448 Gbps High-Speed Interconnect Design:

A Systematic Methodology from Material Modeling to First Tape-out Success; Comprehensive Analysis of Predictable Design for High-Speed PCB Interconnects: Engineering Practices and Verification Processes in the 28-448 Gbps Era

Introduction: Why the "predictability" of interconnection has become a core competitiveness?

Driven by modern data centers, high-performance computing, and AI accelerators, SerDes channel rates have surged from 28 Gbps to 112 Gbps (PAM4) and are now advancing toward 224 Gbps and even 448 Gbps. At such speeds, even the slightest design deviation in PCB and package interconnects—whether it's an error in dielectric loss, a deviation in conductor roughness, or an imprecise via modeling—can cause the entire channel to fail to meet eye diagram margin requirements.

"First-Pass Success" is no longer a mere aspiration but a stringent requirement for engineering cost-effectiveness. The cycle time and cost of a single PCB prototype run can reach several weeks and tens of thousands of dollars in multi-layer HDI board designs. Significant discrepancies between simulations and actual measurements would force design teams into an iterative loop of revisions.

This paper systematically elaborates on a comprehensive methodology for predictable high-speed interconnect design, covering core aspects such as accurate material model identification, via localized modeling, manufacturing deviation sensitivity control, and benchmark platform validation processes, providing in-depth reference for engineers engaged in high-speed signal integrity design.

1. Core Challenges in High-Speed Interconnect Design

1.1 Margin Compression Due to Rate Transition

From the above figure, it can be clearly seen that as the channel rate increases from 10 Gbps to 448 Gbps, the available design margin is rapidly compressed. In the era of 28 Gbps NRZ, the fault tolerance space of the channel is relatively spacious; But in the era of 224 Gbps PAM4, the Nyquist frequency has reached 56 GHz, and the insertion loss budget of the entire channel may have less than 1 dB of margin. This means that if the simulation accuracy deviates by more than 0.5 dB/inch, the design conclusion may change from "pass" to "fail".

1.2 Coupling of multiple physical effects

At a speed of 448 Gbps, engineers must simultaneously address the following dimensions with precision:

Dielectric dispersion and loss: The frequency dependent characteristics of the dielectric constant (Dk) and loss tangent (Df) of PCB laminates must be accurately modeled

Surface roughness effect of conductors: The influence of copper foil surface microstructure on high-frequency loss becomes extremely significant in the millimeter wave frequency band

Discontinuity of via: Impedance discontinuity and resonance effects of signal via, anti solder pad, stub and other structures in a wide frequency band

Manufacturing process deviations: etching tolerances, interlayer alignment deviations, medium thickness fluctuations, etc

2、 Accurate identification of medium and conductor material models

2.1 Medium Model: From Data Manual to Actual Measurement Identification

The Dk/Df values provided by PCB laminate manufacturers are usually only nominal values at specific frequency points (such as 10 GHz), while actual high-speed channel simulations require a wideband causal medium model.

The widely used broadband media models in current engineering practice include:

Djordjevic Sarkar model: an empirical model based on unipolar Debye relaxation, fitting wideband Dk/Df curves with a small number of parameters

Multipole Debye model: achieving higher precision frequency fitting through multiple relaxation poles

Wideband Debye Model: A Wide Band Causal Model Widely Implemented in Commercial EDA Tools

Key practical points: The nominal Dk/Df values of material manufacturers are often obtained under "bare board" testing conditions, which may differ from the actual laminating temperature, pressure, resin flow, and other process conditions experienced by the materials in multi-layer board stacking. Therefore, reverse identification of medium parameters through the S-parameters of actual test boards is the core step to improve simulation accuracy.

The typical approach to identifying the process is to design transmission lines of different lengths (such as 3 inches and 6 inches) on the test board, measure their S-parameters, extract the propagation constant (γ), and then fit the equivalent Dk (f) and Df (f) curves.

2.2 Conductor roughness model: the influence of microstructure on macroscopic properties

The mechanism of the influence of copper foil surface roughness on signal attenuation can be intuitively understood as follows: high-frequency current is constrained by the skin effect to flow in the extremely thin area of the conductor surface, and the rough surface forces the current path to become longer, effectively increasing the resistance loss.

The commonly used roughness models in the industry currently include:

Hammerstad Jensen model: the earliest widely used classical model that describes the increase in loss caused by roughness through a frequency dependent correction factor

Huray's "Snowball" Model: approximates the microstructure of copper foil surface as a collection of hemispherical protrusions, calculates additional losses based on electromagnetic scattering theory, and provides a clearer physical meaning

Revise Huray model: Introduce more parameters based on the original Huray model to adapt to different types of copper foils (standard electrolytic copper, reverse processed copper, low roughness copper, etc.)

