High Frequency PCB

FS Technology has strong PCB manufacturing capabilities, no matter what your project needs, we will do our best to complete it for you. This article will introduce to you a circuit board with the advantages of high efficiency, fast speed, large controllability and strong tolerance - high frequency PCB.

Definition of High Frequency PCB

A high-frequency circuit board is a type of PCB that is divided by function. If the frequency of a circuit board is greater than 1GHz, then it will be defined as a high frequency board, regardless of material and hardness. It is generally used where transmission is required between two points with minimum loss of signal, especially electromagnetic waves. It is usually made of special materials. These PCBs are capable of transmitting a signal at a higher rate, its frequency ranges up to 100GHz.

When the frequency is low, the signal characteristics remain within data characterization and the PCBA performs all of its intended functions. In contrary, higher frequency effect PCBA operations greatly. However, when we raise the system speed, the greater frequency will have an impact on the circuit properties. As, during I/O signaling, this might have a negative impact on data transmission from the sender to the receiver.

It is crucial to remember that many materials are engineered to work at high frequencies. High Frequency PCBs have high-frequency-special-laminates and are very difficult to fabricate as they involve the transmission of sensitive signals while maintaining thermal heat favorable for that signal transmission.

High Frequency PCB Material Selection

High-Frequency PCBs requires special material for their fabrication, especially laminates, to output high-frequency signal. When processing high-frequency circuit boards, many engineers will eventually lead to unsatisfactory performance of the finished product due to the selection of incorrect high-frequency printed circuit board materials. For this reason, FS Technology recommends paying attention to the following points when selecting materials. Does it meet the needs of the project design? Can this high frequency PCB be mass-produced? Have you calculated the break-even point of the project?

Important indicators of high frequency PCB: DK and DF

DK and DF are important indicators to measure high frequency PCB. For the selection of circuit board substrate materials under high-frequency circuits, the change of DK value is used as the first reference factor. For projects that focus on signal transmission or characteristic impedance control, the performance of the DF value under conditions such as frequency, temperature, and humidity needs to be considered.

The change trend of DK and DF values of common substrate materials is obvious, especially when the frequency is from 1MHz to 1GHz. A common model of FR-4 has a DK value of 4.7 at 1MHz and a DK value of 4.19 at 1GHz. When the GHz is greater than 1, its DK changes begin to level off. At this time, the DK trend change is negatively correlated with the frequency increase, that is to say, the frequency increases, and the DK change trend slows down.

At 10 GHz, the DK value of the substrate with high frequency characteristics fluctuates little under the influence of frequency changes. The DK value has a floating value of +/- 0.02 at varying frequencies from 1MHz to 1GHz. The change trend of its DK value under different frequency conditions from low to high is downward.

For general substrates, under the influence of frequency changes, the change curve of DF value is larger than the DK value. The change rule of DF is that it tends to increase. Therefore, when FS Technology evaluates the high-frequency characteristics of materials, it is judged according to the change of its DF. High-quality high-frequency PCBs have the following two characteristics. 1. The change of frequency has little effect on the change of DF value. 2. Its changes are consistent with ordinary materials, but its own DF base is small.

Selection principle of high frequency materials

The commonly used FR-4 has high dielectric losses at different GHz frequencies, which may make it unsuitable for your high frequency projects. Therefore, the issue of materials is extra important when designing high-frequency PCB boards.

As a simple example, the square wave of 10Gb/S high-speed digital signal can be regarded as the superposition of sine wave signals of different frequencies. This square wave includes 5Ghz fundamental signal, 3rd order 15GHz, 5th order 25GHz and 7th order 35GHz signals. The integrity of the digital signal and the steepness of the upper and lower edges are as important to high-frequency circuits as they are to the low-loss and distortion transmission of RF microwaves. Therefore, FS Technology believes that the requirements for high-frequency digital circuit PCB material selection and RF microwave circuits are similar.

In actual engineering projects, the selection of high-frequency PCB seems simple, but as the project leader, you have to understand the characteristics of different boards, and design a reliable high-frequency board through reasonable stacking. For this reason, FS Technology has sorted out the following main considerations for the selection of high-frequency boards:

Manufacturability : such as lamination properties, temperature properties, PCB CAF resistance, heat resistance, fire rating, etc.

Product performance matching : low loss, stable DK/DF performance, material thickness, etc. In addition, for high-frequency PCBs, simulation PCBA testing is required in the early stage of design. FS Technology has closed-loop verification through a large number of simulations and actual tests to ensure the consistency of simulation results and tests.

Material acquisition time : The manufacturing difficulty of high-frequency PCB is not only reflected in its technological process, but also in material procurement. For a general PCBA manufacturer, it may take 2-3 months to purchase, which is very unfavorable for your project. If your project requires this kind of board, FS Technology recommends our turnkey high frequency PCBA service, which will be within your expectations from procurement to delivery.

Cost Factor. The cost of the PCBA industry is relatively transparent. If you are a price sensitive customer you can judge the approximate cost of your high frequency project by reading the PCB assembly cost drivers.

Introduction of several high frequency PCB materials

Rogers :

This is a PCB material company from the United States, which can provide a variety of high-frequency boards with great advantages. In the PCB manufacturing industry, it is often used to compare with FR-4, and even if the price of Rogers PCB is more expensive, it is still the first choice for many projects. Below is the Rogers range that can be used to make high frequency PCBs.

