As electronic systems continue to operate at higher speeds and frequencies, high-frequency PCB design has become increasingly important for achieving stable signal transmission, reliable power delivery, and good electromagnetic compatibility.

High-frequency and high-speed PCB designs are more sensitive to trace geometry, layer stack-up, power distribution, return-current paths, component placement, and electromagnetic coupling than conventional low-speed boards.

When a PCB contains devices such as FPGAs, DSPs, high-speed memory, ADCs, DACs, or RF components, designers should consider signal integrity and power integrity from the schematic stage rather than attempting to solve high-frequency problems after routing is completed.

The following guidelines summarize important considerations for high-frequency PCB design.

1. Verify FPGA Pin Assignments Before Schematic Design

When an FPGA PCB design is part of a high-speed system, FPGA pin assignments should be verified before the schematic and PCB layout are finalized.

FPGA devices often contain dedicated pins for functions such as:

• Configuration
• Clock input
• Differential signaling
• High-speed transceivers
• Memory interfaces
• JTAG
• Power and ground
• Dedicated I/O functions

Some FPGA pins cannot be used as ordinary general-purpose I/O. Therefore, designers should use the FPGA manufacturer’s development software and device documentation to verify pin functions and I/O-bank requirements.

For example, tools such as Quartus Prime can be used for FPGA pin planning and assignment verification. This step helps prevent pin conflicts and avoids costly PCB redesigns later.

2. Plan the PCB Stack-Up for High-Speed Signals

A well-designed PCB stack-up is one of the foundations of high-frequency PCB performance.

For a four-layer high-speed PCB, a practical stack-up may be:

1. Signal
2. Ground
3. Power
4. Signal

For a six-layer PCB, one possible arrangement is:

1. Signal
2. Ground
3. Signal
4. Signal
5. Power
6. Signal

The exact layer arrangement should be determined according to the number of signals, impedance requirements, power distribution, and manufacturing capabilities.

For six-layer and higher-layer-count boards, it is generally preferable to provide high-speed signal layers with closely coupled reference planes.

High-speed traces should not be routed across gaps or splits in their reference planes because this can force return current to take a longer path. The resulting increase in loop area can cause higher electromagnetic radiation, impedance discontinuities, and signal-integrity problems.

Therefore, the PCB stack-up should be planned before detailed routing begins.

3. Design Power Distribution for Multiple Power Rails

A complex FPGA and DSP system may require several power rails, such as 3.3 V, 1.8 V, 1.2 V, and other device-specific voltages.

A common mistake is to treat every power rail as an ordinary routed signal.

For high-density designs, power distribution should be planned according to current requirements, voltage-drop limits, noise sensitivity, and component placement.

3.3 V Power

The main 3.3 V supply may be distributed using a power plane or large copper region when current requirements justify it.

Using a plane can reduce distribution impedance and provide a shorter path between the power source and multiple loads.

Core Power Rails

Lower-voltage rails such as 1.8 V or 1.2 V are often associated with FPGA or processor core power.

These rails may require higher current density and tighter voltage tolerances. Components powered by the same rail should therefore be placed in logically compact regions where practical.

Wide copper areas or dedicated power planes can be preferable to long, narrow traces because they reduce resistance and inductance.

In general, using copper pours and power planes for appropriate power networks can greatly simplify power distribution on a complex high-speed PCB.

4. Route Adjacent Layers in Different Directions

When routing multilayer PCBs, adjacent signal layers should preferably use different routing directions.

For example, one signal layer may be routed predominantly in the horizontal direction while the adjacent layer is routed predominantly in the vertical direction.

This approach reduces long parallel runs between traces on adjacent layers and helps reduce electromagnetic coupling.

However, routing direction should not be treated as an absolute rule. Signal integrity, impedance control, component placement, via transitions, and return-current paths remain the primary considerations.

5. Separate Analog and Digital Circuits Properly

Analog and digital circuits can interact through shared power supplies, ground paths, signal traces, and electromagnetic coupling.

A good analog and digital isolation strategy begins with functional partitioning.

During PCB placement:

• Keep sensitive analog components away from noisy digital devices.
• Keep high-speed digital clocks away from precision analog inputs.
• Separate noisy switching regulators from sensitive analog circuits.
• Keep analog signal paths short and direct.
• Provide appropriate reference planes for high-speed and analog signals.
• Control the return-current paths carefully.

For mixed-signal systems containing ADCs or DACs, the recommended grounding strategy should follow the converter manufacturer’s guidelines.

It is not always correct to physically split the entire ground plane. In many modern mixed-signal designs, a continuous ground plane with carefully controlled component placement and return-current paths provides better performance than aggressive ground-plane segmentation.

Therefore, analog and digital isolation should be implemented according to the actual current-return behavior of the circuit.

6. Treat High-Frequency PCB Design as an Iterative Engineering Process

High-frequency PCB design can be viewed as an iterative engineering process.

Rather than completing the schematic first and discovering problems only after routing, designers should repeatedly verify component footprints, pin assignments, signal connections, placement, and routing throughout the design process.

A practical workflow includes:

Step 1: Verify Component Footprints

Confirm the package dimensions, pad geometry, pin numbering, thermal pads, mechanical clearances, and recommended land patterns.

For high-density packages such as BGA, even a small footprint error can result in serious manufacturing or electrical problems.

