1. Introduction​

Thermal design for opticalmodules.With the rapid iteration of 800G and 1.6T high-speed optical interconnection systems, 224Gbps PAM4 has become the mainstream single-lane transmission standard for next-generation optical modules, silicon photonic chips, and thin-film lithium niobate modulators. Unlike traditional low-speed digital signals that are merely restricted by rise time and impedance matching, ultra-high-frequency signals above 56GHz exhibit prominent electromagnetic wave characteristics, including wavelength compression, dielectric-dependent propagation, resonant interference, and electromagnetic radiation leakage. These physical effects fundamentally determine the ultra-fine linewidth, ultra-narrow spacing, and high-precision layout rules of modern high-speed PCB and packaging substrates.​

In parallel with electrical design optimization, the continuous improvement of signal bandwidth and integration density drastically increases power consumption and heat flux density of DSP chips, EML lasers, and high-speed driving circuits. Thermal-induced wavelength drift, dielectric constant variation, and impedance deviation will severely degrade high-frequency signal integrity. Therefore, thermal management solutions represented by heat sink air cooling and liquid cold plate liquid cooling are no longer independent heat dissipation designs but essential co-design links for high-frequency signal stability. This article systematically elaborates the physical essence of high-speed signal layout scaling, summarizes key high-frequency design pain points, and clarifies the collaborative mechanism between electrical refinement design and hierarchical thermal management.

2. Electromagnetic Wave Foundation: Frequency-Dominated Wavelength Compression in Dielectrics​

All high-speed electrical signals and optical signals conform to the basic electromagnetic propagation formula λ = c/f, where wavelength is inversely proportional to signal frequency. As the signal rate rises to 224Gbps PAM4, the Nyquist fundamental frequency reaches 56GHz, and the actual operating bandwidth needs to be extended beyond 70GHz for engineering margin. Such ultra-high frequency sharply shortens the free-space wavelength, while the dielectric medium of PCB, packaging substrates, and photonic chips further compresses the propagation wavelength via refractive index modulation.​

Different from vacuum propagation, the actual wavelength in dielectric materials is λ_eff = λ₀/√ε_r, where √ε_r is defined as the RF refractive index or compression coefficient. Higher dielectric constant brings more severe wavelength compression, making the electromagnetic wave more sensitive to structural dimensional changes. This is the core reason why high-frequency signals require extremely strict dimensional control of routing, electrodes, and packaging structures.​

A typical physical feature of electromagnetic waves is the 1/4 wavelength resonant interference effect. Laser devices such as VCSEL, DFB, and EML actively adopt 1/4 wavelength grating resonant cavity superposition to amplify optical signals and enhance radiation efficiency. Conversely, high-speed electrical routing must completely avoid approaching the 1/4 wavelength size. Once the line spacing, line length, or dielectric thickness is close to the resonant dimension, electromagnetic waves will interfere and radiate outward, causing severe EMI interference and signal attenuation. The wavelength compression performance of mainstream optoelectronic materials is sorted in the table below.

3. High-Speed Anti-EMI Layout Principle: Spacing Scaling and Electromagnetic Coupling Termination​

The design logic of high-speed electrical signals is completely opposite to that of antenna and laser devices. The core goal of high-density routing is tosuppress electromagnetic radiation and confine electromagnetic fields inside the transmission structure to avoid EMI leakage. Under ultra-short wavelength conditions, any inappropriate structural size will trigger resonance and radiation, so the industry has formed a unified design rule: all routing and electrode spacing must be controlled within 1/12 ~ 1/20 of the compressed dielectric wavelength to eliminate resonant margins.​

To meet ultra-small wavelength dimensional constraints, high-speed layout requires synchronous reduction of three core parameters: signal line width, differential pair line spacing, and dielectric thickness between signal lines and GND reference plane. Reducing the dielectric thickness essentially shortens the return path of high-frequency current, strengthens the electric field and magnetic field coupling between the signal line and the ground plane, and realizes charge termination and field confinement. Compared with microstrip lines, stripline structures have fully symmetric dielectric layers, more uniform field distribution, and better anti-radiation performance, which are more suitable for 70GHz+ ultra-high-frequency scenarios.​

With the upgrading from 112Gbps to 224Gbps PAM4, the engineering bandwidth margin and layout precision requirements are significantly improved. The following table summarizes the industry-standard layout spacing specifications for mainstream high-speed signal rates.

4. Multi-Dimensional High-Frequency Design Challenges and Systematic Optimization Solutions

​ Ultra-fine linewidth and narrow spacing layout are only the basic hardware conditions for high-speed signal transmission. High-frequency and short-wavelength operating conditions bring a series of derivative design challenges involving impedance continuity, dielectric uniformity, copper foil loss, and parasitic resonance. These factors will cause signal reflection, delay skew, scattering loss, and noise amplification, seriously reducing the system signal-to-noise ratio. This chapter systematically sorts out all key pain points and targeted optimization schemes.