Automotive Data HDI PCB Engineering Example

Let package escape justify the HDI structure

An automotive data or telematics board may combine a processor, memory, communications devices and several external connections. HDI becomes a useful option when package escape or route density cannot be resolved cleanly with the proposed through-hole construction. It should answer a specific interconnect problem, rather than serve as a label for a high-performance board.

This is a board-level engineering example, not a vehicle-qualified T-box or a delivered customer design. An eight-layer, single-build-up concept is one optional study configuration. The actual layer count and via structure remain open until placement, signal references and manufacturing review are complete.

Build a connection map before the via map

Identify the processor package, pin escape, memory interface, Ethernet or other wired link, radio modules and power domains. Define which nets need controlled impedance, timing constraints or a short connector path using their selected device documents. Keep antenna and radio-module requirements separate from the digital interfaces.

Draw the package-to-connector routes on the placement plan. Mark every required layer transition and its reference connection. This reveals whether HDI solves a dense escape, an avoidable placement conflict or both. It also prevents an unnecessary buried-via structure from being specified for nets that could remain on one routing layer.

A hypothetical escape-channel calculation

Assume adjacent pads have 0.80 mm center spacing and each is 0.40 mm wide in the direction being considered. The nominal gap is 0.80 − 0.40 = 0.40 mm. If a proposed trace is 0.10 mm wide with 0.10 mm clearance to each pad, one trace needs 0.10 + 2 × 0.10 = 0.30 mm. Its nominal unused gap is 0.10 mm.

Two such traces need 2 × 0.10 + 3 × 0.10 = 0.50 mm, exceeding the nominal 0.40 mm gap. This simplified drawing check suggests investigating another escape arrangement or layer access. It excludes fabrication tolerances, pad geometry, solder-mask constraints and via lands, so it is not a manufacturability approval or a published process limit.

Specify via spans instead of relying on HDI terminology

Würth Elektronik describes laser microvias as short connections through thin dielectric layers. Its published HDI material is useful background for distinguishing such connections from a through-hole spanning the board. It does not establish a KFPCBA process window.

For the proposed board, make a layer-pair table naming every blind, buried and through connection. Show whether via-in-pad features require filling and capping, and identify any stacked or staggered relationships. Include the proposed lamination sequence for manufacturer review. Terms such as single-build-up or first-order HDI are insufficient if the actual connectivity is not drawn.

Keep the communications channel in the review

The DP83TC812x-Q1 data sheet's signal-routing guidance addresses controlled geometry, trace length and avoiding stubs for its Ethernet implementation. This is a reminder to review the completed channel and the exact PHY revision, rather than assuming that short microvias guarantee link quality.

Check the connector, protection network, reference continuity and host-interface configuration alongside the via arrangement. If changing a via construction also changes the stackup, repeat the affected impedance and channel review. Use the HDI PCB review page to frame the construction discussion, with the proposed rules attached to the actual fabrication package.

Match construction tests to functional tests

Agreement on the stress plan and acceptance limits is essential. A successful link startup does not by itself establish long-term interconnect behavior, and a coupon result does not establish the complete vehicle network.

Deliver one consistent layer and revision definition

The handoff should contain stackup, drill files separated by span, via definitions, impedance structures, netlist and fabrication notes. Add assembly data, approved component alternatives, the interface constraint list and the tested software configuration. Mark any density-related exception and its approval on the same revision set.

For an initial review, provide the critical package, board envelope and unresolved escape region. Send the available layout through the engineering RFQ to assess the actual interconnect options and required evidence.