ITEQ Prepreg Selection Guide: Glass Styles, Resin Content & Dk Control

In the architectural phase of a high-speed multilayer PCB, the selection of prepreg is arguably more critical than the selection of the core itself. While the core provides the structural “anchor,” the prepreg is the malleable dielectric that dictates the final impedance, the reliability of inter-layer bonding, and the overall signal integrity of the system.

For the hardware engineer, ITEQ prepreg selection is a balancing act. You aren’t just choosing “glue”; you are selecting a precisely engineered composite of glass fabric and resin. This guide provides a deep-dive into ITEQ’s prepreg parameters—focusing on glass weave styles, resin content (RC%), and the nuances of Dielectric Constant (Dk) control.

1. The Anatomy of ITEQ Prepreg: B-Stage Engineering

Prepreg (short for “pre-impregnated”) is a fiberglass fabric that has been saturated with a resin system and partially cured to what is known as the B-Stage. In this state, the resin is dry to the touch but remains thermoplastic; when subjected to heat and pressure in a lamination press, it liquefies, flows to fill copper topography, and then undergoes a final chemical cross-linking (C-Stage) to become a rigid insulator.

The Role of Resin in Lamination

The resin serves three primary functions:

Encapsulation: It must flow into the “voids” created by etched copper traces on the adjacent core layers.

Bonding: It creates the chemical and mechanical bond between layers.

Dielectric Spacing: It defines the vertical distance between signal and reference planes, which is the primary variable in the characteristic impedance equation ($Z_0$).

2. Glass Weave Styles: The Structural Foundation

ITEQ utilizes standard IPC glass styles, but for high-speed designs, the physical geometry of the weave is just as important as the material properties. The numbering system (106, 1080, 2116, 7628) refers to the fiber count and yarn thickness.

Table 1: Common ITEQ Prepreg Glass Styles & Typical Characteristics

Glass Style	Weave Type	Fabric Thickness (mm)	Warp x Fill (per inch)	Typical RC% Range	Best Application
106	Very Fine	0.033	56 x 56	70% – 76%	Ultra-thin dielectrics / HDI
1080	Fine	0.053	60 x 47	62% – 68%	High-speed signal layers
2113	Medium	0.074	60 x 56	54% – 58%	Balanced impedance control
2116	Heavy	0.097	60 x 58	50% – 56%	General purpose / Rigid boards
7628	Coarse	0.173	44 x 31	42% – 50%	Power planes / Thick boards

The “Glass Weave Effect” and Skew Mitigation

In 112G PAM4 or high-frequency RF designs, standard weaves like 1080 can cause Intra-Pair Skew. Because glass ($Dk \approx 6.1$) and resin ($Dk \approx 3.0$) have different dielectric constants, a signal traveling over a glass bundle moves slower than one traveling over a resin-rich gap.

Engineering Tip: For high-speed layers, specify Spread Glass (e.g., style 1067 or 1078). These weaves flatten the bundles, creating a homogenous Dk environment and effectively eliminating skew.

3. Resin Content (RC%) and Dk Control

The “Effective Dk” of an ITEQ prepreg is a weighted average of the glass and the resin. Since resin has a much lower Dk than glass, increasing the Resin Content (RC%) lowers the overall Dk of the dielectric layer.

Table 2: ITEQ IT-180A Prepreg Dk/Df vs. Resin Content (at 1 GHz)

Glass Style	Resin Content (RC%)	Typical Thickness (mm)	Dk (1 GHz)	Df (1 GHz)
106	75%	0.051	3.75	0.015
1080	64%	0.076	4.05	0.016
2116	54%	0.118	4.25	0.017
7628	45%	0.191	4.45	0.018

Why Dk Precision Matters

Impedance ($Z_0$) is inversely proportional to the square root of the dielectric constant ($Dk$). A 5% shift in Dk can result in a 2.5% shift in impedance. For high-speed interfaces like PCIe Gen 5/6, where the impedance tolerance is often $\pm 5\%$, selecting a prepreg with a stable RC% is non-negotiable.

4. Pressed Thickness Calculation: The Fabricator’s Math

A common error in ITEQ prepreg selection is using the “nominal” thickness from the datasheet. The actual Pressed Thickness ($H_{final}$) is always less than the nominal thickness because resin flows into the etched areas of the copper layers.

The Copper Fill Compensation Formula

To calculate the final dielectric thickness between Layer 1 (Signal) and Layer 2 (Plane):

$$H_{final} = H_{nominal} – [T_{copper} \times (1 – \text{Copper Density \%})]$$

$T_{copper}$: The thickness of the copper foil (e.g., 35μm for 1 oz).

Copper Density %: The percentage of copper remaining on the layer after etching (typically 20-30% for signal layers, 85-95% for plane layers).

5. Specialty ITEQ Prepregs: Low-Flow and No-Flow

Standard prepreg is designed to flow significantly to ensure all voids are filled. However, in specific architectures, “flow” is the enemy.

No-Flow Prepreg: Used primarily in Rigid-Flex designs. It is engineered to bond the rigid and flex sections together without the resin “bleeding” into the flexible window, which would create a rigid “bump” and cause the flex tail to crack.

Low-Flow Prepreg: Often used for bonding heat sinks or in laser-drilled HDI applications where excessive resin squeeze-out would interfere with subsequent plating processes.

6. Sourcing and Design Resources

Selecting the right prepreg requires real-time data from both the material manufacturer and the PCB fabricator.

ITEQ Official Data Portal: Access the latest Dk/Df tables for the ITEQ PCB family, specifically for high-frequency materials like IT-968 and IT-988GSE.

IPC-4101 Slash Sheets: ITEQ materials typically comply with slash sheets /126 (High-Tg) or /131 (High-Speed).

Stackup Calculators: Use tools like Polar SI9000 or ITEQ’s online stackup tool to model the impact of different glass styles on your impedance targets.

Frequently Asked Questions (FAQs)

1. Does 1080 prepreg always have the same thickness?

No. A “1080” designation only refers to the glass weave. ITEQ may offer 1080 prepreg with 62%, 65%, or 68% resin content, each resulting in a different pressed thickness and Dk value.

2. Why is “High Resin” (HR) prepreg used for thick copper?

If you are using 2 oz or 3 oz inner-layer copper, you need more resin volume to fill the “canyons” between the traces. Using a “Standard Resin” (SR) prepreg with thick copper often leads to “resin starvation,” resulting in internal voids and delamination.

3. How does frequency affect the Dk of ITEQ prepreg?

All dielectrics exhibit “dispersion.” As frequency increases, the Dk typically drops. For example, a material might have a Dk of 4.2 at 1 GHz but drop to 3.9 at 10 GHz. Always use the Dk value at your operating frequency for impedance calculations.

4. What is the shelf life of ITEQ prepreg?

Since prepreg is in the B-Stage (partially cured), it is chemically active. It typically has a shelf life of 3 to 6 months when stored in a climate-controlled room (below 20°C and <50% humidity).

5. Can I mix different prepreg styles in one layer?

Yes, it is common to “stack” plies (e.g., two plies of 1080) to achieve a specific thickness. However, mixing different glass styles (e.g., a 1080 and a 2116) in the same dielectric opening requires careful lamination press control to avoid warpage.

Final Engineering Verdict

The success of a high-speed multilayer design rests on the precision of the ITEQ prepreg selection. By understanding that the glass style dictates the structural uniformity and the resin content dictates the electrical performance, engineers can move beyond “standard FR-4” and build boards that are both thermally robust and electrically transparent.

When in doubt, always collaborate with your fabricator’s CAM department to verify the pressed thickness—it is the single most important number in your high-speed link budget.