Every RF programme fixes its laminate earlier than the loss budget is known. A board carried over on FR-4 because the last product used FR-4 can finish a decibel short at 8 GHz, and a board pushed onto PTFE for a 2.4 GHz link pays several times the material cost for margin nobody needs. The decision belongs to the loss budget, the layer count and the mechanical environment, not to habit.
The three families and what each one buys
| Family | Dk at 10 GHz | Df at 10 GHz | What it buys | Where it hurts |
|---|---|---|---|---|
| FR-4, Tg 150 to 170 | 4.2 to 4.6 | 0.015 to 0.025 | Lowest material cost, stiff panels, fine for digital and control layers | Loss climbs quickly with frequency, Dk moves with resin content and weave |
| Hydrocarbon ceramic, RO4003C, RO4350B, RO4835 | 3.38 to 3.48 | 0.0027 to 0.0037 | Stable Dk, FR-4 like drilling and plating, works in multilayer hybrids | Higher material cost than FR-4 |
| PTFE and PTFE glass, RF-35, TLY-5, CER-10 | 2.1 to 3.0 | 0.0009 to 0.002 | Lowest dielectric loss, flattest Dk across frequency | Soft and dimensionally restless, needs plasma treatment before plating |
The Dk spread inside one family matters as much as the gap between families. A laminate quoted at 3.38 and one quoted at 3.48 will not take the same 50 ohm line width, so a late substitution is a new stack up, not a paperwork change.
Start from the loss budget, not the datasheet
Insertion loss is conductor loss plus dielectric loss. Conductor loss follows copper thickness, line width and foil roughness, and grows with the square root of frequency. Dielectric loss follows Df and grows straight with frequency, so the two families cross over.
On a 2 GHz board with lines under 30 mm, FR-4 usually keeps the link inside budget and a low loss laminate buys nothing. Above roughly 6 GHz on longer runs the dielectric term takes over, and the ratio between a Df of 0.02 and a Df of 0.003 is the whole argument. At 77 GHz for automotive radar the laminate and the copper foil matter more than the layer count.
A practical test before quoting: take the longest line and the top frequency, then compute dB per inch for each candidate. If the margin is under about 1 dB, move up one family. If it is over 3 dB, spend the money on impedance control instead.
Dk tolerance is an impedance item
A 50 ohm line is a ratio between trace width, dielectric thickness and Dk. Change Dk by 0.1 and the same width lands roughly 2 to 3 ohms away from target. That is why a laminate published at plus or minus 0.05 is easier to hold than a generic FR-4 whose Dk is quoted as a range.
SUPERB PCB holds impedance to plus or minus 5 percent, and to plus or minus 2 percent on critical nets, with a TDR coupon on each panel so etch and dielectric are measured together. On a mixed stack the coupon has to sit on the same layer pair as the critical line, otherwise it measures a different stack.
Hybrid stack ups: RF on top, FR-4 underneath
Most production RF boards are hybrids: two to six RF layers carry the microwave work and the digital, power and control layers stay on FR-4. The RF layers are bonded to the FR-4 core with prepreg, and the bond is where the problems live, because the two materials expand differently and one press cycle has to satisfy both.
Registration between layers is held to 0.025 to 0.06 mm on these builds, up to 68 layers, with up to seven lamination cycles and a minimum cavity or step of 2.5 mm where a component sits inside the board. Back drilled stubs are held to 0.25 mm or less with depth control at plus or minus 0.05 mm, because at 10 GHz a stub left long enough behaves like a notch filter.
PTFE is a process choice as much as an electrical one
PTFE laminates are soft, they move under heat, and they do not accept plated copper well until the surface has been treated. Plasma treatment before plating is the step that decides whether a PTFE multilayer survives thermal cycling, and it is the step most often skipped. Ceramic filled grades are stiffer, drill more like FR-4, and are the usual answer when a PTFE electrical result is needed on a board that also has to pass vibration.
Board thickness on RF work runs from 0.2 to 12 mm, with mechanical drills down to 0.15 mm and laser drills to 0.075 mm. Lines and gaps go to 0.05 mm, and to 0.035 mm on HDI fan out.
What the RFQ should carry
- Frequency band, the longest line length and the loss allowed in dB.
- Layer count and stack up, or the Dk target if the stack is still open.
- Impedance value and tolerance per net class, and whether a coupon is required.
- Finish by area, plus any press fit holes or bonding pads that must stay unplated.
- Thermal cycling, humidity and vibration environment, and the workmanship class.
- Panel size, quantity and the date the assembly line needs boards.
How SUPERB PCB supports RF builds
SUPERB PCB covers 500 MHz to 77 GHz on Rogers RO4003C, RO4350B and RO4835, Taconic RF-35, TLY-5 and CER-10, and PTFE with plasma treatment before plating. RF and FR-4 can be combined in one hybrid stack, with embedded resistors at 50 ohm plus or minus 10 percent and a TDR coupon on each panel.
Assembly runs at SUPERB Assembly on a twelve zone reflow profile with nitrogen below 500 ppm, 3D SPI before placement, and X-ray with CT scan where a BGA or a thermal pad is involved. Workmanship follows IPC-A-610 Class 2 or 3 per the drawing, and the quality system is ISO 9001 with ISO 13485 applied to medical builds.
- RF boards: laminate range, mixed stack ups and finish options.
- PCB capability: layer counts, copper weights, drill sizes and tolerances.
- Process parameters: thickness, finish and registration tables.
Frequently asked questions
Is RO4350B a drop in for RO4003C?
No. Their Dk differs by about 0.1 at 10 GHz, which moves a 50 ohm line width by a few hundredths of a millimetre and shifts impedance by several ohms. The stack has to be recalculated and the coupon rebuilt.
Can every layer of a 20 layer board be PTFE?
It can be built, but it is rarely right. Cost rises with each PTFE layer and so does lamination risk. Most programmes keep two to six RF layers on the low loss material and put the rest on FR-4, which also gives the board its stiffness.
Do I need a TDR coupon?
If the board has controlled impedance, yes. Without one the first measurement happens on a populated board, the most expensive place to find a stack error.
Next step
Send the stack up or the layer count, the Gerber data, the BOM and the frequency band with the loss target. The reply confirms which laminate family fits the budget, flags the layers that should stay on FR-4, and quotes the build with the impedance and inspection scope attached.
