PCB engineering guide
RF PCB Design Guide: Grounding, Transitions and Material Selection
Prepare an RF layout and fabrication package around the approved stack-up, continuous return paths and the electrical behavior of launches and layer transitions.
Define transmission-line geometry
Choose microstrip, stripline or grounded coplanar waveguide with the routing, shielding and assembly needs in mind. Specify the required impedance for each net class from the interface or RF circuit design. A trace width copied from another board is only a starting estimate.
Record the finished copper, dielectric thickness, relevant design Dk, solder-mask coverage and reference layers. For coplanar structures, also define the gap to adjacent ground. Use the impedance calculator for early exploration, then confirm the geometry with a field solver and the actual material construction.
AstroPCB publishes ±8% controlled impedance with field-solver modeling and TDR coupons. Put the target values and coupon requirements on the drawing; requirements outside the published values need design-specific confirmation.
Keep the return path continuous
Maintain a continuous reference plane beneath critical RF traces. Avoid routing across slots, plane splits or gaps in the reference. Separating RF, digital and power circuitry should not accidentally sever the return path of a signal crossing between zones.
When a signal changes layers, review how its return current moves between reference layers. Ground stitching near a transition can be part of that design, but a fixed via pattern is not a substitute for evaluating the geometry. Keep launches, nearby copper and the enclosure consistent with the modeled structure.
Review launches, vias and stubs
A connector footprint includes its launch geometry, local ground, pad stack and mounting arrangement. Start from the connector manufacturer’s guidance for the intended stack-up and confirm the resulting transition. Model discontinuities where they consume a meaningful part of the channel budget.
Specify signal-via spans, drill sizes, pads, antipads and ground-return connections. Review whether an unused via stub requires back-drilling or a different layer transition. For via fences, choose spacing from the operating band, propagation wavelength and field behavior; there is no single spacing that fits every RF board.
Our published drill references are 0.10 mm minimum mechanical drill and 75 µm minimum laser drill, with laser-via aspect ratio up to 1:1. Review these together with board thickness and via construction.
Select the laminate from the channel requirements
Compare the exact grade and construction, including Dk/Df test frequency and method, foil profile, thickness tolerance and environmental requirements. Avoid choosing a laminate from a universal frequency cutoff or data-rate label.
RO4350B, RO4003C and PTFE-based materials are different starting points for a material discussion. Use current manufacturer data and channel or circuit modeling to choose between them. A hybrid RF/FR-4 stack-up also needs compatible bonding materials and a confirmed lamination process.
What to send for fabrication review
- Fabrication files, drawing, board outline and approved revision.
- Operating band, impedance classes and critical RF nets or structures.
- Proposed stack-up, exact material grades, copper and substitution policy.
- Connector part numbers and launch guidance; via schedule and any back-drill requirements.
- Finish, mask openings, panel requirements, quantities and delivery date.
- Specified electrical tests, impedance coupons and acceptance documents.
Use the high-frequency PCB service and DFM review to align these requirements before release. Fabrication DFM checks producibility; it does not replace RF circuit simulation or system validation.
Engineering references
AMD XAPP1392 PCB Channel Design Guidelines explains channel loss and links to guidance on impedance discontinuities and via transitions. Its numerical limits apply to the interface described in that document, not to every RF design.
For manufacturing planning, use the current AstroPCB capability table and the datasheet for the material actually specified.
Prepare the next build
Send the fabrication package for review.
Include the stack-up, material requirements, impedance table, quantity and required delivery date.