Dk ≤ 3.5, Df ≤ 0.004 and five more indicators with their targets and failure modes — the checklist that turns substrate selection into engineering.
Substrate choice in high-frequency design decides signal integrity, transmission rate and system reliability before a single trace is routed, and the decision gets made on a handful of electrical indicators. Knowing which numbers lead — and what missing each one costs — turns material selection from guesswork into engineering. For brand-level comparisons, the high-frequency material overview picks up where this checklist leaves off.
Dk measures how much electrical energy the material stores, and it sets both propagation speed and impedance. The selection rule is low and stable: a value at or below roughly 3.5 suits most high-frequency work, and — more important than the headline number — variation across frequency should be minimal (on the order of 0.02 or less from 1 MHz to 10 GHz), because a drifting Dk shows up as phase distortion and signal delay that no layout fix can remove.
Df is the energy the material burns as heat under an alternating field — in other words, signal attenuation. Lower is better, with roughly 0.004 as the practical ceiling for serious high-frequency use, and the figure needs checking across frequency, temperature and humidity rather than at one comfortable data point. Standard FR-4 sits near 0.02, which is why it fades fast at high frequency; the material types analysis shows which families stay flat.
Electrical numbers win designs; thermal numbers keep them alive. CTE at or below about 15 ppm/°C keeps the board dimensionally stable through temperature cycles and protects plated through-holes and layer registration. A glass transition temperature of 170 °C or higher survives lead-free reflow without deforming. And thermal conductivity decides where heat goes — metal-based or ceramic-filled substrates that move heat out protect device lifespan in power-dense layouts.
Four quieter properties round out the screen. Flame retardancy must meet UL94 V-0 for safety compliance. Chemical resistance has to survive etching, soldering and the rest of the wet process chain. Mechanical strength and toughness determine whether the laminate endures multilayer lamination and drilling. And copper surface roughness feeds directly into conductor loss and impedance accuracy — ultra-low-profile foil reduces skin-effect losses at the frequencies where it matters.
Beyond the datasheet, five practical checks separate a good material choice from a production problem. Check batch consistency — small Dk/Df spread between lots is what keeps impedance where the simulation put it across a production run. Check process compatibility with your fabricator’s drilling, lamination and plating chemistry, since a mismatched material shows up as blistering or delamination. Weigh cost and supply chain together, favouring suppliers with stable inventory and fast response. Confirm RoHS and REACH compliance if the product ships internationally. And validate: use impedance software to predict line-width and stack-up effects, then verify with real measurements plus thermal-cycling and damp-heat aging before committing volumes. The laminate selection guide works through the brand-level version of this list.
Each indicator carries a target and a failure mode if it slips:
| Indicator | Typical target | If it misses |
|---|---|---|
| Dk | ≤ 3.5, low drift with frequency | Phase distortion, impedance error, signal delay |
| Df | ≤ 0.004 at the operating band | Attenuation, heat build-up, shorter reach |
| CTE | ≤ 15 ppm/°C, Z-axis especially | Via cracking, layer misregistration |
| Tg | ≥ 170 °C | Deformation during lead-free reflow |
| Thermal conductivity | Higher is better for power layouts | Hot spots, shortened device life |
| Surface roughness | Ultra-low profile copper foil | Excess conductor loss at high frequency |
| Flame retardancy | UL94 V-0 | Fails safety compliance |
Material selection is systems engineering in miniature: electrical performance, thermal reliability, processability and total cost all have to hold at once, and no single material wins every column. Teams that screen candidates against the indicators above — and validate with simulation plus physical testing before volume — buy themselves designs that behave in production the way they did in the model.
Frequently Asked Questions
Dk stability across frequency and temperature. A material with a mediocre-but-stable Dk is usually easier to design with than one with a lower headline value that drifts, because stability is what keeps impedance and phase where the simulation put them.
Its dissipation factor sits near 0.02, roughly five times the practical high-frequency ceiling. At GHz-class frequencies that loss becomes significant attenuation and heat, which is why FR-4 works for digital but fails RF budgets.
At high frequency, skin effect pushes current to the conductor surface, so roughness there acts like extra series resistance. Ultra-low-profile foil is one of the cheapest ways to buy back insertion loss in millimeter-wave designs.
Impedance simulation on the real stack-up, followed by thermal cycling, damp-heat aging and high-frequency signal testing on actual coupons. Datasheet numbers describe a lab sample; validation describes your production lot.
Send your frequency band and reliability requirements and we will shortlist laminates against the indicators — with stock and lead time included.
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