Rogers 3010 PCB: The High-Dk Material Explained

A Material Chosen for Its Dielectric Constant

Most laminate decisions are made on temperature, cost and loss. Rogers 3010 is chosen for a different reason: it has a dielectric constant of about 10.2, which is more than twice that of ordinary FR-4 and roughly three times that of the common high frequency materials. That single property makes it useful for a specific class of design, where the wavelength has to be shrunk to fit a structure into a small space.

It belongs to the RO3000 family, a ceramic filled PTFE composite with copper foil bonded to it. The PTFE gives the low loss and the stable electrical behaviour, and the ceramic filler raises the dielectric constant and lowers the expansion. The result is a material that behaves predictably from a few hundred megahertz into the tens of gigahertz, across a wide temperature range.

The Electrical Properties

• Dielectric constant: about 10.2 with a tolerance of plus or minus 0.25 at 10 gigahertz, and stable with frequency and temperature.
• Dissipation factor: about 0.0022 at 10 gigahertz, which is what keeps the insertion loss low on a long line.
• Thermal conductivity: about 0.8 watts per metre kelvin, better than plain PTFE but far below a ceramic substrate.
• Moisture absorption: below 0.02 percent, which matters because absorbed water changes the dielectric constant and therefore the tuning of the circuit.
• Glass transition: above 280 degrees Celsius, high enough that the material is not the limiting factor in the assembly process.
• Expansion: the ceramic filler reduces the expansion in the plane of the board, which helps when a large ceramic component is attached to it.

The stability of the dielectric constant is as important as its value. A material whose Dk drifts with temperature or with the manufacturing batch produces filters and antennas that need retuning, and the whole point of specifying a high frequency laminate is to avoid that.

Why a High Dielectric Constant Helps

The physical size of a resonant structure scales with the wavelength in the material, and the wavelength falls as the square root of the dielectric constant. Raising Dk from about 3 to about 10 shortens the guided wavelength by roughly a factor of 1.8, so a patch antenna, a filter or a coupler built on the same laminate can be about half the area of the same design on a lower Dk material.

There is a second, less obvious benefit. A higher dielectric constant allows a narrower trace to achieve a given impedance for the same dielectric thickness, which is useful where the routing has to be dense. The trade-off is that the tolerances tighten: a small error in the trace width or in the dielectric thickness produces a larger error in the impedance, so the impedance control has to be tighter than on a lower Dk material.

Where It Is Used

• Antennas and antenna arrays. Patch elements and feed networks where the element size has to fit a defined aperture.
• Filters and couplers. Distributed structures that have to be small, with a loss low enough not to spoil the selectivity.
• Satellite and radar front ends. Where a compact, stable transmission line is required across a wide temperature range.
• Automotive radar. Where the board has to work from a cold start to an under-hood soak without drifting out of specification.
• Microwave and medical imaging. Where phase consistency between channels is part of the measurement.

Our notes on telecommunications PCBA describe the wider class of equipment these boards belong to, and our notes on PCB design and layout cover the transmission line design they need.

Processing the Material

The ceramic filler makes the laminate abrasive, and that changes the way it is drilled and routed. Drill bits wear faster, so the bit life is set shorter than for FR-4 and the drill parameters are chosen to produce a clean hole without smearing the PTFE. The panels are routed rather than punched, and the router tooling wears as well.

Lamination is a controlled process. The material needs a specified temperature and pressure ramp, and the bonding layer and the surface preparation are part of the recipe. PTFE is difficult to bond, and the usual approach is to use the correct bonding film for the stack rather than to rely on the resin of the laminate itself.

Storage matters too. The laminate should be kept in a dry environment and used within the shelf life of the bonding film, which is why a fabricator that runs high frequency material regularly is a better choice than one that runs it occasionally. Cutting, cleaning and handling all have to avoid contaminating the surface before lamination.

Surface finishes follow the frequency. Electroless nickel immersion gold and immersion silver are the common choices, because they give a flat, solderable surface with low loss; a thick tin finish is not used where the conductor loss matters. Our notes on PCB manufacturing describe the process controls the material requires.

Hybrid Stacks

Rogers 3010 board is expensive, and a whole board is rarely made from it. The usual approach is a hybrid stack, with the high frequency laminate used only for the radio layers and ordinary FR-4 used for the control, power and digital layers, bonded together in one lamination. That keeps the cost of the speciality material proportional to its function.

The hybrid stack introduces its own problems. The two materials expand by different amounts and have different dielectric constants, so the layer stack has to be symmetrical enough to remain flat and the transition between the two must be designed as a controlled transmission line rather than left to chance. The bond between them is also a place where a poor process shows up as a delamination after reflow, so the fabricator has to have experience with the combination and not just with each material separately.

Cost and Availability

The material is priced per square inch rather than per board. Indicative figures are around 2.50 to 5.00 US dollars per square inch for the laminate, against roughly 0.50 to 1.20 dollars for FR-4 of the same area, so a design that uses a third of the board area for the high frequency section carries a third of the premium.

Board prices follow the same pattern. A one to four layer prototype is typically 3.00 to 6.00 dollars per square inch in small quantity, falling to about 2.20 to 3.80 dollars per square inch above five hundred pieces. Standard lead time is seven to fifteen working days, with an expedited route at three to five days at a premium. Common thicknesses are 0.254, 0.508, 0.762 and 1.524 millimetres, and the price depends on the thickness, the copper weight and the process difficulty.

The material is usually available from the manufacturer and its authorised distributors, and the smaller fabricators buy through distribution. That adds a step to the lead time and sometimes a minimum order quantity, which is worth checking before a project is scheduled around it.

Choosing Between High Frequency Materials

A lower dielectric constant material such as a glass reinforced PTFE in the 3 to 3.5 range is the better choice for a wideband design, for a long transmission line where the loss per unit length matters, and where the structure does not have to be small. A high Dk material such as Rogers 3010 is the better choice where the structure must be compact, where the impedance is high enough that a narrow trace is helpful, or where a low impedance line is needed on a thin laminate.

In practice the decision is made from the structure rather than from the material. Lay out the antenna or the filter for the required size, calculate the impedance and the loss, and let the result choose the laminate. Our notes on quality management describe the incoming inspection and the coupon testing that confirm the material performed as specified, and our notes on PCB assembly cover the assembly of the finished board.

FAQ

What is the dielectric constant of Rogers 3010? About 10.2 at 10 gigahertz with a tolerance of plus or minus 0.25, and it stays stable over frequency and temperature.

Is Rogers 3010 the same as FR-4? No. It is a ceramic filled PTFE material with a much higher dielectric constant, far lower loss and a much higher cost.

Can it be mixed with FR-4 in one board? Yes, and hybrid stacks are common, with the speciality material used only for the radio layers and FR-4 for the rest.

What does it cost? Roughly 2.50 to 5.00 US dollars per square inch for the laminate, and 3.00 to 6.00 dollars per square inch for a small prototype board.

Conclusion

Rogers 3010 is a high dielectric constant, low loss laminate for designs that must be compact, stable and efficient at microwave frequencies. It is the material behind small patch antennas, tight filters and radar front ends, and it earns its price by shrinking the structures and by holding its electrical properties through temperature and frequency. Use it where the geometry demands it, keep the rest of the board on FR-4, and choose a fabricator that handles the material often enough to have the process under control.