High-Frequency PCB: A High-Speed Circuit for Signal Transmission

Are you a technician dealing with signal transmission or wireless networking equipment? One of the critical components in these devices is a high-frequency PCB.

These circuits are also vital in electronics like cell phones because they are responsible for data transmission.

We have looked at these PCBs in detail below, including the high-frequency designs, so read on to learn more.

What is a High-Frequency PCB?

High-frequency printed circuit boards are electronic devices that transmit electromagnetic waves with a frequency range of between 500MHz and 2GHz.

A high-frequency PCB

Since the circuit board has fast signal flow rates and high signal sensitivity, manufacturers build them using high-frequency laminate materials. These end up reducing the dissipation factor and stabilizing the dielectric constant.

The Features of High-Frequency PCBs

• Low and stable Dk (Dielectric Constant)
• Small Df (Dissipation Factor)
• High heat resistance
• Impact endurance
• Chemical resistance
• Peel off resistance
• Low water absorptivity
• Similar thermal expansivity to copper foil
• Low dimensional stability
• Adequate board spacing

Advantages of High-Frequency PCBs

• Reduced moisture rate
• Excellent thermal management
• Impedance control
• Dimensional/mechanical stability
• Excellent signal performance with reduced signal loss

Applications of High-Frequency PCBs

• Automotive, Airborne, and Ground-Based Radar Systems

Radar Systems

• Global Positioning Satellite Antennas
• Cellular Telecommunications Systems
• Direct Broadcast Satellites
• Missile Guidance Systems
• RFID Tags (Radio-Frequency Identification)

A sticker with an RFID chip

• Millimeter-Wave Applications
• Space Satellite Transceivers
• E-band Point-to-Point Microwave Links
• Personal or Healthcare Monitors
• Medical Scanners (MRI, Ultrasound, CT, and more)
• Power Indicators and Control Devices
• Industrial and measuring equipment
• Scientific instruments (photometers, microscopes, and control systems)

Laboratory microscope

Materials Used For High-Frequency PCBs

High-frequency boards require unique PCB materials to produce high-speed signal output. Some of these special materials include:

• FR4: The least expensive laminate material and easy to fabricate. However, it significantly has relatively poor electrical performance when the signal speed exceeds 1.6GHz.
• Rogers 4350B HF: Like FR4, this material has a low fabrication cost, a competitive price, and offers excellent dimensional stability.
• ISOLA IS620 E-fiberglass: Although challenging to fabricate, this material provides excellent thermal and electrical performance.
• Taconic RF-35 Ceramic: This is a low-cost material consisting of ceramic-filled PTFE with woven glass. It is moderately easy to fabricate the material, but it has excellent peel strength, good electrical performance, and low dissipation.
• Taconic TLX: Consisting of PTFE fiberglass, TLX is a dimensionally stable material with excellent mechanical, thermal, and electrical properties. However, it is challenging to fabricate.
• Rogers RO3001: A bonding film with a low loss tangent and dielectric constant. It is also resistant to chemicals and high temperatures.
• Rogers RO3003: The RO3003 consists of PTFE glass materials that provide low Dk loss and economical pricing.
• ARLON 85N: A pure polyamide resin with high thermal resistance

Different manufacturers offer varying material combinations for high-frequency applications, but generally, they are either:

• FR-4
• Ceramic-filled PTFE
• PTFE with woven glass
• Ceramic-filled hydrocarbon
• PTFE with micro-glass fiber
• Ceramic-filled hydrocarbon with woven glass
• Ceramic-filled PTFE with woven glass

Issues With Manufacturing HF Boards

Printed circuit board manufacturing

Weak Binding Force

Since there is a dense placement of buried vias covered in resin in the PCB, there is usually a low binding force between the reinforcing prepreg sheet and the resin.

As a result, some surfaces may experience delamination, especially after soldering at high temperatures.

Manufacturers use resins with a comparable Tg and CTE to the substrate material to prevent delamination.

The goal is to make the two layers contract and expand at a similar rate to ensure they remain bound together even as the temperature fluctuates.

Drilling

High-frequency multilayer boards need multiple through holes for efficient heat dissipation, but this is not easy during PCB fabrication.

Drilling generates heat, and the melted debris from this process can solidify on the hole walls, resulting in dismal heat dissipation, and affecting the high-frequency circuit.

The solution is to use a resin cover instead of an aluminum one, which absorbs heat better when drilling. It also helps to increase the suction and vacuuming pressure to improve the hole quality.

PCB drilling

Back-Drilling

While the through holes help with heat dissipation, they can affect the integrity of the high-frequency signals. The extra stub acts as a reflecting path, increasing insertion loss.

The only way to work around this problem is by drilling a wider hole on the reverse side to eliminate the extra stub.

However, the issue comes from the burrs that remain after drilling. Manufacturers can avoid them by drilling holes before etching.

Surface Finish Selection for High-Frequency PCBs

Surface finishing gives the PCB extra strength and appeal. The options to use include:

HALL

HASL (Hot Air Solder Leveling) involves dipping the PCB in a melting solder bath, then placing it on a path with hot wind. The airflow smoothens and brightens the coat while also making it more uniform.

It is the most commonly used surface finish method due to its low cost and high shelf life.

However, the method increases the chances of solder bridging in fine-pitch PCBs, while the lead component makes it a health risk. There is also the issue of thermal shock.

ENIG

Like HASL, ENG is an abbreviation for Electroless Nickel and Immersion Gold, which involves depositing gold and nickel layers on the copper PCB.

