What Does "M Number" Mean in PCB Materials?

July 7, 2026

When selecting materials for high-speed boards, you’ve likely heard phrases like: "This board can get by with M4," "That backplane should be at least M6," "Do we need M7 for 112G PAM4?" or even some projects that immediately ask, "Can we use M8?"

What exactly does the "M number" in PCB materials mean? Is it a material grade? An IPC standard? Or just a naming convention cooked up by board shops?

1. The Origin of the "M Number"

The terms M4, M6, and M7 commonly used in hardware circles mostly come from Panasonic's MEGTRON series of laminates. For instance, MEGTRON 4 is often abbreviated as M4; MEGTRON 6 as M6; and MEGTRON 7 as M7. Over time, "M number" has gradually become a colloquial way to describe a grade of high-speed, low-loss material. Other manufacturers like ITEQ and TUC also have similar tiered product classifications. Nowadays, when someone says "use M6 material," they might not necessarily mean Panasonic’s original stock, but rather "use a material with a loss level equivalent to M6."

2. What Are the Main Differences Between M-Number Materials?

The primary difference between M-number materials lies in their high-frequency loss performance. Key parameters include:

• Dk (Dielectric Constant): Affects impedance, trace width, and propagation speed. With the same stack-up, different Dk values will result in different trace widths and impedance outcomes.

• Df (Dissipation Factor), also known as Loss Tangent: Closely related to insertion loss in high-speed signals. The higher the data rate and the longer the trace, the more significant the impact of Df.

• Copper Foil Roughness: Many focus solely on Df, but copper loss is also critical at high speeds. There are significant loss differences between standard copper, VLP (Very Low Profile), and HVLP (Hyper Very Low Profile) foils.

• Glass Weave: Standard glass, low-Dk glass, and open-weave fabrics affect Dk uniformity and the glass-weave effect. This cannot be ignored for long differential pairs operating at very high speeds.

• Reliability Parameters (Tg, Td, CTE, T288): These relate to high-layer-count boards, lead-free reflow soldering, thermal reliability, and processing stability.

Therefore, the "M number" is not a single specification but a comprehensive result of an entire material system.

3. How Are Mainstream M-Number Materials Generally Categorized?

• M4: Can be considered the entry-to-mid-tier level for low-loss materials. It offers better loss performance than standard FR-4 at a relatively acceptable cost. This grade is often considered for applications like 10G Ethernet, some PCIe Gen3/Gen4 designs, USB3.x, and various mid-to-high-speed industrial control and communication boards.

• M6: This is a very common high-speed material grade. It is frequently seen in 25G, 28G, 56G PAM4, PCIe Gen4/Gen5 designs, servers, switches, and optical module boards. Its key feature is significantly lower dielectric loss compared to standard FR-4 and M4, making it a workhorse for many high-speed projects.

• M7: This steps up to address scenarios with tighter loss budgets. Examples include 112G PAM4, 400G/800G switching equipment, high-speed backplanes, high-end servers, and AI accelerator card designs. In these cases, the link budget is very tight; excessive loss from any single component (connector, via, trace, copper foil, or material) can strain the equalization capabilities at the receiver.

• M8, M9, etc.: These represent the high-end and leading-edge application tier. They are not casually used for ordinary projects but are intended for next-generation switching, 224G-class channels, and very long backplane links. Selecting materials at this level is not a matter of guesswork; it requires thorough SI simulation, board shop capability assessment, connector modeling, via optimization, and rigorous testing verification.

• Standard FR-4: It's not that FR-4 cannot be used for high-speed designs. For many short links or designs with ample margin, FR-4 can perform perfectly well. Engineering is about balancing performance with cost.

4. How to Choose Between Different M Numbers?

Selecting the right material starts with the link budget.

• For MCU small systems, general power boards, low-speed control boards, relay boards, or small sensor boards: Standard FR-4 is sufficient.

• For USB3.0, Gigabit/2.5G Ethernet, short-distance PCIe, or short-range high-speed ADC/DAC interfaces: Start by evaluating trace length, layer count, and loss budget. If traces are short and margins are high, FR-4 might pass. For longer traces, multiple connectors, or many vias, consider M4 or an equivalent low-loss material.

• For 25G, 28G, 56G PAM4, or PCIe Gen5 designs: M6-grade materials are quite common, especially in scenarios with long channels, high layer counts, and numerous connectors.

• For 112G PAM4, 800G switches, or high-speed backplanes: M7 or higher might be safer. Additionally, factors like stack-up, via design, connectors, copper foil type, trace length, reference planes, return paths, and manufacturing consistency are critical. Simulation and evaluation are indispensable.

5. Cost Differences

A rough cost hierarchy is: Standard FR-4 < High-Tg FR-4 < M4 < M6 < M7 < M8/M9. As you go up the ladder, the raw material cost increases, processing demands are higher, the number of capable board shops shrinks, and lead times may lengthen. High-end materials often come with additional features like HVLP copper, low-Dk glass, tighter impedance control, higher layer counts, and more complex lamination processes—all of which drive up the final board price. It's not about choosing the most advanced material, but the one that fits the requirement best.