A RO4350B Stackup Review from Schematic to Fabrication Drawing
Release a RO4350B stackup only when every critical net has a defined transmission-line structure, reference plane, material model and acceptance method. A layer-count sketch is a starting point. A fabrication-ready stackup connects circuit requirements to physical dimensions and identifies which proposed changes must return to the designer for approval.
This review workflow is intended for an RF board that also carries digital control and power. It complements the hybrid stackup guide by focusing on review evidence and document handoffs. It does not prescribe a universal production stackup or approve a particular fabricator's process.
1. Extract the requirements before assigning layers
Start at the schematic and interface specification. List the RF operating band, required port impedance, allowable insertion loss and any phase-matching requirement. For digital interfaces, record edge rate and the interface manufacturer's routing requirements; clock frequency alone does not describe the interconnect challenge. Identify supply current, allowable voltage drop and sensitive analog nodes separately.
Give each requirement an owner and a revision. A power amplifier output path, radar receiver input and slow configuration bus can share a board while needing very different evidence. RO4350B is not automatically the best material for all of them. An ordinary low-frequency controller may not justify an RF laminate; at millimeter-wave frequencies, compare material loss, copper effects, transitions and fabrication capability before selecting a construction.
Collect the mechanical inputs at the same review: board outline in millimeters, mounting features, connector launch geometry, finished thickness constraints and enclosure clearances. Record whether a thickness requirement refers to copper, dielectric spacing or the complete finished board. Unlabeled dimensions are a common source of disagreements.
2. Make the reference-plane relationship visible
The following original diagram is a conceptual four-layer review example, not a released construction. It assumes an outer RF layer over an uninterrupted ground plane and a separate digital layer adjacent to another ground plane. Dielectric grades, dimensions, copper balance and bonding sequence still require a fabricator's proposal. Power distribution must be designed within the available routing area; an extra power plane is not implied.
Trace each controlled net through its entire route, including connector pads, component escapes and layer transitions. Where a signal changes layers, identify the return-path connection between the associated reference conductors. Do not assume two layers named “GND” provide a short return path at the transition. Review actual geometry, nearby stitching and any plane voids in the layout.
3. Use one shared stackup review table
| Review item | Information to record | Evidence and owner |
|---|---|---|
| RF signal layer | Net class, frequency band (GHz), topology, impedance (Ω), reference layer | RF designer: model and routed cross-section |
| Digital signal layer | Interface, edge rate (ps or ns), routing constraints, reference plane | Digital designer: interface requirements and routing review |
| Reference planes | Plane continuity, splits, antipads and return connections at transitions | Layout reviewer: marked-up plane and via views |
| Each dielectric region | Exact core/bondply grade, finished spacing (mm), tolerance, model Dk and its basis | Fabricator proposes construction; designer approves electrical model |
| Copper on each layer | Starting foil, proposed finished thickness (μm), roughness assumption, plating treatment | Fabricator: process proposal; designer: loss-model assumptions |
| Impedance structures | Trace width/gap (mm), mask condition, target and tolerance, coupon representation | Joint approval: solver report and coupon drawing |
| Mechanical and thermal | Finished board thickness, copper balance, holes, thermal paths and assembly exposure | Mechanical/thermal reviewer and fabricator: compatibility review |
| Release and acceptance | Drawing revision, allowed adjustments, evidence required and change-approval route | Project owner: signed review record and purchase documents |
4. Separate material identity from the electrical model
Rogers' RO4000 processing guidelines direct multilayer bonding decisions to the chosen adhesive system and its processing guidance. They also identify registration requirements and facility capability as inputs to tooling decisions. Consequently, “RO4350B multilayer” alone is not a complete manufacturing instruction. Specify what lies between cores and ask the fabricator to confirm its bonding and registration approach.
The official RO4400 datasheet lists distinct bondply grades, including RO4450F and RO4460G2, with different dielectric properties. Do not assign the core's dielectric constant to every bonding region. The sheet's typical-property disclaimer also matters: published material values do not automatically become finished-board acceptance limits. See the procurement data matrix for a way to separate these records.
In the model, record frequency, temperature assumptions, copper treatment and whether solder mask is present. Obtain proposed pressed dielectric spacings from the fabricator. A nominal unpressed bondply thickness is not sufficient evidence for the finished cross-section. Any supplier substitution needs a review of both manufacturability and electrical impact.
5. Worked geometry check: a dimensional change is a design change
Consider an illustrative microstrip candidate with a trace width w = 0.50 mm and dielectric spacing h = 0.25 mm. Its width-to-height ratio is w/h = 2.00. If the proposed spacing becomes 0.30 mm while width stays unchanged, the ratio becomes 1.67, a decrease of about 16.7%. Preserving the original ratio would require w = 0.60 mm.
This arithmetic is a geometry comparison, not a 50 Ω design or an impedance prediction. It deliberately omits dielectric dispersion, copper thickness and roughness, mask, etch shape, adjacent conductors and launches. Even preserving w/h does not establish equal loss or equal impedance in a real board. The practical review action is to rerun the relevant solver with the proposed finished geometry, then recheck routing clearances and component transitions.
Keep nominal, minimum and maximum scenarios explicit. Do not invent a manufacturing tolerance from the example. Request the fabricator's achievable dimensional ranges and decide which combinations represent useful electrical corners. Document whether a value comes from a material reference, a supplier commitment or a design assumption.
6. Define what the acceptance evidence actually proves
A controlled-impedance coupon should represent the agreed layer, reference structure and fabrication conditions. Define the target, tolerance, test approach, reporting format and disposition of a failure before ordering. The impedance coupon guide provides the related manufacturing context.
A passing coupon does not by itself validate the assembled RF path. Connector launches, vias, matching networks and component variation remain outside a simple uniform-line check. Where system performance depends on loss or phase, define the additional test vehicle or assembled-board measurement separately, including frequency range, calibration/reference planes and fixture treatment. No measured results are claimed in this article.
7. Close the drawing-to-quote loop
Before release, compare the stackup document, fabrication drawing, impedance table and supplier quotation line by line. Resolve mismatched revisions. State whether the supplier may adjust trace widths and require approval for material substitutions or changes outside the agreed construction. Archive the approved proposal with the model revision used to evaluate it.
Primary references and scope
- Rogers: RO4000 series circuit processing guidelines — multilayer processing, bonding and registration context.
- Rogers: RO4400 series bondply datasheet — grade-specific bonding material properties and qualification context.
References checked 8 October 2026. The workflow, worksheet, conceptual diagram and dimensional example are original editorial guidance. Consult the current manufacturer documents and obtain application-specific fabrication approval. This independent engineering resource is not a Rogers Corporation publication.