High-frequency PCB not only for 5G networks
In 2005, 5 Mbps (megabits per second) was considered a typical high-speed transmission speed; today, speeds above 1 Gbps are considered high-speed, and researchers are gradually attempting 25 Gbps. In practice, however, you will only see 5G networks for mobile phones in larger cities, and the average speed is still 200 to 400 Mbps.
What are the requirements for printed circuit boards with the introduction of 5G technologies? Compared to 4G networks, the impending large-scale deployment of 5G networks is forcing designers to rethink PCB design for mobile, IoT and telecom devices.
A 5G network is characterized by high speed, extensive signal bandwidth, and low latency (delay between action and response).
To cope with this fast time, manufacturers will need to pay attention to material selection, considering thermal conductivity and thermal coefficients. A material offering higher heat dissipation, excellent heat conversion and stable dielectric constants is essential to produce a suitable PCB that will support all 5G functions.
5G challenges for printed circuit boards
Compared to 4G networks, fifth-generation mobile technology will offer up to 10 times faster transmission speeds. It can handle 1000x higher traffic density and 10x higher number of connections per square kilometre. The 5G network also aims to provide millisecond latency.
Circuit boards will need to simultaneously support data rates and frequencies much higher than current ones, pushing mixed-signal design to its limits. While the 4G network operates with frequencies below the 6 GHz threshold (from 600 MHz to 5.925 GHz), the 5G network pushes the upper frequency limit much higher, into the wave region with bands centered around 26 GHz , 30 GHz and 77 GHz. This bandwidth places high demands on the base material of high-speed PCBs.
Hand in hand with high speed enters the design of high-speed printed circuit boards the problem of transmission signal interference. The higher the transmission speeds, the more electromagnetic radiation increases. This radiation is relatively harmless in small amounts when in contact with the circuit. However, when it begins to interfere with the operation of the electronic equipment as a whole, the radiation becomes a serious interference. And it is electromagnetic interference (EMI, or the degradation of a device, equipment or system caused by electromagnetic interference) that is becoming a whole new problem that we have not yet addressed.
The geometry of the PCB also plays a crucial role, in which the thickness of the laminate, in accordance with the signal transmission line, is an important value. It is necessary to select a laminate thickness that is typically between 1/4 and 1/8 of the wavelength of the highest operating frequency. If the laminate is too thin, there is a risk of resonance or even wave propagation through the conductors.
PCB materials will need to adequately handle the high speeds required for 5G technology. Higher signal speeds will result in more heat generated from electricity passing through the PCB. If the material is inadequate, the copper traces could flake off, delaminate, reduce and deform, and of course damage the board.
It is also necessary to include rules for the manufacture of high-speed PCBs, namely to keep the paths as short as possible and to check the width and distance between the paths so that the impedance remains, if possible, constant. At the same time, the thermal coefficient of the dielectric constant needs to be monitored.
The dielectric constant is a measure of the arrangement and formation of elementary dipoles in a non-conducting substance (dielectric) under the influence of an electric field. Since there can be no flow of electrons in a non-conducting substance, there will only be a displacement of charges within the atoms or molecules of the substance.
It is true that the smaller the dielectric constant, the smaller the dielectric loss factor and thus also the more stable and better the performance of the high frequency high speed substrate (material).
Successful high-speed PCB design process
1. Fundamental questions at the outset
Work organization – Do we have a visual PCB design that shows how all the subcircuits interconnect and is the backfeed well established? There should be sufficient shielding ground near the corresponding high frequency radio frequency circuit board. Multiple through-holes must be drilled on the shielding ground to ensure a three-dimensional shielding effect, thus ensuring only a small impedance loop. The spacing of the holes is recommended to be less than one twentieth of the wavelength. The holes must not be too dense to avoid creating space on the board with different characteristics.
Signal speed – When designing the PCB, do I know the highest frequency and fastest rise time of each of my signals?
Power – Do I know all the voltage and power requirements for all the integrated circuits?
Sensitive signals – Do I have a solution ready to meet the requirements for differential signals, controlled impedance, length or signal propagation?
Noise and possible tolerances – I can suppress signal noise with appropriate routing and with the help of microstrip. I know the fallback option of minimal tolerance of negative PCB characteristics.
If we have the answers to these questions, then we select the base material.
2. Basic material for high-speed PCB production
In terms of materials, we will probably choose some of the proven materials.
