Everything You Need to Know About High Frequency PCBs.

The High Frequency Printed Circuit Board is designed to meet specific requirements when it comes to a special signal in the electronic components of most devices. The range that high-frequency PCB offers are a range of 500MHz - 2GHz. This is the ideal range for high-speed designs, microwave, mobile applications, and Radio Frequency (RF). The faster the transmission frequencies are, the faster the singla flow rates are as well. High Frequency Printed Circuit Boards are extra beneficial for today?s complex electronic components, especially the switches.


Hight Frequency PCB is at the root of the innovative future, considering that electronic devices with high frequency are the trend nowadays especially in networks and wireless technology. High-Frequency PCB also finds application in satellite communication products that always require high frequency and high speed.
High Frequency Printed Circuit Boards Features
1. The Dielectric Constant (DK) should always be small and stable. It is better if it is smaller because the high dielectric constant can lead to a delay in signal transmission.
2. When the Dielectric Constant is small the quality of the signal is better, because usually there is a smaller signal waste as well.
3. The Thermal Expansivity should be covered with copper foil as good as possible to prevent any copper foil separation in cold and hot environment changes.
4. Low absorptivity, because higher water absorptivity can seriously affect the dielectric constant in wet environmental conditions.
5. High Frequency Printed Circuit Boards have good chemical resistance, heat resistance, impact endurance, and peel off resistance.
Having these advantages in mind we should mention that the high frequency can be also defined as a frequency more than 1GHz. Polyfluortetraethylene (PTFE) is the material that is used mainly for the creation of high-frequency PCBs and you can also find it by its easier name "Teflon". Its frequency is above 5Ghz in normal conditions and additionally, PPO substrate or FR4 is often used for products with the minimum frequency requirements such as 1GHz to 10Ghz.
Speaking of the ways and materials that high-frequency circuit boards are made, here are the substrate differences between them:
Compared with the others, FR4 is the cheapest one among them. Teflon or Polyfluortetraethylene is the most expensive one because of its DK, DF, frequency feature, and water absorption efficiency. Whenever there is a demand for a high-frequency product above 10GHz there is only one choice that the manufacturers are faced against. In this case, the Teflon PCB is the only substrate to manufacture with, which is definitely better substrate performance than the others. However, Teflon also has its disadvantages to illustrate some of them, it has a very large heat resistance property which is not that good and its cost is the highest among other substrates on the market. Manufacturers often use a large number of fiberglass or SiO2 as a material to fill, in order to improve the PTFE stiffness and heat resistance.
There is another big disadvantage or maybe it is more of a production issue. The problem is that due to the molecule inertia of PTFE material it is very difficult to combine it with copper foil. In that regard, it needs special treatment on the surface in order to be combined, which is increasing its production time and cost rapidly. Developers usually use chemical etching on the PTFE surface or a kind of plasma in order to increase the surface roughness and additionally add a film of adhesive between the copper foil and the PTFE. All these operations may have negative influences on the dielectric performance in its end-design.
Radio Frequency RF - PCBs Application
Radio-Frequency RF can be found in many electronics and wireless products that are surrounding us in our daily routine. There are also many applications of radiofrequency circuits in the industrial field of work as well as in handheld devices and advanced communication systems including satellites. High-Frequency Products can also be observed in a variety of different industries including the medical and industrial fields. As we already mentioned high-frequency circuits can be found in cell phones, RF remote control, GPS receiver, and more. In addition, we should mention the high-speed test equipment that is using high-frequency products that are providing the equipment with greater performance and life span of the project. Another great example of radiofrequency application is Ground-based and Airborne radar systems, using high-frequency circuits in order to operate on full speed and efficiency.
As the electronic industry is growing, the demand for high-speed products is also rising, overflowing the market. High-frequency circuits are specifically designed to meet customer requirements whenever the regular PCBs are not fast or efficient enough to deliver on a certain project. In general, these high-speed PCBs can be found almost in all electronic devices that are caprice enough to require higher frequency than the normal PCBs can deliver.
High-Frequency PCBs Production & Materials
Before we can shortlist the general materials that are often used for the creation of high-frequency PCB we should take some aspects under consideration. We already mentioned what dielectric constant is but in simple words this is the ability of the substrate to store energy in the electric field. It is dependent on the material and that means that when the axis of the material is changed the dielectric constant will change as well. The main importance and concern are to be small enough in order to bypass any signal transmission delay.
Furthermore, the dissipation factor DF should be small in order for the quality of the transmitted signal to meet its highest quality. Otherwise, if the DF is high the quality will be interrupted and poor. Another factor that can affect signal transmission is based on the molecular structure of the material. This is, however, not that important for the lower frequency signals.
