Rogers 5870 PCB: The Lowest-Dk PTFE Laminate

Where 5870 Sits

Rogers 5870 is a polytetrafluoroethylene laminate reinforced with glass microfibre, produced in the same family as 5880 and aimed at the applications where the lowest available dielectric constant and the lowest loss are the deciding factors. Its dielectric constant is about 2.33 and its dissipation factor about 0.0012 at ten gigahertz, and both are stable with frequency and temperature.

Those two numbers place it at the top of the general purpose high frequency range. A lower dielectric constant means a wider trace for a given impedance and a shorter electrical length for a given physical length, and a lower loss means the signal survives a longer run. It is the material that appears in radar front ends, satellite communications and millimetre wave modules, and it costs several times what a mid loss laminate costs.

The Properties

• Dielectric constant: about 2.33 with a tolerance of plus or minus 0.02 at ten gigahertz.
• Dissipation factor: about 0.0012 at ten gigahertz.
• Thermal coefficient of the dielectric constant: about minus 115 parts per million per degree Celsius, which is small enough that a filter or a resonator does not drift measurably as the equipment warms up.
• Operating temperature: up to about 250 degrees Celsius, well above any condition the assembled product will see.
• Moisture absorption: about 0.02 percent, which keeps the electrical performance stable in humidity and outdoors.
• Material cost: roughly 400 to 600 US dollars per square metre, against around 40 to 80 for FR-4 of the same area.

The dielectric constant tolerance is worth noting. A tight tolerance on a low value is what allows a printed filter or an antenna array to be produced in volume without tuning each unit, and it is a large part of what the material price buys.

Against FR-4 and the Other High Frequency Materials

Against FR-4. The dielectric constant falls from about 4.5 to 2.33, so the guided wavelength is roughly forty percent longer and the traces are correspondingly wider. The loss tangent falls by more than an order of magnitude, which is the difference between a usable and an unusable link at higher frequencies. The price is roughly five to ten times that of FR-4.

Against Rogers 6002. 6002 has a dielectric constant of about 2.94 and a similar loss. It is easier to process and slightly cheaper, and it is the more common choice where the electrical length does not demand the very lowest dielectric constant.

Against Rogers 3010. 3010 has a dielectric constant of about 10.2 and is used where a compact resonator or a small antenna is needed. It solves the opposite problem: instead of a wide trace and a long wavelength, it delivers a short one.

Against Rogers 4350B. 4350B is a ceramic filled hydrocarbon with a dielectric constant of about 3.48 and a higher loss. It is cheaper, easier to drill and bond, and adequate for a great many designs below the millimetre wave range.

The selection follows the electrical length and the bandwidth. A wideband structure favours the lowest dielectric constant; a compact resonator favours the highest; a design that needs neither can use the cheapest material that meets its loss budget. Our notes on PCB design and layout cover the transmission line and filter design that follows.

Processing

The material is PTFE, and the process has to respect that.

Drilling. The resin smears easily and the glass reinforcement is abrasive, so the drill parameters are chosen for a clean hole and the desmear step is set to remove the smear without damaging the resin.

Lamination. PTFE does not bond readily, and the bonding film and the lamination temperature profile are specific to the material. A temperature that drifts produces a bond that looks sound and delaminates after reflow.

Etching and plating. The copper surface is prepared specifically for a PTFE substrate, and the plating chemistry has to give reliable adhesion to it.

Solder mask. A low acidity, flexible mask is used, because the rigid masks adhere poorly to the PTFE surface and crack at the edges of the pads.

Storage. The laminate is stored dry and used within its shelf life, because absorbed moisture affects both the lamination and the electrical performance.

The practical consequence is that the material should be built by a plant that runs it regularly. A fabricator that processes PTFE occasionally will produce a board, and it may not be a board that survives the assembly and the thermal cycling. Our notes on PCB manufacturing describe the process controls.

Design Considerations

Impedance control is mandatory. The low dielectric constant makes the traces wide, so an absolute etch variation is a smaller proportion of the width and the impedance is comparatively well behaved — but only if the dielectric thickness is held and the geometry is designed against the actual stack. The stack is agreed with the fabricator and the impedance is verified on a coupon built in the same material.

Via transitions need return vias. At millimetre wave frequencies a via without an adjacent ground via is a significant discontinuity. Where the frequency is high enough, the transition is modelled rather than estimated, and the stub is removed by back drilling or by using a blind via from the outer layer.

Thermal design is done in copper. The laminate conducts heat poorly, so the thermal path is built from copper and vias, and a high power amplifier section often sits on a metal core, a ceramic or a metal backed section rather than on the PTFE.

Hybrid stacks are common. The speciality material is used on the radio frequency layers and FR-4 or a mid loss laminate elsewhere, bonded in one lamination. The stack is kept symmetrical and the transition between materials is a controlled transmission line. Our notes on telecommunications PCBA describe the class of equipment these boards belong to.

Cost and Lead Time

Indicative prices for finished boards run around 12 to 18 US dollars per piece for a single sided 1.6 millimetre board with one ounce copper at a hundred pieces, 25 to 35 dollars for a double sided board at fifty pieces, 50 to 70 dollars for a four layer hybrid at ten pieces and 80 to 120 dollars for a single prototype. Above a thousand pieces the unit price typically falls by 15 to 25 percent.

The dominant terms are the laminate, the panel utilisation and the process yield, not the layer count. A design that uses the speciality material only where the loss budget requires it pays a fraction of the price of one built entirely from it, which is why the hybrid stack is the normal answer.

Lead time is longer than for ordinary laminate because the material is ordered in and the hybrid lamination is scheduled. A prototype typically takes five to seven working days once the material is in stock, and longer where a thickness has to be ordered. Our notes on quality management describe the coupon and inspection data that should accompany the order, and our notes on PCB assembly cover the assembly of the finished board.

FAQ

What is the dielectric constant of Rogers 5870? About 2.33 at ten gigahertz, with a tolerance of plus or minus 0.02 and a small thermal coefficient.

Can it be mixed with FR-4? Yes, and hybrid stacks are the usual construction, with the PTFE material used only on the radio frequency layers.

Is it suitable for automotive radar? It is used in millimetre wave radar at 77 gigahertz, where the low loss and the stable dielectric constant are exactly what the design needs.

How should it be stored? Sealed and dry. Within a shelf life of one to two years the electrical performance is unaffected, provided the material is not exposed to humidity.

Conclusion

Rogers 5870 is the material for designs where the electrical length, the bandwidth or the frequency leave no room for a higher loss laminate: a dielectric constant of 2.33, a dissipation factor of 0.0012 and a stability that holds filters and antennas in specification across temperature. Use it on the layers that need it, keep the rest of the board on a cheaper material, control the impedance with a coupon, and build it with a plant that processes PTFE as a routine rather than as an exception.