Table of Contents
What is high frenquency PCB?
A high frequency PCB is a particular circuit board with a high electromagnetic frequency. that utilized for high frequency applications (frequency greater than 300 MHz or wavelength). In general, a high-frequency board is a circuit board having a frequency greater than 1 GHz.
The substrate material must have strong electrical performance as well as chemical stability. The loss on the substrate decreases as the power signal frequency increases. emphasizing the significance of high-frequency circuit boards.
What is high-frequency PCB design?
In high-frequency PCB design, the power supply designed in layers. so that the loop can always follow the path of least impedance. Besides, the power supply board has to provide a signal loop for all signals. That generated and received on the PCB so that the signal loop minimized, thus reducing noise. Omit high-frequency PCB design principles :
- Unity of power and ground, stable.
- Considered wiring and proper termination can cut reflections.
- also can reduce capacitive and inductive crosstalk.
- Noise suppression required to meet EMC requirements.
High Frequency PCB Design Guidelines
High-frequency circuits tend to be more integrated, and the usage of multilayer boards. it is not only important for wiring but also an effective interference-reduction technique. In the PCB Layout, a reasonable choice of a certain number of layers of PCB size . it make full use of the middle layer to set up shielding. It better achieve close grounding. and reduce parasitic inductance,shorten the transmission length of the signal. all which are helpful to the reliability of high-frequency circuits. When uesd the same material for a four-layer board instead of a double-sided board. the noise level is 20dB lower. The higher the number of PCB half-layers, the more complex the manufacturing process . and the higher the unit cost.
which necessitates a PCB Layout to selecting the appropriate number of layers . but also the need for reasonable component layout planning. and the application of the correct wiring rules to complete the design.
A Summary of High-frequency Design Wiring Tips
1. The less lead layer alternation between the pins of high-frequency circuit devices.
“Interlayer alternation of leads is superior”. that refers to the method of connecting components with fewer vias. A hole can contribute around 0.5pF of capacitance. reducing the number of holes could increase the speed and decrease the likelihood of data mistakes.
- The shorter of leads between the pins of high-frequency circuit components .
For signals such as clocks, crystals, DDR data, LVDS lines, USB lines, HDMI lines, and other high-frequency signal lines. the shorter they are, the better they are.
- Between the pins of the lead, high-speed electronic devices should bend as little as possible.
This need in the low-frequency circuit is utilized to improve the attachment strength of copper foil. but in the high-frequency circuit, meeting this need can limit the external emission and coupling of high-frequency signals.
- Pay particular attention to the signal line along the parallel alignment
Where “crosstalk” occurs. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. High-frequency circuit wiring must pay particular attention to signal lines near parallel lines to prevent crosstalk. As the high-frequency signal along the transmission line is transmitted as electromagnetic waves.
The signal line will act as an antenna, the energy of the electromagnetic field will be emitted around the transmission line. The signal caused by the mutual coupling of the electromagnetic field and the resulting unwanted noise signal is known as crosstalk (Crosstalk). Crosstalk affects the path. To avoid crosstalk of high-frequency signals as much as workable. the following points are essential during wiring.
- Insert a ground line or ground plane between two lines with significant crosstalk, which can serve as isolation and reduce crosstalk, if the wiring space permits.
- If parallel distribution cannot be avoided when the region around the signal line itself. it is a time-varying electromagnetic field. a substantial area of “ground” can be provided on the opposite side of the parallel signal line to reduce interference.
- In the wiring space permits the premise of increasing the distance between neighboring signal lines. lowering the parallel length of the signal lines, the clock line to the greatest extent possible. and the key signal lines perpendicular rather than parallel.
- If parallel alignment within the same layer is virtually inescapable. the alignment direction in two neighboring layers must be perpendicular to each other.
- In the digital circuit, the typical clock signal is the edge of a rapidly-changing signal, and the external crosstalk is significant. To reduce crosstalk, the clock line should be encircled by ground wire and there should be more ground holes in the design.
- The high-frequency signal clock must utilize a low-voltage differential clock signal and wrap the ground method. paying close attention to the wrap-around hole punching’s integrity.
- Idle, unused inputs do not overhang . but are grounded or linked to the power supply. the power supply in the high-frequency signal loop is also grounded . as the overhang of the line may be equal to the transmitting antenna. which is grounded to reduce emissions. The application of this method to remove crosstalk. has demonstrated its sometimes instantaneous effectiveness.
- Separate the high-frequency digital signal ground and analog signal ground Analog ground, digital ground, etc., that connected to the public ground . It connected with high-frequency choke beads or direct isolation. and a suitable location for single-point connections must be determined.
The ground potential of the high-frequency digital signal ground is generally inconsistent. there is a voltage difference between the two directions. the high-frequency digital signal ground has a very high proportion of signal harmonic components. when connected to the digital signal ground and analog signal ground. the high-frequency signal harmonics will interfere with the analog signal via ground coupling. So, the ground of the high-frequency digital signal and the ground of the analog signal . that are often isolated using single-point interconnection at a suitable place or high-frequency choke beads.
6. The integrated circuit block’s power supply pins to increase the high-frequency decoupling capacitors.
Each integrated circuit block’s power pins are positioned close to the installation of high-frequency decoupling capacitors. Increasing the power supply , pins of high-frequency decoupling capacitors helps cut the interference . caused by high-frequency harmonics on the power supply pins.
7.Prevent the creation of loops.
All types of high-frequency signal alignment should strive to avoid forming a loop. if this cannot be prevented, the loop region should be as minimal as workable.
