5G mmWave PCB Design, Materials and Manufacturing

Introduction

5G wireless networks operate at mmWave frequencies from 24 GHz to 40 GHz to take advantage of the abundant bandwidth at higher frequencies. However, the wavelengths are only 5-10 mm requiring advanced PCB technologies to implement the antennas and routing. 5G mmWave places stringent demands on PCB materials, stackups, tolerances, and assembly.

In this article, we explore considerations for high frequency PCB design, material selection, and manufacturing techniques for 5G mmWave applications.

High Frequency Design Considerations

Designing PCBs for mmWave frequencies requires paying careful attention to transmission line characteristics. At mmWave wavelengths, even small structures like vias and traces behave as transmission line elements. Key considerations include:

Skin Effect

At mmWave frequencies, current flows mostly near the conductor surface. This skin effect leads to increased losses in traces and vias, requiring larger diameters to reduce loss.

Dielectric Losses

The dielectric materials themselves contribute more loss at mmWave frequencies. Materials must be chosen to minimize dielectric loss tangent for maximum efficiency.

Wave Impedance

With mmWave signals, the characteristic impedance of traces becomes a critical parameter. This must be tightly controlled to prevent reflections and maintain signal integrity.

Cross Talk

At higher frequencies, coupling between adjacent traces increases. Line spacing must be adequate to prevent crosstalk issues between critical signals.

Return Paths

Providing proper return paths is essential to contain fields, control impedances, and minimize radiative losses. A solid ground plane under signals is ideal for return currents.

Passives

Passive components like capacitors and resistors have parasitic inductance and capacitance that affect their behavior and tolerance at mmWave. Components must be selected for optimal high frequency response.

By accounting for these high frequency effects during design, the PCB can deliver maximum performance for mmWave ICs and antennas.

Stackup Selection

The PCB stackup defines the layer materials, sequence and thicknesses. For 5G mmWave, the stackup must deliver:

• Controlled Impedances - Matching trace impedances to ICs requires precise dielectric constants and thicknesses.
• Low Loss - Thin dielectrics with low loss tangent are needed to maximize efficiency.
• Signal Integrity - Layer sequencing and assignments optimized for SI and isolation.
• Manufacturability - Materials and thicknesses compatible with fabrication capabilities.

Some common stackup configurations for mmWave PCBs include:

RF System-on-Chip Package

This stackup integrates an RFIC die into a low profile laminate package providing integration and shielding:

IC Die
Adhesive 
Fine Line Hard Gold Metal Layer
Thin Core Dielectric < 0.2 mm 
Bottom GND Plane        

IC Die Adhesive Fine Line Hard Gold Metal Layer Thin Core Dielectric < 0.2 mm Bottom GND Plane

4-Layer Board with Embedded Components

A 4-layer board allows embedding passives while providing a full ground plane:

Top Signal Layer
Thin Core Dielectric < 0.2 mm
Inner GND Plane 
Core Dielectric 0.4-0.8 mm
Bottom Signal Layer        

Top Signal Layer Thin Core Dielectric < 0.2 mm Inner GND Plane Core Dielectric 0.4-0.8 mm Bottom Signal Layer

6-Layer w/ Buried Capacitance

A 6-layer stackup provides multiple signal layers with a buried capacitance layer:

Top Signal Layer
Prepreg  
GND Plane
Buried Capacitance Layer
Core Dielectric
Inner Signal Layer
Bottom GND Plane        

Top Signal Layer Prepreg GND Plane Buried Capacitance Layer Core Dielectric Inner Signal Layer Bottom GND Plane

10+ Layers for Advanced Boards

For maximum routing capacity, 10 layers or more can be utilized with multiple signal and plane pairings:

Top 3 Signal Layers
Prepreg
GND Plane 1
Prepreg
Signal Layer 4
Core Dielectric
Signal Layer 5
Prepreg  
GND Plane 2
Prepreg
Bottom 3 Signal Layers        

Top 3 Signal Layers Prepreg GND Plane 1 Prepreg Signal Layer 4 Core Dielectric Signal Layer 5 Prepreg GND Plane 2 Prepreg Bottom 3 Signal Layers

The optimal stackup configuration depends on the complexity, layer count budget, and electrical requirements of each unique design.

