Electroless Nickel / Immersion Gold — thickness specs, plating process, design rules, cost and selection tools for hardware engineers.
ENIG (Electroless Nickel / Immersion Gold) is a two-layer metallic surface finish plated over exposed copper to keep pads solderable, flat and corrosion-free.
An ENIG PCB carries 3–6 µm of electroless nickel–phosphorus deposited on the bare copper of every exposed pad, via and hole wall. The nickel is the actual solderable surface and acts as a diffusion barrier that stops copper from migrating into the joint. A final 0.05–0.10 µm layer of 99.9% pure immersion gold caps the nickel, protecting it from oxidation until the moment of soldering. The gold deposit is self-limiting: the displacement reaction stalls once the nickel is fully covered, which is why ENIG thickness is so repeatable.
Because the finish is chemically grown rather than melted on, ENIG pads are flat to within fractions of a micron — no solder domes, no thickness variation across the panel. That coplanarity, plus a 12-month shelf life, makes ENIG the default finish for fine-pitch BGAs, QFNs, HDI boards and anything with via-in-pad construction.
Manufacturing deep-dive: ENIG PCB — fabrication capabilities, stack-ups and instant quoting at PCBSync.
Tap a layer — not to scale (Au shown ~40× thicker)
0.05–0.10 µm of 99.9% pure immersion gold. Protects the nickel from oxidation and dissolves into the solder during reflow. Deposited by galvanic displacement — self-limiting, so thickness is highly repeatable.
ENIG is a fully chemical (electroless) line — no plating current, no bus bars. Every exposed copper feature plates to the same thickness, even electrically isolated pads.
Acid cleaner strips oils, fingerprints and oxide from the exposed copper so subsequent baths wet evenly.
A controlled etch removes ~1 µm of copper, exposing a fresh, uniformly roughened surface for adhesion.
A palladium catalyst seeds the copper surface — the initiation sites the electroless nickel reaction needs to start.
Autocatalytic Ni–P deposition (7–9% phosphorus) builds the 3–6 µm barrier and solderable layer without any external current.
Galvanic displacement swaps surface nickel atoms for 99.9% gold. The reaction self-limits at 0.05–0.10 µm once nickel is covered.
XRF measures Ni and Au thickness on coupons and live pads; solderability is verified per IPC-4552 acceptance criteria.
Engineer's note: because deposition is purely chemical, ENIG coverage does not depend on pad connectivity or location in the panel — a major advantage over electrolytic finishes. Ask your fabricator for the XRF thickness report with each lot; reputable shops provide it by default.
IPC-4552 (Rev B) is the industry specification for ENIG. Put it on your fab drawing and these are the numbers your supplier must hit.
| Layer | Minimum | Typical Range | Function |
|---|---|---|---|
| Immersion Gold (Au) | 0.04 µm / 1.6 µin | 0.05–0.10 µm / 2–4 µin | Oxidation barrier over nickel; dissolves into solder at reflow. |
| Electroless Nickel (Ni–P) | 3 µm / 118 µin | 3–6 µm / 118–236 µin | Solderable surface and Cu diffusion barrier; forms the Ni₃Sn₄ joint. |
Eight rules that separate a clean ENIG build from a respin — collected from fab DFM feedback and assembly-floor failures.
"ENIG" alone is ambiguous. Specify IPC-4552, latest revision and require an XRF thickness report per lot. It costs nothing and makes thickness disputes objective.
ENIG is the safest finish for ≤0.5 mm pitch BGA/CSP and via-in-pad. Pair it with filled-and-capped vias (IPC-4761 Type VII) for a truly flat land pattern.
Nickel is ferromagnetic and lossy at RF: skin-effect current rides in the Ni layer and increases insertion loss. For microwave traces, consider immersion silver, OSP, or selective ENIG that keeps Ni off critical lines.
Specify medium-phosphorus nickel (7–9% P), keep gold ≤0.10 µm, and use fabricators with tight bath control. For safety-critical electronics, step up to ENEPIG.
Aluminium wedge bonding works on standard ENIG. Gold ball bonding does not — the Au layer is too thin. Use ENEPIG or selective soft electrolytic gold for Au-wire COB.
0.05–0.10 µm of gold wipes off in a few insertions. Edge connectors and switch contacts need selective hard gold ≥0.76 µm (30 µin) — mixing ENIG pads with hard-gold fingers on one board is routine.
Solder on ENIG forms Ni₃Sn₄ intermetallic, more brittle than the Cu₆Sn₅ formed on bare-copper finishes. For portable devices, add corner bond or underfill on large BGAs and validate with drop testing.
Sealed ENIG boards hold solderability for 12 months. Handle with gloves (fingerprints etch gold), keep packaging sealed until kitting, and unlike OSP or ImAg no pre-assembly finish bake is needed.
