Special Base PCB

Special Base PCB

A high-frequency PCB is a printed circuit board with a frequency above 1 GHz. These PCBs have strict requirements for physical properties, precision, and technical parameters. They are commonly used in radar systems, military equipment, aerospace applications, and other fields. Designing high-frequency PCBs requires consideration of several parameters to meet specific signal requirements. These parameters include dielectric constant (Dk), dissipation factor (Df), coefficient of thermal expansion (CTE), and thermal conductivity.

Special Base PCB

Characteristics of High-frequency PCB

*Low dielectric constant (Dk): Reduces signal delay and improves frequency transmission. A lower Dk is preferred in most cases, as high Dk can cause signal transmission delays.

*Low loss factor (Df): Minimizes signal loss and improves signal transmission quality. A lower Df results in lower signal loss.

*Coefficient of thermal expansion (CTE): This should ideally match that of the copper foil to prevent separation during thermal fluctuations.

*Low water absorption: High water absorption can affect Dk and Df, especially in humid environments.

*Good thermal resistance, chemical resistance, impact resistance, and peel strength are essential.

Common Materials for HF PCB

Rogers’s RO4000 series has been in a leading position in the industry. This low-loss material is widely used in microwave and millimeter-wave frequency designs. Compared with traditional PTFE material, it is easier to use in circuit manufacturing and has stable and consistent performance. At present, 4003C and 4350B are available for HonLynn. If you need other models, please get in touch with your sales representative.

Material for HF PCBs

Material for HF PCBsRogers RO4003C(Reinforced   Hydrocarbon/Ceramic)Rogers RO4350B (Reinforced   Hydrocarbon/Ceramic)
Z   CTE(ppm/°C)4632
Surface   Resistivity(MΩ)4.2 x 1095.7 x 109
Thermal   Conductivity(W/m·K)0.710.69

The following table shows the relevant parameters of Rogers RO4003C and Rogers 4350B high-frequency materials commonly used by HonLynn.

Check our High-Frequency PCB manufacturing capabilities in the following table:

MaterialRO4003C, RO4350B, Ro3003, Ro3010, RT5880
Min. Hole   Size0.15mm
Board   Thickness0.2-3.2mm
Surface   FinishingImmersion gold, OSP, Hard Gold,Immersion   SIlver,Enepig
Finsih   Cooper0.5-13oz
Solder MaskGreen, Red, Yellow, Blue, White, Black,   Purple, Matte Black, Matte green
SilkscreenWhite, Black
Via ProcessTenting Vias,Plugged Vias,Vias not covered
TestingFly Probe Testing (Free) and A.O.I. testing
Build time7-10 days
Lead time2-3 days

High-frequency PCB Design Considerations

1. Routing Method

In high-frequency PCB routing design, one can choose between a 45-degree bend or a curved turn if bending is required. This approach reduces the chances of high-frequency signal emission and coupling.

2. Routing Length

In high-frequency PCB routing during PCB design, shorter routing lengths are preferred, and shorter distances between parallel traces are desirable. The radiation intensity of a signal is directly related to the length of the signal trace. Longer high-frequency signal traces result in increased coupling with other components. That’s why clock signals, DDR, USB, Gigabit Ethernet, and HDMI prefer shorter routing lengths.

3. Number of Vias

In high-frequency PCB routing during PCB design, fewer vias are preferred. Each via introduces approximately 0.5 pF of distributed capacitance. Reducing the number of vias can improve signal speed and reduce the likelihood of data errors. Via stubs introduce impedance discontinuities that can lead to signal integrity issues.

4. Avoiding Crosstalk

When routing high-frequency circuits, it is essential to consider the “crosstalk” introduced by closely parallel signal traces. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. As high-frequency signals are transmitted via electromagnetic waves along transmission lines, the signal lines act as antennas, emitting electromagnetic field energy around the transmission line. The undesired noise signals generated due to the mutual coupling of electromagnetic fields between signals are called crosstalk.

The parameters of PCB layer stack-up, spacing between signal lines, electrical characteristics of the driver and receiver ends, and termination methods of signal lines all impact crosstalk. Therefore, to reduce crosstalk in high-frequency signal routing, the following points should be considered:

*If the layout space allows, inserting a ground wire or plane between two lines with significant crosstalk can provide isolation and reduce crosstalk.