8.Must guarantee appropriate signal impedance matching
Signal in the transmission process. when the impedance does not match, will result in the signal reflection in the transmission channel. Reflection will cause the production of synthetic signals to overshoot. resulting in signal fluctuations near the logic threshold.
The best way to drop reflections is to match the transmission signal impedance. because the greater the difference between the load impedance and transmission line characteristic impedance. the greater the reflection, so the signal transmission line characteristic impedance should be as close as possible to the load impedance. Also ensure that the transmission line on the PCB has no abrupt changes or corners . and that the transmission line impedance is continuous at all points. otherwise, reflections will occur between the transmission line segments.
Necessitates the observance of the following wiring principles when routing high-speed PCBs.
A. LVDS wiring rules
LVDS signal differential alignment with a line width of 7 mils . a line spacing of 6 mils required to limit the differential signal pair impedance of HDMI to 100 +/-15% ohms.
B.USB wiring standards
USB wiring standards demand differential alignment of USB signals with a line width of 10 mils. a line spacing of 6 mils, and a line spacing of 6 mils between the ground and signal lines.
C. HDMI wiring standards
HDMI cabling regulations mandate differential alignment of HDMI signals with a line width of 10 mils. a line spacing of 6 mils, and a distance greater than 20 mils for every two differential HDMI signal pairs.
D. DDR wiring rules
DDR1 cabling requires that the signal does not go through the hole as far as possible. that the signal line is equal in width, that the signal line is equidistant from the line. and that the alignment must meet the 2W principle to reduce crosstalk between signals. for DDR2 and higher high-speed devices. it also requires that the high-frequency data alignment is equal in length to ensure that the signal impedance matches.
What is High-Frequency Materials?
Driven by a variety of consumer needs for faster Internet connectivity.Such as mobile HD video, and the Internet of Things. high-frequency PCBs must also support the need for high-speed digital data transmission. As IoT, 5G, and commercial applications such as large data centers . a growing number of personal applications continue to refresh the requirements for digital communication system rates.
High frequency pcb Materials
The high-frequency high-speed PCB board and ordinary PCB board production processes are the same. The key point to achieving high-frequency high-speed is the properties of raw materials. that is, the characteristics of raw materials parameters. The main material for high-frequency high-speed PCBs is the high-frequency high-speed copper laminate. whose core need is to have a low dielectric constant (DK) and a low dielectric loss factor (Df). Besides to ensuring a low Dk and Df. the consistency of the Dk parameters is also an important factor in measuring the quality of the PCB board. In addition, there is an important parameter is the impedance characteristics of the PCB board and some other physical characteristics.
The high-frequency high-speed circuit board substrate dielectric constant (Dk) must be small and stable. Generally speaking, the smaller the better. the signal transmission rate, and the square root of the material dielectric constant are proportional to the high dielectric constant. which is prone to signal transmission delays.
High-frequency high-speed circuit board substrate material dielectric loss (Df) must be small. which affects the quality of signal transmission. The smaller the dielectric loss, the smaller the signal loss is also small.
High-frequency high-speed circuit board impedance the resistance and resistance to the parameters. Impedance control is the most basic principle of high-speed design. because the PCB circuit to consider plugging in the installation of electronic components. plugging in after considering the conductive properties and signal transmission performance. so the lower the impedance more better. the major board factories in PCB processing will ensure a certain degree of impedance error.
High-frequency high-speed circuit board substrate to low water absorption. high water absorption will cause dielectric constant,dielectric loss when exposed to moisture.
Which material is best for high-frequency?
The current high-frequency board is more often used with fluorine in the dielectric substrate. such as polytetrafluoroethylene (PTFE), usually called Teflon.
As we mentioned earlier. the production process of high-frequency and ordinary PCB production processes is the same. Its characteristic parameters depend on the PCB material characteristics. Thus, to produce a PCB board to meet the requirements. also to the use of the corresponding process, it is more important to use the appropriate synthetic materials for PCBs. Thus, as a manufacturer of PCBs, R & D designers must first test the PCB material to confirm the characteristic parameters. before making the prototype, the PCB board is modeled by design simulation software . to fit the characteristic parameters to generate the PCB prototype. and then the prototype is verified and tested and iterated to meet the requirements before mass production. In the process of mass production, the PCBs are tested using random or full inspection as needed.
How do you make a high-frequency board?
- Impedance control requirements are more stringent. the relative line width control is very strict, and the general tolerance is about 2%.
- transmission line corners to use a 45-degree angle to reduce the return loss.
- Because of the special plate, so the adhesion of PTH sink copper is not high. usually need to use plasma processing equipment and others. first over-hole and surface roughening treatment to increase the adhesion of PTH hole copper and solder resist ink.
- For signal vias, avoid using the via processing PTH process on sensitive boards. because the process will lead to lead inductance at the vias; use buried blind holes or back drilling.
- Before doing solder resist can not grind the board. otherwise the adhesion will be very poor. only with micro-etching flux and other roughing.
- The solder resists layer can prevent the flow of solder paste. and generally use the solder dam to make the solder resist layer.
- Most of the plates are Teflon-type materials. There will be a lot of burrs with ordinary milling cutter molding. A special milling cutter is needed.
- Special circuit boards with high electromagnetic frequency, the frequency of 1GHz or more.
- In a high-frequency environment. it is best to use surface-mount SMD components to avoid the use of components with leads.
- To provide a rich grounding layer to prevent the three-dimensional electromagnetic field on the board.
- To choose a non-electrolytic nickel plating or immersion gold plating process. do not use the HASL method for plating. this plating can provide a better skin effect for high-frequency currents. that helping to reduce environmental pollution.