Megtron 6 PCB: High Speed Low Loss Laminate

What Megtron 6 Is

Megtron 6 is a high speed, low loss laminate used for boards that carry fast serial links or high frequency signals. It is a filled resin system reinforced with glass fabric, and its value is in two electrical numbers: a dielectric constant in the region of 3.6 to 3.7 depending on the grade and the test method, and a loss tangent low enough, around 0.002 at the frequencies that matter for high speed digital work, that the dielectric loss along a long trace stays acceptable.

It also carries a set of mechanical and thermal properties that make it usable in a normal multilayer process: a glass transition temperature near 200 degrees Celsius, a modest coefficient of thermal expansion, low moisture absorption and good adhesion. That combination is what makes it a production material rather than a laboratory curiosity.

Why the Loss Tangent Matters

Two mechanisms attenuate a signal on a transmission line. The conductor loss comes from the resistance of the copper and grows as the skin depth shrinks with frequency. The dielectric loss comes from the polarisation of the laminate and grows in proportion to the loss tangent, the frequency and the dielectric constant.

At low frequencies the copper dominates. At the data rates used in modern networking and computing, a trace can be tens of centimetres long and the dielectric term becomes the larger of the two, which is why a material change has more effect than a wider trace. The same loss tangent also affects the phase response: loss that varies with frequency distorts the pulse, closes the eye and increases the jitter that the receiver has to tolerate.

A final consequence is skew. In a differential pair or across the lanes of a parallel bus, differences in the dielectric environment and in the propagation constant turn into timing differences. A material with a well controlled dielectric constant and a low loss makes those differences smaller and more predictable.

The Properties That Matter

• Dielectric constant: around 3.6 to 3.7 for the standard grade, which sets the trace geometry for a given impedance and is lower than conventional FR-4, so a wider trace or a thinner dielectric can be used for the same impedance.
• Loss tangent: approximately 0.002 in the standard grade, and higher than the most advanced ultra low loss materials but far lower than FR-4.
• Glass transition temperature: close to 200 degrees Celsius, which gives the board real thermal margin through lead free assembly and multiple lamination cycles.
• Decomposition temperature: high enough to tolerate the thermal excursions of a multilayer build and the reflow profile without degrading the resin.
• Coefficient of thermal expansion: lower than standard FR-4, which reduces the strain on plated holes and on the surface mount joints through thermal cycling.
• Moisture absorption: below 0.1 percent, which keeps the dielectric constant stable and reduces the risk of delamination and of conductive anodic filament growth in a humid environment.

Where It Fits

The material ladder for digital boards runs from standard FR-4 through mid loss and low loss laminates to ultra low loss and PTFE based materials. Megtron 6 sits in the low loss band: noticeably better than a mid loss FR-4 at the frequencies used by 10 and 25 gigabit class links, but not the lowest loss material available.

The decision is a link budget question. If the channel is short, the connector and package losses are small and the receiver has equalisation margin, a mid loss material may pass. If the channel is long, the routing is dense or the loss budget is tight, the low loss material is what makes the link work at all. Choosing it because it is better than FR-4, without a budget that requires it, adds cost and buys nothing measurable. Our notes on PCB design and layout describe how the channel budget is built.

Design Implications

Impedance and geometry. A lower dielectric constant means a wider trace or a thicker dielectric for the same impedance, which is convenient for manufacturability. The trade is that the geometry has to be recalculated rather than copied from an FR-4 design, and the fabricator’s stack-up and the field solver have to be used together rather than separately.

Glass weave and dielectric uniformity. A glass fabric style that produces large openings can create a local variation in the dielectric constant under a narrow trace, which shows up as a small impedance variation and, in extreme cases, as skew between the two halves of a differential pair. A spread glass or a flatter weave reduces it, and rotating a pair relative to the weave is the traditional mitigation.

Hybrid stack-ups. A common approach is to use the low loss material only for the layers that carry the fast signals and standard FR-4 or a mid loss laminate for the rest. The saving is real, but the stack has to be balanced so that it does not warp, the lamination cycles have to be compatible, and the loss of the transitions between the two material types has to be counted.

Vias and stubs. A low loss laminate does nothing for a via stub that rings at the fundamental frequency of the data. Back drilling or a blind via structure removes the stub, and the combination of a low loss material with a properly controlled via is what produces the improvement the budget predicted.

Processing

These laminates are processed in much the same way as FR-4, but the process windows are narrower. The resin is more brittle in drilling, so the drill parameters, the hit count and the entry and exit materials have to be set for the material rather than inherited from an FR-4 program. The desmear chemistry has to match the resin system, or the plated hole adhesion suffers.

Moisture control is important. The panels should be baked before lamination and before any process that heats them, because trapped moisture expands and produces delamination and measling. The lamination cycle itself needs a controlled ramp and an adequate cure, and the material is sensitive to the number of thermal excursions it sees, which is a consideration on an HDI build with sequential laminations.

Conductive anodic filament growth is a failure mode in high humidity and in the presence of a bias voltage, and the low moisture absorption and the fillers in the resin are what make this class of material more resistant to it than standard FR-4. The surface finish and the final coating still matter, and a fine pitch assembly will need a finish that is flat and stable. Our notes on PCB manufacturing describe how these process controls are set.

Where It Is Used

Telecommunications equipment uses it in switches, routers and the backplanes that connect high speed line cards. Data centre hardware uses it in servers and accelerator boards where long traces run at high data rates, and aerospace and defence systems use it in radar, electronic warfare and satellite electronics where the signal loss and the thermal stability both matter.

Automotive radar and advanced driver assistance systems use high frequency boards whose loss budget has to be met over the temperature range of the vehicle, and high speed instrumentation and test equipment use it where signal fidelity is the product. In each case the material is chosen for a measured performance requirement rather than for a general preference.

What It Costs

A low loss laminate costs several times more than standard FR-4 per unit area, and the process adds cost because the yields are lower and the drill and lamination parameters are more demanding. A hybrid stack reduces the premium by using the expensive material where it is needed, at the cost of a more complex build.

The right comparison is not the price per square metre of laminate but the total cost of the channel. If the low loss material allows a design to meet the specification without an additional layer, without a retimer and without a redesign after the first eye diagram, it has paid for itself. Our notes on quality management describe how the material and the process are controlled in production, and our notes on PCBA testing describe how the finished board is verified.

FAQ

What is Megtron 6 used for? High speed digital and high frequency boards where the dielectric loss along a long trace or a tight link budget requires a laminate with a low loss tangent and a well controlled dielectric constant.

How does it compare with FR-4? It has a lower dielectric constant and a much lower loss tangent, a higher glass transition temperature and lower moisture absorption, at several times the material cost.

Can it be mixed with FR-4 in one stack? Yes, and that is a common way to control cost, but the stack has to be balanced, the lamination cycles have to be compatible and the transitions have to be included in the loss budget.

Does a low loss material remove the need for back drilling? No. Via stubs cause reflections and ringing independently of the laminate, and they still have to be controlled.

Is it suitable for a mains or high voltage board? Its use is driven by the signal performance of high speed digital or high frequency analogue circuits. A high voltage board is a different design problem with different material criteria.

Conclusion

Megtron 6 is a production grade low loss laminate: a dielectric constant around 3.6, a loss tangent around 0.002, a glass transition temperature near 200 degrees Celsius and low moisture absorption. It is the material that makes long, fast channels practical when FR-4 losses will not fit the budget, and it is unnecessary when they will. Decide from the loss budget, recalculate the geometry for the new dielectric constant, control the glass weave and the moisture, and the material delivers the performance the simulation predicted.