RO4003C, RO4350B, RO4360, RO4533, RO4535, RO4232, RO4233, RO3001, RO3003, RO3006, RO3010, RO3035, RO3203, RO3206, RO3210, RO3730, RO5780, RO6002, RO3202, RO6006.

Rogers R03001: It has a low dielectric constant and low loss tangent, with excellent chemical and high-temperature resistance.

Taconic:

TACONIC is the world’s largest producer of PTFE CCL. It owns the patent of uniformly coating PTFE on glass woven cloth, and occupies an absolute dominant position in the antenna sheet market, with a market share of more than 80%. The following material families perform exceptionally well at high frequencies.

TLY-5A, TLY-5, TLY-3, HT1.5, TLX-0, TLX-9, TLX-8, TLX-6, TLC-27, TLC-30, TPG-30, TLG-30, RF-30, TSM-30, TLC-32, TPG-32, TLG-32, TLG-34, TPG-35, TLG-35, RF-35, RF-35A, RF-41, RF-43, RF-60A, CER-10.

Taconic RF-35 Ceramic : This material is cheaper usually made up of PTFE which is filled with ceramic along with woven glass. Fabrication is a little bit easier and it provides good electrical properties to PCB along with lesser dissipation.

Taconic TLX : It is stable material dimensionally, made up of PTFE fiberglass. TLX provides excellent thermal, mechanical, and electrical properties, but is relatively difficult to fabricate.

FR4

Very common material for printed circuit board laminate, whenever signal exceeds 1.6 GHz frequency then it has relatively poor electrical performance

ISOLA IS620 E-fiberglass

This material is usually used to provide exceptional electrical and thermal performance.

Features and Benefits of High Frequency PCBs

The selection of PCB needs to be analyzed according to its characteristics and advantages. Below we will conduct a comprehensive analysis of the characteristics and advantages of high frequency PCBs.

Dissipation Factor: High-Frequency boards have usually a low dissipation factor that ranges from 0.0019 to 0.025 which means the transmission rate of the signal will remain unchanged. A low dissipation factor also allows better signal transmission. A lower dissipation factor can aid in reducing signal loss.

Dielectric Constant: Usually HF printed circuit boards have a low and stable dielectric constant. As this ensures frequency transmission at a higher rate with lesser delay.

Chemical Resistance: High-frequency PCBs are chemically resistant. When subjected to a chemical attack, these boards can withstand it. These circuit boards are less likely to corrode as a result of this attribute.

Low Moisture Absorption: Usually, HF PCBs absorb a lesser amount of water. As it does not absorb moisture, it can survive a humid climate.

Low Dimensional Stability: The dimensional stability of frequency PCB is low. With this property, these type of PCBs retains their dimensions even if they are exposed to heat.

Thermal Management Techniques for High-Frequency PCB

As High frequency, PCBs are operating on transferring signals at a higher rate, due to this reason mostly HF PCBs encounter heat management and resulting in rising in PCB temperature, which in some cases becomes fatal for PCBs. If the heat produced in a circuit is within a limit, it can dissipate on its own, but if heat production exceeds and can’t dissipate on its own then heatsinks and cooling fans or other heat management techniques are used. Following are some techniques to manage PCB temperature in order to enhance the lifespan of HF PCBs.

Thermal via Arrays : The integration of via arrays over the copper-filled sections allows for effective heat management. By using these vias, heat can be dissipated into the air. For heat to dissipate properly, the diameter of the via-hole should be big, approximately in the range of 0.1mm. More vias mean, the better the heat dissipation.

Using Wider Traces : As wider traces increases the surface area to dissipate heat more efficiently, which also aids in the reduction of hot spots on the PCB circuit. When creating broader traces, take care not to make them excessively wide, as this can result in crosstalk.

Using Heat-sinks and Cooling Fans : Heatsinks and cooling fans are two effective methods to overcome heat problems in PCB. Application of both depends on available space on PCB, like in CPU and MCU, Heat-Sink is used, while the cooling fan is employed to assist hot air to disperse properly into the surroundings.

Placement Technique : It is good practice to keep heat-producing elements apart from heat-sensitive materials in order to avoid harming them. Potential hot spots in PCBs can be found using PCB analysis software. If the PCBs are mounted vertically, heat generating components must be located near the top.

Issues in Manufacturing High-Frequency PCBs

Weak Binding Force:

More buried vias result in low binding force among sheets of laminates and the resin. Eventually results in delamination.

This issue can be resolved by using laminate and resin sheets that have the same rate of expansion and contraction in order to make sure that they remained bound to each other when temperature varies.

Drilling:

Multiple through holes are required for effective heat dissipation on high-frequency multilayer boards, ad this is the most difficult task during HF PCBs fabrication. Heat produces during drilling, results in hardening of the via-hole wall, which eventually can cause poor heat dissipation and interference with the high-frequency circuit.

This problem can be mitigated by using a resin cover rather than an aluminum cover, which absorbs heat more effectively during drilling. It also helps to enhance hole quality by increasing suction and vacuuming pressure.

Back-Drilling:

Through-Hole vias aid in heat dissipation, but at the same time, they can compromise the integrity of high-frequency transmissions. The additional stub may functions as a reflecting channel, causing insertion loss to increase.

The only option to solve this issue is to drill a larger hole on the opposite side of heat producing components.

This concludes FS Technology’s introduction to high-frequency printed circuit boards. FS Technology has been serving in the PCBA industry for many years, with professional manufacturing experience and PCB assembly experience. If you have any PCBA needs, you can contact us by email at any time, and our salesman will reply you as soon as possible.

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