The green board filled with lines and components in smartphones, computer motherboards, and even AI servers is known as a Printed Circuit Board (PCB). It is not just a carrier for components but also the “nerve center” and “skeleton” of the entire electronic system, responsible for signal transmission, power distribution, and performance assurance.
In cutting-edge fields such as NVIDIA GPUs, 5G communications, or high-speed data centers, the performance of the PCB directly determines the speed and stability of the system. A small PCB hides a series of complex and critical technical parameters, which together guide the PCB towards high-speed, high-frequency, and high-density development.
1. Core Identity of PCB: Three Basic Parameters
These three parameters are the most fundamental indicators for assessing the design and manufacturing difficulty of a PCB.
1. Layer Count
Professional Definition: A PCB is made up of multiple layers of insulating materials and conductive copper foil stacked and bonded together. The layer count refers to the total number of conductive layers (signal layers, power layers, ground layers).
Common Understanding: It is like a multi-story building. The more layers there are, the more space there is for “wiring,” allowing for more complex circuits and functions.
Influencing Factors:
Low-layer boards (2-6 layers): Commonly found in ordinary consumer electronics and simple home appliances.
High-layer boards (8-20 layers): Commonly found in high-end PC motherboards, mid-range servers, and network switches.
Ultra-high-layer boards (over 20 layers): Commonly found in AI accelerator cards, supercomputer backplanes, and core routers, representing the highest technical barriers.
2. Trace Width/Spacing
Professional Definition: Refers to the width of the PCB traces (copper lines) and the minimum distance between the traces.
Common Understanding: It is equivalent to the “road” width and “lane” spacing on the PCB. The narrower the road and the smaller the spacing, the more traces can fit in a limited space.
Influencing Factors: With the explosive growth of chip pin counts, PCBs must achieve extremely small trace widths and spacings (such as 30μm or even smaller), which requires higher manufacturing precision and more complex exposure/etching processes.
3. Via Size
Professional Definition: Refers to the metallized small holes (vias) used to connect traces on different layers.
Common Understanding: It is like the elevators or stairs between different floors of the PCB “building”.
Technical Progress:
Through Via: A hole that goes through all layers, easy to manufacture.
Blind Via: Connects only the outer layer and inner layer.
Buried Via: Connects only inner layers to inner layers.
Microvia: Small holes (diameter less than 150μm) formed by laser drilling, crucial for achieving high-density interconnect (HDI) technology. The more layers and higher the density, the stricter the requirements for microvia technology.
2. Key Parameters Determining Performance: “High Speed” and “High Frequency”
In the era of 5G, AI, and data centers, signal transmission rates can reach dozens or hundreds of Gbps. The PCB is no longer a simple connector; it must ensure the integrity of the signal during transmission on the board. This is determined by two core material parameters.
1. Dielectric Constant (Dk or Er)
Professional Definition: Measures the ability of insulating materials to store electrical energy, indicating the extent to which electric field energy is consumed in the material during signal transmission.
Common Understanding: The speed of signal transmission on the PCB is like the speed of cars on different road surfaces.
The lower the Dk value (for example, high-speed materials range from 2.2 to 3.5), the faster the signal can be transmitted within the PCB, thereby reducing signal transmission delay.
Impact: Especially in high-frequency scenarios, if the Dk of the PCB material is inconsistent or too high, it can lead to inconsistent signal arrival times, resulting in timing errors.
2. Dielectric Loss (Df or Loss Tangent)
Professional Definition: Measures the extent of energy loss as the signal passes through insulating materials.
Common Understanding: The energy consumption of the signal while “running” on the PCB is akin to the attenuation of light signals during fiber transmission.
The lower the Df value (for example, ultra-low loss materials have a Df below 0.002), the smaller the energy attenuation of the signal, allowing for longer and more stable transmission.
Impact: Traditional FR-4 materials (Df around 0.02) are sufficient at low speeds, but when supporting rates of 112 Gbps or higher in AI servers, the signal loss is significant. Therefore, top products like NVIDIA’s GB200/GB300 must use ultra-low loss materials, even employing top materials like M9 quartz fabric to control Df at the level of 0.001.
3. Performance Killers Revealed: “Hidden Factors” Affecting PCB Performance
The actual performance of a PCB is often not determined by a single parameter but is influenced by multiple manufacturing and environmental factors.
1. Temperature and Thermal Management
GPUs and AI accelerator cards generate significant heat during operation (power consumption can reach 1000W). Temperature changes can severely affect the performance of PCB materials.
Coefficient of Thermal Expansion (CTE): When the PCB heats up, the material expands. If the CTE difference between the material and copper foil is too large, it can cause stress between layers, and in severe cases, lead to the fracture of vias, resulting in circuit failure.
Glass Transition Temperature (Tg): The critical temperature at which PCB materials transition from a hard state to a soft state. Once the operating temperature approaches or exceeds Tg, the mechanical strength and electrical performance of the material deteriorate sharply. High-end PCBs must use high Tg materials (above 180°C).
2. Impedance Control and Signal Integrity (SI)
Every trace on a high-speed PCB must be treated as a transmission line.
Impedance Control: Engineers must strictly control the characteristic impedance of signal lines (usually 50Ω or 100Ω) to ensure that signals are transmitted without reflection between the source, PCB, and receiving end.
Signal Integrity: Any discontinuity in the lines (such as right-angle bends, poorly designed vias, solder joints) can cause impedance changes, leading to signal reflection and distortion. This is a signal integrity (SI) issue and is the primary cause of performance degradation in high-speed systems.
3. Manufacturing Precision and Consistency
Even with a perfect design, small manufacturing deviations can have fatal impacts.
Alignment Precision: The alignment precision of the traces during multilayer PCB lamination is crucial. If the alignment is inaccurate, vias cannot connect traces accurately, leading to open or short circuits.
Copper Thickness Uniformity: The thickness of the copper traces after etching must be uniform; otherwise, it will affect the impedance and current-carrying capacity of the traces. This relies on advanced exposure, etching, and plating equipment (such as DI/AOI equipment).
A small PCB carries the ultimate pursuit of speed, stability, and reliability in electronic systems. From ultra-low loss M9 substrates to high-density HDI microvias, to strict impedance control, every improvement in parameters represents a significant investment in technology and capital.