Step 2: Validate the PCB Library

After verifying the footprint, add appropriate verification information and ensure that the approved footprint is stored in the controlled PCB library.

This reduces the risk of using incorrect component packages during layout.

Step 3: Review Power and Ground Connections

Carefully inspect all power and ground connections in the schematic.

Power and ground networks are fundamental to the entire system and should receive the same level of attention as critical signal networks.

Step 4: Route Critical Networks First

Critical power and high-speed signal networks should be considered before less-sensitive signals.

While routing, continuously check:

• Trace width
• Trace spacing
• Differential impedance
• Reference-plane continuity
• Via transitions
• Return-current paths
• Crosstalk risk

Step 5: Import the Netlist and Optimize Signal Assignment

After schematic verification, import the netlist into the PCB design environment.

Signal assignments can be optimized to make routing more efficient, especially for connectors, buses, FPGA interfaces, and high-density component areas.

Component reference designators should not be changed unnecessarily after placement simply to make routing easier.

The goal is to coordinate schematic design, footprint selection, placement, and routing throughout the entire development process.

7. Place Crystal Oscillators Close to the IC

Crystal oscillators and clock sources should generally be placed as close as practical to the device they serve.

Clock signals are particularly sensitive because they have fast edges and may contain significant high-frequency harmonics.

Keep the crystal and its associated components compact, minimize unnecessary trace length, and avoid routing unrelated signals underneath or through sensitive oscillator regions when recommended by the IC manufacturer.

A solid reference plane should normally be maintained beneath the clock circuitry to provide a controlled return path.

For systems with multiple clock domains, clock distribution should also be carefully planned to minimize skew, coupling, and unwanted radiation.

8. Optimize Connector Signal Assignment

The arrangement of signals on a connector can have a major influence on PCB routing difficulty.

Before finalizing the PCB layout, designers should review the connector pin assignment and optimize the signal order where possible.

Important considerations include:

• High-speed signal grouping
• Differential-pair continuity
• Ground-pin placement
• Power-pin placement
• Signal return paths
• Crosstalk reduction

Changing the signal assignment in the schematic may be preferable to forcing complicated routing on the PCB.

However, any change must remain consistent with the system architecture and connector definition.

9. Consider Connector Orientation in Multi-Board Systems

When multiple PCBs are connected through board-to-board connectors, connector orientation and pin assignment should be considered during the schematic stage.

For direct plug-in connectors, the upper and lower interfaces may use complementary or mirrored arrangements depending on the mechanical structure.

For flat cables, identical pin ordering on both ends may be appropriate when the cable and connector orientation require a straight-through connection.

Correct connector planning can significantly reduce crossed traces and unnecessary vias.

10. Optimize Inter-Module Signal Connections

When two functional modules are placed on the same PCB, the signal assignment between them should be optimized according to their physical arrangement.

For example, control signals and data buses can be assigned in an order that matches the physical orientation of the two modules.

This can reduce trace crossings and simplify routing.

If two modules are placed on opposite sides or in mirrored orientations, the signal order may need to be adjusted accordingly.

These techniques are not universal rules. The best approach depends on the actual PCB architecture, component placement, connector configuration, and signal requirements.

The key principle is to consider physical placement and electrical connectivity together rather than treating schematic design and PCB layout as completely independent tasks.

11. Minimize Power and Ground Loop Areas

One of the most important principles in PCB grounding and high-speed routing is to minimize current-loop area.

Power current and its return current should flow through paths that are as close together as practical. A smaller loop area generally reduces loop inductance and electromagnetic radiation.

For high-speed signals, the return current normally follows the path of lowest impedance, which is often concentrated near the signal trace on the adjacent reference plane.

Therefore:

• Keep power and return paths close together.
• Use continuous reference planes.
• Avoid unnecessary plane splits beneath high-speed traces.
• Minimize large current loops.
• Avoid unnecessary routing detours.
• Keep high-speed signal paths short and direct.
• Provide sufficient ground vias around high-speed transitions when appropriate.

For example, placing a signal trace directly above a continuous ground plane provides a controlled return path and helps maintain predictable impedance.

Signal Integrity and Power Integrity Should Be Designed Together

A successful high-speed PCB is not achieved simply by making traces shorter.

Designers must consider the interaction between:

• Signal integrity
• Power integrity
• Electromagnetic compatibility
• PCB stack-up
• Component placement
• Trace impedance
• Return-current paths
• Crosstalk
• Via structures
• Decoupling networks

A PCB may pass a basic connectivity check while still suffering from severe signal-integrity problems at operating speed.

For complex FPGA, DSP, memory, RF, and communication systems, simulation and laboratory measurements can be used to validate critical interfaces.

Conclusion

Modern high-frequency PCB design requires a system-level approach. FPGA pin planning, stack-up design, power distribution, signal routing, connector assignment, grounding, and component placement must be considered together.

A carefully designed PCB stack-up provides stable reference planes and controlled impedance. Proper PCB grounding minimizes return-current loops, while appropriate analog/digital partitioning reduces unwanted coupling. At the same time, careful power distribution improves power integrity, and optimized routing helps maintain signal integrity.

Kingda can support customers with PCB design and manufacturing requirements for high-speed, high-density, and high-frequency applications. By combining appropriate materials, controlled manufacturing processes, and design-for-manufacturing principles, Kingda helps customers develop reliable PCB solutions for demanding electronic systems.