The chemical electroplating method creates a coat with a long shelf life and is perfect for plating through holes. Also, it complies with RoHS regulations, so it is safe to use.

However, it is expensive and can increase signal losses if not applied correctly.

ONE PIG

Electroless Nickel Electroless Palladium and Immersion Gold are similar to ENIG but eliminate the creation of a metal compound (black pads) between the gold and nickel layers. It does so by introducing a stable palladium layer.

The finish also results in better soldering, makes the surface smoother, and enhances oxidation & heat resistance.

Immersion Tin (IT)

Immersion Tin is one of the more traditional methods and involves depositing tin on the circuit board base via chemical displacement. The deposited layer protects the copper underneath from oxidizing and does not have any lead, so it is safe.

However, the process is prone to creating tin whiskers. While IT is ideal for press-fit insertion PCBs, it is unsuitable for multiple assembly processes.

An Immersion Tin PCB

OSP

OSP (Organic Soldering Preservative) uses a water-based organic compound to prevent copper oxidation.

The process applies a thin, flat layer cost-effectively, but the finish has a short shelf life, and it is impossible to measure its thickness. OSP is also not ideal for board layouts with plated through holes.

High-Frequency PCB Design Guidelines

PCB design

As a circuit designer, you need to follow these guidelines:

Plan Your Design

Before beginning the process, lay out your ideas on a checklist to avoid forgetting vital aspects that might cause drawbacks later.

Determine the PCB Signal Frequency

Note down the power and voltage requirements for the integrated circuits, and determine if you will be dividing any power planes.

Try to accommodate all the different signals, trace lengths, and controlled impedance while minimizing the noise of the HF signals.

Draw Up Board Stack Up Plan For Production

After planning the board design, note down the requirements of the stack-up layer while consulting your PCB manufacturer to determine the specific materials.

Plan the Floor

Next, plan the floor by dividing the PCB into logical sections and deciding whether to place the sub-circuits into a larger layout or separate them.

This step is vital if you have analog and digital signals that should remain isolated to reduce interference.

Determine the Power and Ground Planes

After defining the board layout, examine it to understand the ground plane and check whether it is complete.

If not, you must divide the ground plane by including a resistor along the signal trace. The goal is to have a bridge to enhance the return path.

Prototype printed circuit design isolated

Minimize the Size of Land Patterns

One characteristic of HF boards is small pads, so the next step is minimizing the land patterns' size.

While other PCBs have a capacity of 30% of the component pins, HF PCBs usually have 0 - 5%, so reduce their size to make them more useful. A smaller board is also mechanically more robust and minimizes parasitic capacitance.

Route the Frequency Signals

High-frequency signals emit high radiation, so routing them increases the shielding benefits of your PCB, which helps to reduce interference.

Design an Efficient Current Return Path

Each signal in a high-frequency board needs a route, which starts at the source and terminates at the sink. In between the two should be a smooth path with minimal obstruction.

Use 3W Rule to Reduce Trace Coupling.

Line coupling can severely affect signal integrity, but the 3W rule can help by increasing the distance between the lines. It states that the distance between the traces should be 3X the width of one line.

Apply the 20H Rule to Reduce Plane Coupling

Coupling between the ground and power planes can damage your HF circuit board.

The 20H rule helps solve this problem by suggesting you keep the dielectric thickness between the ground and adjacent power 20X thicker than the power plane. 

Fabrication Process for High-Frequency PCBs

After designing, the next stage is PCB fabrication, which involves these steps.

Make a PCB Design

Begin by laying out the PCB blueprint on your preferred design software. You can use Extended Gerber to encode the blueprint, then send the file to the fabricator for manufacturing.

Print the PCB Design

Also, print the PCB design, and you can use a plotter printer to produce a PCB film, which is a negative for the blueprint. It has two inks: black shows the conductive copper lines, while clear ink indicates the non-conductive sections.

Print Copper for the Inner Layers

Manufacturing begins at this stage, and after printing the PCB on the laminate, copper gets pre-bonded on the same laminate, which acts as the PCB structure. Next, engrave the copper to reveal the initial blueprint.

Alignment of Layers

Since a high-frequency PCB has different layers, manufacturers usually use optical punches to align them by forcing pins down the holes. Another machine checks whether the layers are in order and without defects.

PCB Layers Lamination

The next stage involves fusing or laminating the layers and layup steps if there are no defects. Prepare the inner and outer sections, then use a metal clamp to help join them.

Drilling

Use a computer-guided drill with the guidance of the Extended Gerber design to do the drilling. But before that, use an X-ray machine to determine the drilling spots. After drilling, file off the excess copper to leave the surface smooth.

A homemade PCB drilling machine

Plating of PCB

Plating involves joining the layers together using chemicals. But first, you must clean the boards thoroughly before dipping them in the chemicals.

Solder Masking

Next, clean the panels again before applying the solder mask, then use epoxy ink with the solder film. Expose the board to UV light to remove the solder mask parts.

Screening and Finishing of Silk

As described earlier, the finishing process involves plating the board with gold, silver, or HASL to prevent oxidation. Afterward, silkscreen the PCB to print all the essential details, such as the manufacturer name and warning labels.

Testing

This process checks the circuit isolation and continuity to ensure no shorts or disconnections exist.

An operator checks a finished PCB

Cutting

The last step is cutting the PCB using a CNC machine or a V-a groove.

PCB cutting on a CNC machine

Your Expert High-Frequency PCB Manufacturer: Our Capabilities

Summary

As you can see, high-frequency printed circuit boards are critical components in modern electronics, and we make them using the highest standards. If you need such a circuit for your project, contact us to place your order today.