Rogers 6002 PCB: Properties, Design and Selection

Where 6002 Sits in the Range

Rogers 6002 is a polytetrafluoroethylene based laminate reinforced with a woven glass fabric, part of the RO6000 family and designed for microwave and high speed digital circuits where loss has to be very low. Its dielectric constant is around 2.94 at ten gigahertz, which is low, and its dissipation factor is around 0.0012, which is among the lowest available in a laminate that can still be processed on ordinary equipment.

The combination matters. A low dielectric constant makes a wide trace and a loose impedance for a given dielectric thickness, which is convenient for a thick transmission line and for a wideband structure. A low loss makes a long line usable, and it is why the material appears in antenna feed networks, radar and satellite front ends and long high speed serial links.

The Electrical Properties

• Dielectric constant: around 2.94 with a tolerance of plus or minus 0.05 at ten gigahertz, and stable with frequency over the useful range.
• Dissipation factor: around 0.0012 at ten gigahertz, so the insertion loss of a long line is dominated by the copper rather than by the dielectric.
• Thermal coefficient of the dielectric constant: small, which means a filter or a resonator built on the material does not drift as the equipment warms.
• Moisture absorption: very low, which keeps the electrical performance stable in humid conditions and prevents the dielectric constant from moving with the environment.
• Thermal conductivity: low, in the region of 0.2 watts per metre kelvin, so the thermal design is done in copper and in the mechanics rather than through the laminate.

Two of these numbers deserve emphasis. The low dissipation factor is the reason the material exists, and it is what distinguishes it from a mid loss laminate. The stability of the dielectric constant with temperature is the reason it appears in filters and in phase critical circuits, where a drift in the electrical length changes the performance of the whole system.

The Low Dielectric Constant and Trace Geometry

A low dielectric constant means a given impedance needs a wider trace for the same dielectric thickness, or a thicker dielectric for the same trace width. In practice the designer takes the wider trace, because it reduces the conductor loss and improves the manufacturing tolerance: a wider trace has a proportionally smaller variation for the same absolute etch variation.

The trade is that a wide trace occupies more routing area. On a board where the routing is dense, the density is bought by thinning the dielectric rather than by narrowing the trace, which brings its own constraints: a thin dielectric is more difficult to laminate uniformly and has a lower breakdown voltage. Where a very thin dielectric is required, the fabricator uses a bonded film or a coated foil rather than a glass reinforced sheet.

The width of a 50 ohm line on this material is substantial, so a design that moves from FR-4 to 6002 without re-computing the geometry produces an impedance that is far from the target. This is the most common error when a board is migrated to a low loss laminate and is a good reason to agree the stack with the fabricator before the layout is committed.

Processing

The material is PTFE based, and that shapes the process in four ways.

Drilling. PTFE is soft and prone to smearing, and the woven glass reinforcement is abrasive. The drill parameters are set to produce a clean hole with a controlled entry and exit, and the desmear step is chosen to remove the smear without attacking the resin.

Bonding. PTFE does not bond easily. Multilayer construction uses a bonding film or a specialised prepreg, and the lamination cycle is set for the material rather than for the plant default.

Dimensional stability. The material moves during lamination, and registration has to be compensated. In a thin, large panel the movement is significant.

Surface preparation and plating. The copper surface is treated before imaging and before plating, and the plating chemistry is chosen so that the adhesion to the PTFE is reliable. Our notes on PCB manufacturing describe these controls.

Where the board has both RF and digital sections, a hybrid stack with 6002 on the RF layers and a mid loss or FR-4 material elsewhere is common, and the same cautions apply as on any hybrid: a symmetrical stack, a controlled transition between the materials and a fabricator with experience of the combination.

Choosing Between the High Frequency Materials

• Rogers 6002. The lowest loss of the common PTFE laminates, with a low and very stable dielectric constant. Chosen for long lines, wideband structures, filters and phase critical circuits.
• Rogers 4350B. A ceramic filled hydrocarbon with a dielectric constant around 3.48 and a higher loss. Cheaper, easier to process, and the default for many sub ten gigahertz designs.
• Rogers 3010. A high dielectric constant, around 10.2, for compact resonators and small antennas where area matters more than loss.
• FR-4 with a low loss variant. Adequate below a few gigahertz and for short traces, at a fraction of the price.

The selection is made from the electrical length and the loss budget. A short line at a moderate frequency does not need a PTFE laminate; a long line, a wideband match or a filter with a narrow bandwidth usually does. Our notes on PCB design and layout cover the transmission line design that follows from the choice, and our notes on telecommunications PCBA describe the equipment class.

Cost and Availability

The material is priced by area and is significantly more expensive than FR-4, with the laminated sheet itself often costing several times more per square inch than a mid loss material. The finished board price follows, and it is driven by the laminate, the number of lamination cycles, the panel utilisation and, on a PTFE material, the process yield.

Availability is the practical issue. The material is stocked in a smaller range of thicknesses than FR-4 and the smaller fabricators buy it through distribution, which adds a lead time and sometimes a minimum order quantity. The copper thickness, the panel size and the bonding film all have to be confirmed before the order, and a design that assumes a thickness which turns out not to be stocked will have to be re-laminated with a different stack, which changes the impedance.

For a first order, the useful practice is to buy a prototype panel with the stack the production will use, build the impedance coupon, and measure it. The difference between the designed and the measured impedance on a low dielectric constant material is the number that decides whether the design meets its specification. Our notes on quality management describe the coupon testing requirements, and our notes on PCB assembly cover the assembly that follows.

FAQ

What is the dielectric constant of Rogers 6002? About 2.94 at ten gigahertz with a tight tolerance, and it stays stable with frequency and temperature.

How does 6002 compare with 4350B? 6002 has a lower dielectric constant and a much lower dissipation factor, which matters on long lines and in filters. 4350B is cheaper and easier to process.

Can it be mixed with FR-4 in one board? Yes, and hybrid stacks are common, with the low loss material used only where the loss budget requires it.

Why is the impedance harder to hold on this material? Because the low dielectric constant makes the trace wide, so a given absolute etch variation is a smaller proportion of the width, and the dielectric thickness has to be held tightly.

Conclusion

Rogers 6002 is a low loss PTFE laminate with a low and stable dielectric constant, chosen where the electrical length or the loss budget rules out a cheaper material. Use it where the line is long or the bandwidth is wide, recalculate the trace geometry rather than reusing the FR-4 dimensions, expect a PTFE process with its own drilling and bonding requirements, and order a prototype with the production stack so that the impedance can be measured before the design is fixed.