From the trend chart, it can be seen that at 56 GHz (corresponding to the Nyquist frequency of 224 Gbps PAM4), the loss difference between standard electrolytic copper and ultra-low roughness copper can reach over 0.7 dB/inch. For a 10 inch backplane channel, this means a difference of 7 dB - which is almost a key factor determining the success or failure of the design in a tight loss budget.

2.3 GMS Parameters: A New Tool for Material Model Identification

The Generalized Modal S-parameters (GMS) mentioned in this article are an advanced characterization method that has received widespread attention in the field of signal integrity in recent years. Unlike traditional S parameters, GMS parameters can more effectively separate from measurement data:

Intrinsic propagation characteristics of transmission lines (dielectric and conductor effects)

The effect of discontinuous structures such as connectors and vias

Embedding residual errors in test fixtures

The core value of this method lies in providing cleaner input data for the identification of medium parameters and roughness parameters, avoiding the contamination of transmission line characteristics extraction caused by via and connector discontinuities.

Specifically:

The through-hole diameter, solder pad diameter, and ground via spacing collectively determine the characteristic impedance of the through-hole segment

By adjusting these geometric parameters, the impedance of the via section can be made as close as possible to the target impedance of the transmission line (such as 85 Ω or 100 Ω differential)

The arrangement of grounding vias determines the cutoff frequency and higher-order mode suppression capability of the waveguide

4、 Low sensitivity layout technology

4.1 Statistical characteristics of manufacturing deviations

The inevitable deviations in PCB manufacturing process include:

Deviation type/ typical range/  impact on performance

Dielectric thickness/  ± 10%~15%/  Impedance offset, coupling variation

Copper foil etching width/  ± 0.5~1.0 mil/  impedance offset

Drill hole position/  ± 2-3 mil/  impedance discontinuity

Interlayer alignment/  ± 2-3 mil/  differential asymmetry

Copper foil thickness/  ± 10%/  loss variation

4.2 Low sensitivity design principles

The core concept of "Low Sensitivity Layout Techniques" is to actively select topology and geometric parameters that are insensitive to manufacturing deviations during the design phase, rather than relying on tightening process tolerances afterwards to ensure performance.

The key strategies include:

Choose line width/spacing combinations that are insensitive to etching deviations: for example, under certain stacking conditions, there are certain line width/spacing combination points near which the impedance has the lowest sensitivity to line width changes

Reasonably plan the routing bends and fan out methods around the via: avoid forming tight coupling sections with narrow spacing at the exit of the via

Symmetry guarantee of differential pairs: Ensure that the two lines of the differential pair maintain as symmetrical a path length and environment as possible in the via conversion area

5、 Sink or Swim "benchmark verification platform

5.1 Design concept of benchmark platform

The "Sink or Swim" verification process proposed in this article is a highly valuable concept in engineering. The core idea is to validate the accuracy of the entire simulation methodology through a specially designed benchmark testing platform before formal product design.

This benchmark platform typically includes:

Transmission line segments of different lengths: used to identify medium parameters and roughness parameters

Various via configurations: single via, differential via, different solder pad sizes, different residual pile lengths, etc

Typical Interconnection Topology: Simulating Possible Routing Paths and Transition Structures in Actual Products

Repeatable measurement detection points: facilitating one-to-one comparison with simulation results

6、 Design process oriented towards practical constraints

6.1 Balancing in Engineering Reality

In actual product development, engineers must weigh multiple conflicting constraints:

Performance vs manufacturability: Finer linewidth can increase wiring density, but it will increase manufacturing difficulty and yield risk

Simulation accuracy vs computational cost: Although full channel 3D full wave simulation has high accuracy, the computation time may take several days

Design cycle vs design quality: Market pressure requires rapid iteration, but insufficient validation may lead to chip failure

6.2 Layered simulation strategy

In response to the above contradictions, a layered simulation strategy is usually adopted in practice:

Simulation level tool types are suitable for typical accuracy scenarios

2D cross-section simulation, 2D field solver, high transmission line impedance/loss (when material model is accurate)

2.5D Simulation Plane EM Solver Pad/Wire Transition Medium High

3D Localized Simulation 3D Full Wave Solver via/connector height

Cascade model S parameter cascade complete channel depends on the accuracy of each segment

The final validation of the full channel 3D simulation 3D full wave solver is the highest (but computationally expensive)

Key design suggestion: Use 2D and cascaded models for rapid iteration and parameter scanning in the early stages; Perform final validation using 3D full wave simulation before freezing the design. This "coarse screening fine screening" strategy can achieve a good balance between design cycle and design quality.

