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(1) Material Selection: From “General-Purpose” to “Customized”
The most common material for standard PCBs is FR-4 epoxy glass cloth substrate. This material is low-cost and easy to process, meeting the low-frequency, low-power requirements of everyday electronic devices. FR-4 exhibits a dielectric constant (Dk) of approximately 4.2 and dielectric loss (Df) around 0.02, delivering stable performance for signal transmission below 10Gbps. However, signal attenuation increases dramatically when data rates exceed 50Gbps—akin to sound becoming progressively muffled in noisy environments.
AI server PCBs must employ “high-frequency, high-speed specialized materials,” with core requirements being “low loss and high stability.” Currently, board manufacturers predominantly use specialty substrates like PTFE (polytetrafluoroethylene) and Rogers RO4350B. These materials maintain a dielectric constant between 2.2 and 3.0, with dielectric loss below 0.002, effectively minimizing transmission attenuation for high-speed signals. To illustrate: For a 100-meter signal transmission, FR-4 material experiences approximately 20dB attenuation, while PTFE material only suffers 5dB—equivalent to “voice clarity improved by 4 times.”
However, applying specialty materials poses significant challenges for PCB manufacturers: PTFE's high melting point of 327°C requires precise temperature control during processing to prevent substrate deformation. Additionally, PTFE exhibits weak adhesion to copper foil, necessitating specialized processes like “plasma treatment” and “chemical etching” to enhance bonding strength and prevent delamination during use. To meet diverse cost requirements, manufacturers also offer “hybrid substrate solutions”—using PTFE for critical signal layers and modified FR-4 for standard layers—striking a balance between performance and cost.
(II) Routing Design: From “Loose Layout” to “Precision Placement”
Comparing standard PCBs with AI server PCBs reveals a striking difference: standard PCBs feature sparse traces and large vias, while AI server PCBs display densely packed traces resembling “spider webs,” with vias so small they require magnification to see. This stems from significant differences in routing design.
Standard PCBs typically feature a routing density of 100-200 points per square inch, with 4-8 layers and via diameters around 0.3-0.5mm. Design priorities focus on “uninterrupted connectivity” with minimal signal interference concerns. For instance, home router PCBs may have trace spacings exceeding 0.2mm, eliminating the need for complex shielding.
In contrast, AI server PCBs feature routing densities as high as 500-800 points per square inch, with 12-30 routing layers and via diameters as small as 0.1-0.2mm—or even 0.05mm microvias. This is because AI servers integrate multiple GPUs, memory chips, and interface chips. A single PCB may contain tens of thousands or even hundreds of thousands of solder joints. “High-Density Interconnect (HDI) technology” is essential to achieve effective connections for all components within limited space.
More critically, AI server PCB routing must address “signal interference.” High-speed signals transmitted through dense traces are prone to ‘crosstalk’ (mutual interference between adjacent traces) and “reflection” (signal bounce caused by impedance discontinuities). To counter this, board manufacturers' design engineers employ three key techniques: First, “differential pair routing” pairs positive and negative signals with strictly matched trace widths and spacing to cancel interference. Second, “ground plane optimization” places a complete ground plane beneath signal layers, acting like a shield to block interference. Third, “impedance matching” uses simulation tools to precisely calculate trace width and dielectric thickness, ensuring stable impedance during signal transmission.
3) Thermal Design: From “Natural Cooling” to “Active Cooling”
Standard PCBs feature low power density and minimal heat generation, typically relying on natural air cooling without specialized thermal design. For instance, PCBs in keyboards and mice operate at temperatures only 3-5°C above ambient, requiring no additional heat management.
AI server PCBs, however, are major heat generators—a single high-end GPU consumes over 400W. A PCB integrating four GPUs can reach total power consumption exceeding 1600W, equivalent to 16 100W light bulbs operating simultaneously. Without timely heat dissipation, component temperatures on the PCB can rapidly rise. Performance throttling may occur above 85°C, and components can be destroyed outright at temperatures exceeding 125°C. Thus, thermal design represents the “life-or-death line” for AI server PCBs.
To address these thermal demands, PCB manufacturers employ multiple customized solutions:
1. Metal-based substrates: Replacing traditional FR-4 substrates with aluminum or copper bases boosts thermal conductivity from FR-4's 0.3W/(m·K) to 2-10W/(m·K), enabling rapid heat transfer to heat sinks.
2. Thickened copper foil: Increasing standard PCB copper foil thickness from 1oz (approx. 35μm) to 2-3oz, or even 5oz. Copper foil serves not only as an electrical conductor but also aids in heat dissipation;
3. Heat-dissipating vias: Densely arrange vias beneath heat-generating components, with diameters of 0.2-0.3mm and quantities reaching hundreds, forming “heat channels” to conduct heat to the PCB's reverse side;
4. Embedded heat sinks: Embed copper blocks or heat pipes within the PCB, directly contacting high-heat-generating chips to enhance heat dissipation efficiency.
(IV) Reliability Requirements: From “Daily Use” to “Extreme Environments”
Standard PCBs operate in relatively mild conditions, typically within 0-60°C, with low vibration and humidity requirements. Reliability testing mainly includes “conductivity testing” and “high/low temperature cycling (50 cycles)”, with a typical lifespan of 3-5 years.
AI server PCBs face far more complex scenarios: Cloud data center servers require 24/7 continuous operation with an 8-10 year lifespan; autonomous vehicle AI servers endure extreme temperatures (-40°C to 85°C), constant vibration, and electromagnetic interference; edge computing device PCBs may face outdoor exposure to wind and sun. Consequently, reliability standards for AI server PCBs far exceed those of ordinary PCBs.
PCB manufacturers implement stricter quality control processes when producing AI server PCBs:
• Environmental Testing: 1,000 cycles of high/low temperature cycling, 500 hours of salt spray testing, and 96 hours of humidity/heat testing to ensure stable performance under extreme conditions;
• Mechanical Testing: Vibration testing (10-2000Hz frequency range), shock testing (50G acceleration) to simulate mechanical stresses during transportation and operation;
• Electrical Testing:
Signal integrity testing (eye diagram analysis, crosstalk measurement), power integrity testing to guarantee stable high-speed signal transmission and power delivery;
• Life Cycle Testing:
Accelerated aging tests that simulate 8-10 years of operational lifespan by elevating temperature and humidity levels.