HDI Special Feature - The Hidden Variable in Impedance Control: The Ultimate Guide to HDI Dielectric Constant (Dk) and Signal Attenuation

This article will reveal these hidden variables to help you precisely avoid pitfalls during PCB prototyping.

1. Dielectric Constant (Dk): The Dual Influence of Frequency and Resin Content

The Physical Truth Behind Dk Decrease with Frequency

Many engineers are accustomed to entering a fixed Dk value (e.g., 4.2 for FR-4) in tools like Polar Si9000. However, in the world of high frequencies, Dk is not a constant.

• Physical Phenomenon: As frequency increases, the polarization of material molecules cannot keep up with the changes in the electric field, leading to a decrease in Dk.
• Measured Data: Taking the mainstream high-frequency material Panasonic Megtron 6 (R-5775) as an example, its Dk is approximately 3.71 at 1GHz, dropping to 3.61 at 10GHz.
• Impact: If you use Dk parameters from 1GHz to design impedance for 10GHz, the actual impedance will be higher, leading to increased reflection.

The Hidden Variable: The Trap of Resin Content (RC%)

Even for laminates of the same grade/model, the Dk can differ significantly based on the Glass Style. This is due to the differing mix ratio between glass fabric (Dk~6.0) and resin (Dk~3.0).

| Glass Style | Resin Content (RC%) | Dk @ 10GHz (Megtron 7) | Application Suggestion | | :--- | :--- | :--- | :--- | | 1027 / 106 | High (~75%) | 3.15 | HDI buildup layers, easier for laser drilling | | 1078 / 1080 | Medium (~63%) | 3.35 | General signal layers | | 7628 | Low (~45%) | 3.60 | Core layers, high mechanical strength |

Practical Advice: In high-frequency HDI design, never mix different types of Prepreg (PP) within the same impedance-controlled layer. This will cause inconsistent propagation delay along a trace, resulting in severe Skew (timing skew).

2. Signal Attenuation (Loss): The Trade-off Between Df and Copper Foil Roughness

Above 10GHz, Dielectric Loss becomes the key factor determining how far a signal can travel. We've compiled key data for three mainstream high-frequency materials for your reference:

| Material Vendor | Product Series | Dk @ 10GHz | Df @ 10GHz | Loss Characteristics | Typical Application | | :--- | :--- | :--- | :--- | :--- | :--- | | Panasonic | Megtron 6 | 3.61 | 0.004 | Ultra-Low Loss | Servers, Networking | | Rogers | RO4350B | 3.48 | 0.0037 | Ceramic / RF | 5G PA, Antennas | | Isola | Tachyon 100G | 3.02 | 0.0021 | Extreme Low Loss | 100G Backplanes |

Key Decision: Copper Foil Roughness (Surface Roughness)

Even if you select a top-tier material like Tachyon 100G, pairing it with standard foil (STD Copper) will still cause signal attenuation. This happens because the "Skin Effect" forces current to travel a longer, more lossy path on the rough surface.

• Supporting Data: At 28GHz, using HVLP (Hyper Very Low Profile) copper foil can reduce conductor loss by approximately 15-20% compared to standard foil.
• Prototyping Note: Be sure to specify "HVLP Copper" or "VLP2" in your fabrication notes. This is a high cost-performance method to improve performance without changing the base laminate.

3. The Golden Rule of HDI Stack-up: Symmetry of L3 and L6

In a standard 8-layer HDI (2+N+2) design, L3 and L6 are typically designated as high-speed Stripline layers. Sandwiched between two ground planes (GND), they offer the best EMI shielding.

Impedance Consistency Challenge

• Resin Flow: During lamination, the resin from the PP flows to fill the gaps between inner-layer traces. If the copper distribution on L3 is uneven (dense on one side, sparse on the other), it leads to inconsistent dielectric thickness, causing impedance variations of 3-5 ohms.
• Solution: Implement "Dummy Copper" fill during the Layout phase to balance the copper distribution across all areas, ensuring uniform dielectric thickness after lamination.

Stub Effect

A traditional Through Hole connecting L1 to L3 leaves a dead-end section from L3 to L8, creating an antenna-like Stub. For 25Gbps signals, a stub longer than 10mil can create a fatal resonant notch. The advantage of HDI lies in using Blind Vias and Buried Vias to precisely connect L1-L2-L3, completely eliminating the Stub effect. This is the fundamental physical reason why high-speed designs must adopt HDI.

4. Glass Fiber Weave Effect

This is a commonly overlooked "phantom" issue. The glass fabric in PCBs is woven from warp and weft threads, creating "Bundles" and "Gaps."

• Problem: The Dk of a glass bundle is ~6.0, while the Dk of the resin in the gap is ~3.0. If one trace of a differential pair runs over a bundle and the other over a gap, their propagation speeds differ, causing Intra-pair Skew and severely degrading the signal eye diagram.
• Solutions:
  1. Zig-Zag Routing: Deliberately route traces at a 10-degree angle (wastes space, difficult for layout).
  2. Spread Glass Fabric: This is the optimal solution. Specify the use of 1067 / 1078 spread glass fabric (flattened fibers) to create a more uniform Dk distribution. Always specify "Use Spread Glass" when requesting prototypes.

5. Summary and Prototyping Recommendations

In high-frequency PCB design, impedance control is a precise game of material science. To ensure your design yields high-quality prototypes with Taiwanese PCB manufacturers, pay attention to the following points when outputting Gerbers and specifying requirements:

1. Precise Material Selection: Clearly specify materials like "Panasonic Megtron 6 (R-5775)" or "Rogers RO4350B," avoiding vague terms like "High Speed Material."
2. Stack-up Symmetry: Ensure L3/L6 are mirror-symmetrical and use Dummy Copper fill in empty areas.
3. Specify Processes: For applications above 10GHz, annotate "HVLP Copper" and "Spread Glass."
4. Leverage HDI: Utilize blind/buried via technology to eliminate Stubs. This is the physical foundation for ensuring signal integrity.

Taiwan possesses the world's most complete PCB supply chain. Choosing a prototyping partner who understands the technology and can provide DFM (Design for Manufacturability) advice is key to your product's success from R&D to mass production. We hope this guide serves as a strong support for your R&D journey, helping your signals travel faster, more stably, and farther.