RF PCB

01

Choose hydrocarbon-ceramic or PTFE laminate

Rogers says the range of usable laminates narrows once operating frequencies reach 500 MHz and above. Its RO4000 hydrocarbon-ceramic laminates are rigid thermosets processed with standard FR-4 techniques and, unlike PTFE materials, need no special via preparation such as sodium etch. RO4350B lists process Dk 3.48 ± 0.05 at 10 GHz, Df 0.0037 at 10 GHz and 0.0031 at 2.5 GHz, thermal coefficient of Dk +50 ppm/°C, CTE 10, 12 and 32 ppm/°C in X, Y and Z, Tg above 280 °C, thermal conductivity 0.69 W/m·K, moisture absorption 0.06%, copper peel 0.88 N/mm after solder float and UL 94 V-0. [1]

Rogers' RT/duroid 5880, a glass-microfiber-reinforced PTFE composite for stripline and microstrip work to Ku-band and above, lists Dk 2.20 ± 0.02 and Df 0.0009 at 10 GHz, thermal coefficient of Dk −125 ppm/°C, CTE 31, 48 and 237 ppm/°C in X, Y and Z (0–100 °C), moisture absorption 0.02%, thermal conductivity 0.20 W/m·K and UL 94 V-0. [2] Rogers lists RO4000 for base-station antennas and amplifiers, RFID tags, automotive radar and LNBs, and RT/duroid 5880 for millimetre-wave, radar and point-to-point radio antennas. [1] [2]

Editorial calculation: at 10 GHz RO4350B's Df of 0.0037 is about 4 times RT/duroid 5880's 0.0009, while 5880's Z-axis CTE of 237 ppm/°C is about 7 times RO4350B's 32 ppm/°C, though the two are measured over different temperature ranges. [1] [2] Editorial note: lower loss and lower Z-axis expansion pull in different directions, so ask the fabricator for plated-through-hole reliability data on the laminate you choose.

A third option is a low-loss thermoset that is neither RO4000 nor PTFE. Isola lists Astra MT77 for millimetre-wave and automotive radar boards with Dk 3.00 and Df 0.0017 in its typical-values table (rows labelled 2 GHz and 10 GHz, with the value entered against 2 GHz; Df by Bereskin stripline), and states its Dk is stable from −40 °C to +140 °C at up to W-band frequencies. Typical values include Tg 200 °C by DSC, Td 360 °C, Z-axis CTE 50–70 ppm/°C below Tg and 250–350 ppm/°C above it, X/Y CTE 12 ppm/°C, thermal conductivity 0.45 W/m·K, peel strength 1.0 N/mm with 1 oz EDC foil, moisture absorption 0.1%, UL 94 V-0 and RTI 130 °C. Isola describes it as FR-4 process compatible with no plasma desmear required, offers laminate from 2.5 to 60 mil (0.0635–1.50 mm) with HVLP copper of ≤2.5 µm Rz JIS in ½, 1 and 2 oz (18, 35 and 70 µm), and cites IPC-4103/17 and UL file E41625. [3]

Editorial calculation: Astra MT77's Df of 0.0017 sits between RT/duroid 5880's 0.0009 and RO4350B's 0.0037 (about 1.9 times the first and 0.46 times the second), though Isola's figure is not stated at the same 10 GHz test point as the Rogers values; compare loss at your own frequency from each maker's tables. [3] [2] [1]

Give the fabricator the laminate brand and grade (or approved equivalents), dielectric thickness and tolerance, copper foil type and weight, stack-up including any hybrid FR-4 layers, controlled-impedance lines with target and tolerance, operating frequency, insertion-loss limits if any, via and ground-fence requirements, surface finish, IPC class, and quantity.

02

Hold the Dk, copper foil and verification

Rogers publishes two Dk values: process Dk from the clamped-stripline test at 10 GHz, used for quality acceptance, and design Dk, an average over multiple tested lots from 8 to 40 GHz by differential phase length. For RO4350B the design Dk is 3.66 against a process Dk of 3.48, and in its LoPro notes Rogers adds that design Dk falls by about 0.1 as core thickness drops from 0.020 in to 0.004 in. [1] For RT/duroid it states that process Dk values may be incorrect for design purposes, especially microstrip, and recommends verifying prototype boards for new designs. [2]

Editorial calculation: RO4350B's design Dk of 3.66 is 0.18 above its process Dk of 3.48, more than three times the ±0.05 process tolerance. [1]

For reduced insertion loss Rogers offers LoPro reverse-treated foil on RO4000, which adds about 18 µm per core; RO4350B LoPro does not share the standard UL designation. Rogers also notes that prolonged oxidative exposure can change the dielectric properties of hydrocarbon materials, faster at higher temperatures, and recommends evaluating each material and design for the product's life. [1] Rogers lists RT/duroid 5880 as supplied with electrodeposited copper of ½ to 2 oz/ft² or reverse-treated foil, can be clad with rolled copper for more critical electrical applications, and should be ordered by dielectric thickness, tolerance, foil type and weight. [2]

Hybrid stack-ups pair an RF laminate with a low-loss multilayer material for digital and control layers, and that material's figures need the same reading. Panasonic's MEGTRON property table lists MEGTRON 8 R-5795(U) at Dk 3.08 and Df 0.0012 at 14 GHz with Tg 220 °C (DMA), z-axis CTE 50/270 ppm/°C (α1/α2), T288 with copper above 120 min and peel strength 0.7 kN/m on H-VLP3 copper; MEGTRON 7 R-5785(N) at Dk 3.31 and Df 0.0023 at 14 GHz with Tg 200 °C; MEGTRON 6 R-5775(N) at Dk 3.34 and Df 0.0037 at 13 GHz with Tg 185 °C; and the HIPER V R-1755V at Dk 4.3 and Df 0.020 at 10 GHz. Dk and Df were measured by the balanced-type circular disk resonator method on 0.75 mm samples for MEGTRON 8, 7 and 6 (IPC-TM-650 2.5.5.5 for the 10 GHz values), and Panasonic states the data are "typical values and not guaranteed values". [4]

Editorial calculation: across Panasonic's table Df rises about seventeen-fold from 0.0012 (MEGTRON 8 at 14 GHz) to 0.020 (HIPER V at 10 GHz); MEGTRON 6's 0.0037 at 13 GHz equals the 0.0037 Rogers lists for RO4350B at 10 GHz, though the test methods and frequencies differ, so compare laminates only on values measured the same way and ask the fabricator which test method its stack-up model uses. [4] [1]

03

Compare what suppliers offer

Suppliers linked to this page list RF or high-frequency PCBs; each card links to the page reviewed.

Listings include high-frequency and Rogers PCBs alongside HDI, rigid-flex and metal-core boards; ask each which laminates it stocks, its impedance tolerance and test method, and whether it builds hybrid stack-ups.

04

Trace the RF board construction

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your board

An RF PCB quotation needs values for your board. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the laminate, Dk basis and RF limits together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Laminate: [brand and grade; thickness; tolerance; equivalents allowed]
Dk basis: [design Dk used; frequency]
Copper: [weight; ED, reverse-treated or rolled]
Stack-up: [layers; hybrid FR-4]
RF: [impedance targets; frequency; insertion/return loss]
Vias and finish: [ground vias; finish]
Quality: [IPC class; prototype verification; quantity]

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