HDI & High Tg PCB Solutions for High-Performance Electronic Design

In the race to develop electronics with higher performance, smaller form factors, and greater reliability, High-Density Interconnect (HDI) technology and high Glass Transition Temperature (Tg) materials have become indispensable core tools for engineers. These two technologies address the ultimate challenges of "density" and "thermal stability," respectively, and together define the physical limits of next-generation high-end electronic products. This blog aims to provide an in-depth analysis of the technical aspects, design trade-offs, and cutting-edge applications of HDI and high Tg PCB solutions, offering hardware engineers a precise reference guide from theory to practice.

Design and Process for High-Density Interconnection

High-Density Interconnect (HDI) printed circuit boards are the core enablers of miniaturization and high performance in modern electronics. Their essence lies in utilizing precision processes to maximize wiring density and connection efficiency within a limited area, thereby meeting the stringent demands of smartphones, high-speed communication equipment, and AI hardware for high-frequency, high-speed signal transmission.

The Core of HDI: The Interconnection Revolution Beyond Traditional Vias

Unlike traditional multi-layer PCBs that primarily use "through-holes" penetrating all layers for interconnection, the core of HDI technology lies in the application of micro-blind vias, buried vias, and their stacking techniques. This "three-dimensional" routing strategy allows signals to travel between different layers via the shortest paths, significantly reducing signal delay and crosstalk, and enhancing overall electrical performance.

• Blind Vias: Connect the outer layer to an inner layer without penetrating the entire board thickness, providing valuable fan-out space for high-density components like BGAs on the surface.
• Buried Vias: Completely hidden between internal layers, not extending to the surface, they are key to achieving efficient interconnection within inner layers and further increasing routing density.
• Stacked and Staggered Vias: These are hallmarks of advanced HDI. Through multiple lamination and laser drilling cycles, vias on different layers are precisely stacked or staggered, creating more complex and higher-density interconnection networks. For example, high-layer-count HDI boards used in AI servers can have up to 104 layers, with costs several times higher than traditional designs.

HDI Design Rules and Process Challenges

Adopting HDI design means adhering to a more stringent set of physical rules. These rules directly determine design feasibility and final cost.

This precise dimensional control relies on advanced processes like laser direct drilling, vacuum lamination, and precision alignment. Consequently, the inherent attributes of HDI boards are complex manufacturing processes, higher costs, and longer lead times. Designers must find the optimal balance between performance, size, and cost.

The Stability Cornerstone for High Thermal Loads

As power density in electronic devices continues to rise, so does the heat generated by PCBs during operation. When temperatures exceed the substrate material's limit, a series of reliability issues arise. Here, the Glass Transition Temperature (Tg) becomes the key metric for evaluating a PCB substrate material's heat resistance.

Understanding Tg: The Critical Point from Rigidity to "Softening"

Tg is the temperature threshold at which a resin-based material transitions from a hard, glassy physical state to a soft, rubbery state. When a PCB's operating temperature approaches or exceeds its Tg, the material undergoes:

• Sharp decline in mechanical properties: The substrate softens, potentially leading to warpage and deformation.
• Dramatic increase in Coefficient of Thermal Expansion (CTE): Excessive expansion in the Z-axis direction subjects plated through-hole (PTH) walls to immense stress, ultimately leading to barrel cracking (CAF failure).
• Deterioration of electrical properties: Dielectric constant (Dk) and dissipation factor (Df) become unstable, affecting signal integrity.

Therefore, a rule of thumb is: the PCB's long-term operating temperature should be at least 20-25°C below its Tg value. For applications with ambient operating temperatures above 130°C, high Tg materials are essential.

Performance Advantages and Application Scenarios of High Tg Materials

High Tg PCBs typically refer to materials with a Tg value above 170°C (measured by DSC method). Compared to standard FR-4 (Tg ~130-140°C), high Tg materials excel in the following areas:

• Superior Thermal Stability and Heat Resistance: Maintains shape and dimensional stability during lead-free soldering (peak temperatures can reach 260°C+) and in high-temperature operating environments, preventing delamination and blistering.
• Higher Mechanical Strength and Dimensional Stability: Provides robust support for high-density mounted components and better matches the CTE of components like chips, reducing thermal stress.
• Better Chemical Resistance and Moisture Protection: Low water absorption reduces the risk of performance degradation in humid environments, enhancing long-term reliability.
• Stable Electrical Properties: Dk and Df remain stable at high temperatures, which is crucial for high-speed digital and high-frequency RF circuits.

Based on these advantages, high Tg PCBs are the ideal choice for the following applications:

• Multilayer and HDI Boards: The lamination process itself generates high heat, and internal heat buildup is more severe.
• High-Power Devices: Such as power modules, motor drives, and industrial control equipment, where I²R losses generate sustained high heat.
• Automotive and Aerospace Electronics: Must withstand high under-hood temperatures or harsh environmental temperature cycling.
• Next-Generation Computing and Communication Hardware: Such as AI servers and 5G base stations, where high-power chips place extreme demands on thermal management.

