A high frequency printed circuit board (PCB) is designed to operate at high frequencies typically above 1 GHz. As frequencies increase into the GHz range, the electrical properties of the materials used and the layout of the traces become increasingly important. High frequency signals require controlled impedance paths to minimize unwanted radiation, crosstalk, and signal reflections. Special design techniques are required for high frequency PCBs including impedance matching, bypassing, decoupling, and ground planes. The materials used must also have stable electrical properties across frequency. Popular materials for high frequency PCBs include FR-4, polyimide, Rogers, and ceramic substrates.

In this article, we will provide an overview of high frequency PCB design and discuss the key considerations including:

• Electrical properties of materials
• Controlled impedance routing
• Signal integrity
• EMI control
• Stackup design
• Component placement
• Manufacturing capabilities

We’ll also briefly touch on some high frequency PCB applications and provide a comparison to low frequency PCBs.

Key Electrical Properties for High Frequency Materials

When signals operate at higher frequencies, the electrical properties of the materials used in the PCB become increasingly important. Key material properties to consider for high frequency PCB design include:

Dielectric Constant (Dk)

The dielectric constant measures how much a material decreases the speed of an electrical signal compared to the speed in a vacuum. All non-conductive materials have a dielectric constant greater than 1. The lower the dielectric constant, the less the material will slow down electrical signals. Most standard PCB materials like FR-4 have a dielectric constant around 4. For high frequency boards, materials with lower Dk are preferred such as polyimide (Dk3.5) or ceramic (Dk3-10).

Dissipation Factor (Df)

The dissipation factor indicates how much energy from an electrical signal will be lost in the dielectric material. Lower dissipation factor means less signal loss. Standard FR-4 materials have a dissipation factor around 0.02 while advanced materials used for high frequency boards can have Df less than 0.003.

Thermal Conductivity

This measures how well heat transfers through a material. High frequency circuits can generate significant heat that needs to be dissipated. Using a dielectric material with higher thermal conductivity improves heat transfer and cooling.

Controlled Impedance Routing

One of the main challenges in high frequency PCBs is maintaining the characteristic impedance of traces and minimizing reflections. When traces have an uncontrolled impedance, it can lead to ringing, crosstalk, and EM radiation.

To maintain proper impedance, high frequency boards use controlled impedance routing where traces are a consistent width and separated from a reference plane by a specific dielectric thickness. Common controlled impedance values are 50 Ohms and 75 Ohms.

By matching the trace impedance to the impedance of the circuits and cables, this minimizes signal reflections. The stackup, trace dimensions, and materials must be precisely controlled to achieve the target impedance across the entire path length.

Signal Integrity

In additional to controlled impedance, high frequency boards must be carefully laid out to ensure good signal integrity. Two key signal integrity techniques used are:

Bypassing and Decoupling

Bypass and decoupling capacitors are placed near integrated circuits and devices to filter noise from the power distribution network. This helps maintain a clean power supply for sensitive high frequency circuits.

Proper Layer Stackup

A well designed layer stackup isolates critical signals using ground planes and power planes to shield noise. Sensitive traces should be routed on inner layers while outer layers are used for power distribution. Differential signaling should be contained between solid ground plane layers.

EMI Control

Electromagnetic interference (EMI) occurs when the high frequency signals radiate and couple to other circuits on the board or to external devices. Controlling EMI is a key concern for high frequency PCB layout. Common EMI mitigation techniques include:

• Embed traces between ground or power planes to contain signals
• Minimize trace lengths
• Route traces orthogonally to avoid coupling
• Use shielding covers or cans
• Add ferrite beads to suppress common mode noise

Careful component placement is also needed to isolate noise sources and sensitive circuitry. It is also critical to properly terminate traces to eliminate resonances and reflections.

Stackup Design

The layer stackup defines the materials used in the PCB and the position and function of each layer. A properly engineered stackup is critical for signal integrity, EMI control, and manufacturability. Here are key guidelines for a high frequency PCB stackup:

• Place a solid reference plane next to each signal layer
• Use thicker dielectrics between layers to reduce capacitive coupling
• Keep dielectrics consistent whenever possible to simplify impedance control
• Ensure symmetry from the center of the board for even thermal expansion
• Allow for several power and ground layers to distribute power
• Use thicker copper weights (2 oz +) to allow high current power distribution

Here is an example 6-layer high frequency PCB stackup:Copy code

| Signal 1 | 1 oz copper | Ground Plane | 1 oz copper | Power Plane | 2 oz copper | Power Plane | 2 oz copper | Signal 2 | 1 oz copper | Ground Plane | 1 oz copper

This provides good isolation between the two signal layers while allowing robust power distribution in the middle. The ground planes shield EMI and provide an impedance reference.