7、 Industry technology trends and market insights

7.1 Technological layout in the era of 224G/448G

Several key trends in the current high-speed interconnection field deserve close attention:

Low loss materials continue to evolve: as the speed advances towards 224G/448G, higher requirements are placed on PCB laminate materials. Ultra low loss materials such as Megtron 7 and Tachyon 100G are becoming standard in high-speed backplate designs. Domestic material manufacturers such as Shengyi Technology and South Asia New Materials are also actively expanding into the field of high-speed materials, but there is still a gap between them and international leading manufacturers in terms of performance consistency and frequency characteristic characterization of ultra-low loss materials.

Advanced Packaging Interconnection: With the popularity of Chiplet architecture, the battlefield of signal integrity is extending from PCB board level to package level. The interconnect design in Silicon Interposer and Organic Substrate also requires the precise modeling methodology described in this article.

Intelligentization of EDA tools: AI/ML assisted simulation optimization and parameter identification are becoming a new direction for EDA tools, which is expected to significantly reduce the engineering threshold for material model identification and via optimization.

7.2 Reference value for domestic industries

Material characterization capability building: Domestic high-speed PCB design teams should establish independent material parameter identification capabilities and cannot rely solely on supplier data. Suggest investing in the construction of a material characterization laboratory that includes high-frequency testing boards, vector network analyzers, and specialized identification software.

Standardization of simulation methodology: hardware teams of large communication equipment enterprises and Internet enterprises should establish a standardized process of simulation methodology based on the benchmark verification platform, similar to the "Sink or Swim" verification concept proposed in this paper, to ensure the simulation credibility of each generation of products.

Talent cultivation direction: The talent reserve in the field of signal integrity in China is still insufficient, especially the lack of composite engineers who are proficient in electromagnetic field theory, materials science, and PCB technology. Universities and enterprises should strengthen talent cultivation in this interdisciplinary field.

8、 Summary of Key Technologies Know How

For engineering and technical personnel engaged in high-speed interconnect design, the following are the most practical technical points:

Key points of material modeling practice

Never directly use the Dk/Df values in the data manual for simulation - material parameters applicable to specific stacking structures must be identified through actual measurements

The selection and parameter settings of roughness models are crucial for predicting losses in frequency bands above 20 GHz - it is recommended to use the Huray model

Material parameter identification requires a "clean" testing structure - the transmission line length difference method is currently the most reliable solution

Key points of through-hole design practice

The control of through hole stubs is the primary task in the frequency band above 56 GHz - back drilling or HDI blind holes are necessary means

The number and distribution of grounding vias in differential vias directly affect mode conversion (differential to common mode) - at least 4 symmetrically distributed grounding vias are required

The position definition of ports in 3D simulation must be consistent with the cascading model - otherwise, the cascading results will exhibit systematic bias

Key points of verification process practice

Step by step verification and layer by layer troubleshooting are key methods for efficiently diagnosing the root cause of simulation bias

The embedding quality of measuring fixtures directly affects the comparative conclusion - it is recommended to use TRL or multiline TRL calibration methods

The Monte Carlo analysis of manufacturing deviations should be included in the design approval process, rather than just simulating nominal values

Conclusion

From 28 Gbps to 448 Gbps, the core challenge of high-speed interconnect design has evolved from 'can it be achieved' to 'can it be accurately predicted'. The methodology systematically outlined in this article - from precise identification of material models, waveguide via design, low sensitivity layout, to multi round verification processes based on benchmark platforms - represents the current level of engineering practice in interconnect predictability design in the industry.

It is worth noting that this methodology does not rely on a single 'magic tool', but rather a complete engineering chain that runs through materials science, electromagnetic field theory, PCB technology, and system level validation. In the era of 448 Gbps, any weakness in any link will become a bottleneck in the entire channel design.

For domestic related industries, the most practical reference does not lie in a specific simulation technique, but in the systematic investment in establishing a "simulation measurement identification verification" closed-loop capability. Whether in the design of communication devices, server motherboards, or AI acceleration cards, teams that master this methodology will gain significant competitive advantages in product iteration speed and first-time chip success rate. With the continuous breakthroughs in high-speed chips and advanced packaging in China, the synchronous improvement of interconnect design capabilities is no longer an option, but a necessary condition.