The next important aspect that we should consider is the proper spacing. Speaking of skin-effect and cross talk it is essential. Crosstalk is likely to occur when the board starts interacting with itself, coupling its own components. In this regard, the manufacturers leave minimal space between the plane and trace in order to prevent crosstalk from happening. As far as the skin-effect, it is connected directly with the resistance that the traces have. It will increase along with the resistance increasing, which leads to warming board results. The conclusion is that the trace length and width have to be selected in a specific way that does not allow the board to be affected in higher frequencies.
Whenever the frequency goes higher, if the diameter is smaller it is comfortable for maintaining the low conductance. The quality of the signal is also better whenever the value of peel-off resistance is higher as well as the impact of endurance and heat resistance. The temperature plays a vital role when it comes to the coefficient of thermal expansion during the drilling and assembly process. It depends on the size of the material as it widely affects the process of manufacturing the PCB. Even the slightest change in the temperature can change severely the size of the material. Whenever the copper foil is set the thermal expansion should be the same as the material, if not there is a high risk the copper foil will separate from the metal due to heat expansion.
Another concern will be the environment where the device is operating. I am speaking of moisture here because if your device is an operation in an outdoor environment, moisture will be a huge problem. The recommendation here is to install the device in a lab or at least indoor space. With all these concerts in mind it is now time to review the materials used for the high-frequency circuit:
1. Rogers RO3001
2. Rogers 4350B HF
3. Rogers RO3003
4. Taconic TLX
5. Taconic RF-35 Ceramic
6. Arlon 85N
7. ISOLA IS620 E-fiberglass
Once the design has been selected and a proper material is chosen for the high-quality assembly, manufacturers initiate the key process of the production. During the manufacturing process of high-frequency circuits, the width and dielectric spacing are considered as a high priority.
The laminate materials for the circuit board design are crucial when it comes to the overall functionally of the final product. Every single aspect including thermal, electrical, and mechanical properties, has to be carefully considered when determining the best fit for an ordered PCB assembly project. The selection of material is not an easy job and requires extreme precision and caution.
The high precision and extensive expertise are not enough for the assembly process of the high-frequency circuit board. Many years of experience in PCB production are required for the creation of high-profile end-design.
In high process availability, an etching tolerance of  ?15 ?m is possible. In order to do so, each process is specifically controlled by a 3rd party for designing the layout of each individual printed circuit board. There is also an additional inspection during and after the production stage to check the consistency of the quality.
In many of the RF applications, you can see multi-layer affairs with a wide range of different properties. Whenever ordering you can mix different materials with the purpose of strengthening the substrate and increasing the electrical performance plus elevating the thermal properties and cost.
Often the hybrid multi-layer PCBs are referred to as composite boards. They are sometimes difficult to build because layer to layer manufacturing process is taking many aspects into account before it is completed and many of them usually cause technical issues. There are many more additional challenges arising within the production process such as mixed-signal PCB designs that are combining digital and analog components. The list of issues is pretty long, and the conclusion is that the high frequency printed circuit is very difficult to assemble, especially whenever the customer has custom orders.
The Problems of High-Speed PCBs
As we already mentioned, designing a high-frequency PCB can bring many unpredicted issues. Here we will illustrate the 3 main difficulties that the manufacturers are experiencing during the production process.
Integrity - In general, if the signal will perform as good as it should whenever it arrives at his final destination. If it doesn?t that means that the singla encountered some interference on its way, which unfortunately corrupted its integrity.
Noise - This is the interference that the signal may encounter during its journey from the transmitter to the receiver. Every single PCB emits some amount of noise, but if it is much the chase of data corruption is high.
Timing - This is the 3rd big problem and in other words if the signal is coming on time. All high-speed signals are being controlled by a clock and if the timing is off, the receiver will arrive corrupted data.
Conclusion:
As you can tell, learning about high frequency printed circuit boards is like learning about a whole new engineering world. There are a handful of people that are capable of understanding the electronics at this deep level. In this article, we have covered everything you need to know about the high-speed or high-frequency circuits but this is only the surface of all the information available. We just uncovered the most important and included a detailed explanation, materials, production, advantages, and disadvantages.
High-frequency PCBs are the beginning of a whole new electronic era. Currently, high-speed PSBs are used in the military and in the development of a 5g wireless network. This is something remarkable and it is on its way of changing the way we use electronics forever.
In this article, we tried to warn you about the future. If you find yourself developing or purchasing a high-frequency circuit board, be ready for extra consideration before you choose a material or before you combine two of them. Take notes, try to truly understand the field of work, and only after that, you are ready to make a purchase. What we know today, at last, allows us to assemble the finished end design of a high-speed board but there is a long journey ahead of us before we can proudly say that we know it all.
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