Component Placement

Like PCB routing, component placement on high frequency boards requires special attention to signal and power integrity. Some guidelines include:

• Place decoupling capacitors as close as possible to IC power pins
• Orient components to minimize trace lengths
• Keep high-speed traces short with minimal vias
• Group components with similar functions or frequency ranges
• Separate analog and digital sections to reduce noise coupling
• Keep noisy circuits isolated from sensitive ones

Taking advantage of symmetries in the stackup and component placement is key. This balances timing, impedance, and thermal issues across the board.

Manufacturing Capabilities

In additional to material properties and design, manufacturing capabilities also factor into high frequency PCBs. Key manufacturing considerations include:

Number of Layers – High layer counts help isolate signals and contains EMI. High frequency boards often use 12-32 layers.

Trace/Space – Thinner trace widths and spaces are needed for fine-pitch components and tightly routed controlled impedance traces. Machines with 5/5 mil capabilities are preferred.

Registration – Tight registration accuracy prevents shorts/opens with HDI manufacturing. 0.5 mil accuracy or better is recommended.

Impedance Control – The process must support tightly controlled dielectric thickness and trace dimensions to maintain target impedance across the board.

Via Construction – Small microvias are usually needed between layers to reduce parasitic inductance and capacitance.

So selecting a manufacturer with advanced PCB capabilities suitable for high speeds is key.

Comparison to Low Frequency PCBs

It’s worth briefly comparing some of the key differences between low frequency and high frequency PCBs:

Parameter	Low Frequency PCB	High Frequency PCB
Frequency Range	<1 GHz	>1 GHz
Materials	FR-4	Low loss laminates
Trace Width	8-15 mils	5-8 mils
Spacing	>10 mils	5-6 mils
Layer Count	2-8 layers	12+ layers
Impedance Control	Not critical	Controlled impedance
EMI Control	Optional	Extensive shielding
Bypassing/Decoupling	Minimal	Extensive
Signal Integrity	Good	Very tightly controlled

So in summary, high frequency PCB design requires special attention to material selection, stackup design, layout techniques, manufacturing capabilities, and simulation/modeling to ensure optimal signal and power integrity at microwave and mm-wave frequencies.

Applications of High Frequency PCBs

Here are some common applications and products that utilize high frequency PCB technology:

• Cell phones
• WiFi/Bluetooth
• Radar and sensors
• Satellite communications
• High speed data links
• USB 3.0
• Computer servers
• Networking equipment
• Wireless base stations
• Automotive electronics
• Aerospace avionics

So high frequency boards support a wide range of cutting edge wireless and high speed applications that require operating at GHz frequencies and beyond. The specialized design techniques allow these RF/high speed signals to function properly and maintain signal integrity through the PCB.

Conclusion

To summarize, high frequency PCB design requires careful engineering to account for electrical properties, controlled impedance, signal integrity, EMI control, manufacturability, and other factors that become critical at high frequencies. When designed properly using the right materials and layout techniques, high frequency PCBs enable bleeding edge applications like 5G, high speed computing, radar, and beyond. The specialized expertise and constraints relative to conventional PCBs allow high frequency boards to properly route and function at GHz and higher frequencies.

Frequently Asked Questions

What frequency range is considered “high frequency” for PCBs?

Generally frequencies above 1 GHz are considered high frequency for printed circuit boards where the wavelengths become short enough that special design techniques are required. However, certain applications like high speed digital logic may require special layout considerations even at frequencies below 1 GHz.

What are some key high frequency PCB design rules?

Some key design rules include controlled impedance routing, proper layer stackups, extensive bypassing/decoupling, minimizing trace lengths, isolating critical signals, and minimizing vias. Maintaining careful control over dimensions and materials is critical for proper performance.

Why is impedance control important for high frequency PCBs?

At higher frequencies, uncontrolled impedance will lead to increased signal reflections, ringing, and EM radiation. By properly matching trace impedance to terminations and cabling, reflections can be minimized to maintain signal integrity and reduce EMI.

How many layers are typical for a high frequency PCB?

High frequency boards typically use higher layer counts than conventional PCBs. Layer counts of 12 or more are common, with high end boards using 20 layers or more. This helps isolate critical signals, provide shielding planes, and distribute power across the board.

What types of materials are used for high frequency boards?

Some popular dielectric materials used are polyimide, FR-4, Rogers laminates, Taconic, and ceramic substrates. These provide stable electrical properties at high frequencies. Many special high frequency laminates are also available from PCB manufacturers