Integrated Design for AI and High-Frequency Applications

In today's most advanced electronic systems, particularly AI servers, high-speed switches, and high-frequency communication modules, the demand for high density and high thermal stability coexists. This makes the combination of HDI technology and high Tg materials transition from "optional" to "mandatory."

Integration Challenges: Coordinating Material and Process Upgrades

When using high Tg materials in an HDI structure, designers face a dual challenge:

1. Material Processability: High Tg resin systems often require higher curing temperatures and longer lamination times, which must be precisely coordinated with the complex, multi-cycle lamination process inherent to HDI manufacturing.

2. Signal Integrity: High Tg materials (e.g., Shengyi S1000-2M, Tg≥170°C) often possess superior dielectric properties (low Dk/Df), which is crucial for the high-speed signal transmission that HDI serves.

Taking the NVIDIA Rubin AI platform as an example, its design represents the pinnacle of this integration. Key interconnect components (such as the Switch Tray) employ a 24-layer HDI design and are fully upgraded to M8U-grade low dielectric constant (Low-Dk2), low-loss materials with extremely low-profile HVLP4 copper foil. This combination of "high-density interconnect structure + top-tier high-frequency high Tg material" aims to minimize signal loss and delay to meet the massive data exchange demands between GPUs.

Future-Oriented Design Strategies and Supply Chain Considerations

When designing next-generation high-performance PCBs, engineers should adopt a systematic approach:

• Early Collaborative Design (DFM): At the conceptual stage, involve PCB designers, thermal management engineers, signal integrity engineers, and crucially, the PCB manufacturer in discussions. Manufacturers can provide key advice on material selection (e.g., well-known high Tg grades like IS410, IT-180A), stack-up structure, and pad design to avoid manufacturability issues.
• Simulation-Driven Design: Utilize advanced electromagnetic simulation (SI/PI) and thermal simulation (CFD) tools to evaluate the performance and temperature rise of different material, stack-up, and layout scenarios during the early design phase. This replaces traditional empirical estimation, significantly reducing trial-and-error costs and cycle times.
• Monitor the Upstream Supply Chain: The performance ceiling of high-performance PCBs is determined by upstream materials. For instance, the supply of Q-glass or Low-Dk2 fiberglass cloth for high-speed materials, and HVLP4 copper foil for reducing skin effect, may become tight and hold stronger bargaining power. Understanding these dynamics helps in formulating long-term material strategies.

HDI and high Tg PCB solutions are the two pillars supporting modern electronic engineering in tackling the "Three Highs" challenge (high density, high frequency, high power). They are not isolated technologies but are deeply integrated in advanced applications, collectively defining the physical and performance boundaries of circuit boards. For engineers, the key to success lies in a profound understanding of their underlying principles, mastery of precise design rules, and deep collaboration with process experts and the supply chain from the earliest stages of product development. Only in this way can these advanced technologies be translated into truly reliable and competitive end products.

One-Stop HDI PCB Manufacturer and Its PCB Via Filing Capabilities

If you're looking for turnkey HDI electronics manufacturing services (EMS) from hardware development to PCBA fabrication and box-build assembly, you can work with the one-stop HDI PCBA manufacturer PCBONLINE.

Founded in 1999, PCBONLINE has R&D capabilities for HDI projects and EMS manufacturing capabilities, including via filling for stacked vias. It provides 4-to-64-layer HDI PCB fabrication, assembly, and PCBA box-build assembly. You can order various HDI PCBs from PCBONLINE, such as FR4, polyimide (flexible PCB), polyimide + FR4 (rigid-flex PCB), and PTFE/Rogers (high-frequency PCB).

Here'e the PCB via filing capabilities at PCBONLINEL:

• Micriavia filling with copper: laser via size 0.1-0.125mm, priority 0.1mm
• Finished hole size for via-in-pad filling with resin: 0.1-0.9mm (drill size 0.15-1.0mm), 0.3-0.55mm normal (drill size 0.4-0.65mm)
• Max aspect ratio for via-in-pad filling with resin PCB - 12: 1
• Min resin plugged PCB thickness: 0.2mm
• Max via-filling ith resin PCB thickness: 3.2mm
• Making different hole sizes with via filling in one board: Yes
• Via filling with copper/silver: Yes

If you need HDI PCBAs or any other PCBAs requiring via filling, please send your email to PCBONLINE at info@pcbonline.com. We will provide one-on-one engineering support to you.

Conclusion

Via filling is used for creating stacked vias in HDI PCB fabrication, BGA/CSP/QFN IC packaging, and filling PCB via-in-pad with resin during multilayer PCB fabrication. If you need one-stop electronics manufacturing for your HDI PCBA project, contact the one-stop advanced PCB manufacturer PCBONLINE for high-quality PCBA and box-build solutions tailored to your project's needs.