Ceramic PCB for Microwave and 5G mmWave: Alumina, AlN, LTCC or Laminate?

Ceramic substrates are used in microwave and 5G mmWave hardware where a low, stable dielectric loss has to come together with good heat removal, hermetic packaging or very fine thin-film lines: power amplifier modules, filters, radar front ends and packaged antenna modules. Alumina (Dk about 8.5–9.8) is the default for thin-film microwave circuits, aluminum nitride conducts heat seven to nine times better for high-power parts, and LTCC (Dk around 5–8) builds multilayer packages with buried passives and cavities. For larger antenna arrays and boards, a low-loss RF laminate is usually cheaper and easier. Choosing a ceramic PCB for microwave and 5G mmWave starts with the frequency, the loss budget, the heat to remove and the package functions you need.

Ceramic values are from Kyocera’s ceramic options table and MARUWA’s AlN data. They are typical values at the stated frequency, not design values at 28 or 39 GHz.

Which 5G Bands Are mmWave?

The 5G bands above 24 GHz, defined by 3GPP as Frequency Range 2. The common ones are n258 (24.25–27.5 GHz), n257 (26.5–29.5 GHz), n261 (27.5–28.35 GHz) and n260 (37–40 GHz). mmWave 5G is therefore a subset of 5G; most 5G service runs in the sub-6 GHz bands of Frequency Range 1.

At these frequencies a wavelength in alumina is only a few millimeters, so line widths, substrate thickness, surface roughness and dielectric tolerance all show up directly in loss and phase. That is the main reason ceramics and thin-film processes are used for the most critical parts of an mmWave front end.

What Does Ceramic Change in an RF Module?

Three things: loss, heat and packaging. Ceramics have low and stable dielectric loss, they conduct heat far better than laminates, and they can form hermetic packages with cavities and lids. They also have high Dk, which shrinks circuits but makes lines narrower, and they are brittle and limited in panel size.

For a power amplifier or radar transmitter, AlN’s thermal conductivity keeps the junction temperature down; for a filter or a packaged transceiver, LTCC’s multilayer structure lets passives and interconnects sit inside the package. For an antenna array covering a large area, a laminate usually wins on size, cost and weight. Our ceramic PCB guide covers the ceramic routes in general.

Which Ceramic Should You Choose?

Pick by the dominant requirement:

• Alumina when you need a stable, low-loss microwave substrate for thin-film lines, couplers or carriers and heat is moderate. It is the most established and least costly ceramic.
• Aluminum nitride when the part dissipates real power. MARUWA lists its AN grades at 180–230 W/m·K at 25 °C, against about 24 W/m·K for 96% alumina; CoorsTek lists AlN at 140–180 W/m·K (CoorsTek AlN data). Its expansion, around 4.6 ppm/K, is close to GaAs and GaN devices.
• LTCC when the package needs several layers, buried resistors or capacitors, cavities for dies and a lower Dk than alumina. Its thermal conductivity is low, so hot devices need thermal vias or a metal base.
• HTCC when you need a rugged, hermetic multilayer package and can accept tungsten or molybdenum conductors with higher loss than LTCC’s copper or silver.

Ask for dielectric data at your band, not only at 1 MHz or 2 GHz. Datasheet values at low frequency say little about loss at 28 GHz, where surface finish and conductor roughness add their own share.

When Should You Use an RF Laminate Instead?

When the circuit is large, multilayer or cost-sensitive and its heat load is modest. Phased-array antenna boards, beamforming boards and most mmWave PCBs in phones and small cells are built on low-loss PTFE or hydrocarbon laminates, often hybrid-pressed with FR-4 for the digital layers. Rogers’ high-frequency circuit materials are a common reference for that class, and ceramic-filled PTFE laminates such as the one in our Shengyi mmWave77 article sit between the two families.

A common split is to put the power amplifier or the RF die on a ceramic carrier or in an LTCC package and mount that package on a laminate antenna board. The finished laminate board is then accepted to IPC-6018, the performance standard for high-frequency (microwave) printed boards (IPC-6018D contents, 2022), while the ceramic part follows its own drawing and the maker’s specification.

When Is LTCC the Right Choice?

When the module needs layers and integrated passives in a small, sealed package. LTCC is fired at low temperature, which allows silver or copper conductors, and many tape layers can be stacked with vias, buried capacitors and resistors, and cavities for dies. Orbray, one ceramic maker, describes LTCC packages built this way (Orbray LTCC).

LTCC shrinks during firing, so layout must allow for shrinkage tolerance, and very fine lines are usually done by thin film on top. Volume tooling is significant, so LTCC suits modules made in quantity rather than one-off boards. For the comparison with HTCC, see our HTCC vs LTCC guide.

What RF Data Belongs in the RFQ?

The thermal and package rows often decide the material more than the dielectric rows. Our article on ceramic PCB design for RF covers layout rules and DFM for these substrates.

How Should the Substrate Be Qualified?

With RF measurements on coupons or the module itself at the operating band, plus thermal and mechanical checks. Material test methods such as IEC 60672-2 give typical properties of the ceramic, but the standard itself notes that real parts differ with size, shape and process (IEC 60672-2). Ask for S-parameter data on transmission-line coupons from the same process, dimensional reports on critical lines, and thermal cycling suited to the application. For a packaged module, add hermeticity and lid-seal tests if the package is sealed.

How Can We Help?

QueenEMS makes ceramic boards on alumina and aluminum nitride by DPC, DBC and AMB, as well as HTCC and LTCC, and builds RF laminate boards on our RF PCB line. Send the frequency, the heat load, the drawing and any package features; we will tell you which route we would quote and the dielectric data we can supply for it, and bare-board pricing comes back within 2 hours. Our ceramic PCB manufacturing page lists the substrates and metallizations we run.

Conclusion

Decide from frequency, heat and package needs, and ask for dielectric data at your band. Contact us with the drawing, operating band, power and quantities.

FAQ

Which 5G bands are mmWave? The FR2 bands above 24 GHz, mainly n257, n258, n260 and n261.

Is mmWave 5G the same as 5G? No. mmWave is one part of 5G; most 5G networks run in sub-6 GHz bands.

What is an RF PCB? A circuit board designed for radio-frequency signals, built on low-loss materials with controlled impedance and dimensions.

What is a ceramic substrate? A fired ceramic base, such as alumina, AlN or LTCC tape, on which RF lines and pads are metallized; it insulates the circuit and spreads its heat.

Is ceramic better than an RF laminate for 5G? Not always. Ceramic suits small, hot or hermetic parts; laminates suit larger antenna and RF boards.