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Count usable channels, not empty space

Between two obstacles, traces need width and separation from each other and from the obstacles. The usable corridor is reduced by pads, vias, mask-related constraints where relevant and local keepouts. A channel count is a geometric screening calculation, not a routing completion guarantee.

The worksheet uses a straight corridor width, trace width, inter-trace spacing and an edge clearance. For n traces, the required width is n times trace width plus n minus one spaces plus two edge clearances. It reports the largest nonnegative integer that fits this ideal arrangement.

Example: a small rule change can remove a channel

An illustrative 0.80 mm corridor with 0.10 mm traces, 0.10 mm spaces and 0.10 mm clearance at each edge holds three straight traces. Four would require 0.90 mm. If the corridor shrinks to 0.65 mm, only two fit under those same assumptions. This helps explain why a small pad or antipad change can force another routing layer.

Real escape paths turn, cross layer boundaries and compete with power and ground connections. Do not multiply a local straight-channel count across an entire package and call the board routed. Use the result to identify which areas deserve actual CAD trials.

Assign reference planes deliberately

Signal layers need appropriate references, and transitions need suitable return paths. Power distribution requires sufficient copper and connection access. A stackup that maximizes nominal signal layers can still be poor if reference planes are fragmented or too far from critical traces.

Coordinate the functional layer map with the impedance proposal. Prepreg and core choices affect physical spacing, and the finished construction may differ from a preliminary arithmetic stack. Review the actual material and copper assumptions before treating the routing rules as final.

Straight routing-channel screen

Choose the interconnect architecture after the trial

Compare conventional through connections, selected blind or buried connections, and build-up microvias where they solve a demonstrated problem. Use the HDI multilayer architecture guide to describe the construction and the any-layer guide for broader access requirements. Avoid automatically escalating every difficult BGA to the most complex route.

Mark transitions that are optional. Changing pin assignment, placement or a layer destination may remove a stacked path. Those opportunities are easier to identify when the layer map follows functional groups rather than a collection of isolated traces.

Include fabrication and assembly constraints

Review drill-to-copper clearances, land sizes, via protection, solder-mask geometry and component escape together. A fine-pitch land pattern can consume room needed for a robust interconnect. Refer to the 0201 and micro-BGA tolerance guide when small assembly features become the limiting factor.

Large boards add dimensional and registration considerations across a longer span. Local success near one package does not demonstrate that the same geometry has adequate margin everywhere on a large panel. Ask the fabricator which compensation and tolerances apply to the proposed construction.

What to bring to a stackup review

Provide critical package drawings, a functional layer map, draft escape patterns, impedance targets, total thickness constraints and the expected manufacturing volume. Identify whether the layout is still flexible or already released. Include the areas that fail to route so the review addresses a specific decision.

Does more copper always mean more routing capacity? No, it can change etching and spacing constraints. Can the tool choose the final layer count? No, it only screens a local corridor. The useful next step is a small set of CAD escape trials and a proposed fabrication stack that can be reviewed together.

Review the architecture through its complete build sequence

Create a layer-access map before optimizing individual dimensions. For each important net group, show where it starts, where it must reach and which reference structure accompanies the route. Then identify the physical interconnect used at each step. A line drawn between two layer numbers is only a connectivity requirement until the actual sequence of holes, lands and intermediate copper is defined.

Ask the fabricator to review the architecture while placement and routing choices are still flexible. A small early change to an escape pattern may avoid an additional specialized operation. A late change can affect the stackup, impedance, fabrication data and assembly land design at the same time. Record the dependencies so a cost reduction does not accidentally become an electrical or assembly regression.

Review the whole material and copper construction. The same nominal layer count can use different dielectric thicknesses, foil choices and internal connections. Material trade names alone do not describe the complete build. Keep the exact grade, relevant construction and approved alternatives with the stackup proposal. If a substitution changes the dielectric or thermal behavior, revisit the affected engineering requirements.

Inspection and qualification need a representative structure. Ask which geometries, layer spans and process stages a sample actually covers. A continuity check at room temperature and a test under relevant thermal exposure address different questions. A polished section is useful for understanding geometry at its location, but it cannot establish the behavior of every interface across all production conditions. Keep those limits visible when reviewing evidence.

The commercial comparison should use the same construction and acceptance scope. Separate a feasibility sample from a qualified recurring production route. Ask what is included in setup, coupons, inspection and any additional evidence work. Where the architecture remains open, request alternatives with their assumptions stated rather than allowing a low total to hide a different interconnect design.

Before release, reconcile the layer-access map with the actual CAD outputs. Confirm that each hole span exists where intended and that no obsolete pad stack remains in a dense region. Preserve the approved stackup, manufacturing proposal and open-item disposition together. The outcome should be a coherent construction that the design, fabrication and assembly teams can each interpret, not just a page of ambitious feature sizes.