Hyper-corroded nickel under the gold — joints wet fine, then fracture
SURFACE FINISH: ENIG (ELECTROLESS NICKEL / IMMERSION GOLD) APPLY PER IPC-4552, LATEST REVISION. ELECTROLESS NICKEL: 3–6 µm [118–236 µIN] IMMERSION GOLD: 0.05–0.10 µm [2–4 µIN]; 0.04 µm [1.6 µIN] MIN SOLDERABILITY PER J-STD-003. SUPPLIER SHALL PROVIDE XRF THICKNESS REPORT WITH EACH LOT.
ENIG costs more than HASL because you are buying real gold — but on a populated assembly the finish is a small slice of total cost, and one fine-pitch respin erases years of "savings".
What moves the price: gold spot price, total plated surface area, panel utilisation, order quantity and region. As a planning rule of thumb, an ENIG bare board runs about 1.2–1.45× the cost of the same board in HASL. The premium shrinks as a percentage on multilayer and HDI boards where laminate and drilling dominate.
INDEX VS. SnPb HASL BASELINE = 1.00 · TYPICAL BARE-BOARD RANGES — ACTUAL QUOTES VARY WITH GOLD SPOT, AREA, QTY & REGION. ALWAYS CONFIRM WITH YOUR FABRICATOR.
Side-by-side against the seven finishes you will actually be quoted. Then let the selector below score them against your requirements.
| Property | ENIG | HASL (SnPb) | LF-HASL | OSP | Imm. Silver | Imm. Tin | ENEPIG | Hard Gold |
|---|---|---|---|---|---|---|---|---|
| Coplanarity / flatness | Excellent | Poor | Fair | Excellent | Excellent | Excellent | Excellent | Excellent |
| Fine pitch ≤0.5 mm | ✓ | ✗ | ✗ | ✓ | ✓ | ✓ | ✓ | ✓ |
| Shelf life | 12 mo | 12 mo | 12 mo | 3–6 mo | 6–12 mo | 3–6 mo | 12 mo | 12+ mo |
| Reflow cycles | 3+ | 2–3 | 2–3 | 1–3* | 2–3 | 1–2 | 3+ | 3+ |
| Al wire bonding | ✓ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓ | △ |
| Au wire bonding | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓ | △ |
| Wear / sliding contacts | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | △ | ✓ |
| RF > 10 GHz | △ | △ | △ | ✓ | ✓ | △ | △ | △ |
| RoHS / lead-free | ✓ | ✗ | ✓ | ✓ | ✓ | ✓ | ✓ | ✓ |
| Cost index (vs HASL) | 1.2–1.45 | 1.00 | 1.0–1.1 | 0.85–0.95 | 1.1–1.25 | 1.05–1.2 | 1.45–1.75 | 1.5–2.2 |
* High-temperature OSP grades survive 3 cycles. △ = marginal / conditional — verify against your exact requirement.
Anywhere flatness, fine pitch and long-term solderability matter more than the last few cents of bare-board cost.
HDI stack-ups, 0.35–0.5 mm pitch BGAs and via-in-pad — the territory ENIG was made for.
Implantables and diagnostics value ENIG's corrosion resistance, reliability and long storage life.
Thermal cycling, vibration and decade-long service lives demand a stable, inspectable finish.
ADAS, ECUs and sensor modules built to AEC-Q / IATF expectations under hood-level temperatures.
Light-duty carbon-key contacts and probe targets — gold stays conductive and oxide-free.
Bare die attached with aluminium wedge bonds go straight onto standard ENIG pads.
ENIG stands for Electroless Nickel Immersion Gold — a two-layer metallic surface finish in which 3–6 µm of electroless nickel is plated over the copper pads and sealed with a thin 0.05–0.10 µm layer of immersion gold.
Per IPC-4552, electroless nickel is 3–6 µm (118–236 µin) and immersion gold is a minimum of 0.04 µm (1.6 µin), with most fabricators targeting 0.05–0.10 µm (2–4 µin).
ENIG typically adds roughly 20–45% to bare-board fabrication cost versus HASL. The exact premium depends on board area, order volume and the current gold price, and it is usually a small share of total assembled-product cost.
Black pad is hyper-corrosion of the nickel layer that occurs during the immersion-gold displacement reaction. It appears as a dark, mud-cracked nickel surface and causes brittle solder joints. It is prevented by tight control of nickel bath chemistry, keeping gold at or below 0.10 µm, and specifying IPC-4552 with XRF reports.
Yes. ENIG is completely lead-free and is one of the most widely used RoHS-compliant PCB surface finishes.
About 12 months before assembly when stored sealed in normal warehouse conditions, because the gold layer prevents the nickel and copper underneath from oxidizing.
ENEPIG inserts a 0.05–0.1 µm electroless palladium barrier between the nickel and gold. This makes the finish gold-wire-bondable and essentially immune to black pad, at roughly 15–25% higher cost than ENIG.
Generally yes up to roughly 5–10 GHz. Above that, the ferromagnetic nickel layer adds measurable conductor loss, so mmWave designs often use immersion silver, OSP, or a selective ENIG-free finish on critical RF traces.
