Introduction to high-frequency PCB circuit board losses

　　In high-frequency (usually hundreds of MHz or higher, especially GHz level) PCB design, signal loss becomes the core limiting factor that determines system performance. Loss means that signal energy is irreversibly attenuated and distorted during transmission, directly leading to insufficient system link budget, increased bit error rate, shortened communication distance, and even functional failure. Understanding and controlling losses is of utmost importance in high-frequency PCB design.

　　The total loss of high-frequency PCB (usually expressed in decibels dB/length units) is mainly composed of three parts:

　　Total loss ≈ conductor loss+dielectric loss+radiation loss

　　1、 Conductor loss

　　Conductor loss mainly refers to the loss of energy converted into thermal energy caused by the inherent resistance and skin effect of PCB wiring (usually copper), as well as surface roughness.

　　Skin effect:

　　Mechanism: As the frequency increases, the current is no longer evenly distributed across the entire cross-section of the wire, but is "squeezed" into a thin layer on the surface of the conductor, which is called the skin depth. The sharp decrease in effective conductive cross-sectional area leads to a significant increase in AC resistance.

　　Formula influence: Skin depth δ ≈ 6.6/√ f (cm) (copper, 20 ° C). For example, at 10GHz, δ is only about 0.66 μ m.

　　Design inspiration: Simply increasing line width cannot infinitely reduce high-frequency resistance. High frequency current only flows on the surface "skin layer".

　　Surface roughness:

　　Mechanism: PCB copper foil is not necessarily smooth. At the microscopic level, the surface of copper foil is rough (such as common STD, RTF, HVLP copper foils). The actual path of high-frequency current on rough surfaces is elongated and distorted, like driving on a "rugged mountain road", further increasing effective resistance and losses.

　　Impact: This is one of the main sources of conductor loss in modern high-performance PCBs, especially in the high frequency range. The rougher the surface, the greater the loss.

　　Strategies for reducing conductor losses:

　　Choose low roughness copper foil: Use Ultra Low Profile (HVLP) or Reverse Processing (RTF) copper foil, which has a smoother surface.

　　Optimize line width: Under the premise of impedance control, increase the line width appropriately to increase the effective conductive surface area.

　　Increasing copper thickness: beneficial for low-frequency components and direct current, but limited improvement for high-frequency skin effect areas.

　　Surface treatment selection: Choose processes that have less impact on surface roughness, such as immersion gold and OSP, which are better than tin spraying.

　　2、 Dielectric loss

　　Dielectric loss is the main contributor to high-frequency PCB loss (usually dominant above 1GHz), which refers to the conversion of electrical energy into thermal energy when the signal electric field passes through the PCB insulation substrate due to the asynchronous polarization of dielectric molecules and the alternating electric field (relaxation phenomenon).

　　Key parameter: Loss factor

　　Mechanism: The loss factor is an inherent characteristic of the dielectric material itself, representing its "appetite" for absorbing electromagnetic energy. The larger the loss factor, the higher the dielectric loss. The loss factor of ordinary FR-4 is about 0.02 at 1GHz, while high-frequency boards (such as Rogers and Taicony) can be as low as 0.002-0.005.

　　Impact: There is a linear relationship between dielectric loss and frequency, and the higher the frequency, the more severe the loss.

　　Design and Material Influence:

　　The selection of substrate is fundamental: using low loss high-frequency boards. This is a direct and effective method to reduce dielectric loss.

　　Field distribution influence: In microstrip lines, about 80% of the electric field is distributed in the air and 20% in the medium; In a strip line, the electric field is completely surrounded by the dielectric. Therefore, under the same material and frequency, the dielectric loss of strip lines is usually higher than that of microstrip lines.

　　Strategies for reducing dielectric loss:

　　Select low loss boards: Based on frequency and cost requirements, choose well-known brands of high-frequency materials such as Rogers (RO4000 series), Taconic RF series, Isola (FR408HR), etc.

　　Optimize the stacked structure: Whenever possible, prioritize using microstrip line structures with partially exposed electric fields in the air for extremely high frequency signals.

　　Control the consistency of dielectric constant: Select plates with stable dielectric constant (small variation with frequency/temperature) to ensure impedance and phase stability.

　　3、 Radiation loss

　　Radiation loss refers to the leakage of signal energy in the form of electromagnetic waves from the wiring without reaching the receiving end.

　　Main scene:

　　Impedance discontinuities: through holes, connectors, line width changes, corners, and other locations can trigger signal reflection and modal conversion, and some energy is radiated out.

　　Resonant structure: A specific length of wire may form a resonant antenna at a specific frequency, generating strong radiation.

　　Incomplete reference plane: When the wiring crosses gaps or slots on the reference plane, the return current path is disrupted, resulting in distorted field distribution and enhanced radiation.

　　Strategies for reducing radiation losses:

　　Maintain impedance continuity: Optimize via design (using back drills, short pile holes), avoid 90 degree right angle routing, and use gradient lines or rounded corners.

　　Provide a complete reference plane: Ensure that there is a complete and unbroken ground plane below the high-speed signal line.

　　Reasonable use of shielding: For particularly sensitive or radiation prone lines, grounding shielding via walls or shielding covers can be considered.

　　4、 Measurement and characterization of total loss: insertion loss

　　In practical engineering, we characterize the total loss of a transmission line by measuring the "insertion loss". It is the S21 parameter (dB) directly measured on the network analyzer, which includes all the loss mechanisms mentioned above.

　　Typical insertion loss curve characteristics:

　　The low-frequency band is mainly dominated by conductor loss, and the curve is relatively flat.

　　As the frequency increases, the proportion of dielectric loss increases, and the curve begins to decrease at a composite slope close to √ f (conductor) and f (dielectric).

　　At extremely high frequencies, the reflection and radiation losses caused by discontinuities can cause fluctuations and additional attenuation in the curve.

　　Summary and Design Philosophy

　　The loss control of high-frequency PCB is a systematic engineering:

　　Material first: board selection is the foundation. Invest in suitable low loss substrates and low roughness copper foils based on target frequency and performance indicators.

　　The design aims to minimize impedance discontinuity and radiation through precise stack design, impedance control, complete reference planes, and optimized vias.

　　Simulation verification: During the design phase, professional SI (Signal Integrity) simulation tools (such as HFSS, ADS) are used to predict and optimize losses, impedances, and eye diagrams.

　　Test benchmark: Compare the measured insertion loss with the simulation results, continuously calibrate the model and design specifications, and form a closed loop.

　　In short, to deal with high-frequency losses, designers need to shift their thinking from "circuit connection" to "waveguide transmission", treating PCB as a precise microwave transmission system and designing and controlling it from the perspectives of material physics and electromagnetic field theory. This is a necessary condition for implementing cutting-edge technologies such as 5G communication, millimeter wave radar, and high-speed data centers.