Rogers RO4003C: High Frequency Laminate Guide

What RO4003C Is

RO4003C is a high frequency laminate made from a ceramic filled hydrocarbon thermoset resin reinforced with woven glass. It is not a PTFE based material, and that is the point: it delivers a low, stable dielectric constant and a low loss tangent while being processed on a standard FR-4 line.

Its published figures are a dielectric constant of approximately 3.38 with a tight tolerance and a loss tangent of about 0.0027 at ten gigahertz, together with a glass transition temperature above 280 degrees Celsius and a moisture absorption below 0.1 percent. Those numbers put it in the low loss band, and the processing similarity to FR-4 is what made it one of the most widely used RF materials rather than a specialist option.

Why It Became a Standard

The alternative at this performance level has traditionally been a PTFE based laminate, and PTFE solves the electrical problem but creates a manufacturing one: it is soft, it is chemically inert, it needs special drilling and a hole wall preparation before plating, and the solder mask adhesion is weaker.

A thermoset hydrocarbon laminate behaves much more like FR-4. It can be drilled with conventional parameters, it accepts a plated hole without a sodium etch, it laminates in a standard press cycle and it accepts solder mask. The result is a material that an ordinary high performance PCB shop can produce reliably, at a lower cost and with a shorter lead time than a comparable PTFE stack. For a base station antenna, a radar sensor or a microwave radio, that combination is usually better than the last fraction of a decibel.

The Properties That Matter

• Dielectric constant: approximately 3.38, with a tight tolerance and a value that stays close to constant across frequency and temperature.
• Loss tangent: approximately 0.0027 at ten gigahertz; not as low as a PTFE composite, but far below FR-4.
• Glass transition temperature: above 280 degrees Celsius, which gives a large process window and survives multiple lamination cycles.
• Moisture absorption: below 0.1 percent, so the electrical performance stays stable in a humid or outdoor environment.
• Coefficient of thermal expansion: low in the z direction for a laminate in this class, which reduces the strain on plated holes during assembly and thermal cycling.
• Thermal conductivity: modest, so heat spreading still relies on copper and on the mechanical design.
• Thickness range: available in a range of standard thicknesses, which allows the impedance and the mechanical requirements to be met without a custom build.

Design Implications

Geometry changes, but less than with PTFE. The dielectric constant of 3.38 is lower than FR-4, so a given impedance needs a wider trace or a thinner dielectric, but the change is small enough that a design is easier to translate than a PTFE one. The design should still be calculated for the actual stack, and the fabricator’s dielectric thickness and the field solver should be used together.

The dielectric constant is the selling point. Its stability with frequency means a wideband circuit keeps its phase response, and its tolerance means the impedance can be held without a wide trim allowance. For a filter, a coupler or a corporate feed network, that predictability is worth more than a marginal loss improvement.

Hybrid stacks are the common use. Most designs use RO4003C only on the layers that carry the RF, with a standard FR-4 or a mid loss material for the digital and power layers, bonded together into a single board. The stack has to be balanced so it does not warp, the lamination cycles have to suit both materials and the transition between them has to be designed, because the impedance changes at the boundary and the plated holes pass through it.

Thermal design is unchanged. The material does not conduct heat particularly well, so a power amplifier or a high duty cycle device still needs copper, thermal vias and a mechanical path to the enclosure. Our notes on PCB design and layout cover the layout considerations.

Processing

The material processes like FR-4 up to a point. Drilling uses conventional equipment, and the hole wall can be plated with a standard desmear and activation sequence rather than the special preparation a PTFE board requires, which is one of the reasons the cost and the lead time are more attractive.

Two differences deserve attention. The first is moisture: the panels should be baked before lamination and before any high temperature process, as with any laminate, but the consequence of trapped moisture is delamination and measling, so the bake schedule should be specified rather than assumed. The second is the lamination cycle itself: the resin needs an adequate cure to develop its final properties, and a stack that is laminated too fast will have a lower glass transition temperature than the data sheet promises.

Solder mask, legend and surface finish are all applied with standard processes, and the finish can be chosen for the assembly rather than for the material. Because the material is used in hybrid stacks, the fabrication house has to be able to laminate the two material types together and to control the finished thickness across both. Our notes on PCB manufacturing describe how these processes are controlled.

Where It Is Used

Cellular base station antennas and the feed networks behind them are the largest application, because the material gives a low loss, a stable dielectric constant and a cost that a volume product can carry. Automotive radar and advanced driver assistance sensors use it where the performance has to hold across the temperature range of a vehicle.

Microwave and millimetre wave radios, satellite communication terminals, GPS and navigation antennas, and power amplifiers for communications use it where the loss budget is tight but a full PTFE process is not justified. Test and measurement boards and high frequency instrumentation use it for the same reasons, and hybrid boards for mixed RF and digital products use it on the RF layers only.

What It Costs

The laminate costs several times more than standard FR-4 for the same area, but less than a PTFE material, and the processing premium is smaller than for PTFE because the material runs on a conventional line. A hybrid stack reduces the cost further by limiting the expensive material to the RF layers.

The comparison that matters is against the alternative, not against a plain FR-4 board. If the design needs a stable dielectric constant and a low loss, the choice is between this class of thermoset laminate and a PTFE material, and the thermoset usually wins on cost, on lead time and on the availability of fabricators that can build it. Our notes on PCBA testing describe how the finished board is verified, and our notes on quality management cover the process control around it.

FAQ

Is RO4003C a PTFE material? No. It is a ceramic filled hydrocarbon thermoset with woven glass reinforcement, which is why it processes like FR-4 instead of like PTFE.

How does it compare with FR-4? A lower dielectric constant, a loss tangent roughly an order of magnitude lower, a much higher glass transition temperature and far lower moisture absorption, at several times the material cost.

Can it be mixed with FR-4 in one board? Yes, and that is the usual construction. The RF layers use the high frequency material and the digital layers use FR-4, bonded into a single stack with the transition designed to handle the impedance change.

Does it need special drilling or plating? No. That is its main practical advantage: conventional drill parameters and a standard plating sequence without the surface preparation a PTFE laminate requires.

What is it used for? Base station antennas, automotive radar, microwave radios, satellite terminals, GPS antennas, power amplifiers and any RF design that needs a stable dielectric constant with a cost that a volume product can carry.

Conclusion

RO4003C is a low loss thermoset laminate with a dielectric constant of about 3.38 and a loss tangent of about 0.0027, and it earns its place by being processable on a standard FR-4 line. It does not have the lowest loss available, and it is not the cheapest material, but for most antenna, radar and microwave designs it delivers the electrical performance the specification needs at a cost and a lead time that a volume product can accept. Use it on the RF layers of a hybrid stack, specify the bake and the lamination cycle, and the material delivers the stable, predictable behaviour the design was built around.