*When there is a time-varying electromagnetic field in the space surrounding the signal lines, if parallel distribution cannot be avoided, placing a large area of “ground” on the opposite side of the parallel signal lines can significantly reduce interference.

*Increase the spacing between adjacent signal lines and reduce the parallel length of the lines if the layout space permits. Clock lines should ideally be perpendicular to critical signal lines rather than parallel.

*If parallel traces within the same layer are nearly unavoidable in adjacent layers, the routing directions of the traces must be mutually perpendicular.

*In digital circuits, clock signals typically have fast edge transitions and significant external crosstalk. Therefore, ground lines should surround clock lines in design, and multiple ground via holes should be used to reduce distributed capacitance and crosstalk.

*it is advisable to use low-voltage differential clock signals with a grounded enclosure for high-frequency clock signals. Attention should be paid to the integrity of the ground via holes.

*Unused input terminals should not be left floating but grounded or connected to the power supply (which serves as the ground in high-frequency signal circuits) because floating lines can act as emission antennas. Grounding can suppress emissions. In practice, this method has proven to be effective in eliminating crosstalk.

5. Add High-frequency Decoupling Capacitors to the Power Supply Pins of Integrated Circuit Chips

Each integrated circuit (IC) chip should have a high-frequency decoupling capacitor near its power supply pins. Adding high-frequency decoupling capacitors to the power supply pins effectively suppresses the formation of high-frequency harmonic interference on the power supply pins.

6. Isolate the Ground of High-frequency Digital Signals and the Ground of Analog Signals

Analog ground, digital ground, and other connections to the common ground should be isolated using high-frequency choke beads or direct isolation, and appropriate single-point interconnection should be chosen. The ground potential of high-frequency digital signal lines is generally inconsistent, often resulting in a particular voltage difference between the two. Moreover, the ground of high-frequency digital signals often carries rich harmonic components of high-frequency signals. When the digital signal ground and analog signal ground are directly connected, the harmonic components of the high-frequency signals can interfere with the analog signals through ground coupling.

Therefore, in most cases, isolation is required between the ground of high-frequency digital signals and the ground of analog signals. This can be achieved by single-point interconnection at suitable locations or by using high-frequency choke beads for interconnection.

7. Avoid the Formation of Loops Resulting from Routing

Try to avoid the formation of loops in the routing of various high-frequency signals. If unavoidable, minimize the area of the loop as much as possible.

8. Adopt Fly-by Topology/daisy Chain Routing For DDR4

This wiring technique reduces reflections during high-speed data transmission. It also reduces the number and length of stubs, thereby improving signal integrity and timing of loaded signals.

9. Adopt the 20H Rule to Minimize Plane Coupling

In high-speed PCB designs, there is typically mutual coupling between the power and ground planes, causing RF energy to leak as edge magnetic flux. Moreover, RF energy (RF current) radiates along the PCB edges. To reduce this coupling effect, all power plane physical dimensions should be smaller than the adjacent ground plane dimensions by 20H.

At 10H, significant changes in magnetic flux leakage can occur. At 20H, 70% of magnetic flux leakage can be suppressed. At 100H, 98% of magnetic flux leakage can be suppressed. Although 100H provides better suppression, increasing the edge inset of the power plane and ground plane beyond 20H increases the physical distance between layers without significantly reducing the radiated current.

10. Necessary to Ensure Proper Signal Impedance Matching

During signal transmission, if there is an impedance mismatch, signal reflections occur in the transmission channel. These reflections cause signal overshoot, leading to fluctuations near the logic threshold.

The fundamental solution to eliminate reflections is to ensure proper impedance matching of the transmission signal. The more significant the difference between the load impedance and the characteristic impedance of the transmission line, the greater the reflection. Therefore, it is essential to make the transmission line’s characteristic impedance as close as possible to the load impedance. Additionally, care should be taken to avoid abrupt changes or corners in the transmission lines on the PCB. Maintaining continuous impedance at each point of the transmission line is advisable to prevent reflections between different segments of the transmission line.

Mass production lead time

lead   time
Lead Time (Wroking   days,ex-factory
10L and upTo be determined based on case

The above lead time is based on:

1. Conventional Material

2. All engineering consultations are confirmed.

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