Dielectric Materials

Selecting the right dielectric material is critical for optimizing loss tangent, consistency, and manufacturability. Common materials for mmWave PCBs include:

• PTFE Composites - Woven glass-reinforced PTFE offers excellent performance from 3-70 GHz with low loss, tight impedances and stability.
• Ceramic Filled PTFE - Provides ultra-low loss tangent <0.002 with improved thermal conductivity over pure PTFE.
• Liquid Crystal Polymer (LCP) - With a loss tangent of only 0.002, LCP is ideal for mmWave but difficult to manufacture.
• RF Materials - Specialty materials like Taconic TLY and Rogers RO4000 series offer advanced electrical performance.
• Circuit Board Materials - FR-4 can be used for lower cost carrier layers that do not carry mmWave signals.

Selecting the right dielectric depends on the frequencies, performance requirements, board thickness, and fabrication capabilities. Advanced materials like LCP or ceramic-filled PTFE are attractive but face manufacturability and cost challenges.

Design Guidelines

To translate the stackup into an optimal board layout, certain guidelines must be followed:

• Maintain constant dielectric thickness under traces to ensure uniform wave impedance.
• Minimize use of vias which introduce discontinuities.
• Provide ground plane shields between sensitive signals.
• Use blind and buried vias to route between layers.
• Allow adequate spacing between traces and planes.
• Minimize crossing splits in ground planes.
• Use impedance matched transmission lines for critical routes.
• Include test points for validation and troubleshooting.

Simulation and analysis using 3D EM tools are essential to model the high frequency behavior and optimize the layout.

Manufacturing and Assembly

Fabricating mmWave boards requires tight process controls and parameters:

• Impedance Tolerances - Must maintain tight impedance control usually ±5% or less.
• Registration - Excellent layer-to-layer registration around ±0.5 mil typical.
• Trace and Space - Capability for fine line traces and spaces less than 2 mils.
• Via Construction - Small microvias with low aspect ratios ideal.
• Surface Finish - Smooth fine line gold or copper finish.
• Solder Mask - Thin LF or LDI masks preferred.
• automation - Automated optical inspection needed.

Many standard FR4 shops lack the specialized processes needed for mmWave boards. An experienced RF PCB manufacturer is highly recommended.

Testing and Inspection

Once fabricated, mmWave boards require thorough inspection and testing:

• Automated Optical Inspection - Inspects bare boards for material and fabrication defects.
• X-Ray Inspection - Verifies internal layers, vias, alignment.
• Network Analyzer Testing - Validates dielectric constant, trace impedances.
• High Frequency Probing - Confirms via transitions, crosstalk and signal integrity.
• Thermal Cycling and Shock Testing - Stresses boards to identify latent defects.
• SAM Analysis - Scans acoustic microscopy to identify subsurface flaws.

A robust post-fab verification methodology ensures the fabricated boards achieve 5G mmWave performance requirements prior to integration of expensive ICs.

Conclusion

Designing and manufacturing PCBs for 5G mmWave demands special attention to electrical, mechanical, and material considerations impacting signal integrity and efficiency. By selecting low-loss materials optimized for mmWave, following high frequency layout techniques, and working with specialized fabrication processes, boards can deliver the quality and performance needed for 5G systems.

Frequently Asked Questions

Here are some common questions about 5G mmWave PCB design and manufacturing:

What dielectric materials work best for mmWave boards?

PTFE composites, ceramic-filled PTFE, and LCP offer the lowest loss tangents. Rogers and Taconic RF materials are also excellent choices. FR-4 can be used for lower frequency layers.

What stackup configurations are common?

4 to 6 layer is typical for cost-sensitive designs. High complexity boards may use 10+ layers. Integrated packages are also popular. Buried capacitance layers help decouple.

What tolerances are important?

Impedance tolerances around ±5% are needed. Registration and trace/space of 2 mils or below preferred. Low aspect ratio microvias ideal.

Why is surface finish important?

A smooth copper or fine line gold finish reduces surface roughness losses at mmWave frequencies. Unsupported copper can oxidize.

How can boards be tested post-fabrication?

Network analyzer electrical validation, high frequency probing, x-ray inspection, SAM acoustic microscopy and thermal stress testing are commonly used.