Choose materials with a low dielectric constant because dielectric losses increase proportionally with frequency. And it is necessary to choose materials with the lowest dielectric constants.
FR-4. This is a great material if you will be working with clock speeds < 5 Gbps. The material is cheaper and has decent impedance stability.
Other brands might be Nelco or Megtron. Each of these are suitable for speeds of 5-25 Gbps.
If a high speed design requires 56 Gbps speeds you will probably end up using Rogers laminate.
3. PCB layering strategy
Once you know the material, you need to know some strategies for additional layers.
First at all, you always need to have a signal layer placed next to the planar layer to give the electrical signals an effective return path space.
Also consider routing any high-speed signals on the inner layers of the board between planes. Avoid external radiation.
Utilize multiple ground planes in the layers to help reduce reference impedance, and also reduce common-mode thermal radiation that otherwise affects the operation of the overall circuit.
4. Layout of PCB subcircuits
The digital and analogue sections need to be carefully isolated to avoid any potential interference.
In theory, the routing of the HF radio circuit board paths should avoid a 45° angle as much as possible, preferring instead to route in an arc. Also avoid U-shaped paths to avoid the effect of parasitic capacitance.
If it is necessary to split the ground plane, a 0 Ohm resistor should be designed into the plan next to the signal path to create a bridge for the return signal path.
When routing signals on different layers, be sure to route them perpendicular to each other. That is, route the paths horizontally on one signal layer and vertically on the other.
An efficient solution for actual signal return are vias. Otherwise, the signals will propagate around the splits in your ground plane, resulting in a loss of signal integrity.
Rule 3W for communication between tracks
In order to reduce signal noise between PCB circuit paths, the spacing of the placement of the next line of traces should be large enough. When the center track spacing is not less than 3 times the track width, most of the electric fields can be under mutual interference. This is the 3W rule. Compliance with the 3W principle can reduce inter-signal loss by 70%, and compliance with the 10W principle can reduce inter-signal crosstalk by nearly 98%.
Useful recommendations for high frequency PCB design
- Use a small layer of solder mask: most solder masks have a high ability to absorb moisture. If this occurs it is very likely that the board will be damaged and cause losses.
- Use perfectly smooth copper traces and paths: the actual depth of the surface layer is actually inversely proportional to the frequency, and therefore very shallow on a circuit board with high frequency signals. If the copper surface is irregular, the current will also cause an irregular path. And again it will induce losses.
- Signal integrity: High frequencies present one of the most difficult challenges for integrated circuit designers. In order to maximize performance, a high-density insertion (HDI) board requires thinner traces. However, these can cause signal degradation. Thin traces cause RF signal transmission delays of even a few milliseconds.
In the high-frequency region, signal integrity is based almost entirely on impedance control. Traditional PCB manufacturing processes are unsuitable due to the creation of trapezoidal cross-sectional paths. Therefore, it is essential to follow the theoretical rules for high-frequency PCB design and avoid the formation of paths with 45°.
- Automatic control: The creation of PCBs for high frequencies requires an enormous increase in product quality and frequent production control with warnings of possible errors or circuit inefficiencies.
Advances in automatic PCB inspection and testing are leading to significant time savings and cost reductions associated with manual verification and testing. The use of new automated inspection techniques helps overcome the pitfalls of high-frequency PCB manufacturing, including global impedance control in high-frequency systems.
- Teflon: A Teflon-based laminate is used for high-frequency circuits. Its cost is higher, but its long-life nature makes it cost-effective. Teflon resists oxidation, ultraviolet rays, embrittlement or discoloration. Such a plate can operate from -270 to 315 °C. Copper plating is an essential stage in the production of Teflon printed circuit boards. The manufacturer must use copper with high tensile strength to prevent cracks and, more importantly, bending and deformation of the boards. A Teflon printed circuit board or also PTFE printed circuit board is a high frequency printed circuit board that uses polytetrafluoroethylene (PTFE) as a substrate.Teflon printed circuit boards transmit signals of 5 GHz and above, and are therefore the best choice for the manufacture of high frequency printed circuit boards.
The name Teflon is a trademark used by manufacturer Dupont Corporation for its PTFE materials.
Nowadays, the complexity of electronic components and electrical devices is constantly increasing and there is a need to speed up the signal flow. Therefore, higher transmission frequencies are required, and increasingly so. High-frequency printed circuit boards find applications mainly in radio and high-speed digital applications such as 5G wireless communications, in automotive and aerospace, in satellites, and generally in missile and military technologies.
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