At the 76–81 GHz allocation used by long-range automotive radar, a radome skin contributes both reflection loss and transmission attenuation. A nonpolar low-Dk/Df hydrocarbon resin is melt-compounded into a 25 wt% short-glass PBT/PPE radome compound at 6–12 wt% to lower the dielectric constant without introducing the polar groups that raise dissipation factor under high-humidity ageing. Compounding takes place on a co-rotating twin-screw extruder with 40:1 L/D, ten-barrel configuration, side-fed glass at barrel 6, vacuum venting at -0.08 MPa, melt temperature 270–285 °C, and screw speed 350–450 rpm. Pellet pre-drying at 105 °C for 4 h to a moisture content below 0.02 wt% is required before injection moulding. Moulded plaques are tested at 79 GHz in a WR-12 waveguide fixture per ASTM D5568-22; the target for the compound is Dk ≤ 3.20 and Df ≤ 0.0035 after 168 h at 85 °C/85% RH. The hydrocarbon resin reduces melt viscosity enough to allow 2.5 mm wall sections at holding pressure 45–55 MPa, but strand pelletising shows static build-up above 8 wt% resin, requiring an ionised air bath at the pelletiser inlet. Above 12 wt% resin, notched Charpy impact falls below 4.0 kJ/m² per ISO 179-1:2020, and HDT at 1.8 MPa drops below 180 °C per ISO 75-2:2013. The moulded components are therefore specified for front bumper radomes and grille-integrated radar brackets where creep under warm bonnet airflow must remain below 0.5 mm after 500 h at 90 °C. Table 1 summarises the dielectric and mechanical response across the formulation gradient.

What Limits Dk Homogeneity in Hydrocarbon-Resin-Based Copper-Clad Laminates at 79 GHz?

Radar module antennas fabricated from resin-rich prepregs require in-plane Dk variation below ±0.02 at 79 GHz to keep patch resonance within 200 MHz. The low-Dk/Df hydrocarbon resin is used as a co-binder at 18–25 phr of the laminate resin-solids fraction, combined with fused silica at 25–35 wt% to control CTE. A peroxide cure package at 1.5–3.0 phr dicumyl peroxide and a triallyl cyanurate co-crosslinker at 5–12 phr of resin solids are blended into a MEK/toluene varnish. Varnish is cast onto 1035 woven quartz glass; B-staging proceeds through a vertical treater with zone temperatures 90 °C, 130 °C, and 160 °C until residual solvent is below 0.3 wt%. Lamination is carried out at 190–210 °C and 2.0–3.5 MPa for 120 min, followed by post-cure at 220 °C for 60 min. Dk and Df are measured at 10 GHz by split post dielectric resonator per IEC 61189-2-721:2015; 79 GHz verification is performed with a WR-12 waveguide per ASTM D5568-22. Copper peel strength is evaluated after 288 °C/10 s solder stress per IPC-TM-650 2.4.8. Production-scale issues include microvoid formation at peroxide loadings above 2.5 phr and silica agglomeration when mixing speed exceeds 800 rpm. The laminate stack is specified for 76–81 GHz MIMO antenna boards and radar module feedlines, where the resin-rich layer must not exceed 100 µm to avoid excessive Z-axis thermal expansion. Published data for this specific resin-filler combination is limited; therefore initial qualification must include a 1000-cycle thermal shock test at -40 °C to +125 °C with Dk drift below 0.03.

For decorative radar-transparent front fascia emblems, the moulded component must maintain a bright metallic appearance without creating the conductive continuity that backscatters 77 GHz energy. A PC or PC/ABS base is modified with 8–15 wt% low-Dk/Df hydrocarbon resin to reduce polar carbonate dipole density and moisture uptake. Metallisation is replaced by a discontinuous aluminium pigment at less than 0.5 vol%, with flake diameter below 10 µm and spacing above 100 µm, or by an indium tin oxide sputtered film with sheet resistance controlled above 1 kΩ/sq. Injection-compression moulding is preferred over straight injection to reduce birefringence in the 2.5 mm wall; melt temperature is held at 260–280 °C, mould temperature at 80 °C, compression stroke 0.5–1.0 mm, and holding pressure 50–60 MPa. Coating adhesion after plasma pre-treatment is tested by ISO 2409 cross-cut tape pull, and weathering is assessed by SAE J2527 with 2500 kJ/m² UV exposure. The hydrocarbon resin must be fully saturated because residual aromatic unsaturation yellows below 80 °C and raises Df above 0.004 after UV ageing. Above 15 wt% resin, pencil hardness falls below F per ASTM D3363 and orange peel increases during sputter deposition. Terminal parts include front fascia emblems, badgeless radomes, illuminated ADAS logos, and painted radar covers with a 12 dB two-way insertion-loss budget.

PPS-Based ADAS Sensor Housing and Low-Warpage Collimator Components

Where the sensor housing also functions as a dielectric window, PPS is selected for dimensional stability, but its crystalline morphology and high mineral loadings increase dielectric anisotropy. A low-Dk/Df hydrocarbon resin at 5–10 wt% is melt-compounded with a 40 wt% glass-fibre/mineral PPS compound. The resin lowers melt viscosity by 15–25% measured per ISO 1133-1:2022 at 315 °C and 5 kg, allowing fill of 0.8 mm wall ribs without flash. Moulding uses an 1800 kN injection machine, barrel temperature profile 300–340 °C, mould temperature 130–150 °C, screw speed 80 rpm, and injection speed 80 mm/s. Dielectric performance at 79 GHz is measured by ASTM D5568-22; the acceptance window for a housing window is Dk ≤ 3.3 and Df ≤ 0.004 after 1000 h at 85 °C/85% RH. The hydrogenated resin only is permitted because standard C9 resin exhibits TGA onset below 380 °C per ISO 11358-1, while the housing must tolerate 150 °C continuous under-bonnet exposure. Warpage is assessed by 3D optical scanning per ISO 1101 after 24 h at 23 °C/50% RH; the housing datum plane must remain within ±0.3 mm across a 150 mm span. Table 2 lists the qualification matrix applied to moulded PPS sensor housings.

When Hydrocarbon Resin Shifts the Mould Shrinkage Balance in a Hidden Radar Bracket

Hidden radar brackets behind the front fascia require post-mould dimensional tolerance to maintain the antenna plane orthogonal to the longitudinal axis within ±0.3°. A mineral-filled polypropylene compound containing 10–15 wt% low-Dk/Df hydrocarbon resin reduces anisotropic mould shrinkage from 1.1% in the flow direction to 0.7% and from 0.9% transverse to 0.6% as measured per ISO 294-4. Sequential valve-gate injection is used on a 2500 kN machine with gate open delays of 0.5–1.0 s to equalise cavity filling pressure; melt temperature is 220–240 °C, mould temperature 40–60 °C, holding pressure 35–45 MPa, and cooling time 25 s for a 2.0 mm nominal wall. After ejection, brackets are fixture-cooled for 60 s to prevent post-mould warpage. Dimensional stability is verified after 500 h at 90 °C by coordinate measurement per ISO 1101; total fixture drift must remain below 0.2 mm. The low-polarity resin also reduces moisture regain to below 0.15 wt% per ISO 62 after 24 h water immersion, limiting dielectric drift in the nearby antenna field. The bracket is not a primary dielectric element, but its Df at 79 GHz is kept below 0.005 to avoid resonance shifts when the radar module is mounted directly to it. Terminal use is hidden radar brackets, antenna carrier frames, and sensor alignment brackets for forward-facing ADAS units.