide. Mechanical handoff examples are illustrative, not reported customer projects.
