Gold Surface Finishes for Hardware, RF, PCB & Microassembly Engineers
Gold surface finishes can be extremely confusing:
ENIG, plating gold, nickel-gold, immersion gold, thick gold, thin gold, palladium-gold, DIG…
One tiny difference in name leads to huge gaps in cost, reliability, high-frequency performance, solderability, and bondability.
I will break down every finish clearly:
Name → Structure → Process → Pros & Cons → Applications → Restrictions → Cost.
After reading, you will never choose wrong again.
- Two Basic Categories of Gold Finishes
All gold processes fall into just two types:
1. Chemical Gold (ENIG / Immersion Gold)
No electricity, chemical deposition only
→ Excellent flatness, ideal for BGA.
2. Electroplated Gold (Hard Gold / Soft Gold)
Electroplating with current
→ Thick, wear-resistant, suitable for bonding & contacts.
Nickel-gold, palladium-gold, thick gold, thin gold are all sub-types.
- ENIG (Electroless Nickel Immersion Gold)
Common names: Immersion Gold, ENIG, Chemical Gold, Nickel-Gold
Structure
Cu → Electroless Ni (3–5 μm) → Au (0.05–0.1 μm)
Key Features
- Extremely flat, perfect for BGA / QFN / fine-pitch ICs
- Stable soldering, low voiding
- Long shelf life
- Medium cost, mainstream for mass production
Critical Disadvantage (For RF & High Frequency)
CONTAINS NICKEL.
Nickel is ferromagnetic (μr ≈ 100–600), conductivity only 1/4 of copper.
At high frequency, skin effect causes signals to penetrate thin gold into the nickel layer, causing:
- Hysteresis loss
- Eddy current loss
Loss explodes at higher frequencies.
RF / mmWave boards MUST avoid ENIG.
Performance Data (Microstrip)
- 1 GHz: Insertion loss 40–80% higher than copper
- 10 GHz: Loss 100–200% higher
- 28 GHz / 77 GHz: Loss 3–5x higher
Applications & Restrictions
- Use: Consumer electronics, BGA, high-volume production
- Avoid: RF, high-frequency, mmWave, connectors, gold fingers
- ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold)
Common names: Palladium Gold, ENEPIG, Black-Pad-Free ENIG
Structure
Cu → Ni → Pd (0.05–0.1 μm) → Au
Key Features
- Eliminates black pad issues
- Extremely high reliability
- Cost: 30–50% higher than ENIG
RF Performance
Still contains nickel → still very high RF loss.
Only reliability improves; RF performance does NOT.
Applications & Restrictions
- Use: Automotive, medical, military, high-reliability
- Avoid: RF, high-frequency boards
- Electroplated Gold (Plated Gold, Thick Gold, Thin Gold)
Common names: Gold Plating, Electroplated Gold
Standard Structure
Cu → Electroplated Ni (5–8 μm) → Electroplated Au (0.1–3 μm)
Important Fact:
All standard PCB electroplated gold contains nickel.
Nickel is required for adhesion and as a barrier layer.
Gold without nickel is DIG, not plating gold.
Thick Gold vs Thin Gold
- Thin Gold: 0.1–0.3 μm, low cost, test points, simple gold fingers
- Thick Gold: 0.5–3 μm, used for wire bonding
Hard Gold vs Soft Gold
- Hard Gold (Co-doped): Wear-resistant → connectors, contacts
- Soft Gold (Pure): For wire bonding → die bonding
Why Plated Gold Is Bad for RF?
NOT the gold — THE NICKEL.
Even thin gold still has a nickel layer, causing massive RF loss.
Why Thick Gold Is Bad for SMT?
- Poor wettability
- Forms brittle Au-Sn IMCs
- Poor flatness → BGA head-in-pillow issues
Thick gold = for bonding only, NOT soldering.
Recommended for RF/Microassembly
- Bonding pads: Local thick gold plating
- RF traces: NO gold plating — use OSP, Immersion Ag, or DIG
- If full-board plating is mandatory: use ultra-thin gold (0.05–0.1 μm)
- DIG (Direct Immersion Gold) — RF-Specialized
Common names: DIG, Nickel-Free Gold, Pure Immersion Gold
Structure
Cu → Au
NO NICKEL, NO PALLADIUM.
Features
- Best RF / mmWave performance
- Good flatness
- Very thin gold (0.02–0.05 μm)
- Not bondable, not for contact/switch
Applications & Restrictions
- Use: RF, microwave, mmWave, high-speed transmission lines
- Avoid: Wire bonding, connectors, high-reliability harsh environments
- Other Common Surface Finishes
OSP (Organic Solderability Preservative)
- Cu → organic coating
- Nickel-free, lowest cost, excellent RF performance
- Use: RF boards, low-cost, BGA
- Avoid: Frequent contact / mating
HASL (Hot Air Solder Leveling)
- Cu → Sn
- Low cost, good solderability, poor flatness
- Avoid: BGA, RF / high frequency
Immersion Silver
- Cu → Ag
- Nickel-free, excellent RF performance
- Use: RF, high-frequency PCB
- Avoid: Harsh environments (tarnish risk)
Immersion Tin
- Cu → Sn
- Nickel-free, flat, good solderability
- Use: Low-cost high-frequency boards
- Avoid: Extreme high-reliability
- Full Comparison Table (Copy-Paste Ready)
Surface Finish Common Name Structure Nickel? Flatness Solderability Bondability RF/HF Performance Typical Applications Restrictions
ENIG Immersion Gold Cu→Ni→Au Yes Very High Excellent No Poor (3–5x loss) BGA, Consumer Electronics RF, mmWave
ENEPIG Palladium Gold Cu→Ni→Pd→Au Yes Very High Excellent No Poor (Same as ENIG) Auto, Medical, Military RF, High Frequency
Electroplated Gold Plated Gold Cu→Ni→Au Yes Medium Poor Fair Poor Bonding, Gold Fingers Full-board, BGA
Thick Plated Gold Thick Gold Cu→Ni→Thick Au Yes Medium Very Poor Excellent Poor Wire Bonding RF Traces, Solder
Thin Plated Gold Thin Gold Cu→Ni→Thin Au Yes Medium Poor Fair Poor Test Points, Fingers RF Traces
DIG Direct Immersion Gold Cu→Au No High Good No Best (Near Cu) RF, mmWave Bonding, Mating
OSP Anti-Oxidation Cu→Organic No Very High Good No Best RF, Low Cost Contact, Mating
Immersion Ag Immersion Silver Cu→Ag No High Good No Best RF, High Frequency Harsh Environment
HASL Spray Tin Cu→Sn No Low Good No Poor Power, Through-Hole BGA, High Frequency
Immersion Sn Immersion Tin Cu→Sn No High Good No Excellent Low-Cost HF High-Reliability
- Final Summary (Must Remember)
1. ENIG = Nickel-Gold → contains Ni → NOT for RF
2. ENEPIG = more reliable, but still Ni → still bad for RF
3. Electroplated Gold = always Ni → high RF loss
4. Thick Gold = bonding only; Thin Gold = contacts only
5. DIG is the only Ni‑free gold → best for RF
6. Standard RF/Microassembly stackup:
- RF Traces: DIG / Immersion Ag / OSP
- Bonding Pads: Local thick gold plating
7. Plated Gold ≠ DIG - Plated: Ni, thick, bondable
- DIG: No Ni, thin, RF-only
8. Nickel destroys high-frequency performance
10 GHz: loss doubles
mmWave: loss 3–5x higher