High-Speed PCB Technology: Principles, Design Considerations and Challenges
This article focuses on high-speed printed circuit board (PCB) technology, delving deeply into its principles, carefully examining the key points of design, and analyzing the application challenges and corresponding coping strategies. It aims to provide professional and detailed technical references for electronic engineering practitioners and help them overcome the difficulties in high-speed PCB design and application.
I. Introduction
Currently, electronic technology is advancing rapidly towards high frequency, high speed, and high integration. High-speed PCBs play a major role in fields such as computers, communications, and consumer electronics. The requirements for PCBs in high-speed signal transmission are constantly increasing, and traditional design concepts are no longer sufficient. Therefore, it is of great practical significance to conduct in-depth research on high-speed PCB technology.
II. Principle
1．Characteristics of High-Speed Signal Transmission
High-speed signals have high frequencies and short edge times. Their transmission no longer follows the laws of low-frequency signals, and the characteristics of distributed parameters become prominent. The capacitance and inductance between lines cause signal delays, reflections, and crosstalk, which seriously threaten the integrity and accuracy of signals.
2．Transmission Line Effects
(1) Characteristic Impedance: It is the "key factor" of high-speed PCBs and depends on the line geometry and dielectric constant. The calculation formulas for microstrip lines and striplines are different. Whether the impedance is matched or not is related to signal distortion and bit error rate. If the characteristic impedance is not matched, the signal will be reflected at the end and interfere with subsequent signals, resulting in signal distortion.
(2) Signal Delay and Propagation Velocity: The propagation velocity of signals is related to the dielectric constant (the formula is [formula not provided in the original text]). Different media have different propagation velocities, and the length of the transmission line also causes delays, affecting the system timing. In the design of the high-speed bus on a computer motherboard, the delay must be accurately calculated to ensure data synchronization.
III. Design Considerations
1．Material Selection
(1) Dielectric Materials: Materials with low dielectric constants and loss tangent values are required. Polytetrafluoroethylene (PTFE) has excellent performance, with a dielectric constant ranging from 2.2 to 2.6 and extremely low loss, but it is costly and difficult to process. Rogers RO4350B strikes a balance between performance and cost and is widely used.
(2) Copper Foil: Rolled copper foil has high flatness and low roughness, resulting in small scattering and loss at high frequencies, which is beneficial for high-speed signal transmission. When selecting the thickness of copper foil, both current and high-frequency characteristics should be taken into account.
2．Stacking Structure Planning
(1) Signal Layer Layout: Multilayer boards are the "standard configuration" of high-speed PCBs. Signal layers are sandwiched between ground layers and power layers to shield crosstalk. A six-layer board often adopts the "signal - ground - signal - power - ground - signal" structure, and high-speed signal layers are placed close to the ground layer to reduce inductance.
(2) Interlayer Parameters: The thickness of the interlayer dielectric is accurately calculated according to the characteristic impedance. The stability of the dielectric constant and thickness of the prepreg affects the electrical performance. In the design of high-speed differential signals, parameters are adjusted to ensure impedance matching and isolation.
3．Wiring Strategies
(1) Line Width and Spacing: At high frequencies, the skin effect causes the current to flow on the surface. The line width should be controlled within an appropriate range to reduce losses and ensure current-carrying capacity. The line spacing should prevent crosstalk, and sufficient spacing should be left between adjacent lines. For high-speed signals above 10 Gbps, the line width is approximately 0.1 - 0.15 mm, and the line spacing is 0.2 - 0.3 mm. 2. Differential Signal Wiring: Differential signals are widely used in interfaces such as USB and HDMI. They require equal lengths and constant line spacing, avoiding strong interference sources and ensuring the return path. In the design of the PCIe bus, the length matching accuracy of the differential pair is within a few mils.
4．Via Design
(1) Parasitic Parameters: Vias connect different layers but introduce parasitic parameters such as inductance and capacitance, resulting in signal distortion and delay. Irrational via design in high-speed multilayer PCBs can lead to impedance discontinuity.
(2) Optimization Strategies: Blind vias and buried vias are used to reduce penetration, the diameter and length of vias are reduced, and they are reasonably arranged to avoid a large number of vias on critical paths. High-density and high-speed PCBs use tiny blind vias to reduce the parasitic effects.
5．Power Integrity Design
(1) Impedance Control: The power integrity of high-speed PCBs is crucial. The impedance of the power distribution network (PDN) is controlled to ensure a stable power supply for chips. The power and ground layer structures are reasonably designed, and decoupling capacitors are added. On the CPU motherboard, a capacitor array is arranged beside the chips to filter power noise at different frequencies.
(2) Ground Bounce Noise Suppression: In high-speed digital circuits, ground bounce noise is generated during switch transitions, affecting the operation of chips. The thickness of the ground layer is increased, the ground layer is divided, or multi-point grounding is adopted to suppress it. In the design of high-speed FPGAs, the analog ground and digital ground are divided, and magnetic beads or 0-ohm resistors are used to connect them to reduce interference.
IV. Challenges and Responses
1．Signal Integrity Issues
(1) Reflection, Crosstalk, and Delay: High-speed signals are prone to reflections, crosstalk, and delays, threatening the operation of the system. Professional software is used for simulation and analysis in the design process to calculate the characteristic impedance and timing, and strict control of wiring parameters is implemented.
(2) Timing Analysis and Adjustment: High-speed circuits have strict timing requirements, and deviations can lead to system failures. In the design of the DDR memory interface, the signal timing is accurately calculated, and the line length is adjusted or delay lines are added to meet the requirements of the chips.
2．Electromagnetic Interference (EMI) Control
(1) Radiation Mechanism and Hazards: High-speed PCB signals have high frequencies and rapid current changes, resulting in strong electromagnetic radiation that interferes with themselves and surrounding equipment, especially in communication equipment.
(2) Suppression Measures: The layout of PCBs separates analog and digital circuits, and special treatments are applied to strong radiation sources, such as grounding shielding and adding shielded lines. A shielding layer is set at the edge, and filters are selected for filtering. In the PCB of mobile phones, the radio frequency module is strictly shielded and filtered.
3．Heat Dissipation Management
(1) Causes and Impacts of Heat Generation: High-speed PCBs generate heat due to the high power consumption of chips and signal transmission losses. If the heat cannot be dissipated, the performance of chips will decline or even be damaged. Poor heat dissipation of the motherboard of a high-performance server will lead to system instability and crashes.
(2) Heat Dissipation Solutions: Materials with good thermal conductivity, such as aluminum-based and copper-based PCBs, are selected. Space for heat dissipation is reserved in the layout, and heat dissipation vias, heat sinks, or heat pipes are added. On the PCB of a computer graphics card, a heat sink is installed on the GPU chip, and heat pipes conduct heat to the fins.
V. Conclusion
High-speed PCB technology is a "pillar" of the electronics industry. Only by thoroughly understanding its principles, making comprehensive designs, and overcoming challenges can high-performance and reliable high-speed PCBs be created. In the future, with the progress of electronic technology, high-speed PCBs will continue to be optimized in materials, designs, and processes to meet the development needs of electronic equipment and promote the take-off of the industry.