A layer-by-layer breakdown of the four-layer ENIG RO4350B construction — stackup, thermal behavior, and the specification habits that keep spins predictable.
A four-layer stack on RO4350B — signal, ground, ground or power, signal — covers the majority of production RF designs: antenna feed networks, radar IF modules, and power amplifier stages up to the point where heat, not layers, becomes the constraint. The construction is thick enough to shield both signal layers and thin enough to laminate predictably, and the hydrocarbon-ceramic resin system processes on standard FR-4 lines, which keeps the four-layer economics genuinely competitive.
A representative build pairs two 20 mil (0.508 mm) RO4350B cores with RO4450B prepreg (Dk 3.30 ±0.05) bonding them around two copper foils — inner layers typically 1 oz for power handling, outers 0.5 oz finished to 1 oz. The impedance reference planes sit directly beneath each signal layer, so trace geometries stay wide and forgiving: a 50 Ω line on 20 mil 4350B comes out near 28 mils, wide enough that etch tolerance consumes a small fraction of the impedance budget. The full parameter reasoning follows our controlled-impedance design workflow.
| Layer | Material | Thickness | Role |
|---|---|---|---|
| L1 | RO4350B core, 0.5 oz + plating | — | RF signals / components |
| L2 | RO4350B core, 1 oz | 0.508 mm | Ground reference |
| L3 | RO4450B prepreg + core, 1 oz | 0.508 mm | Power / ground |
| L4 | RO4450B prepreg, 0.5 oz + plating | — | RF signals / routing |
On a 4350B panel, ENIG contributes a flat, aluminum-wire-bondable surface and oxidation-free pads that survive multiple reflow cycles — useful when the same board carries both RF connectors and fine-pitch control ICs. Gold thickness is held at 0.05–0.10 μm, thin enough that nickel’s higher resistivity stays outside the significant field for most microstrip at these dielectric heights. The finish trade space is compared directly in our surface finish comparison; on four-layer 4350B the assembly-yield argument usually settles it.
RO4350B’s Z-axis CTE of roughly 40 ppm/°C below Tg keeps plated through-holes mechanically safe across −50°C to +150°C cycling, and Tg near 280°C means the stack does not soften during lead-free assembly. Salt-spray exposure of the ENIG surface above 95% conductivity retention after 96 hours is the kind of evidence a qualification file should contain — the reliability-test methods behind such numbers are the subject of our HALT and thermal shock analysis.
Three deployment families dominate. 5G massive MIMO feed networks use the four-layer symmetry for consistent phase across ports. 77 GHz automotive radar IF modules value the low loss and the automotive-grade process pedigree. Military frequency-hopping radio power stages take the construction with heavier copper — the high-power variant of the same platform is analyzed in our 4350B power RF article. Across all three, IPC-6012 Class 3 workmanship and 0.2 mm microvia capability are reasonable baseline expectations.
Two specification habits prevent most four-layer 4350B surprises. First, lock the stackup in the fabrication drawing — core and prepreg thicknesses, copper weights, and the impedance table — rather than letting the shop infer them; the Rogers manufacturing process guide shows how each choice propagates. Second, demand per-lot impedance coupons with TDR traces. With those two controls, the construction delivers exactly what its parameters promise, spin after spin.
Frequently Asked Questions
It balances shielding and cost: both signal layers get solid reference planes, the stack laminates predictably, and the material processes on standard FR-4 lines.
RO4450B prepreg, Dk 3.30 ±0.05, whose dielectric constant is matched to the core for predictable impedance through the bond layers.
At the standard 0.05–0.10 μm gold thickness, nickel's resistivity stays outside the significant field for most microstrip geometries on 20 mil cores.
Yes — the Z-axis CTE near 40 ppm/°C and Tg around 280°C hold up across automotive thermal cycling, and IPC-6012 Class 3 workmanship is standard.
Send your layer count, impedance table and connector footprint and we will return a stackup review with prepreg and copper recommendations.
Get a Fast Quote Fast Reply on WhatsApp